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 a bit, 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. (If you’re completely new to Blower Door testing, make sure you read our section about testing for air-tightness.)
Then 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). (A cfm50 is a cubic foot per minute at 50 Pascals.) 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, and only researchers use 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 easy one: new construction. The 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 was at 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 – 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).
The places to pay careful attention to in new homes are funky transitions in the building envelope, band joists, top plates, bottom plates, and myriad other details. The simpler the building envelope, the easier it is to find and seal the air leakage sites. We’ve published quite a few articles on this topic here, so click on the air sealing, air barrier, or building envelope tags to tags to the right to read about some of these details.
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. (Hmmmm. Do worms even like light?) 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 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. The cost a fortune! Check out Martin Holladay’s recent post called The High Cost of Deep Energy Retrofits at Green Building Advisor.
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 see if you can reduce the number by 20% or more.