What a blower door test actually reveals — and why the number isn't the whole story.

A blower door test tells you how tight a building is. But airtightness isn’t won on test day.  It’s won at the drafting table — before a single wall is framed.  The failure points we find most often aren’t random. Electrical penetrations, top plates, band joists, attic hatches — they show up on the punch list because nobody identified them as risk areas during design. A good airtightness strategy maps those details in advance, specifies the right materials, and gives the crew a clear target before they ever pick up a nail gun.  Material selection matters just as much as the plan. Tapes, membranes, and sealants designed for both airtightness and moisture management do double duty — they keep conditioned air in and handle vapor drive so you’re not trading one problem for another. Specifying the wrong product in the right location is a common and costly mistake. (Check out 475 High Performance Building Supply for some excellent options and training videos) By the time we’re on site running the blower door, we’re confirming what was already planned — not discovering what got missed. But design alone doesn’t win the test. Sequencing matters: air barrier work has to happen in the right order, with inspections built in before walls close up. And the whole crew has to be bought in — framers, plumbers, electricians — because every trade that penetrates the envelope is part of the airtightness system. Someone on site has to own that responsibility and hold the line when the schedule gets tight.  A poor blower door result is rarely just one thing. It’s usually a combination of under-detailed design, wrong material specs, missed sequencing, and no single person accountable for the outcome. That’s exactly where a building science consultant earns their fee.  Have a blower door story — good or bad? We’d love to hear it.