Posted by: Li Ling Young | September 9, 2012

The Blower Door Test

One of the foundations of efficiency work is, Test In, Test Out.  Meaning, know the condition going into a project, measure the condition after the work is done.  You want to know how much improvement was made; you want to be able to show that the goal is met; and you want to know that you’re not leaving a dicey situation behind.  There are dangerous but avoidable situations that arise when one ignores physics, but that’s for another post.

After we’d been living in the house for a couple of months it was time to do a blower door test in which I measure the air tightness of the building.  The blower door is a fundamental tool of building-energy-saving.  For this test you close up the house and use a big, calibrated fan to depressurize the house.  When you’ve reached the test pressure, you measure how much air the fan is moving, and that’s your test result.

Existing homes like ours (modestly-sized, simple shape) have blower door numbers between 1000 and 3000 cfm50 (the unit means cubic-feet-per-minute at 50 pascals of pressure).  New homes have blower door numbers between 200 and 1000 cfm50.  As a very rough guideline, homes that have been sealed up to the point where the blower door number is about the same as the square footage of the house are considered cost-effectively sealed.  In my opinion, that’s some pretty outdated thinking.  My goal for this house is less than 500 cfm50.  It’ll take a while to get there because there are parts of the house that won’t be accessible for air sealing until a later stage in the work, and that’s a pretty aggressive goal anyway.
I want to know what the original leakage of the house is before we do any work.  Not having done the blower door test was actually holding up our air sealing work.  One of the first things we did was add two recessed light fixtures to the living room (there wasn’t ANY lighting in the living room!)  So, I had to seal those up just to get the house back to the original leakage condition.  By the same token I couldn’t wait too long to do the blower door test because I’d pretty soon start making changes to the house that couldn’t be returned to the original condition even if I wanted to.

An important consideration is the presence of vermiculite in the home.  The best, safest thing to do in homes that are known to have vermiculite is not to depressurize them.  You don’t want to cause any of the vermiculite dust to become airborne and get sucked into the house where people will breathe it.  Since we had the vermiculite in the attic removed I felt ok about doing a blower door test, but I wanted to do it when no one else was home.  Finally, my opportunity arrived: no one home all day long.  It only takes about 10 minutes to do the test.  Our blower door number was around 2300 cfm50: pretty leaky.

There are two known features of this house that contribute to high leakage.  A linen closet in the original bathroom has a dropped ceiling.  From the attic it looks like a sink hole in the attic floor, and the lower ceiling leaves three walls open to the attic in a way that they would not be if the ceiling were at the same height as the rest of the house.
The foundation of the addition is also leaky.  There is a leaky access door to the crawlspace, and the “shade” porch on the back of the house shares the same foundation, so air can move from the porch into the crawlspace and up through the floor of the 2nd bathroom.

Between the two of these things I would have expected the house to leak almost, if not quite as much as it does.  There are numerous smaller leaks too: the perimeter of the floor, which is uninsulated and unsealed; the wiring holes in all the top plates of the walls (visible from the attic), the fuel pipe into the garage, etc.  One thing you can do with the blower door is find the leaks.  I didn’t spend any time doing that because we’re going to seal everything up that we can reach, whether it’s a big leaker or not.  Over time, as our motivation, budget and time allow we’ll get to almost all of the thermal envelope of the building.  Knowing that it leaks (or doesn’t) in a particular location probably won’t change the order in which we get to insulating and air sealing that part of the building nor how well we air seal it.  Everything gets the same treatment: sealed as well as we can.

A couple of days after the blower door test I got into the linen closet in the hallway to discover potting soil all over the towels…  But no, it’s vermiculite!  I had sucked vermiculite into the closet and there was a lot of it.  I had broken the rule about depressurizing homes with vermiculite, and the worst had happened.  I just closed the door and told everyone not to use any new towels.  I was so demoralized I couldn’t even deal with it.  Eventually (weeks later, in fact) I pulled the towels out (very carefully) and carried them outside where I dumped the vermiculite into a garbage bag.  The towels got washed and dried in the usual way.  I took the shelves out of the closet and hosed them off in the backyard.  The floor and walls of the closet got wiped down.  I’m still not sure how the vermiculite got into the closet, but Nicholas discovered that the trim on the back of the closet door was missing, and that left a gap between the wall board and the door jam.  I sealed that up from inside the closet before I put everything back.  I’ve drawn the geometry of that part of the building as well as I understand it.

Any place where the ceiling is a different height than the rooms around it can be a bad air leaker and can allow stuff from the attic to fall down into the house.

Now I had a little conundrum.  I definitely did not want to depressurize the house again, so no test-out blower door test.  That means we wouldn’t be able to say how much of an improvement in air leakage we’d managed to accomplish.  There is another way, and it involved doing another blower door test, but this time PRESSURIZING the building instead of depressurizing.  It’s almost the same, but the overall numbers tend to be slightly higher.  I think it’s because the pressures in the fan are a little funny, but other building scientists have different explanations for the higher positive pressure test results.  No matter, because what I’m really interested in is the difference between the starting condition and the ending condition.  At test-out we’ll do another positive pressure test so we’ve got comparable numbers.  I still want to beat 500 cfm50 when all is said and done, and if I can do it with a positive pressure test, then hurrah for me.

I did the positive pressure test and the result was about 2400 cfm50.  Now that I’ve pegged the starting condition leakage of the house, we’re free to start air sealing everything we can get our hands on.  It’s about time!


  1. Always been fascinated by the blower door test, and this is a perfect description of what, how it works, and why it matters. Thanks, Li Ling.

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