Glossary

Absorptivity a measure of the amount of radiation a surface absorbs, relative to a notional black body. Values range from 0–1.
Admittance a measure of how rapidly heat will pass between the surface of an element and the interior of the building.
Advection the transfer of heat by means of movement of a material, for example, air movement cause by differential wind pressure.
Air permeability a measure of the rate of air infiltration though the external envelope of a building. Expressed as the volume of air passing through one square metre of the building surface per hour, at a pressure difference of 50 Pa (m³/m2.h at 50 Pa).
Airborne transmission the transmission of sound around a building, either directly through gaps and openings, or indirectly as sound waves generate vibrations on one side of building element which then generate sound waves on the other side.
Becquerel (Bq) the unit of radioactivity, defined as one transformation or decay per second. Named after Henri Becquerel, a pioneer researcher of radioactivity.
Candela (cd) the measure of luminous intensity. One candela is 1/683 watt per steradian.
Capillary action the movement of water through narrow spaces in materials, often upward, produced by surface tension in the liquid and the adhesive forces between the liquid and the materials.
Centre pane U-value the U-value of an insulated glazing unit, measured perpendicularly through the middle of the unit.
Combined method a simplified method of calculating the U-value of a building element.
Compressive strength a measure of how a material performs under load. Often quoted as a pressure which will produce a 10% deformation, or a 2% long-term deformation.
Conduction heat transfer within materials resulting from energy being passed from atoms with high internal energy to those with lower internal energy.
Convection the movement of atoms in a fluid as a result of variations in density, often produced by temperature differences.
Cylindrical illuminance a measure of light distribution on cylindrical objects. A value of 150 lx or greater will give lighting which provides good illumination of faces.
Decrement delay the time taken for heat to pass from one side of the structure to the other.
Decrement factor (f) the ratio of the internal temperature range (minimum to maximum) to the external temperature range.
Dew point temperature the temperature at which air at a given vapour pressure would be saturated.
Diffuse lighting lighting conditions where light reaches surfaces after reflection from several surfaces.
Directional lighting light reaching a surface directly from a light source. Sunlight provides directional lighting.
Dry bulb temperature the ambient temperature of air, measured out of direct solar radiation.
Dynamic simulation modelling a method of analysing the energy performance of buildings which simulates the performance on an hourly basis.
Dynamic stiffness a measure of the rigidity of materials.
Electromagnetic radiation energy waves of photons generated by sub-atomic forces.
Emissivity a measure of the effectiveness of a body as a source of radiation.
Expressed as a fraction of the performance of a perfect source of radiation (a ‘black body’). Values range from 0–1.
Enthalpy the total of the sensible and latent heat in the air, which will vary as the air is heated or cooled, or the moisture content changes. Used when calculating the energy involved in humidification and dehumidification.
Equivalent area (vents) the area of a single opening which would provide the same air flow as the vent, taking account of air turbulence at the vent openings.
Flanking transmission the transmission of sound through a building element which is adjacent to one being treated.
Free area of vent the total unobstructed area of a vent.
Gas laws the laws of physics which describe the behaviour of ideal gases.
Heat the internal energy possessed by matter.
Heat capacity the amount of energy required to raise the temperature of a layer or construction by 1 K.
Horizontal illuminance the illuminance on a horizontal surface.
Illuminance the amount of light received by a surface, measured in lux.
Impact transmission sound produced and transmitted by an impact on the building fabric, for example, footsteps on a floor.
Infiltration the movement of air through the fabric of a building.
Interstitial condensation condensation which occurs within building elements.
Intrusive sound sound reaching a room or space from an adjoining space or from outside the building. Typically it is a nuisance.
Ionising radiation radioactivity which results in atoms losing or gaining electrons and becoming ionised, (ie. positively or negatively charged).
Kappa-value (κ-value) a measure of the heat capacity of a building element.
Latent heat the energy required to change a material from solid to liquid or liquid to gas. It does not result in temperature change.
Linear thermal transmittance rate of heat transfer through a junction between two building
ψ-value, (psi-value) elements, or at the perimeter of an opening.
Lumens (lm) the measure of light emitted by a source. A light source which radiated 1 cd in all directions would have a luminous flux of 12.6 lm.
Luminous flux the total amount of light given out by a source, measured in lumens.
Luminous intensity the strength of a light source, defined as the power of visible light emitted for a solid angle of one steradian. Measured in candela.
Lux (lx) the measure of the amount of light received by a surface. 1 lx is = 1 lumen/m2.
Metabolic gains gains in heat resulting from the presence of people in a building.
Modelling the balance of direct and diffuse light in a space. A good balance enhances perception of the space and of human faces.
Moisture content the amount of water vapour in air. Expressed in grams per kilogram of dry air (g/kg).
Numerical modelling computer-based simulation of heat flows through building elements and components. More accurate than the combined method, but slower.
Operative temperature a temperature value which incorporates the radiant and air temperatures and the effects of air movement.
Partial pressure in a mixture of gases that part of the total pressure which is produced by one of the gases. For example, water vapour in air.
Percentage saturation the ratio of moisture content of a body of air to its moisture content at saturation. Similar to, but not identical to relative humidity.
Phase change materials (PCM) composite materials which include a material which melts at a set temperature, typically just above normal room temperature.
Photometric data sheets data sheets which describe the performance of a luminaire.
Pressure (of a gas) the force exerted by its atoms and molecules as they collide with each other and with solid surfaces. Expressed in pascals (Pa) or kilopascals (kPa).
Psychrometric chart a chart which graphically expresses the relationships between air, vapour pressure and temperature.
Psychrometric connected with the physical and thermodynamic properties of gas/air mixtures.
Radioactive decay the change of a radioactive element to another element as a result of charged particles being emitted.
Radioactivity the emission of sub-atomic charged particles from a material.
Relative humidity the vapour pressure in air at a given temperature, expressed as a percentage of the saturated vapour pressure at that temperature.
Reverberation the lengthening of sound duration as sound waves reflected from a surface reaching a listener after those coming directly from the source.
Saturation vapour pressure (SVP) the maximum possible vapour pressure at a given temperature.
SAP the Standard Assessment Procedure (generally known by its abbreviation) is the methodology used by the UK Government to assess and compare the energy and environmental performance of dwellings
SBEM the Simplified Building Energy Model is a computer program developed for the UK Government to assess a building’s energy consumption.
Sensible heat the heat which results in a change of temperature in a material. ‘Sensible’ in the sense of being sensed or felt.
Solar gain an increase in energy resulting from solar radiation falling on a surface.
Specific heat capacity a measure of the amount of energy required to raise the temperature of a material. Measured in J/kgK.
Stack effect the vertical air flow produced by temperature differences in an air column and wind action.
Standard temperature a defined temperature and pressure used when examining the and pressure behaviour of gases. Usually 0ºC and 100 kPa.
Steady state analysis a method of assessing interstitial condensation risk in a building element using monthly average data. Also known as the Glaser method.
Steradian a solid angle which is roughly 1/12.6 of the surface of a sphere.
Surface factor (F) the ratio between the range of heat flow out of a surface to the range of heat flow into the surface.
Temperature the fundamental unit of measure of heat.
Temperature factor a measure of the cooling which takes place at the surface of a building element.
Thermal bridging the regular interruption of one material in a construction by another with a different thermal conductivity or thermal resistance.
Thermal conductivity a measure of the rate of heat transfer through a solid material. Expressed in W/mK.
Thermal diffusivity a measure of how rapidly heat travels in a material.
Thermal effusively a measure of the thermal inertia of a material.
Thermal mass the capacity of the building fabric to absorb heat.
Thermal resistance a measure of the resistance to heat transfer in a specified thickness of a material or cavity. Expressed in m² K/W.
Thermal transmittance see U-value.
Time factor the time delay between the maximum flows into and out of an element’s internal surface.
Transient analysis a method of condensation risk analysis which examines moisture movement on an hourly basis.
Troposphere the lowest layer of the earth’s atmosphere.
U-value the rate of heat transfer through a building element, expressed in W/m2K. Also known as the thermal transmittance.
Vapour permeability the measure of how rapidly water vapour can diffuse through a material. Measured in gm/MNs.
Vapour permeance the rate at which water vapour can diffuse through a given thickness of a material. Measured in g/MNs.
Vapour pressure the pressure exerted by water vapour in a volume of air, or in a material. Usually measured in kilopascals (kPa).
Vapour resistance the degree to which a material resists the passage of water vapour m easured in MNs/g.
Vapour resistivity the measure of a material’s resistance to the diffusion of water vapour. Measured in MNs/gm.
Wet bulb temperature air temperature measured using a bulb which is covered in wet muslin and moved rapidly through the air. With a corresponding dry bulb temperature it allows vapour pressure to be determined.
Wind load the forces acting on a building which are produced by differences in air pressure.
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