How to Calculate Air Infiltration

Calculating air infiltration rates involves understanding the air leakage characteristics of the building envelope and measuring the difference in air pressures between the interior and exterior. Here's a step-by-step guide to help you calculate air infiltration:

Step 1: Determine the Air Leakage Characteristics

- Identify and catalog all potential sources of air infiltration in the building envelope, such as windows, doors, cracks, and gaps.

- Gather information on the size and shape of each infiltration point.

- Assess the airtightness of the building by conducting a blower door test or pressurized fan test. This test pressurizes or depressurizes the building and measures the air leakage rate.

Step 2: Calculate Air Leakage Area

- Based on the information collected in step 1, calculate the total effective leakage area (ELA) of the building. ELA represents the total area through which air can potentially infiltrate.

Step 3: Measure Pressure Difference

- Install differential pressure sensors or gauges at strategic locations inside and outside the building to measure the difference in air pressure between the interior and exterior.

Step 4: Calculate Air Infiltration Rate

- Use the following formula to calculate the air infiltration rate (Q):

- Q = ELA x P

- Where:

- Q = air infiltration rate in cubic feet per minute (CFM) or cubic meters per hour (CMH)

- ELA = effective leakage area in square feet (sq. ft.) or square meters (sq. m)

- P = pressure difference between interior and exterior in Pascals (Pa)

Step 5: Adjust for Wind Speed

- Air infiltration rates can be influenced by wind speed, so it's important to adjust your calculations accordingly. Use the following formula:

- Q_adj = Q x [1 + (C x W)]

- Where:

- Q_adj = adjusted air infiltration rate

- Q = air infiltration rate calculated in step 4

- C = a constant that depends on building height, typically between 0.05 and 0.15

- W = wind speed in miles per hour (mph) or meters per second (m/s)

Step 6: Analyze and Interpret Results

- Compare the calculated air infiltration rate with industry standards and benchmarks to assess the air tightness of the building.

- Identify areas of significant infiltration and consider implementing weatherstripping, caulking, or other sealing measures to improve the building's airtightness.

Remember, calculating air infiltration rates requires careful attention to detail and accurate measurements. If you're not confident in your ability to perform these calculations, it's best to consult with a qualified professional, such as an energy auditor or building scientist.