Glossary

Air Infiltration

Air ingress into a building through cracks or gaps.

Blown Wool

Loose mineral wool insulation applied by blowing, usually into cavity walls or lofts.  

Breather Membrane

A breather membrane repels water that penetrates an outer cladding material but is permeable to water vapour escaping from the structure. It is defined as a material with a vapour resistance between 0.1 and 2.0 MNs/g, although the norm is for breather membranes to have a maximum vapour resistance of 0.6 MNs/g.

Cavity Tray

A damp-proof membrane or pre-formed tray crossing the cavity of a wall at interruptions in the cavity. Rises from the outside of the cavity upwards at least 150mm.

Condensation

When water vapour from the air comes into contact with cold surfaces, the vapour condenses on the cooler surface.

Conduction

The transmission of heat through, along or from one material to another material in contact with it.

Convection

The transfer of heat by movement of air.

Dewpoint

A calculated temperature at which water vapour will condense.

Emissivity

Emissivity is a measure of the radiation emitted from a surface. 'Black bodies' (materials with matt black surfaces) have a high emissivity & emit large amounts of radiation. In contrast shiny surfaces, such as polished aluminium, have a low emissivity. This characteristic can be exploited in buildings by providing aluminium foil facings on insulation materials, facing out towards cavities to reduce the radiated heat loss. Matt black has a typical emissivity of 0.95, compared to 0.08 for polished aluminium. But most building surfaces, even white painted surfaces, have high emissivities, of around 0.90 to 0.95.

Fire Resistance

The ability of a building element or component to act as a barrier to the spread of fire and confine it to the area in which it started. Contains flames and inflammable gas and prevents not only the transmission of flames and smoke but also of heat to the other side of the construction.  

Fire Resistance:

Heat Transmission Contains flames and inflammable gas for a short period of time, but does not prevent the transmission of heat to the other side of the construction (example: wired glass).   Frequency of Sound The frequency of sound is measured in Hz (hertz). The higher the value, the lighter the tone (bass - treble). The frequency of speech lies primarily between 125 and 8000 Hz, while audible sound lies between 20 and 20 000 Hz.  

Heat Loss

Heat transfer from internal spaces to external areas through conduction, convection and radiation.  

Interstitial Condensation

Interstitial condensation is condensation that occurs within a building element. It can be very harmful if interstitial condensation occurs on surfaces within the structure, as this can lead to the rot of timber or oxidation of metal surfaces, and other problems. There are many instances when condensation can form on the surface or interface of two materials without causing problems i.e. the cavity side of an external brick or block leaf.  

Kelvin

An absolute temperature scale in which the temperature intervals correspond to those of the Celsius scale and the freezing point of water is 273 Kelvin (written 273K). When referring to temperatures on the Kelvin scale, refer to "Kelvin" and not "degrees Kelvin". Zero Kelvin (0K) is known as "absolute zero".

Noise Reduction Coefficient (NRC).

A single value for sound absorption, derived as the mean value of 4 frequencies in the range 250-2000 Hz.  

Radiation

The process by which heat is emitted from a body through open space, for example, sunlight.

Sound Absorption

Sound energy converted into mechanical vibration energy and/or heat energy. Sound absorption is usually expressed as a sound absorption coefficient.  

Sound Insulation

The ability of a building element or building structure to reduce the sound transmission through it. The sound insulation is measured at different frequencies, normally 100-4000 Hz. Airborne sound insulation is expressed by a single value, Dn,c,w , Rw or R'w. Impact sound insulation is expressed by a single value Ln,w or L' n,w.  

Sound Strength (dB)

Measured in dB (deciBel). dB is measured at different frequencies.  

Surface Resistance

Surface resistance (Rs) is the reciprocal of surface coefficient. Its units are m2/K/W.   Temperature Temperature levels are commonly measured in degrees Celsius (°C) whereas temperature differences are measured in Kelvin (K). Each unit Celsius is the same size as each unit Kelvin, so that, if the outside temperature was 3°C and the internal temperature was 18°C the difference in temperature would be 15K. Note that the degree symbol ° is used with degrees Celsius but not with Kelvin.  

Thermal Conductivity

The rate at which heat is transmitted through a material, measured in watts per square metre of surface area for a temperature gradient of one Kelvin per metre thickness, simplified to W/mK.  

Thermal Mass

The mass in a structure which is used to absorb solar heat during the day and then release the heat in the evening.  

Thermal Resistance

A measure of the thermal resistance of a particular thickness of material. It is measured in m2K/W and is equal to the thickness of the material (in metres) divided by the conductivity of that material. Surfaces and cavities also provide thermal resistance and there are standard figures for these resistances that must be taken into account when calculating U-values. The resistances of each material within an element are added together to determine the overall resistance of the element. The reciprocal of the overall resistance is the U-value.  

Thermal Resistivity

Resistivity is a measure of a material's thermal resistance and is the reciprocal of its conductivity. It is a measure of the thermal characteristic of a material and is independent of its thickness.  

U-value (Thermal Transmittance)

Commonly known as the U-value, it is a measure of the rate of heat loss of a building component. It is expressed as watts per square metre, per degree Kelvin, W/m2K. The U-value is calculated from the reciprocal of the combined thermal resistances of the materials in the element , air spaces and surfaces, also taken into account is the effect of thermal bridges, air gaps and fixings.  

Vapour Control Layer

A vapour control layer is defined as a material that substantially reduces the water vapour transfer through a building element into which it is incorporated i.e. polythene sheet materials or foil backed plasterboard. Vapour control layers are sometimes required on the warm side of the insulation, to reduce the possible risk of interstitial condensation within the construction element.  

Vapour Permeable Underlay

A vapour permeable underlay repels water that penetrates a roofing finish but is permeable to water vapour escaping from the structure. It is usually defined as a material with a vapour resistance of not more than 0.25MNs/g.  

XPS - Extruded Polystyrene

A cellular plastics insulation material that has been extruded, with or without a skin and which has a closed cell structure, for example, Polyfoam