House Blower Door Test

2010 February 6
by ottopohl
Mike Mcpherson runs our blower door test

Mike Mcpherson runs our blower door test

There are three ways that energy is transferred: radiation (that’s what makes your face feel warm when the sun is shining on it), thermal (that’s what burns your hand when you touch a hot stove), and convective (like when hot air rises). And here’s the thing about insulation: insulation is tested by how it slows down thermal transfer. When you’ve got a leaky house, then the stated insulation value written on the package when you bought it won’t mean a whole lot.

And all houses leak. Most of them leak a lot.

I don’t think there are any national standards when it comes to how airtight a house has to be built, but recently, a test called the Blower Door Test has become popular. We just did one on our house and learned some remarkable facts. Read on!

The idea behind a blower door test is to close all the windows and doors, putting a fan in the front door and depressurizing the house by blowing air out. Then an attached computer calculates the rate at which air seeps back in and presents it as the number of times all the air in the house would be replaced per hour. The Air Changes per Hour (ACH) is done at 50 Pascals of depressurization, so you’ll see the “ACH50″ number quoted when comparing houses.

Mike McPherson, the guy who did our house, compared the test to seeing how the house performs when a 20-mile-an-hour wind is blowing at the house from all sides. So the test does two things: one, you see how leaky your house is. And two, perhaps more importantly, you can walk around the house and feel where the air is blowing in.

Crappy houses might have an ACH50 of over 10. Well-built houses these days, Mike says, average around ACH50 of 5 to 8. Energy Star, the government program to highlight “energy-efficient” homes, require an ACH50 of 4 to 7 depending on the climate zone. Below 3.5 you need active air handling (ie the house is so tight that the air would get too stuffy without mechanically ventilating the house). Anything below 2.0 is really getting somewhere. The tightest standard I’ve come across is the Passive House standard, which requires a 0.6 ACH50.

When Mike turned on his machine, it said our house was 1.78, which Mike said was the equivalent of having roughly a 10×10 inch hole in my house open 24/7. The point of doing the test now, while the drywall isn’t up yet, is so you can walk around, plug the leaks, and make that hole smaller. And so we set out.

Check out this short video I made of the test:

As you can see, we spent the bulk of our time walking around behind Mike’s infrared camera and caulking all the holes he found. We went through three big tubes of caulk in an hour. The most common gaps were around the windows, although there were other things we found: nail holes, air gaps around ceiling can lights, and a few places where the foamers hadn’t filled wall cavities completely.

On a philosophical note, I was struck by how “smart” air was. The moment the fan was switched on air found every last single crack in the house and came streaming in. I imagined how complex a computer program you would need to model the entire house and the air flow so that you could find the tiny little nail hole a careless carpenter left in the upstairs middle bedroom. But the air molecules had no such worries. They just followed their nature and poured in.

So if the average house has an ACH50 of 5 to 7, how often does the air actually leak out per hour when the house isn’t pressurized? Dividing the ACH50 number by 20 is a good rule of thumb. That means that if your house has an ACH50 of 5, then all of the air in your house leaks out six times a day. And double that is quite common. If you have an ACH50 of 10, then all of the air leaks out every other hour! Talk about heating the great outdoors.

After plugging all the leaks, Mike turned on his machine again. 1.55. About 12% better, but I was disappointed, to be honest, that we didn’t make a bigger difference. Mike didn’t know what else to do. Usually, he said, it’s easy to make more progress, but sometimes all the work doesn’t do much. He claimed, plausibly, that our double-hung windows might be a culprit. Casement windows, since they clamp shut, are much tighter than double-hung windows, where the two windows just slide past each other.

We’ll do another blower-door test when the house is done. Mike thinks that we should get down to about 1.2 once all of the drywall, doorknobs, window trim, etc are installed. Let’s hope he’s right!

2 Responses leave one →
  1. casey dudley permalink
    February 8, 2010

    Otto. This video is great. What a super informative 2 minute video. We need to link your blog to the MTUSGBC Chapter website. I think lots of folks would get great value from your journal. Would that be okay?

  2. May 23, 2011

    I feel so much happier now I uendsratnd all this. Thanks!

Leave a Reply

Note: You can use basic XHTML in your comments. Your email address will never be published.

Subscribe to this comment feed via RSS