Airtightness
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Air leakage is a description of the airtighness of a building but it does not give an accurate description of the problem. Un-designed holes with the fabric of a building can allow cold air to enter the house or warm air to escape; a two way street so to speak.
Attempting to quantify the cost is difficult but research suggests that leakage of air from a building can be responsible for up to 40% of heat loss (Webb – Barton 2002, BRE report BR448 Airtightness in commercial and public buildings).
In years past we had buildings which had numerous un-designed holes in the fabric of the building allowing air leakage such as chimneys, suspended timber floors with gaps in the floorboards, one reason why carpets became so popular – to stop the draughts through the floorboards.
As the house building industry strives towards lower carbon emissions un-designed holes within the fabric of the building need to be reduced by improved design and improved labour. For this reason attention to airtightness must be paid at design stage and the early phase of construction. The air tightness is checked at a fairly late stage in the construction process and if this highlights a problem then rectifying the problem can be destructive and expensive.
The airtightness test is measured in M3 of air lost from a dwelling per hour divided by the internal floor area when pressurised at a pressure of 50 pascals. All very technical but to relate this to air speed the Lawrence Berkeley Institute has made comparisons between 50Pa air pressure and air speed and have concluded that taken as an average this relates to a wind speed of 4.4 MPH.
The UK building Regulations give a maximum air leakage of 10m3/(h.m2) @ 50Pa, Holland has a maximum allowed value of 6, Germany around 3.6. The Passive House Standard of 0.6ACH @ 50 Pa. is a different metric and is not a direct comparison to m3/(h.m2) @ 50Pa, however, the AECB gives guidance that the Passive House standard relates to between 0.75 and 0.6m3/(h.m2) @ 50Pa.