Blower door tests are often done near the end of the building process, when the paint is done, and the final doors and windows are in place.
When is the best time to perform blower door testing?
It depends. Doing blower door tests at different construction stages can help diagnose air infiltration issues allowing them to be fixed before the interior finishes have been completed. The key to early testing is that you must seal up openings in the building envelope; otherwise, you may not be able to depressurize the building to look for leaks.
Recently, Enercept did a blower door test on a Habitat for Humanity duplex under construction in Watertown, SD, using Enercept SIP wall panels. The outside temperature was minus 11 degrees F, and the units were heated with a temporary heat source that was vented through a board over a window opening - as shown here:
Also, there were utility closets in the garages and beneath the stairs that did not have 
doors. So our first line of business was to seal up these areas the best we could with poly and painter's tape. We were able to depressurize the home - shown here.
Then the fun began! We toured the unit with thermal imaging, looking for cold spots, and we found some:
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