Alpha (a) - value

The overall thermal performance of the entire building envelope, taking into account the positive effect of the U-value of all plane elements (roof, rooflights, walls, etc) and the negative 'heat draining' thermal bridging effect of junctions, details and interfaces, which act as direct heat conductors from the inside to the outside of the building. To achieve compliance under Part L of the Building Regulations, two a-values must be calculated, one for a notional building and one for an actual building. The objective is to establish that the a-value of the actual building is lower than the notional building.

Abiotic components

The non-living components of the biosphere. Chemical and geological factors, such as rocks and minerals, and physical factors, such as temperature and weather, are referred to as abiotic components.

Abiotic depletion

Refers to the exhaustion of natural (non-living) resources such as iron ore, copper and fossil fuels.

ACH (Air Changes per Hour)

Measures ventilation as the number of times per hour that the air inside a building is changed. Units m3 hr/ m3.

Acid rain

See Acidification


The result of acidifying pollutants emissions, such as SO2 or NOx, to the air. These emissions have negative impacts on soil, groundwater, surface waters, biological organisms, ecosystems and materials


The sustainable building association (see also: )

Aerobic digestion

The bacterial process of decomposition or rotting occurring in the presence of oxygen - aka composting.


The planting of trees in areas that have not previously held forests.

Air barrier / airtightness membrane

Membranes that prevent the flow of air from the inside to the outside of a building (see ‘airtightness layer’ below). They are available in different materials according to their function. In its basic form, an air barrier / airtightness membrane is nothing more sophisticated than polythene sheeting that can also double as a vapour barrier; A more sophisticated membrane is one that permits the flow of moisture through the material which is allowed to diffuse through the structure to the exterior (‘breathing wall’). Air / vapour barriers can also include materials such as OSB and plasterboard. (see also: Airtightness membranes)

Air film resistance

Results from convection currents at the surface of a material. As the surface heats up or cools down, it affects the temperature of the air immediately adjacent. This then starts to rise or fall depending on whether it is hotter or colder. This has the same effect as increasing the resistance of the material to the flow of heat.

Air infiltration

Another term for air leakage / permeability

Air leakage index

The measure of air leakage per unit thermal envelope area including the roof and walls – but not the ground floor. Units are m3/m2hr at 50 Pascals or m/h @ 50 Pa.

Air permeability

Determined by Part L of the Building Regulations as the uncontrolled air leakage through the building envelope including the roof, walls and ground floor. Air permeability is defined by BS EN 13829. Units are in m3/m2hr at 50 Pascals or m/h @ 50 Pa.


Part L of the Building Regulations specifies maximum values for the air permeability of dwellings, commercial and public buildings. 

Airtightness layer

Essential to the energy performance of a building, the airtightness layer is the method by which the flow of air through the building structure is controlled. The layer represents an unbroken / un-penetrated envelope encompassing the interior of the building which prevents warm air ‘leaking’ to the exterior. In timber frame buildings the airtightness layer usually comprises of a membrane integrated into the structural element (located more often on the ‘warm’ side of the insulation), whereas in masonry construction plaster usually acts as an effective barrier.

Airtightness line

A design technique that defines the location of the airtightness layer, usually represented by a line on the drawing. Having defined the line, details can be developed to ensure the continuity of the airtightness layer around junctions / connections / penetrations within the structure. 

Airtightness test

A requirement of Building Regulations, the test establishes the effectiveness of a building’s airtightness through measuring the extent to which air is lost through leaks in the building, in units of air changes per / hour at a test pressure of 50 pa. The current regulations require a maximum of 10 whereas the more demanding Passivhaus standard requires 0.6.
The most common method of testing is through use of a ‘Blower Door Test’ which uses a fan to ‘blow’ air into the building in order to achieve the necessary indoor air pressure.