air leakage

Air leakage is the uncontrolled transfer of air in and out of a building; the main cause of draughts. Not only does this lead to uncomfortable living and working conditions, left unchecked, it can also represent as much as 60% of the total heat loss, resulting in higher heating energy bills. The main causes and locations of air leakage are:

  1. Gaps around doors and windows or other penetrations through the building envelope
  2. Services routes for pipes and cables, particularly at outlet locations.
  3. Joist connections with suspended or intermediate floors.
  4. Absorbent construction materials: blockwork, mortar joints, timber, etc.

Pressure differentials between the outside and the inside of a building will cause the transfer of air between the internal warm space and outside. This is often caused by the wind creating a negative pressure inside as it blows over the house. Whilst the house is under negative pressure, warm air will be pulled out through the gaps and cracks in the building structure. Other contributing forces that give rise to heat loss are the natural stack effect, i.e. warm air rising and then finding its way out at high level (e.g. loft), and ventilation systems creating negative or positive pressures inside the house.

Why should air leakage be measured?

Air leakage can affect the thermal performance of a building leading to:

  • Uncomfortable conditions for the occupants due to draughts from areas such as, poorly sealed windows or, gaps around suspended floors.
  • Higher bills – uncontrolled air leakage represents a heating energy loss.
  • Unsatisfactory control of heating and ventilation installations to maintain comfortable conditions.
  • Deterioration of the building fabric resulting from interstitial condensation.
  • Poor air quality as a result of pollution entering the house from the outside.

Airtightness guidelines

The result of an air leakage test is expressed as the rate of leakage per square metre of the external building elements (walls/floors/roof) at a pressure differential of 50 Pascals between the inside and outside of a building (m3.h-1.m-2). A theroetical result of 0 m3.h-1.m-2 represent a hermetically sealed box. For new dwellings, the upper limit is 10 m3.h-1.m-2, although the target CO2 calculation may dictate this value to be much lower. By contrast, Passivhaus requires a value of <1.0 m3.h-1.m-2.

The following are air permeability guidelines given by the Energy Savings Trust, Improving airtightness in dwellings (GPG224):

Building Type Good Practice Best Practice
Dwellings 7 3
Dwellings with whole house ventilation 4 3