Blower-door-guided air sealing isn’t real

I really like blower doors. There are a lot of building diagnostic tools most people have never heard of that I like: duct blasters and flow plates, to name a couple more.

Blower doors are mainly used to measure a home’s air leakage at a fixed pressure, nearly always 50 pascals. That lets us do two important things:

1. Compare houses. When people ask “How leaky is my house?” they usually mean “How leaky is my house compared with other houses?”
2. Track changes (hopefully reductions) over time.

We can do a lot of other things with a blower door, but we shouldn’t care nearly as much about zone pressure diagnostics as we do about that big-picture stuff.

I run a blower door several times a week, and I’ve worked on the shells of hundreds of homes. My crew has had tremendous success at making buildings tighter. One of our earliest discoveries was that a blower door is pretty much useless at directing air sealing. That runs counter to conventional wisdom in most energy-geek circles, so here is my case:

1. We can’t multitask very well (thanks, Daniel Kahneman). Either we’re running a blower door or we’re air sealing the house. We can’t do both at once.
2. A small area of sealing that doesn’t move the numbers doesn’t mean the rest won’t, and vice versa. Suppose we scamper down to check the blower door after finishing a quarter of the attic and the numbers haven’t changed much. Should we skip the other three quarters? Of course not. Get back in the attic and finish the job!
3. Walking back and forth between the work and the front door takes time away from the important work of sealing.
4. Blower door tests run sporadically through the day probably aren’t very accurate. Occupants and workers open and close windows and doors, which makes the numbers useless.

None of this is aimed at high-performance new construction, where homes are tight enough to find minor leaks with a fog machine.

The practical upshot makes air sealing existing homes fairly simple. Start with the accessible stuff up high (attics), then down low (basement and crawlspace rim joists). When other areas become accessible, say exterior walls during a re-siding project or a vaulted ceiling during a roof replacement, make those tighter too. Run a blower door test at the start and finish of each project.

Now get back in the attic!

Originally published on the Tay River blog. Lightly edited.