Whether you want to build a new home or fix an old one, the way to ensure that you get the best performance is to do the building envelope right. That means installing the right amount of insulation and installing it well, and it means having an air barrier with minimal leakage. But how do you know when you’ve done enough air sealing? How tight is tight enough?
I get asked this question every once in a while, and I love to talk about the measures for air leakage anyway, so let’s dive in. First, of course, you have to be able to measure how much air leakage the house has. That’s what a blower door is for.
About air leakage units
First, you have to choose how you want to specify the air leakage. The most common unit used by blower door operators is ACH50, which stands for Air Changes per Hour at 50 Pascals. I prefer cfm50 per square foot of building envelope, or better, cfm50 per hundred square feet of building envelope (sfbe). (One cfm50 is a cubic foot per minute with a pressure difference of 50 Pascals between inside and outside.)
The two reasons for that choice are that (i) air leakage happens at the surface, not in the volume, and (ii) it’s the best unit, in my opinion, to express what a Blower Door is really good at — measuring the amount of air moving across the building envelope at elevated pressure.
Please don’t talk to me about ACHnat (‘Natural’ Air Changes per Hour). I loathe that measure! If you’re using a blower door, you can’t measure it; the only ones who can are researchers using tracer gas analysis.
Now we’re ready to discuss the actual question: How much air leakage should you aim for? OK, we’re not really there yet. I lied. First, we have to know about your house. Are you building new or fixing an existing home? If the latter, what’s your budget and how complex is the building envelope?
Let’s start with the easier one: new construction. The first rule here is that a house can never be too tight. The Passive House program takes houses about as far as you can go with air tightness, and their threshold is 0.6 ACH50. I tested a net-zero house a couple of weeks ago that had less than 200 cfm50 of air leakage, or about 0.5 ACH50. That’s really tight!
A target that’s more achievable for anyone — and which the 2012 International Energy Conservation Code (IECC) will require for most climate zones (CZ 3 and higher) — is 3 ACH50. That’s also the level that Joe Lstiburek identified as a good target in his great article on blower door testing new homes, Just Right and Airtight.
That number — 3 ACH50 — translates to about 0.25 cfm50 per square foot of envelope, or 25 cfm50 per hundred square feet of envelope. Since roofers have already abbreviated 100 square feet as 1 square, I like the latter form the best. It gets it into a whole number form and is easy to remember. Get your blower door number down to 25 cfm50 per square (or below), and you’ve got a tight house. The house I built ten years ago came in at 14 cfm50 per square (1.7 ACH50).
But wait. I can hear you wondering, “Is he saying that this one number, 25 cfm50 per square, is good for every climate zone?” My friend David Butler says that we should consider cost effectiveness and not just apply a “one-size-fits-all” threshold. “We should always ask ourselves which combination of efficiency measures will provide the lowest energy usage within a given budget,” he wrote recently. “The cost-benefit of achieving 3.0 ACH50 would look very different in Buffalo than in Tucson, on multiple levels.”
He does have a point, He went on to explain that “in hot climates, I’d rather fight to bring ducts inside (always challenge in non-basement homes) than to arbitrarily specify a 3-ACH50, especially in dry climates where latent loads are negligible.”
Lstiburek, a Canadian who lives in New England, thinks the roughly equivalent 3 ACH50 is good because it’s achievable, and it solves a lot of comfort and efficiency problems associated with air leakage. I understand David’s point, but here in Georgia, home builders now have to pass a blower-door test and get their houses 7 ACH50 or below. What we’re finding is that a significant number of builders are getting down to 3 ACH50 in their attempt to meet the code target. Once builders get their crews trained, 3 ACH50 should cost them about the same as 5 or 7 ACH50. Now, if we can just get them to change the target to 25 cfm50 per square!
This can of worms is really too big for the little article I’m writing here, but let me try to shed at least a little light on it. The amount of air sealing you’re going to be able to do in an existing home is limited — unless your budget isn’t. Generally, with a good attack on the holes, you can get about a 20% to 30% reduction in your Blower Door number (whether you specify it in cfm50, ACH50, or cfm50/sfbe). Sometimes you can get much more when you add surface area or volume by moving the building envelope.
The first thing you want to know here, though, is: How leaky is your home to start with? If you’re already at 25 cfm50 per square, it’s going to be really hard to get a 20% reduction. If you’re at 100 cfm50 per square, it should be a snap to reduce it to 75 cfm50 per square or even lower. The higher that starting number is, the more big holes you probably have in your building envelope.
If you’re starting at 100 cfm50 per square, however, don’t count on getting down to 25 cfm50 per square unless you’re doing a deep-energy retrofit. Sometimes deep-energy retrofits are called deep pockets retrofits, for obvious reasons. They cost a fortune! Check out Martin Holladay’s recent post called The High Cost of Deep Energy Retrofits here at GBA.
I’ve done a number of air-sealing jobs when I was in the contracting business, and the results varied. Sometimes it was a frustrating 5-10% reduction, and other times it was an easy 20%. The most reduction I ever achieved was about 40% in a house that started with about 120 cfm50 per square. We encapsulated the crawl space and did a lot of work on the kneewalls and can lights on the upper floor.
If you’re looking to make your home tight and you really want some numbers to go after, here are my recommendations:
New Homes: 25 cfm50 per hundred square feet of building envelope (or 3 ACH50)
Existing Homes: Get a blower door test, and try to reduce the number by 20% or more.
Allison Bailes of Decatur, Georgia, is a RESNET-accredited energy consultant, trainer, and the author of the Energy Vanguard blog.
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