Air Infiltration: Definition, Rates & Calculation

Instructor: Dina El Chammas Gass

Dina has taught college Environmental Studies classes and has a master's degree in Environmental and Water Resource Engineering.

This lesson defines the term 'air infiltration' and provides an understanding of air infiltration rates. The lesson also covers the way air infiltration rates are calculated.

What is Air Infiltration?

Air infiltration is the unintentional flow of air into a building. It is air that's entering your home through cracks in the home's envelope. Window frames, door frames, and areas in the home where the ceiling meets the wall or your walls meet the floor are common areas for leaks. Places where wiring or pipes enter the house from the outside are also common infiltration sites.

You want some air infiltration to occur so that you're getting fresh air into a household even when windows and doors are shut, but too much infiltration is undesirable. If you're cranking up the heat you sure don't want cold air seeping through the cracks. It's like trying to fill a container with water when there is a hole in the bottom.

What Causes Air Infiltration

Air infiltration is caused by wind, mechanical equipment, and something called the stack effect. Keep in mind that when air infiltrates a home, the pressure builds up. When the pressure builds up, air is pushed out at some other place in the home. This also holds for air flowing the other way as well; if air is being pushed out of the home, thereby lowering air pressure, then air is likely being drawn into (infiltrating) the house somewhere else. Pushing air out of a home is called air exfiltration. It's the same concept as using a straw. You suck the air out the straw, which in turns sucks liquid into the straw. Exfiltration and infiltration go hand in hand.

Wind can blow right through cracks and end up inside a house (or other building). Air infiltrates where the wind hits, and leaves from the other end of the house, downwind. Mechanical equipment, such as bathroom vent fans or stove-top fans, work by pushing air from inside the home to the outside. This causes a reduction in pressure inside the house, which in turn causes air from the outside to be sucked in. The final culprit is the stack effect. This has to do with the simple concept of air rising when it gets hot. When hot air rises in a house it leaves a low pressure zone behind, at the lower level. Air from the outside then rushes in to the low pressure zone to fill the gap. The hot air exfiltrates out of the house from above.

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