Gaines Group Architects

Green Terms Defined: Air Changes per Hour

Green Terms Defined: Air Changes per Hour

Air Changes per Hour (ACH) is the measure how many times the air within a defined space is replaced. This measure is determined by how the house / building allows for air to be pulled in or pushed out. It is important to have the right ACH to keep the indoor-air-quality healthy and your energy bills low.


There are many places for air leakage that negatively impacts the right ACH for a home / building. The key is to stop the unwanted leaks from happening and only use the designed ventilation system for air changes. The goal is to make the home / building as air tight as possible rather than allowing it to “not be too tight.” This approach allows you to protect indoor-air-quality, reduce energy bills, and maintain the right ACH for your particular space.

attic insulation

The only way to determine your ACH is through using a blower door test. The 2012 building code that is used in Virginia has a requirement to measure the ACH for both commercial and residential construction. In Virginia, you can do a visual air leakage test for residential construction (this is not possible), but for commercial you have to do the blower door test on all new construction.