Absolute humidity – the weight of water vapour per unit volume.

Absorbent – a material which due to an affinity for certain substances, extracts one or more such substances from a liquid or gaseous medium with which it contacts, and which changes physically or chemically or both during the process. Calcium chloride is an example of a solid absorbent, while solutions of lithium chloride, lithium bromide, and ethylene glycol are liquid absorbents.

Absorber – the blackened surface in a solar collector that absorbs solar radiation and converts it to heat.

Absorptance – the ratio of the radiation absorbed by a surface to the total energy falling on that surface.

Active solar energy system – a system that requires auxiliary energy for its operation, e.g., energy to operate fans and pumps.

Activated alumina – a form of aluminium oxide which adsorbs moisture readily and is used as a drying agent.

Adfreezing – the process whereby wet soils freeze to below grade materials such as fountains, walls or insulation, forcing movement of the material.

Adiabatic process – a thermodynamic process during which no heat is extracted from or added to the system.

Adsorbent – a material which has the ability to cause the molecules of gases, liquids, or solids to adhere to its internal surfaces without changing itself physically or chemically. Certain solid materials such as silica gel and activated alumina have this property.

Air barrier – a material carefully installed within a building envelope assembly to minimize the uncontrolled passage of air into and out of a dwelling.

Air change – the replacement of a quantity of air in a volume within a given period of time. This is expressed in number of changes per hour. If a house has 1 air change per hour, all the air in the house will be replaced in a 1-hour period.

Air change per hour (ach) – a unit that denotes the number of times a house exchanges its entire volume of air with outside air in an hour.

Air cleanser - a device used to remove airborne impurities.

Air leakage – the uncontrolled flow of air 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 air leakage.

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 air. This may refer to static (atmospheric) pressure, or dynamic components of pressure arising from airflow, 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 buildings structure.

Air, ambient – surrounding air.

Air, saturated – 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 co-exist at the same dry-bulb temperature.

Air, standard – dry air at a pressure of 101.325 kPa at a temperature of exactly 20 °C. Under these conditions, the density is 1.2041 kg/m³

Altitude angle – the angular height of a point above the horizontal plane, i.e. solar altitude – the angle between the line joining the center of the sun and its projection on the horizontal plane.

Anemometer – an instrument for measuring the velocity of air.

Angle of incidence – the angle that the sun’s rays subtend with a line perpendicular to a surface.

Atomize – reduce to fine spray.

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. Compare with semi-enclosed sunspace.

Awning – an exterior, movable and usually flexible element. Protects detaining or diffusing solar radiation at certain angles.

Azimuth angle, solar – the angle on a horizontal surface between true south and the projection of sun’s ray on the horizontal surface (negative before noon, positive after noon.)

Backdraft – (flow reversal) the reverse flow of chimney gases into the building through the barometric damper, draft hood, or burner unit. This can be caused by chimney blockage or it can occur when the pressure differential is too high for the chimney to draw.

Beam or direct radiation – radiation coming directly from the sun without its direction undergoing any change.

Berm – a man-made mound or small hill of earth.

Bimetallic element - an element formed of two metals having different coefficients of thermal expansion, used as a temperature control device.

Black body – a perfect absorber and emitter of radiation. A cavity is a perfect black body. Lampblack is close to a black body, while aluminium (polished) is a poor absorber and emitter of radiation.

Brightness – the subjective human perception of luminance.

Building orientation – the siting of a building on a plot, generally used to refer to solar orientation.

Calorific value – the energy content per unit mass (or volume) of a fuel, which will be released in combustion. (kWh/kg, MJ/kg, kWh/m³, MJ/m³)

Candela (cd) – an SI unit of luminous intensity . An ordinary candle has a luminous intensity of one candela.

Chimney 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 of the surrounding air or gas. In buildings, the tendency towards displacement (caused by the difference in temperature) of heated internal air by unheated outside air, due to the difference in their densities.

Clear sky – A sky condition with few or no clouds, usually taken as 0-2 tenths covered with clouds. Clear skies have high luminance and high radiation, and create strong shadows relative to more cloudy conditions. The sky is brightest nearest the sun, whereas away from the sun, it is about three times brighter at the horizon then at the zenith.

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.

Clo – clothing factor , a measure of the insulating value of clothing. For example, 0.3 clo is typical for light summer clothing and 0.8 is typical for heavy winter clothing.

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

Collector, focusing – a collector that has a parabolic or other reflector which focuses sunlight onto a small area for collection. A reflector of this type can obtain considerably higher temperatures but will only work with direct beam sunlight.

Collector, solar – a device for capturing solar energy, ranging from ordinary windows to complex mechanical devices.

Combustion air – the air required to provide adequate oxygen for fuel burning appliances in the building. The term ‘combustion air’ is often used to refer to the total air requirement of a fuel burning appliance including both air to support the combustion process and air to provide chimney draft (dilution air).

Comfort chart – a chart showing dry-bulb temperatures and humidities (and sometimes air motion) by which the effects of various air conditions on human comfort may be compared.

Comfort zone – on the bioclimatic chart , the area of combined temperatures and humidities that 80% of people find comfortable. People are assumed to be in the shade, fully protected from wind, engaged in light activity, and wearing moderate levels of clothing that increases slightly in winter.

Condensation – the process of vapour changing into the liquid state. Heat is released in the process.

Conditioned and unconditioned spaces - conditioned spaces need air treatment such as heat addition, heat removal, moisture removal, or pollution removal. Unconditioned spaces do not need such air conditioning, and no effort is made to control infiltration.

Conductance (C) - a measure of the ease with which heat flows though a specified thickness of a material by conduction. Units are W/m² °C.

Conduction – the process by which heat energy is transferred through materials (solids, liquids or gases) by molecular excitation of adjacent molecules.

Conductivity – the quantity of heat that will flow through one square metre of material, one metre thick, in one second, when there is a temperature difference of 1ºC between its surfaces.

Convection – the transfer of heat between a moving fluid medium (liquid or gas) and a surface, or the transfer of heat within a fluid by movements within the fluid.

Cooling load – a load with net cooling required.

Cross ventilation – ventilative cooling of people and spaces driven by the force of wind. When the outside air is cooler than the inside air, heat can be transferred from the space to the ventilation air. Cross ventilation also removes heat from people by convection and by increasing the rate of sweat evaporation. The cooling rate from cross ventilation is determined by wind speed, opening sizes and temperature difference between the inside and outside. See also, stack ventilation.

Daylight – illuminance from radiation in the visible spectrum from the diffuse sky, reflected light, and direct sun that lights a room.

Daylight envelope – the maximum buildable volume on a site that will not unduly restrict daylight available to adjacent buildings.

Daylight factor (DF) – the proportion of interior horizontal illuminance (usually taken on the work plane) to exterior horizontal illuminance under an unobstructed sky . It is sum of the sky component and the internal reflected component. The range is 0-100%, but for most rooms it is usually limited to 1-10%.

Decrement factor – ratio of the maximum outer and inner surface temperature amplitudes taken from the daily mean.

Density – the mass of a substance, expressed in kilograms per cubic metre.

Diffuse radiation – radiation that has travelled an indirect path from the sun because it has been scattered by particles in the atmosphere such as air molecules, dust and water vapour. Indirect sunlight comes from the entire skydome.

Direct gain – the transmission of sunlight through glazing directly in to the spaces to be heated, where it is converted to heat by absorption on interior mass surfaces.

Direct sunlight – the component of visible spectrum radiation that comes directly from the sun without being diffused or reflected.

Direct radiation – the component of solar radiation that comes directly from the sun without being diffused or reflected.

Diffuse reflectance – reflectance is the ratio of reflected radiation to incident radiation. Diffuse reflectance spreads the incident flux over a range of reflected angles/directions.

Diurnal – relating to a 24-hr cycle. A diurnal temperature swing is the cycle of temperatures over the course of one 24-hr period.

Downdraft evaporative cooling tower - a cooling system that humidifies and cools warm dry air by passing it though a wetted pad at the top of a tower. The cooled air being denser, falls down the tower and into the occupied spaces below, drawing in more air through the pads in the process. Consequently, no distribution fans are required.

Dry bulb temperature – the temperature of a gas of mixture or gases indicated by an accurate thermometer after correction for radiation.

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