When to Perform a Blower Door Test (And when not to) – GreenBuildingAdvisor

By Randy Williams | January 24, 2025

The code requirements for blower door testing are found in the International Residential Code (IRC), Chapter 11, Energy Efficiency. In the 2021 IRC, the codes discussing testing are found at R402.4.1.2. This section tells us the maximum allowable air leakage rates under any compliance path, (prescriptive paths and non-prescriptive paths), the testing standards allowed, who can perform testing, and the requirement of a testing report. One additional piece of information, and I quote: “Testing shall be performed at any time after the creation of all penetrations of the building thermal envelope have been sealed.” 

An interpretation of this code language could result in a final blower door test happening before drywall and the rest of the interior finishing. That’s not what I would recommend. With that being said, when is the best time to perform a final blower door test? When is the best time for a “mid-build” test? And how many tests should be performed? Let’s see if we can answer some of these questions, plus one more: when shouldn’t you do a blower door test?

How a blower door works

Before we get into discussing timing for blower door testing, we should have a quick review of what a blower door test is. A blower door tests the continuity and integrity of the air barrier, a test of the “air tightness” of a structure. Testing is performed using a fan that creates a pressure difference between inside and outside the building, usually 50 Pascals, about the same pressure as a 20 mile per hour wind blowing on all sides of the structure at the same time.

The tool calculates the air flow rate in cubic feet per minute (CFM) moving through the fan opening. From there, we can manipulate the CFM number to indicate a leakage rate based…