Building airtightness
Building airtightness (also called envelope airtightness) can be defined as the resistance to inward or outward air leakage through unintentional leakage points or areas in the building envelope. This air leakage is driven by differential pressures across the building envelope due to the combined effects of stack, external wind and mechanical ventilation systems.
Airtightness is the fundamental building property that impacts infiltration (the uncontrolled inward leakage of outdoor air through cracks, interstices or other unintentional openings of a building, caused by pressure effects of the wind and/or stack effect).
An airtight building has several positive impacts when combined with an appropriate ventilation system (whether natural, mechanical, or hybrid):
A number of studies have shown substantial energy savings by tightening building envelopes. The ASIEPI project technical report on building and ductwork airtightness estimates the energy impact of envelope airtightness in the order of 10 kWh per m2 of floor area per year, for the heating needs in a moderately cold region (2500 degree-days). Experimental data showing the energy savings of good airtightness were also published by the Building Research Establishment in the UK as well as REHVA journals' special issue on airtightness. They conclude 15% of the space conditioning energy use can be saved in the UK context going from 11.5 m3/(m2·h) @50 Pa (average current value) down to 5 m3/(m2·h) @50 Pa (achievable).
Given its impacts on heat losses, good building airtightness may allow installation of smaller heating and cooling capacities. Conversely, poor airtightness may prevent achieving the desired indoor temperature conditions if the equipment has not been sized with proper estimates of infiltration heat losses.
From an energy point of view, it is almost always desirable to increase air tightness, but if infiltration is providing useful dilution of indoor contaminants, indoor air quality may suffer. However, it is often unclear how useful this dilution is because building leaks cause uncontrolled airflows and potentially poorly ventilated rooms although the total building air exchange rate may be sufficient. This adverse effect has been confirmed by numerical simulations in the French context which has shown that typical mechanical ventilation systems yielded better indoor air quality with tighter envelopes.
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