Infiltration is the unintentional or accidental introduction ashrae standard 154 pdf outside air into a building, typically through cracks in the building envelope and through use of doors for passage. Infiltration is sometimes called air leakage.
The leakage of room air out of a building, intentionally or not, is called exfiltration. Infiltration is caused by wind, negative pressurization of the building, and by air buoyancy forces known commonly as the stack effect. ACH is the hourly ventilation rate, divided by the building volume. It can be calculated by multiplying the building’s CFM by 60, and then dividing by the building volume.
In many smaller buildings, ‘forced’ or ‘mechanical’ ventilation is not used to introduce ventilation air. Instead, natural ventilation, via openable windows and other openings, exhaust fans, and infiltration are used to provide ventilation air. Typically, at least one-third air change per hour is considered the minimum. The Standard specifies that forced ventilation is required in houses with infiltration less than 0.
This is typically accomplished with heat recovery ventilation or exhaust fans running constantly or periodically. Because infiltration is uncontrolled, and admits unconditioned air, it is generally considered undesirable except for ventilation air purposes.
Typically, infiltration is minimized to reduce dust, to increase thermal comfort, and to decrease energy consumption. For all buildings, infiltration can be reduced via sealing cracks in a building’s envelope, and for new construction or major renovations, by installing continuous air retarders. In buildings where forced ventilation is provided, their HVAC designers typically choose to slightly pressurize the buildings by admitting more outside air than exhausting so that infiltration is dramatically reduced. HVAC energy consumption is due to infiltration.
Another third is to ground-contact, and the remainder is to heat losses and gains through windows, walls, and other thermal loads. As such, reducing infiltration can yield significant energy savings, with rapid payback.
In cold climates, with a 15 MPH wind, residences often have air exchange rates of 1. 5 ACHs, far in excess of the ventilation air needs and are thus called loose construction. It is very easy to reduce infiltration rates to less than 1.
Smoke candles and blower-door tests can help identify less-than-obvious leaks. The Weatherization article describes methods for energy savings further.
If infiltration is reduced below 0. Fundamentals volume of the ASHRAE Handbook, Ch. Ventilation for Indoor Air Quality, ASHRAE Standard 62-89, American Society of Heating, Refrigerating and Air Conditioning Engineers, Atlanta, GA.
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