Insulation Terms

Air Curtain

An air curtain is a device used for separating two spaces from each other. It usually happens at the exterior entrance. The most common configuration for air curtains is a downward facing fan mounted over an opening. Normally this opening is an entrance to a building and the air curtain is intended to help keep outside air out and avoid cold draught by mixing in warm air from the air curtain.

Air Permeability

Air permeability is the physical parameter used to quantify air tightness of the building fabric. It measures the resistance of the building envelope to infiltration. It is defined as the average volume of air (in cubic metres per hour) that passes through unit area of the building envelope (in square metres) when subject to an internal to external pressure difference of 50 Pascals. The envelope area of the building is defined as the total area of the floor, walls, and roof separating the interior volume from the outside environment. It is measured with ventilators closed. For a newly constructed building the default value is 10m3/h/m2 at 50PA, this is used as the default value in the calculation of an asset rating if not known.

Air Tightness Test

In its simplest form, an air tightness test measures the rate at which air leaks into or out of a building. The test takes into account the air leakage from windows, service entry points and cladding. The most common test method used is to pressurise a building using a mechanical fan connected to a temporary sealed aperture (i.e. a door or window) and measure the resulting pressure difference.

Controlled Fitting

A window, door or roof-light which separate a thermally conditioned space from the external environment, and includes all parts of the element (i.e Frame and glazing)

Heat Loss

A decrease in the amount of heat contained in a space, resulting from heat flow through walls, windows, roof and other building surfaces and from infiltration of warm air.

Solar Gain

Solar gain refers to the increase in temperature in a space, object or building that results from solar radiation. The amount of solar gain increases with the strength of the sun, and with the ability of the building fabric to transmit or resist the radiation.

Thermal Element

A wall, floor or roof (but not including windows, doors or roof-lights) which separate a thermally conditioned space from the external environment, and include all parts of the element.


The use of infrared thermography whereby temperatures of a wide variety of targets can be measured remotely and without contact. This is accomplished by measuring the infrared energy radiating from the surface of the target and converting this measurement to an equivalent surface temperature.

Transmittance Factors

Transmittance factors are applied specifically to glazed controlled fittings and have two parameters, T-Solar and L-Solar. T-solar is the total solar energy transmittance defined as the time-averaged ratio of energy passing through the un-shaded element to that incident upon it. L Solar is the light transmittance (gL).It is the amount of visible solar energy that passes through a glazing system, expressed as a fraction of the visible solar energy incident on the window. It is the L-Solar that is used for day lighting calculations.


The U-Value is the measure of the amount of energy (heat measured in watts) that is transferred through an area (measured in m2) of material when there is a 1oC difference in the temperature between the inside and outside. The lower the U-Value the better the material is at preventing heat loss. This has a significant effect on the overall energy rating. This is required for both thermal elements and controlled fittings; in addition, for glazing the light and solar transmittance factors are required.