Glossary

Absolute pressure: pressure referred to a perfect vacuum. It is the sum of gauge pressure and atmospheric pressure.

• Absolute viscosity : the force per unit area required to produce unit relative velocity between two parallel areas of fluid distance apart; also called coefficient of viscosity .
• Absolute zero temperature : the zero point on the Kelvin temperature scale, (= -273.16^oC)
• Absorbent: a material which extracts one or more substances form a liquid or gaseous medium which it contacts and charges physically or chemically, or both (e.g. soled absorbent: Calcium chloride, liquid absorbent: solutions of lithium chloride, lithium bromide, and ethylene glycols).
• Absorber: an object absorbing another object or radiation (e.g. the blackened surface within a solar collector, which absorbs the solar radiation converting it to heat. These surfaces usually absorb solar radiation but do emit a limmited amount of radiative heat because of the so-called spectral selective coatings applied to the surface).
• Absorptance: the ratio absorbed objects or radiation. (For solar energy the ratio of absorbed to incident solar radiation on a surface).
• Absorptance of a glazing material: in this context the absorption of solar energy in the glazing material occurring throughout the thickness of a glazing material. The absorptence is the fraction absorbed related to the incident overall radiation (invoIving multiple reflections). The absorptence is different for different wavelengths (coloured glazing) and angles of incidence. The transmittance + absorptance + reflectance = 1.
• Absorptance of an opaque surface: the ratio of the radiation absorbed at a surface relative to the incident radiation on that surface. The absorptance is different for different radiation waveIengths and angles of incidence. The absorptance of normally incident radiation however is always numerically equal to the emittance of the surface for the same wavelength. 'bIack body' is a theoretical perfect absorber with an absorptance of 1 for all wavelengths: a typical matt black paint has an absorptance of 0.97 for normal soIar radiation and 0.95 for long-wave (thermal) radiation. For any wavelength and angle of incidence, for an opaque surface, absorptance + reflectance = 1.
• Absorption: a process where an object absorbs another object or radiation for example a process whereby a material extracts one or more substances present in an atmospheric or mixture of gases or liquids accompanied by the material's physical and/or chemical changes.
• Absorption Factor: For solar radiation the fraction of solar radiation, transmitted through a glazing system that is absorbed inside the building.
• Adaptive model (adaptive approach): For thermal comfort this is an approach to the study thermal comfort starting from the observation that there are a range of actions which occupants can take to achieve thermal comfort, and that discomfort is caused by constraints imposed on the range of actions by social, physical or other factors (see also Adaptive Principle). Over time this means that people are comfortable at the 'usual' or 'average' temperature which they experience.
• Adaptive opportunity: For thermal comfort: the ability of a building to allow adaptation to broaden the band of temperatures at which the person could be comfortable. Actions might include adding or removing a layer of clothing, changing posture, or adjusting the metabolic rate. Spatial variation of temperature across the room, availability of a desk fan, and the presence of sun shading or openable windows would make the adaptive opportunities still greater. These actions either alter the temperature which is required for comfort, or alter the room temperature towards that which the occupants will find comfortable
• Adaptive Principle: A principle which can be stated as: If a change occurs (in the thermal environment) such as to produce discomfort, people react in ways which tend to restore their comfort.
• Adaptive processes: The range of actions that man takes to achieve thermal comfort.
• Air barrier: for buildings a material carefully installed within a building envelope assembly to minimise the uncontrolled passage of air into and out of a dwelling.
• Air change: One air change is an exchange of one volume of air equal to internal volume of a room or building (see also Air Change Per Hour).
• Air changes Per hour (ach): ventilation unit denoting the number of times a room exchanges its own total air volume with outside air per hour. For the calculation of the air permeability of a room or building this is determined (1) under natural conditions and (2) under a 50 Pascal pressure difference. (The R-2000 Home Program requires a house with 1.5 ach at 50 Pascal).
• Air conditioning: Control of the indoor environment by controlling the thermal characteristics (cleaning, changing the water content but mainly heating and cooling) the air supplied to the room.
• Air leakage: the uncontrolled flow of air for instance through a component of the building envelope, or the building envelope itself, when a pressure difference is applied across the component. Infiltration refers to inward flowing air leakage and exfiltration refers to outward flowing airleakage.
• Air movement: the movement of the air past a point, generally represented as the speed or velocity of the air measured with an anemometer.
• Air permeability: the property of a building component to let air pass when it is subjected to a differential pressure.
• Air pressure: the pressure exerted by the air. This may refer to static (atmospheric) pressure, or dynamic components of pressure arising from air flow, or both acting together.
• Air sealing: the practice of sealing unintentional gaps in the building envelope (from the interior) in order to reduce uncontrolled air leakage.
• Air tightness: the degree to which unintentional openings have been avoided in a building's structure.
• Albedo: The proportion of the global solar radiation that is reflected by the surface of the Earth. Will clearly vary with materials and topography.
• Altitude angle: the angular height of above the horizon, i.e. the solar altitude is the angle between the centre of the sun and the horizon.
• Ambient air: Generally, the air surrounding an object or building.
• Ambient temperature: See: Air Ambient
• Anemometer: Instrument to measure the air speed; Common electrical types are hot wire or and the hot-body anemometers. The principle used is to apply a fixed electrical current to a wire and the voltage over the wire is measured. The current heats the wire. Moving air cools the wire, decreasing the electrical resistance. This change is measured through the voltage across the wire.
• ANSI: American National Standards Institute
• ASHRAE: The American Society of Heating Ventilating and Air-conditioning Engineers, based in Atlanta Georgia, USA.
• ASHRAE scale: a set of seven descriptors of subjective warmth. Usual wording is: Cold (-3), cool (-2), slightly cool (-1), neutral (0), slightly warm (+1), warm (+2), and hot (+3).
• ASHRAE/ANSI Standard 55: A temperature standard proposed by the American Society of Heating, Refrigeration and Air-conditioning Engineers formerly based on the Gagge's Standard Effective Temperature, but due to come in line with ISO7730 (see below) in using Fanger's PMV/PPD.
• Attached sunspace: -solar collector that doubles as useful building space also attached greenhouse, solarium. The term 'attached' specifically implies a space that shares one common wall with the associated building, such as a conservatory.
• Auxiliary heating: the conventional (i.e. non-solar) contribution to the total load (e.g. gas boiler etc.).
• Awning: an exterior, movable and usually flexible sun shading device. Protects by deflecting or diffusing solar radiation at certain angles.
• Azimuth angle: The horizontal angle (in degrees) from the North point measured clockwise. Thus North = 0^o, East = 90^o, South = 180^o etc.

  

• Base temperature: a fixed temperature in the definition of degree-days.
• Beam or direct radiation: the solar radiation directly from the sun without changed angle or diffusion. Instruments for measuring direct radiation require for practical reasons a wider aperture so the measured radiation also contains radiation from the immediate region of the sky around the sun (called circum solar; see also diffuse radiation).
• Bedford scale: A set of seven descriptors of subjective thermal response from Much Too Warm (7), Too Warm (6), Comfortably Warm (5), Comfortable, neither warm nor cool (4), Comfortably Cool (3), Too Cool (2), Much Too Cool (1). This differs from the ASHRAE scale in defining the central three categories as Comfortable.
• Black body: a perfect absorber and emitter of electromagnetic radiation (light, heat etc). A black body absorbs all radiation without any reflection (see also absorption). A cavity is a perfect black body. Lampblack is close to a black body , while aluminium (polished) is a poor absorber and eminer of radiation.
• Bright sunshine: sunshine above a certain level sufficient to be registered on a Campbell-Stokes sunshine recorder .The threshold is imprecisely defined and varies according to the type of recorder, but generally refers to sunshine bright enough to cast a shadow. Burning typically starts when the beam intensity is 200W /m².g
• BSI: British Standards Institute
• Building orientation: Orientation of tihe building, generally used to refer to solar orientation which is the siting of building with respect to solar access. Although any building will have different orientations for its different sides, the orientation can refer to a particular room, or to the most important facade of the building.

  

• Calorific value: the energy content per unit mass, per unit volume or released if reacting with usualy oxigen (burned).
• Candela (cd) : SI (Standard International) unit of luminous intensity: the candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and has a radiant intensity in that direction of 1/683 watt per steradian. (16th General Conference of Weights and Measures, 1979) 
• Candela per square metre (cd/m²): SI unit of luminance. (Note: this unit was sometimes called the nit [nt] (name discouraged). Other units of luminance: 10-4 metric, non-SI: Lambert (L)
  
• Casual gain: see internal gains
• CEN: The European Standards Institution
• Chimney effect: see stack effect.
• CIBSE: The Chartered Institution of Building Services Engineers.
• CIE Standard Overcast Sky: completely overcast sky for which the ratio of its luminance Ly in the direction at an angle X above the horizon to its luminance Lz at the zenith is given by the relation Ly=Lz(1+2sinX)/3
• CIE standard sources: artificial sources specified by the CIE, whose radiations approximate CIE standard illuminants A, B and C, (see CIE Publication N015).
• Clear sky (Standard): cloudless sky for which the relative luminance distribution is described in CIE Publication N022 (1973).
• Clear Sky: Cloudless Sky - clear sky in this context has two separate meanings. In the radiation Tables clear sky refers to days producing the mean maximum monthly daily global radiation on a horizontal surface. These days on average contain some cloud but the precise cloud amount is not known. In the temperature Tables, the term clear skies refers to summaries of measured temperature data for days when the mean total amount of cloud during hours of daylight was less then 2/8.Night-time temperature data for such days are related to the same 'clear' days i.e. night-time cloud cover was not taken into account.
• Clerestory: a window that is placed vertically (or near vertical) in a wall above one's line of vision to provide natural light in a building.
• Climate chamber: A chamber usually in a laboratory in which the environmental conditions can be changed by the experimenter, used to investigate the effects of the thermal environment on subjects.
• Climate control : The control of the indoor climate.
• Clo unit: a unit for the insulating effect of clothing on a person. 1 clo = 0.155 (K.m²)/W.
• Clothing insulation: The effective insulation of clothing worn characterized as a single layer covering the whole body surface; measured in Clo Units or K.m²/W
• Clothing surface temperature: Average temperature of the surface of the clothes for use in heat exchange calculations.
• Colour rendering: Effect of (not nescessarily white) light on the colour appearance of objects compared to their colour appearance under a reference illuminant.
• Colour Rendering Index (R): measure of the degree to which the psychophysical colour of an object illuminated by the test illumihant conforms to that of the same object illuminated by the reference illuminant, suitable allowance having been made for the state of chromatic adaptation.
• Comfort chart: a chart showing effective temperatures with dry-bulb temperatures and humidities (and sometimes air movement), by which the effects of various thermal conditions on human comfort may be compared.
• Comfort line: a line on the comfort chart showing relation between the effective temperature and the percentage of adults feeling comfortable.
• Comfort temperature (neutral temperature): 1)The temperature (or environment) judged by a population to be neutral on the ASHRAE scale, or Comfortable, Neither Warm Nor Cool on the Bedford scale: assumed by most workers in the field to be the desired temperature or
• 2) the temperature at which the greatest number of people feel comfortable
• Comfort vote: The subjective response given by a subject on a comfort scale such as the ASHRAE or Bedford scales (see above).
• Comfort zone: The range of temperature within which a subject will feel comfortable, though not necessarily neutral.
• Condensation: condensed liquid - in this context beads, drops or a film of water (or frost) that accumulates on the inside of facade elements, roofs or floors (usually on windows) when warm, moisture air from the interior reaches a point where the temperature (the 'dew point') no longer permits the air to sustain the moisture it holds.
• Conduction: the heat moving from a warmer to a colder region in the same substance without mass transfer, this type of heat transfer depends on the thermal conductivity of the material [measured in W/m².K].
• Conductive heat transfer : Process of conduction through a material. Materials like copper or silver are good conductors.
• Constraints: factors in the physical or social environment which prevent people from taking actions to achieve thermal comfort.
• Controlled ventilation: ventilation brought about by mechanical means through pressure differentials induced by the operation of a fan.
• Convection: heat transfer between a surface and adjacent fluid (usually air or water) and by the flow of fluid from one place to another. See also Forced convection, Natural convection.
• Convective heat transfer : Process of heat transport by convective flow. With free convection the flow is caused by temperature differences is the liquid or gas. For example the air inside double glazing transports heat due to natural convection: the glazing layer on the inside is warmer than the one on the outside; the air inside the cavity moves because of this temperature difference and transports the heat. With forced convection energy is transferred due to a mechanically driven gas flow or liquid flow.
• Convector: an agency of convection. In heat transfer, a surface designed to transfer its heat to a surrounding fluid largely or wholly by convection. The heated fluid may be removed mechanically or by natural convection (gravity convector). Such a surface may or may not be enclosed or concealed. Apparatus for heating, in which the heat is passed on to the room by convection, due to natural air movement sometimes helped by a fan.
• Cooling: The process of active heat extraction for the benefit of climate control. see also direct cooling, evaporative cooling, indirect cooling, regenerastive cooling, surface cooling.
• Cooling effect, sensible: difference between the total cooling effect and the dehumidifying effect, usually in watts.
• Cooling effect, total: difference between the total enthalpy of the dry air and water vapour mixture entering the cooler per hour and the total enthalpy of the dry air and water vapour mixture leaving the cooler per hour , expressed in watts.
• Cooling element: heat transfer surface containing refrigeration f1uid where refrigerating effect is desired.
• Cooling load: The thermal energy extracted from a building to maintain a pre-determined indoor setpoint temperature.
• Cooling medium: any substance whose temperature is such that it is used, with or without a change of state, to lower the temperature of other bodies or substances.
• Cooling range: in a water cooling device, the difference between the average temperatures of water entering or leaving the device.
• Cooling water: water for condensation of refrigerant; condenser water
• Core temperature (deep body temperature): The temperature of the internal organs of the body, especially the brain, on the basis of which the thermoregulatory system of the body is controlled.
• Correlation:: An indication of the strength of the relationship between two variables characterised by the correlation coefficient (r). The value of r varies between zero (no relationship) and one (linear or exact relationship).
• Corresponding values: values of parameters corresponding to other parameters.
• Counterflow: flow in the opposite direction in heat exchange between two fluids; the coldest portion of one meeting the coldest portion of the other.
• Critical temperature: the saturation temperature corresponding to the critical state of the substance at which the properties of the liquid and vapour are identical.
• Cross ventilation: flow of air through a building caused by wind-generated pressure drop across it. 

Data-logger: An instrument which accepts and stores environmental or other data electronically ready for subsequent downloading into a computer for analysis.

Data set: A set of subjective and physical data obtained simultaneously in a comfort survey.

Dampproofing: the process of coating the interior or exterior of a foundation wall, f1oor, etc., with bituminous emulsions or plastic cements. The purpose of dampproofing is to prevent or interrupt the capillary draw of moisture into the wall or floor system and to the interior of the foundation.

Daylight: visible part of global solar radiation. (Note: when dealing with the thermal effects of optical radiations, this term is commonly used for radiations extending beyond the visible region of the spectrum.)

Daylight Factor : ratio of the illuminance at a given point on a given plane due to the light received directly or indirectly from a sky of assumed or known illuminance distribution, to the illuminance on a horizontal plane due to an unobstructed hemisphere of this sky. The contribution of direct sunlight to both illuminances is excluded. (Notes: (1) glazing, dirt effects, etc. are included. (2) when calculating the lighting of interiors, the contribution of direct sunlight must be considered separately.)

Declination of sun: the angle of the sun above or below the equatorial plane. This is plus if north of the plane and minus if below and varies day by day throughout the year from +23.47^o to June 21 to -23.47^o to December 21.

Degree days for cooling calculations: the sum, over a stated period of days, e.g. month, of positive values of the arithmetic difference between a stated reference 'base temperature' and the daily mean dry bulb air temperature. The base temperature for assessing cooling requirements is normally set above that used for assessing heating requirements to allow for the fact that acceptable comfort temperatures in summer are higher than those adopted for winter heating assessments.

Degree days for heating calculations: the product of the number of degrees below a given base temperature and the number of days when that difference occurs. The base temperature is usually defined between 15.5^o to 21^oC. Different base temperatures are required to estimate heating demands, frost protection, and for relating insulation standards to internal gains.

Dehumidification: (1) condensation of water vapour from air by cooling below the dew point, (2) removal of water vapour from air by chemical or physical methods.

Dehumidifier, surface: an airconditioning unit, designed primarily for cooling and dehumidifying air through the action of passing the air over wet cooling coils.

Dehumidifying effect, air cooler: product of the weight of moisture condensed in the cooler by the constant 1060.

Dehydration: (1) removal of water vapour form air by the use of absorbing or adsorbing materials, (2) removal of water from stored goods.

Density (D): the mass of a unit of volume of material. [Kg/m³]

Descriptive scale: A subjective scale in which the subject is asked to choose between a given list of descriptions in casting a comfort vote

Design heat losses: a term expressing the total predicted envelope losses over the heating season for a particular house design in a particular climate.

Dewpoint: the temperature at which a given air/water vapour mixture is saturated with water vapour (i.e. 100% relative humidity). Consequently , if air is in contact with a surface below this temperature, condensation (dew) will form on the surface.

Diffuse lighting: a form of lighting where the light is received from a number of different directions such that a precise shadow is not formed.

Diffuse solar radiation: the component of solar radiation that has been scattered by atmospheric molecules, aerosol particles, and by cloud or other particles. The diffuse radiation is somtimes assumed to be isotropic, that is, equally intense from all points of the sky . Also solar radiation scattered by transmission through diffusing glazing, See diffuse transmission.

Diffuse transmission: the type of transmission through a diffusing or translucent glazing or other material. Transmited radiation that is scattered by interaction with the glazing material.

Diffuser: a device object or surface used to alter the spacial distribution of light by diffusing it.

Diffusion, scattering : process by which the spatial distribution of a beam of radiation is changed when it is deviated in many directions by a surface or by a medium, without change of frequency of its monochromatic components. (Note: a distinction is made between selective diffusion and non-selective diffusion according to whether or not the diffusion properties vary with the wavelength of the incident radiation.)

Dilution air: the air required by some combustion heating systems in order to isolate the furnace from outside pressure fluctuations and to maintain an effectively constant chimney draft.

Direct method of cooling : a system in which the evaporator is in direct contact with the material or space refrigerated or is located in aircirculation passages communicating with such spaces.

Direct gain: the directly transmitted solar radiation absorped within a room.

Direct glare: glare caused by selfluminous objects situated in the visual field, especially near the line of sight.

Direct solar gains: see Direct gain.

Direct solar radiation: solar radiation which reaches the Earth's surface after selective attenuation by the atmosphere as a collimated beam (i.e. the solar radiation which is not absorbed or diffused).

Direct sunlight: direct solar radiation in the visible wavelengths.

Directional lighting: a form of lighting where light is received from a single direction.

Draught: air movement causing potential discomfort . 

Dry-bulb temperature : the temperature of a gas or mixture or gases indicated by an accurate thermometer.

Ecliptic: the great circle cut in the celestial sphere by an extension of the plane of the sun's apparent orbit around the earth. The great circle drawn on a terrestrial globe makes an angle of about 23.27^o with the equator.

Effective Temperature, : the temperature of a black enclosure at 50% relative humidity (sea level), in which a solid body or occupant would exchange the same heat by radiation, convection, and evaporation as in the existing nonuniform environment.

Electromagnetic radiation: (1) emission or transfer of energy in the form of electromagnetic waves or photons. (2) these electromagnetic waves or these photons.

Emissivity: the ratio of the radiant energy emitted from a surface at a given temperature to the energy emitted by a black body at the same temperature.

Emittance : the ratio of the radiant energy emitted (in the absence of incident radiation) from a given plane surface at a given temperature to the radiant energy that would be emitted by a perfect black body at that same temperature.

Energy demand: Amount of energy required (e.g. for indoor climate control).

Energy use: The amount of energy used (e.g. for indoor climate control), including losses of the installation.

Energy use for cooling: see cooling load.

Energy use for heating: The amount of energy needed to cover the heating demand, including losses of the installation.

Enthalpy: Total energy in a fluid (usually air) including both sensible energy (dry heat) and the latent energy associated with the water vapour in the air.

Environmental controls: means by which the physical environment can be controlled: these may be active, e.g. heating or cooling systems, fans etc which use energy, or passive, such as openable windows, blinds, etc.

Environmental variables (thermal environment): Thermal characteristics of the environment; generally the air temperature (T_a), the radiant temperature (T_r), the water vapour pressure (P_a) (or humidity) and the air velocity (V_a).

Envelope: the exterior surface of a building including all external additions e.g. chimneys, bay windows, etc.

Evaporation: change of state from liquid to vapour.

Evaporative cooling: involves adiabatic heat exchange between air and water spray or wetted surface. The water is cooled by evaporation and assumes the wet-bulb temperature of the air.

Evaporative equilibrium (of a wetbulb instrument): the condition attained when the wetted wick has reached a stable and constant temperature. When the instrument is exposed to air at velocities over 4.6m/s, this temperature may be considered to approach the true wetbulb temperature.

Exfiltration: the uncontrolled leakage of air out of a building e.g. through cracks.

Extinction coefficient: a property of glazing material that characterizes the solar absorption in the material, namely, the fraction of radiation that is absorbed per unit of path length through the material.

Extraterrestrial radiation: in this context solar radiation impinging on the earth's outer atmosphere.

Flat plate collector: This is a solar collector, a flat assembly containing a panel of metal or other suitable material, usually a mat black colour on its sunny side, that absorbs sunlight and converts it into heat. This panel is usually in an insulated box, covered with glass or plastic to take advantage of the greenhouse effect. In the collector , this heat transfers to a circulating fluid, such as air, water or oil.

Floor heating: Heated floor, which gives off heat to the room.

Focussing collector: a collector that has a reflector which focuses sunlight onto a smaller area for collection. A reflector of this type can obtain relatively high temperatures. The system is most effective for solar radiation not diffused by clouds.

Forced convection: convection heat transfer between a surface and an adjacent fluid (air in the present context)resulting from forced circulation of a liquid or a gas, as by a fan, jet or pump.

Free convection : See Convective heat transfer.

Free-running building: A building which is neither being mechanically heated nor cooled.

Gaseous state: one of three states of matter charactemed by the greatest freedom of molecules and lack of any inherent fixed shape or volume.

Glare: condition of vision in which there is discomfort or a reduction in the ability to see details or objects, caused by an unsuitable distribution or range of luminance, or to extreme contrast.

Glare, disability: glare that impairs the vision of objects without necessarily causing discomfort.

Glare, discomfort: glare that causes discomfort without necessarily impairing the vision of objects.

Glazing: glass used in a window.

Global illuminance (g): illuminance [W/m²] produced by daylight on a surface.

Global radiation: the total solar radiation projected on a surface, (i.e. the sum of the direct and diffuse radiation) [W/m²].

Heat: form of energy that is transferred by virtue of a temperature difference. see latent heat, radiant heat, sensible heat.

Heat accumulation : Heat Heat stored in the thermal mass (of a building). See Thermal mass.

Heat Balance Model (of thermal comfort): A model of human thermal response based on the assumption that a necessary condition for thermal comfort is a balance between the metabolic heat production and the heat loss from the body (generally an analytical model).

Heat Capacity: the amount of heat necessary to raise the temperature of a body by one degree. Numerically, the mass multiplied by the specific heat.

Heat demand : The amount of energy needed to keep the temperature of a room or building at a given value.

Heat Exchanger: a device, usually consisting of an arrangement of metal tubing used to transfer heat from one fluid to another, often used to conserve energy by heating incoming air using the heat from outgoing air.

Heat gain : Sum of internal heat gains (like heat gains from persons, lighting and apparatus) and solar heat gains.

Heat load: The heat flux which has to be removed from a room to maintain (or prevent to exceed) a required air temperature or humidity.

Heat loss (from the body): the loss of heat from the surface of the human body by conduction, convection, radiation and evaporation.

Heat loss (from a building): heat flow through building envelope components (walls, windows, roof).

Heat pump: Apparatus that transfers heat from a low (unusable) temperature level to a (usable) higher temperature level.

Heat recovery: the process of extracting heat, (usually from a fluid, gas or air) that would otherwise be wasted. For example heat recovery using a heat exchanger between incoming and exhaust air.

Heat transmission : Heat transfer through a construction, due to conduction, convection and radiation.

Heat transmission coefficient: heat transmission per unit temperature difference per area unit (usually used for total constructions).

Heating: The process of active heat supply for the benefit of climate control.

Heating degree-days (DD): see degree days.

Heating season: the period of the year during which heating the building is required to maintain comfort conditions.

Humidifying/Dehumidifying effect: latent heat of water vaporization at the average evaporating temperature times the number of pounds of water evaporated or condensed in watts.

Humidity: the "dampness" of the air expressed either as a subjective sensation or as a physical measure of the water vapour in the air (see relative humidity, water vapour pressure).

HVAC (heating ventilating and air conditioning): A term generally applied to the hardware or the industry concerned with the supply of environmental control in buildings.

Hybrid solar heating system: solar heating system that combines active and passive techniques.

Illuminance (at a point of a surface): quotient of the luminous flux dX incident on an element of the surface containing the point, by the area dA of that element. Equivalent definition. Integral, taken over the hemisphere visible at a given point, of the expression Lv.cosY.dX. where Ly. is the luminance at the given point in the various directions of the incident elementary beams of solid angle dX and Y is the angle between any of these beams and the normal to the surface at the given point.

Impermeable not permitting gas or water pass through.

Incident angle: in this context the angle between the beam of solar radiation and a line perpendicular (normal) to the irradiated surface.

Indirect cooling : a system in which a liquid, such as brine or water, cooled by the refrigerant, is circulated to the material or space refrigerated or is used to cool air so circulated.

Indirect gain - the indirect transfer of solar heat into the space to be heated from a collector that is coupled to the space by an uninsulated, conductive or convective medium (such as thermal storage wall or roof pond).

Indirect lighting: lighting achieved by reflection, usually from wall and/or ceiling surface.

Induced draft flue system: a term referring to a type of gas heating system equipped with a fan downstream of the fumace. The fan pulls gases from the fumace and propels them to the outside, thereby eliminating the requirement for dilution air.

Infiltration: the uncontrolled movement of outdoor air into the interior of a building through cracks around windows and doors or in walls, roofs and floors. This may work by cold air leaking in during the winter, or the reverse in the summer.

Infrared radiation: electromagnetic radiation with wavelengths longer than that for light (>700 nanometers). Infrared is often divided into the near infrared (wavelength 800-2000 nm) and the far infrared (>2000nm). Almost all energy radiated by surfaces with a temperature beneath 6000 K is heat radiance. Glass, which is most transparent for visual light radiance (400 nm to 800 nm), is not always transparent to heat radiance. This is the preponderant form of radiation emitted by bodies with moderate temperatures such as the elements of a passive building.

Insulation material : A material with a low thermal conductivity (e.g. below 0.2 W/m²K) applied to reduce thermal losses through building envelope (e.g. walls, roofs, floors).

Internal (heat) gains: the energy dissipated inside the heated space by people (body heat) and appliances (lighting, cooker, etc.). A proportion of this energy contributes to the space heating requirements (kWh).

Internal sources: the sources of internal heat such as appliances, lights and people . See internal heat.

Intrinsic Heat: heat from human bodies, electric light bulbs, cooking stoves, and other objects not intended specifically for space heating.

Irradiance (at a point of a surface): quotient of the radiant flux dX, incident on an element of the surface containing the point, by the area dA of that element. Equivalent definition: Integral, taken over the hemisphere visible from the given point, of the expression Le cos Y.dX. where Le is the radiance at the given point in the various directions of the incident elementary beams of solid angle dX where Y is the angle between any of these beams and the normal to the surface at the given point.

Irradiation: see radiation.

Isentropic: an adjective describing a reversible adiabatic process a change taking place at constant entropy.

ISO:The International Standards Organisation, based in Geneva.

ISO/CEN 7730:The Standard for calculating the effect of indoor environments using the PMV/PPD indices of Fanger, produced by the International Standards Organisation, based in Geneva.

Iso-: The (l) prefix meaning constant: as in isothermal (constant temperature); isentropic (constant entropy); isobaric (constant pressure); etc.; and (2) in chemicals, one having different characteristics but with the same number and kind of atoms.

Isobaric: an adjective used to indicate a change taking place at constant temperature.

Isolated gain: the transfer of heat into the space to be heated from a collector that is thermally isolated from the space to be heated by physical separation or insulation (such as an attached sunspace with an insulated common wall).

Isothermal: an adjective used to indicate a change taking place at constant temperature

Kata Thermometer: A special thermometer whose purpose is to measure the cooling power of the environment. Often used as an anemometer

Lag: Time delay between, say, the peak outdoor temperature and the indoor temperature, also applied to the time required for the sensing element of a control system to reach equilibrium with the property being controlled.

Latent heat: the energy required to change the state of a unit mass of material from solid to liquid (fusion) or liquid to gas (evaporation), without a change in temperature. This energy is then released, again without change of temperature (provided super cooling does not occur), when the material reverts from gas to liquid or from liquid to solid. (Wh/kg, kJ/kg)

Latitude: the angular distance north (+) or south (-) of the equator, measured in degrees of arc

Light: Electromahgnetic radiation within the visible spectrum (wavelengths between approx 400-800 nm)

Light, perceived: Universal and essential attribute of all perceptions and sensations that are peculiar to the visual system. (Notes: light is normally, but not always, perceived as a result of the action of a light simulation of the visual system.)

Lightwell absorption fraction: the fraction of solar radiation that directly heats the air after it is transmitted through the glazing and after each reflection from interior surfaces. It is intended to simulate the presence of lightweight objects that absorb solar radiation and rapidly convect heat to the air.

Liquid state : one of three states of matter characterized by limited freedom of molecules and by substantial incompressibility .

Load: see heat load.

Load Collector Ratio (LCR): the ratio of the building load coefficient to the projected area.

Load factor: the ratio of actual mean load to a maximum load of maximum production capacity in a given period.

Longitudinal sampling: A method of sampling a population in which a large number of comfort votes is sought from each of relatively few subjects (cf Transverse sampling).

Long-wave radiation: radiation emitted between roughly 5000 and 30000 nm wavelength, as in thermal radiation from the surfaces of a room, or from the outside surface of the roof.

Longitude: the arc of the equator between the meridian of a place and the Greenwich meridian measured in degrees east or west.

Louvre (system): an assembly of sloping vanes intended to permit air to pass through and to inhibit transfer of water droplets. Also used as an solar control, either external to the building or internally (e.g. venetian blinds)

Low energy building : A very good insulated and passive solar building, with a very low remaining heating and cooling demand.

Low-e glass : Glazing with a low thermal transmittance (U-value), due to a heat reflecting coating(s) applied in the gap of double glazing. The applied coatings are visually transparant.

Lumen (lm): SI unit of luminous flux: luminous flux emitted in unit solid angle (steradian) by a uniform unit source having a luminous intensity of 1 candela. Equivalent definition: luminous flux of a beam of monochromatic radiation whose frequency is 540 x 1012 hertz and whose radiant flux is 1/683 watt.

Luminance: The physical measure of the brightness of a surface such as a lamp, a reflecting material or the sky in a given direction. Luminance is the luminous intensity emitted by an area of a surface. Units : candela per square meter (cd/ m²)

Luminous efficacy of radiation: quotient (K) of the luminous flux by the corresponding radiant flux.

• Luminous environment: lighting considered in relation to physiological and phychological effects.
• Luminous flux: quantity derived from radiant flux evaluating the radiation according to its action upon the CIE stand photometric observer.
  
  
• Luminous intensity (of a source in a given direction): quotient of the luminous flux dFv leaving the source and propagated in the element of solid angle containing the given direction, by the element of solid angle.
• Lux: SI unit of illuminance: illuminance produced on a surface, area 1 square metre by a luminous flux of 1 lumen uniformly distributed over that surface lm/m²

  

• Masonry: concrete, concrete block brick, adobe, stone and other similar materials.
• Mass: the quantity of matter in body as measured by the ratio of the force required to produce a unit acceleration.
• Mass-area-to-glazing-area ratio: the ratio of the total surface area of massive elements to the glazing area. Massive elements including in this definition are all floors, walls ceilings, or other interior objects with densities comparable to high-density concrete provided their surfaces are exposed and located in a room that is at least partially illuminated by direct solar gains.
• Mean radiant temperature (MRT): the area weighted mean temperature of all surrounding surfaces, i.e. sum of (s.t)/ sum of (s), where 't' is the temperature of each surface of angular area 's'. Also the temperature of a uniform black enclosure in which a solid body or occupant would exchange the same amount or radiant heat as in the existing nonuniform environment. It is an approximate indication of the effect that the surface temperatures of surrounding objects have on human comfort.
• Mechanical systems: a term widely used in commercial and industrial construction, referring to all the mechanical components of the building. i.e. plumbing, heating, ventilation, air conditioning and heat recovery.
• Meta-analysis: a method of analysis which combines the results of a number of surveys to investigate the underlying processses.
• Metabolic rate: The rate of heat production by the body when engaged in various tasks, often defined in terms of the resting metabolism (Mets).
• Micro-climate: The climate in the immediate vicinity of a building.
• Mixed systems: a solar heating system that combines two or more passive solar heating types.
• Mixed-mode systems: heating or cooling systems which combine passive and mechanical systems

  

• Nano-meters (nm): unit used to measure short wavelengths in electromagnetic radiation 1 nm = 10^-9m. See also infra-red radiation, visible radiation.
• Natural convection: heat transfer between a surface and adjacent fluid (usually air) and by the flow of fluid from one place to another, induced by temperature differences rather than by mechanical means, also called free convection. Compare with forced convection.
• Natural convection air cooler: an air cooler depending on natural convection for air circulation.
• Natural light: visible part of direct solar radiation. (Note: when dealing with actinic effects of optical radiations, this term is commonly used for radiations extending beyond the visible region of the spectrum.)
• Natural lighting system: component or a series of components joined in a specific building for natural daylighting.
• Natural ventilation : Air which enters a building controlled, through open windows or other openings, due to wind pressue or temperature differences between the outdoor and indoor air.
• Negative pressure: a pressure below atmospheric. In residential construction negative pressure often refers to pressures around the house that is less than the indoor or the pressures on e.g. the windward side of the house : such variations in pressure will encourage infiltration by cross ventilation.
• Net load coefficient (NLC): net reference load per degree of indoor-minus-outdoor temperature difference per day
• Net reference load: steady-state heat loss from a building, excluding the solar wall, assuming constant indoor temperature. Compare with net load coefficient (J).
• Neutral temperature: see Comfort temperature.
• Night insulation: movable insulation that covers a glazing at night and is removed during the day.
• Night radiance : Heat radiance which appears at night: When the sky is clear at night a surface facing the sky sends and receives heat radiance to and from the sky. The radiant temperature of a clear sky is usually well below zero, particularly at night. Because of this the surface temperature can easily descend below the ambient outdoor air temperature.
• Night ventilation: Ventilation which takes place outside the normal operating time of the building.
• Normal distribution: A frequency distribution that predicts the probability that a variable will take a certain value; the distribution is symmetrical about the mean value. Based on statistical theory, the normal distribution underlies much statistical analysis

  

• Obstruction: (in daylight calculations) anything outside a building which prevents the direct view of part of the sky .
• Orientation: The azimuth of a particular surface or the major axis of a building. see Azimuth
• Outside air: external air , atmosphere exterior to refrigerated or conditioned space, ambient (surrounding) air.
• Overcast sky: A sky in which there is no direct solar radiation. NOTE: The measured temperature data and solar radiation tables for overcast skies within this Handbook refer to days when the cloud cover was 8/8 during all the daylight hours. Night-time temperature data are related to the same 'overcast' days, i.e. night-time cloud cover was not taken into account. 

  

• Passive solar energy : Solar energy collected by non active systems e.g. windows, atria etc..
• Pascal: unit of pressure. House air tightness tests are typically conducted with a pressure difference of 50 Pascals between the inside and outside. 50 Pascals is equal to 5.008 mm (0.2 in) of water at 12.9^oC.
• Percent of possible sunshine: the actual number of hours of sunshine expressed as a percentage for the month (or other period of time).
• Percent radiation received: the monthly mean Global Radiation of a horizontal plane, expressed as a percentage of the monthly mean radiation on a horizontal plane outside the atmosphere.
• Permeance: water vapour permeance is the rate of water vapour diffusion through a sheet of any thickness of material (or assembly between parallel surfaces). It is the water vapour flow to the differences of the vapour pressures on the opposite surfaces. Permeance is measured in perms (m².K/W).
• Permeability: water vapour permeability is a property of a substance which permits passage of water vapour. When permeability varies with psychrometric conditions, the 'spot' or 'specific permeability' defines the property at a specific condition.
• Phase-change materials: materials with a phase change point close to the desired room temperature which can be used either to reduce temperature fluctuations within a room or to store energy in the phase change process (usually melting) and re-releasing it to the system when its temperature falls below the transition point. Phase change materials may be surface mounted as in phase change tiles or storage based as in latent heat stores for active solar systems (see latent heat).
• Physical environment (thermal environment): The environment in a room or building defined by its the environmental variables.
• PMV - predicted mean vote: The average comfort vote predicted by a theoretical index (generally that of Fanger) for a group of subjects when subjected to a particular set of environmental conditions.
• Positive pressure: a pressure above atmospheric. In residential construction positive pressure often refers to pressures around the house that is above than the indoor or the pressures on e.g. the leeward side of the house or above the roof: such variations in pressure will encourage infiltration by cross ventilation.
• PPD - predicted percentage dissatisfied: The percentage of subject population who will be dissatisfied (uncomfortable) in a given environment as predicted by a theoretical index (generally that of Fanger). see PMV
• Preference vote: The response by a subject as to their preference at the time of asking; this may be a preference for a warmer or cooler environment, or for a particular response on a comfort scale.
• Preheating: in air conditioning: to heat the air before other processes.
• Pressure: the normal force exerted by a homogeneous liquid or gas, per unit of area, on the wall of the container .
• Pressure difference: the difference in pressure of the volume of air enclosed by the building envelope and the air surrounding the envelope.
• Probit analysis: A statistical analysis whereby the proportion of people casting different comfort votes at a particular temperature can be estimated for a population from the results from a sample.
• Profile angle (vertical shadow angle): the angle between the direction of the sun resolved in a vertical plane perpendicular to the face of the solar aperture, on the normal of the solar aperture. This angle is used to predict the effect of shading devices in sectional drawings.
• Psychophysics: The branch of psychology dealing with the relationship between our sensations and the stimuli from the physical world.
• Psychrometry: the study of the interactions between temperature and water vapour in air. Includes relative humidities, wet and dry bulb temperatures, enthalpy, and so on

  

• 'R' Value : the thermal resistance to heat flow across a sample material of a unit area and known thickness caused by a temperature difference across it. This is often used to give a comparative value of the effect of different insulating materials. (m².K/W)
• Radiance of the sky (Ls) : the radiant flux emitted in a small solid angle from a specific area of the sky in the direction of the observer expressed as the flux received normal to the beam per unit solid angle. (W/m².sr)
• Radiant flux, radiant power: power emitted, transmitted or received in the form of radiation. (Unit: W)
• Radiant heat transfer : the transfer of heat energy from a location of higher temperature to a location of lower temperature by means of electromagnetic radiation. Radiant heating is due to infrared radiation and its use is very prevalent in passive systems. see also infra-red radiation
• Radiative heat transfer : Heat transfer by electromagnetic radiation. Often divided into short-wave radiation - generally taken as those wavelengths associated with the solar spectrum (300-3,000nm) and longwave radiation (>3,000nm).
• Radiator: Apparatus for heating of a room, which gives off heat to the space by radiation and convection.
• Recirculated air: return air passed through the conditioner before being resupplied to the conditioned space. The air contditioning unit takes in outdoor air but also a part from the rooms.
• Reference design: a detailed specification of the passive solar features of a hypothetical passive solar building used as the subject of performance analysis.
• Reference temperature: a fixed indoor temperature in the definition of the net reference load. also the fixed indoor temperature in the reference nonsolar building used in the definition of solar savings.
• Reflectance: ratio of the reflected radiant or luminance flux to the incident flux in the given conditions. Can be defined for a incident radiatior of given spectral composition polarization and geometrical distribution)
• Reflectance (visible): the ratio or percentage of the amount of light reflected by surface to the amount incident upon it. Good light reflectors are not necessarily good heat reflectors.
• Reflectance of a glazing material: radiant energy is reflected from both inside and outside surfaces of a sheet of parallel sided glazing. Multiple inter-reflections occur within the glazing. The ratio of the overall reflected radiant energy to the incident radiant energy is known a the reflectance. Its value depends on the angle of incidence. Energy interreflected from the inside face is dependent on the absorbitivity of the material and so is also dependent on wavelength. For any wavelength and angle of incidence, absorptance reflectance + transmittance = 1.
• Reflectance of opaque materials: the proportion of radiation reflected by surface. the reflectance varies for different wavelengths and angles of incidence: for instance white paint has an average reflectance of 0.71 for normal incidence solar radiation and 0.11 for long-wave (thermal) radiation. For any wavelength and angle of incidence, absorbance + reflectance = 1.
• Reflected (global) so1ar radiation: radiation that results from reflection of the global solar radiation by the surface of the Earth and by an surface intercepting that radiation.
• Refractive index: a property of glazing materials that determines the reflection/refraction characteristics of the glazing.
• Regenerative cooling: process of utilizing heat which is rejected or absorbed in one part of the cycle to function usefully in another part of the cycle by heat transfer .
• Regression (linear): A statistical technique by which the variation of a dependent variable can be predicted from the value of an independent variable (or variables) through a Regression Coefficient (or coefficients) plus a constant term. (y = a.x + b)
• Reheating of air : in an air conditioning system, the final step in treatment in the event the temperature is too low.
• Relative humidity (RH): the water vapour pressure in the air expressed as a proportion of the saturated water vapour pressure (ie the highest possible value) at the current air temperature.
• Relative sunshine duration: ratio of sunshine duration to possible sunshine duration within the same period.
• Resistance value (RSI): thermal resistance value. Measurement of the ability of a material to resist heat transfer
• Resistivity : the thermal resistance of unit area of a material of unit thickness to heat flow caused by a temperature difference across the material. (m²K/W)
• Respiration: production of carbon dioxide and heat by ripening of perishables in storage; also the breathing process of animals.
• Room temperature : used colloquially to mean the ordinary temperature one is accustomed to find in dwellings. Technically it can mean the temperature of (l) a room in which a refrigerator is being operated or tested; (2) a room being conditioned for the occupant's comfort.

  

• Saturated air: moist air in which the partial pressure of water vapour equals the vapour pressure of water at the existing temperature. This occurs when dry air and saturated water vapour coexist at the same dr -bulb temperature.
• Saturated temperature : of a fluid, the boiling point corresponding to a given pressure; evaporation temperature, condensation temperature.
• Saturation ratio : the ratio of the specific humidity of humid air to that of saturated air at the same temperature and pressure, usually expressed as a percentage .
• Saturation pressure: the saturation pressure for a pure substance for any given temperature is that pressure at which vapour and liquid, or vapour and solid, can coexist in stable equilibrium.
• Sealants : flexible materials used on the inside of a building to seal gaps in the building envelope thereby preventing uncontrolled air infiltration and exfiltration.
• Selective coating : finishes applied to materials to improve their performance in relation to radiation of different wavelengths: for example those applied to solar absorbers would seek to have a high absorptance of solar radiation and a low emittance of long-wave (thermal) radiation, while those for glazing have a high transmittance to solar radiation and high reflectance of long wavelengths.
• Selective surface (absorber) : a surface absorbing essentially all incident solar radiation (short wave high temperature source), while emitting a small fraction of thermal radiation (long wave low temperature source).
• Semantic differential: A technique for obtaining a value for subjective response in which the subject is asked to denote the intensity of a stimulus by choosing a subdivision between two extremes.
• Semi-enclosed sunspace: a sunspace that shares common walls with the associated building.
• Sensible heat: heat which is associated with a change in temperature in contrast to a heat interchange associated with a change of state (latent heat).
• Sensitivity date: date that express the dependence of the heating performance of a passive solar heating system on individual parameters of the system design.
• SI units: (Systeme International d'Unites) the International System of Units being adopted throughout the world.
• Skin temperature: The average temperature of the skin surface.
• Skin wettedness: A theoretical measure of the coverage of the skin by sweat needed to explain the rate of evaporation of sweat as a proportion of the total skin surface area.
• Skylight: horizontal or sloping window in the ceiling of a room.
• Sol-air temperature: the hypothetical temperature which would give the same temperature distribution and heat flow in a building element that would result from the impact of the actual combination of the absorbed short wave radiation, long wave exchange with sky and ground, forced convection due to wind at the given external air temperature . It is used to assess transfer of heat through a specific building element of a given surface colour. Different sol-air temperatures are encountered on differently orientated surfaces for materials of the same surface properties.
• Solar absorptance : the fraction of incident solar radiation that is absorbed upon striking a surface.
• Solar aperture : that portion of the solar wall covered by glazing.
• Solar collection area : the projected area of a collector normal to the direction of the sun.
• Solar collector: -a device for capturing solar energy, ranging from ordinary windows to complex mechanical devices.See also flat plate collector, focussing collector
• Solar constant : the irradiance of solar radiation beyond the earth's atmosphere at the average earth-sun distance on a surface perpendicular to the sun's rays. The value for the solar constant is 1.353 kW/m².
• Solar fraction (or percentage solar) : the percentage of the total heat load supplied by the solar heating system, including useful losses from the storage .
• Solar gain: Heat from the sun, entering a room through transparent surfaces (kW/m²).
• Solar heat gain : in passive solar heating a term referring to the amount of heat gained through windows over the heating season. Net solar gain refers to the solar heat gain less the heat losses through the windows (kWh/m²).
• Solar load ratio (SLR) : ratio of solar gain to building load used in SLR correlations.
• Solar radiation : radiation emitted by the sun, including infrared radiation, visible light and ultraviolet radiation.
• Solar savings fraction (SSF) : the ratio of the solar savings to the energy requirement of the reference nonsolar building.
• Solar spectrum : the radiation emitted by the sun outside the atmosphere approximates to that emitted by a black body at 6000^oK. At the surface of the earth, the majority of the radiation falls within a range of wavelengths from 300 to 2,500 nm and typically about ha1f the energy comes in the form of visible light (380 to 700 nm wavelength). Ultra violet radiation has wavelengths shorter than 380 nm and only forms a very small proportion of the spectrum. Due to water vapour and carbon dioxide and other absorbing gases in the atmosphere, several bands of long wave radiation are substantially removed.
• Solar time : a time scale based on the movement of the sun, such that noon is when the sun is at the highest of its arc and is true south, as opposed to local standard time.
• Solid state : one of three states of matter characterized by stability of dimensions, relative incompressibility , and molecular motion held to limited oscillation. [note the term solid state is often used to talk about solid-state semiconductors].
• Sound insulation: The ability of building elements to reduce the sound level between internal spaces, or between outdoors and indoors. In HVAC systems: acoustical treatment of fan housings, supply ducts, space enclosures, and other parts of system and equipment to isolate vibration or to reduce noise transmission
• Specific heat: the amount of heat required to raise the temperature of a unit mass of any substance one degree as a fraction of that required to raise the temperature of unit mass of a standard substance one degree (usually water at 15^oC).
• Specific heat capacity : a measure amount of energy required to raise a unit mass or volume of a material through a unit temperature change. (kWh/kg.K, J/kg.K. kWh/m³.K, J/m³.K)
• Specific volume : the volume of a substance per unit mass; the reciprocal of density.
• Spectrum (of radiation) : display or specification of the components of the radiation considered in terms of intensity and wavelength. (Notes: (l) there are line spectra, where the spectrum consists of a finite number of 'lines' at particular wavelength(s) and continuous spectra where there is an infinite number of wavelengths with no sudden discontinuities of intensity. Some spectra exhibiting both these characteristics. (2) this term is also used for spectral efficiencies [excitation spectrum, action spectrum].
• Specular reflectance : proportion of incoming radiation which undergoes specular reflection.
• Specular reflection : reflection at a surface where the angle of reflection (more or less) equals the angle of incidence (as happens in a mirror)
• Specular surface : a surface with specular reflection properties. For example finishes such as polished aluminium, stainless steel or tin.
• Spherical irradiance, radiant fluence rate (at a point): quantity defined by the formula
  
  where d_ is the solid angle of each elementary beam passing through the given point and Le its radiance at that point. (Unit: W/m²)
• Spray pond: arrangement for lowering the temperature of water in contact with outside air by evaporative cooling of the water. The water to be cooled is sprayed by nozzles into the space above a body of previously cooled water and allowed to fall by gravity into it.
• Stack effect: the tendency of air or gas in a duct or other vertical passage to rise when heated due to its lower density in comparison with that the surrounding air or gas. In buildings, the tendency towards displacement (caused by an increase in temperature) of internal heated air by unheated outside air, due to the difference in density of outside and inside air. See Convective heat transfer.
• Standard air: dry air at a pressure of 101.325 kPa at a temperature of 20^oC. The density is than 1.2041 kg/m³
• Standard conditions: a set of physical, chemical or other parameters of a substance or system which defines an accepted reference state or forms a basis for comparison.
• Steady conditions: an operating state of a system, including its surroundings, in which the extent of change with time of all the significant parameters is so small that no important effects on the performance can be observed or measured.
• Steady-state model (of thermal comfort): A theoretical model of thermal response based on climate chamber measurements in conditions which are held constant in time.
• Subject A person taking part in a comfort survey.
• Subject population: A group of people who form the basis for a comfort survey.
• Subject sample: A sample of the subject population who participate in the survey.
• Subjective response: The sensation caused by a physical stimulus (generally a comfort vote).
• Sun effect : solar energy transmitted into interior spaces through windows and building materials.
• Sunlight: visible part of direct solar radiation. (Note: when dealing with actinic effect of optical radiations, this term is commonly used for radiations extending beyond the visible region of the spectrum.)
• Surface film conductance: the rate of heat flow per unit area under steady conditions between a surface and a fluid for unit temperature difference between the surface and fluid [in W/m².K].
• Sunshine (possible percentage): the actual number of hours of sunshine expressed as a percentage of the maximum possible duration for the month (or other period).
• Sunshine duration [S] : sum of time intervals within a given time period (hour, day, month, year) during which the irradiance from direct solar radiation on an plane normal to the sun direction is equal to or greater that 200 W/m².
• Sunspace : see attached sunspace or semienclosed sunspace.
• Suntempered building : a minimal solar building derived from a conventional building by orienting its long axis east-west and placing a substantial fraction of its window area on the south side.
• Surface cooling: method used to cool air or other gas by passing it over cold surfaces.
• Surface resistance : With the transition from air to material and material to air a surface resistance has to be overcome. It has two components, the surface resistance for convection/conduction Rc whose value depends on the rate of air flow over the surface and radiative surface resistance for long wave radiation heat transfer Rr whose value depends on the surface emittance and the surface temperature. These two components are assumed to act in parallel so that the overall surface resistance Ro is give by
• Survey (comfort survey, field survey): An experimental investigation of subjective responses of a group of subjects in the field, generally assumed to be undertaken in such a way as to disrupt the normal pattern of the subject's lives as little as possible, and to leave subjects to decide their own dress and activity, use of environmental controls and so on
• Sweat rate: the rate at which sweat is produced by the body.
• Systematic errors: Errors in the underlying assumptions of a theoretical comfort model which are temperature-dependent, and will therefore give rise to errors in its predictions

  

• Temperature : the thermal state of matter with reference to its tendency to communicate heat to matter in contact with it
• Temperature difference, diffusion : temperature difference between the air temperature at supply opening and design outdoor temperature.
• Temperature difference, effective : difference between the room air temperature and the supply air temperature at the outlet to the room.
• Temperature difference, mean: mean of difference between temperatures of a fluid receiving and a fluid yielding heat.
• Temperature swing (indoor) : the range of indoor temperatures in a building (K).
• Temperature swing (outdoor) : the range of outdoor temperature between day and night.
• Temperature standards: Recommended values for temperature (and other environmental parameters) in buildings or rooms, values generally defined by the expected use of the room.
• Thermal break : a material of low conductivity used in an assembly to prevent flow of heat by conduction from one side of the assembly to the other . Often used to refer to materials used for this purpose in the frame of metal windows.
• Thermal bridge : Non- or relative low-insulated connections between indoor and outdoor environment and may compromise the effect of thermal insulation: for example, framing members in insulated frame walls or metal ties in cavity wall and panel construction.
• Thermal comfort: The study of the relationship between man's thermal sensation and the stimulus in the form of the thermal environment in conditions of moderate heat stress (generally taken to include thermal discomfort).
• Thermal conductance: rate of heat flow through a body per unit area from one of its bounding surfaces to the other for a unit temperature difference between the two surfaces, under steady conditions [e.g. W/m².K]. See also U-Value.
• Thermal conduction: process of heat transfer through a material medium in which kinetic energy is transmitted through the material from particle to particle without displacement of the particles.
• Thermal conductivity: rate of heat flow through unit area and unit thickness of a homogeneous material under steady conditions when a unit temperature gradient is maintained in the direction perpendicular to area. Materials are considered homogeneous when the value of the thermal conductivity is not affected by variation in thickness or in size of the sample within the range normally used in construction. (W/m².K)
• Thermal controls: see Environmental Controls
• Thermal experience: The different thermal environments experienced by a subject taking account of the order in which they occurred.
• Thermal insulation: a material having a relatively high resistance to heat flow and used principally to retard heat flow.
• Thermal mass: the ability of a building to accumulate heat effectively. A construction with a low thermal mass is heated quickly; a construction with a big thermal mass is heated slowly. This way construction with a big thermal mass, like concrete or brick, can store heat or cold and release it many hours later. 'Primary thermal mass' receives direct sunlight; 'secondary thermal mass' is in sight of the primary thermal mass and so receives radiative and convective energy from the primary thermal mass; 'remote thermal mass' os hidden from view of both the primary and secondary thermal mass and so receives energy by convection only.See Heat accumulation.
• Thermal performance: the characteristic way in which a building reacts to the thermal climate.
• Thermal radiation : energy transfer in the form of electromagnetic waves from a body by virtue of its temperature, including infrared radiation, ultraviolet radiation, and visible light.The heat transfer between objects is according to difference of the fourth power of their temperatures. See also Infra-red radiation.
• Thermal resistance : The thermal resistance (the reciprocal of thermal conductance - see above), denoted by R, of a construction layer is the quotient of thickness and conductivity. A construction layer with a high thermal resistance is a good insulator; one with a low thermal resistance is a bad insulator (m².K/W).
• Thermal resistivity : the reciprocal of thermal conductivity. (m².K/W)
• Thermal storage mass : building elements, usually masonry or water in containers, designed to absorb solar heat during daytime hours for release later when heat is needed.
• Thermal storage volume ratio: ratio of the volume of thermal storage material to the projected area.
• Thermal storage wall : a wall of massive material (masonry or water in containers) placed between the solar aperture and the heated space. Heat is transferred into the space by conduction through the masonry or conduction and convection through the water , and, if openings are provided, by natural convection. See also Trombe wall and water wall.
• Thermal transmittance : the thermal transmission through 1 square metre area of a given structure (e.g. a wall consisting of bricks, thermal insulation, cavities, etc .) divided by the difference between the environmental temperature on either side of the structure. Usually called 'U-value'. (W/m².K).
• Thermocirculation : free convection from a warm zone (sunspace or Trombe-wall air space) to a cool zone through openings in a common wall.
• Thermocirculation vents : openings in a common wall between cool and warm zones through which thermocirculation occurs. The vents are arranged in pairs, one of each pair near the floor and one near the ceiling.
• Thermodynamic properties: basic qualities used in defining the condition of a substance, such as temperature, pressure, volume, enthalpy, entropy.
• Thermo-regulation: the various physiological means by which the core temperature is regulated: vasoregulation, sweating and shivering.
• Thermosiphon : the convective circulation of a fluid which occurs in a closed system where warm fluid uses and is replaced by a cooler fluid in the same system.
• Tilt : the angle of a plane relative to a horizontal plane.
• Time sampling: sampling by time of day; a wide time sample is necessary if the full experience of subjects is to be represented,
• Toplighting : light which enters through the top part of intrior space such as clerestories, light wells or skylights.
• Transmission (light) : passage of radiation through a medium without change of frequency of its monochromatic components.
• Transmission : in thermodynamics, a general term for heat travel; properly , heat transferred per unit of time.
• Transmittance, radiant : the ratio of the radiant energy transmitted by parallel sided plane sheet of a given material to the radiant energy incident on the outside surface of that material: the value depends on the angle of incidence and wavelength. For any wavelength and angle of incidence, absorptance + reflectance + transmittance = 1.
• Transverse sampling: A method of choosing a population sample for a survey by taking one or a few comfort votes from a large number of subjects (cf longitudinal sampling).
• Trombe wall : a thermal storage wall of masonry placed between the solar aperture and the heated space. Heat is transferred into the space by conduction through the masonry and, if vents are provided, by natural convection.

  

• U-Value : the thermal conductance of a composite building element: see Thermal Transmittance.
• Ultraviolet radiation :electromagnetic radiation having wavelengths shorter than visible light (<380nm). This invisible form of radiation is found in solar radiation and plays a part in the deterioration of plastic glazings, paint, and furnishing fabrics.It is also the major carcenogen in sunlight. 
• "Usual" temperatures (thermal comfort): If the adaptation processes are working satisfactorily then we expect people to be comfortable in the environments they are accustomed to, for they will have had adequate opportunity to adjust the temperatures to suit their requirements, and to modify their living patterns to suit the temperatures. It follows that knowledge of these 'usual' environments would provide thermal comfort information of a basic kind.
• Useful solar energy : the amount of solar energy contributing to the total heat load. It is expressed in absolute figures (kWh) or per unit collector area (kW/m²).

  

• Vapour pressure: the pressure exerted by the molecules of a given vapour (e.g. water).
• Vasoregulation (vasoconstriction, vasodilation): The physiological mechanism by which the body regulates the supply of blood to the periphery of the body to reduce heat loss from the skin (vasoconstriction) or enhance it (vasodilation).
• Ventilation : the process of supplying or removing air by natural or mechanical means to or from any space. Such air may or may not have been conditioned.
• Ventilation losses : the heat losses associated with the continuous replacement of warm, stale air by fresh cold air.
• Viscosity : that property of semifluids, fluids and gases by virtue of which they resist an instantaneous change of shape or arrangement of parts. It is the cause of fluid friction whenever adjacent layers of fluid move with relation to each other.
• Visible radiation: see below
• Visible spectrum : that part of the solar spectrum which is visible to the human eye: there are not precise limits for the spectral range of visible radiation since they depend upon the amount of radiant power reaching the retina and the responsivity of the observer. The lower limit is generally taken between 360 and 400 nm and the upper limit between 760 mm and 830 nm.
• Visual performance : performance of the visual system as measured for instance by the speed and accuracy with which a visual task is performed.
• Volumetric heat capacity (pc) : a measure of the ability of a unit of volume of material to store heat, namely, the heat stored in a unit of volume of material per degree of temperature rise (J/m³).
• Volumetric heat loss coefficient (G-value) : the total heat loss of a dwelling (through the fabric and ventilation), divided by the heated volume and the temperature at which the loss occurs. (W/m³.K) 

  

• Warmth sensation: Subjective response to the thermal environment.
• Water vapour pressure: The partial pressure of the water vapour in a sample of air
• Water cooling :water used for condensation of refrigerant; condenser water.
• Water wall : a thermal storage wall of water in containers placed between the solar aperture and the heated space . Heat is transferred into the space by conduction and convection through the water.
• Wet-bulb temperature, : thermodynamic wet-bulb temperature is the temperature at which liquid or solid water, by evaporating into air, can bring the air to saturation adiabatically at the same temperature. Wet-bulb temperature (without qualification) is the temperature indicated by a wet-bulb psychrometer constructed and used according to specification.see also evaporative equilibrium
• Wind speed : the speed of the air measured in accordance with the recommendations of the World Meteorological Organisation, normally measured ten metres above ground level (m/s).
• Work plane : reference surface defined as the plane at which work is usually done. (Note: in interior lighting and unless otherwise indicated, this plane assumed to be a horizontal plane 0.85 m above the floor and limited by the walls of the room. in the USA the work plane is usually assumed to be 0.76 m above the floor, in the USSR 0.8 m above the floor.)

  

• Zenith Angle - the angular distance from the sun to the zenith (the point directly above the observer) (at noon: latitude - solar declination).