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The average blower door test result depends largely upon the age of the home, the insulation levels, and the age of the windows and doors. Generally, a new home should test less than 3.5 ACH50, as the insulation levels and efficiency of the windows/doors would make for a fairly airtight home.

To determine the percent reduction in CFM50, subtract the after-tightening test result from the before-tightening test result. Divide this difference by the before-tightening result and multiply by 100.

The average blower door test result depends largely upon the age of the home, the insulation levels, and the age of the windows and doors. Generally, a new home should test less than 3.5 ACH50, as the insulation levels and efficiency of the windows/doors would make for a fairly airtight home.

The formula is the rate of airflow in cfm x 60 minutes in an hour / volume of the home = number of air changes per hour. As an example, a home had a blower door test of 750 cfm and a calculated volume of 12,000 cubic feet. Using the formula, 750 cfm x 60 minutes / 12,000 cu.

To calculate the ACH50 from the CFM50 given by the blower door we need to know the volume of the building. Multiply the flow by 60 to convert to hours, then divide by the volume. (ACH50 = airflow (CFM50) X 60 / conditioned volume) If the number is under 7 then it is a pass.

Every 10 points on the blower door is about 1 square inch of air leak. So a 1400 cfm50 blower door number is about 1 square foot of air leakage across the whole home. That's not that much to chase, so every little bit counts. (1400/10 = 140 square inches/144 square inches/square foot = 1 square foot)

The Blower Door measurement is a pressure difference measurement in which the influence of the wind at wind force 5 on a building is simulated. This corresponds approximately to the conditions on a stormy autumn day. The air exchange rate is determined and leaks in the building envelope are documented.

A building's ACH50 number indicates how tightly a building was originally constructed (or later air-sealed) and is an excellent gauge for comparing leakiness between buildings. The lower the ACH50 values, the tighter the building.