The Blower Door Test

I had a home energy audit this morning.  It was really educational.  Not because the guy was so informative, but because I got to look over his shoulder and see the numbers on his gauges.

The major test he did was a blower door test to measure air infiltration.  A blower door is a plastic seal put over your front door, with a large fan on the bottom (see picture to the left).  The fan blows air out of the house to create a pressure differential between the inside and outside.  This causes air to be sucked into the house via numerous cracks and leaks.  While the fan is on, you can go around the house and feel the areas that have obvious leaks.  For example, there was air coming out of my light switches - not much, but it was definitely there.

There were several big (and obvious) drafts.  First, the chimney flue doesn't really close all the way, so that's a gigantic hole letting my conditioned air escape.  Second, I have ten recessed lights that are not sealed, so they're just a bunch of conduits between the living space and the attic (and the attic is likely damn hot during the summer).  Third, I have a few holes in the wall where coaxial cable is poking through but there's no cover.  So it's just a ~ 1-inch diameter hole where air comes right in.

These were all fairly obvious leaks.  What I didn't expect was that my floor leaked.  Seriously.  I have hardwood floors on a slightly raised foundation and when I put my hand near the floor, there was a very subtle, but real, draft.  Apparently, hardwood floors (on raised foundations) are typically more drafty than mine, but still, this opened my eyes a bit.

For the subtle leaks, the guy (inspector?) had a cool contraption that was basically a large plastic hemisphere that you put over something you want to inspect for leaks (a vent, recessed light, etc).  The rubber around the edge forms a seal and it reads out the pressure difference between the room and the sealed inside of the hemisphere.  The blower door was maintaining 50 Pa (pascals) pressure difference between the inside and outside.  If this contraption also measured a large differential between the room and the object of interest, that means it's leaking a lot.  For example, he put it over one of my AC vents, and it only read 3 Pa.  Not a big difference.  However, when he put it over a recessed light, it read 33 Pa.  Basically, there was almost nothing holding the air back.

On top of this, you can use an infrared camera to scan the walls and openings to spot obvious places where heat is being transported.  It was a very cold morning and the joints between the wall and ceiling were colder than the rest, indicating that some of the cooler air was leaking in there (apparently this is quite common).  Unfortunately, the temperature differential between the inside and outside was pretty low, so this stuff was hard to see.  I felt a little ripped off because of that.

Anyway, the whole experience was fascinating.  But here's the thing - I could have done it myself.  I'm a smart guy with basic physics knowledge.  I could have figured out how to do this in 5 minutes on the internet.  The inspector didn't really have a deep understanding of what he was measuring.  When cranking up the blower door fan, he kept saying he was going to crank it up high enough so that there was a "50% difference" between inside and outside, while pointing to the meter.  The meter was actually reading the pressure difference in units of pascals, not percentage difference in pressure.  A 50% drop in pressure would have been like transporting us to 18,000 ft. elevation in a matter of seconds.  That would have done some nasty things to my ears - and it would have really pissed off my cat.

So, yes, I could have done this myself.  It looks like it might be possible to rent these blower doors.  That would be awesome!  It makes so much more sense.  Seal up the recessed lights, take the measurement again, seal the fireplace, take the measurement again.  You could actually see how much you're improving your home's insulation.  The way it works now is, you get the test, they tell you what will help, and then they claim to fix it.  But I want to see a before and after.  How much have you really helped me out?  This is all just making me want to do it myself.

Side note:  The inspector was useful in that he helped me realize some quirks about the way my city provides efficiency rebates.  That might save me a ton of money.




Ok, onto some technical stuff that was kind of fun.  As I said above, the pressure difference maintained by the fan was 50 Pa.  It turns out that the fan was displacing 2100 cubic feet of air per minute in order to maintain that pressure difference.  The inspector claimed that for a house of my size (~1500 sq. ft.), that is pretty good.  Really drafty houses apparently have values double that.

I looked this all up, and there are some fairly standard units for measuring how drafty a home is.  The first is the number I just gave, cubic feet per minute at 50 Pa, CFM50.  But this unit doesn't tell you much.  If your home is ten times bigger than mine and we have the same CFM50, well then you've got a lot fewer leaks per room or per volume than I do.  The second unit tries to correct for this.  It is air changes per hour (at 50 Pa), ACH50 - or how many times per hour the fan displaces the entire volume of the house in order to maintain a 50 Pa differential.  This is easy to calculate.

ACH50 = CFM50 * 60 / Volume

My house volume is about 11,712 cu. ft (1464 sq ft times 8 ft. in height).  Therefore my ACH50 = 10.75.  Now, this is an interesting number to compare to other homes, but there is another number that comes into play.  This air flow takes place under a contrived situation.  The "natural" air flow is based on several factors, including the number of stories, wind,  and outdoor temperature, which are highly dependent upon location, on scales both small (exposed hill top vs. shielded valley) and large (northern Maine vs. the Mojave Desert).  Fortunately, the government provides conversion factors between ACH50 and ACH_nat (called LBL factors, I think after Lawrence Berkeley Lab) for each region.   I'm in region 3 (on a 1-4 scale, see map below).

The higher the zone number the lower the natural air flow will be in the same house.  Windy cold places are lower zones, where better insulation and seals are more important.

Anyway, the punchline is that my natural ACH (ACH_nat) = 0.5.  This is exactly the cutoff for Energy Star certification for air infiltration from the government.  woohoo!

But hold on a second.  The only reason I made this cut is because I live in the right climate.  That's a bit of a cop out.  I want to have an efficient house, regardless of where it's located.  In order to be Energy Star certified for any of the above zones, I need to get my house down to ACH50 < 7.8.  I think I can do that quite easily.

And after I'm done dealing with infiltration, it's on to insulation!