How to air seal the attic floor to the wall sheathing?
ranson
| Posted in Green Building Techniques on
I’m designing a house that will be built near Rochester, NY in zone 5. My primary air barrier will the plywood attic floor and plywood wall sheathing. What’s the best way to seal these together? For structural reasons, I would prefer if the sheathing were continuous between the first floor and attic.
Cheers,
John
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Replies
John,
There are lots of ways to do what you propose. One method was described by Carl Seville in one of his recent blogs, Topping Out.
It's far more common to use the ceiling drywall as the air barrier than the attic floor. For details on connecting a drywall ceiling air barrier with a wall sheathing air barrier, see A Practical Air-Sealing Sequence. An illustration from that article is reproduced below.
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If you're framing the roof with regular lumber (instead of trusses), then the process couldn't be simpler. Frame the ceiling joists, lay down the plywood deck (just like building a floor!), seal all the seams in the plywood on the flat AND at the corner (plywood joist deck to plywood wall sheathing). Use a high quality air-sealing tape (Siga, 3M, Pro-Clima, Zip) or a joint/seam filler (Prosoco or Duralink). Then, install your ridge beam and rafters...business as usual.
You can then run all the lighting, wiring (even plumbing and ductwork!) that you want in the ceiling joists...underneath the air barrier, underneath the insulation. In one house I consulted on, we used open web trusses for the ceiling joists, in order to accommodate the relatively extensive heating/cooling ductwork that was needed for the top floor.
See attached details - one for the air barrier staging, one for a typical insulation detail for this kind of construction.
Here's a beautiful photo example from Daniel Ernst at Promethean Homes just prior to the attic insulation going in - http://www.prometheanhomes.com/uploads/1/4/8/4/14843776/4123868_orig.jpg
John,
Your detail will work, but I prefer a detail that raises the rafter tails so that the full depth of the attic floor insulation extends all the way to the attic perimeter.
Your detail shows that the top of the insulation is beveled at an angle. To make up for this reduced insulation thickness, you've provided some extra insulation in the 2x8 ceiling joists -- a solution that works, but which (for me anyway) is a little inelegant and unsatisfying.
I don't think there's an "ideal" solution either way, Martin. To get full depth attic insulation (20") at the eave in my detail, it seems to me you'd either have to switch to a trussed rafter with a built-up heel, or frame the heel on site if using a dimensional lumber rafter. Issues of eave overhang depth, window head height and aesthetics certainly also come into play. My detail might result in too much shading of the windows if there's a desire for a relatively deep overhang.
Any way you slice it, though, the air barrier detail can remain the same, and is, in my opinion, as straightforward as you can get.
In the house I am almost done building, I put 2x6 joists on top of my walls, sheeted it with OSB, covered with plastic, then used energy heel trusses right above the 2x6 joists. Then I screwed the 2x6's to the trusses from below with 7 1/2 inch screws. The 2x6 chase was used for wiring and HRV ducting, etc. Blown cellulose in the attic.