With all of the discussion around air infiltration these days, we thought we would take a few steps back to review the basics.
Air infiltration is the unintentional introduction of outside air into a building. The building envelope (exterior faces of a building) is the first and best defense against unwanted air infiltration.
Most air infiltrates a building through cracks, gaps, openings along the building envelope, and particularly in and around windows or doors. There are three typical types of air infiltration:
- stack effect
- combustion & ventilation
When wind blows across a building, it creates positive pressure on the windward face and negative pressure on the non-windward facing walls (also called leeward).
Because of the positive pressure on the windward side, air infiltrates through gaps in the envelope into the building. On the leeward side of the building, the negative pressure pulls air out through any openings in the building.
When warm air rises within a tall structure, it is called the “stack effect.” Warm air, being lighter than cold air, rises. When warm air rises in a building, it escapes through the upper levels and any gaps or openings. The rising warm air reduces pressure at the base of the building, drawing in higher pressured outside air. Air infiltration from the stack effect is more pronounced during the winter.
Finally, mechanical equipment can stimulate air movement within the building and through the building enclosures. This movement of air can create pressure differences and can introduce unwanted airflow through the building envelope. Exhaust fans and other forced air ventilation creates a negative pressure inside the building and, in turn, draws in air from the outside. Similarly, combustion in furnaces or boilers exhausts gas to the outside and draws in air from the outside.
Obviously, its key to maintain proper indoor air quality standards through appropriate ventilation and fresh air intake. However, unwanted air infiltration can be extremely detrimental to your wallet and the building’s components.
Besides overworking HVAC equipment and racking up energy costs, infiltration brings humid summer air into the building and warm, moist indoor air into envelope cavities in the winter. In either case, the resulting condensation can turn into mold or rot, producing future structural headaches.