What is a reasonable air leakage target for a new home?
ranson
| Posted in Energy Efficiency and Durability on
I’m writing up the specs for a new house. I’m not aiming for any particular efficiency standard, but I want the best envelope I can economically build. What’s a reasonable target for ACH50? The primary air barrier will be taped plywood. I would like a goal that a conscientious builder can meet with diligent air sealing without heroic measures. Any thoughts?
–John
Rochester, NY
Zone 5
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I should additionally add that the house will be well insulated. I will have 12" cellulose filled double stud walls, 18" of loose fill cellulose on the attic floor, and four inches of EPS under the slab.
--John
1 ach50
John,
Although I stated a number, the real answer is more complicated. You need to talk to your builder. If you can find a builder who has built several new homes that were tested with a blower door, such a builder may well be happy to sign a contract with a binding target of 1 ach50.
If your builder has never before built a home that was tested with a blower door, the builder may be justifiably reluctant to sign such a contract.
IRC code max is 3 ACH/50, which is dead-easy to hit, and not an option- it's in the code- it MUST be met.
The Canadian R-2000 performance-home standard calls out 2 ACH/50 max, also not difficult if all trades are paying attention to air sealing during construction.
You might try 1 ACH/50 as a target, (perhaps with a bonus if they make it?) with 2 ACH/50 max.
Poor sealing at the foundation sills and under the bottom plates of framed walls are commonly missed. EPDM sill gaskets on the foundation are pretty good. A bead of polyurethane caulk under the bottom plates (and between doubled up top plates or any other doubled up framing around windows, doors, corners, etc) of framed walls as they are assembled are cheap solutions, easier and more effective than sealing them later. There are dozens, maybe hundreds of other details, they're not hard or time consuming.
If going through all the troubles of double studding your walls and putting insulation under your slab why not add some more loose fill on your attic floor? A pretty easy and cheap way to increase thermal resistance (R).
We found it very useful to have a blower door test at a point where it was still easy to fix any leaks. Doors and windows in, insulation in, no drywall. The blower door operator cranked up the fan to max and we walked around looking for drafts. Turned out to be pretty tight but we found a few leaks and taped or foamed as needed. Whole process was only a couple of hours.
We taped every joint we could, not just sheathing but also housewrap and the membrane we used as an air barrier on the outside of our inner stud wall. We ended up at .59ach50 and it was our contractor's first try at a tight house.