Air Sealing Interior Attic Ceiling Options
spb2015
| Posted in General Questions on
My builder and I have been trying to come up with some air sealing options for the attic floor / 2nd floor ceiling area.
1. Be extremely detailed and use a sealant around every penetration and seam.
2. Staple Tyvek (with the exterior face facing upwards) to bottom chord of roof trusses.
3. Do a combination of both.
Are there other options available for a cost effective air sealing system for this area? The attic is unconditioned and it is not feasible to make the attic a conditioned space.
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Replies
Is this a new build or a renovation project? With a new build, there are more options available.
Assuming this is a renovation project, and assuming you have access to the attic spaces, the usual way to air seal is canned foam in every hole or large gap, and caulk in small ones. Most of the effort will go around the top plate areas, since that's usually where penetrations are, and you also want to get the area where the wall drywall ties into the top plate of the studwalls (which helps seal air from leaking into electrical boxes and getting up into the wall through the drywall/framing gap).
The uppermost floor's ceiling is usually your air barrier. If you'll have drywall there, then you don't need tykvek -- the drywall itself is your primary air barrier. You only need to seal any penetrations or gaps that the drywall doesn't cover. If you're going to use a tongue and groove ceiling, then you need something behind it to act as an air barrier. Drywall is often used for this purpose, but it's not the only way. I'm doing a small project in the next few weeks where I'm using 1/2" polyiso behind a T and G ceiling for an air barrier. Polyiso also adds some insulating value here. The plan for that project is to remove the T and G ceiling, beef up the batt insulation in the small attic space (accessible only from below), then put up the polyiso with seams taped and the perimeter caulked to the framing. After that, the T and G ceiling gets reinstalled.
Bill
Bill,
Thank you for your response! This is new construction. My builder and I are working on some cost saving solutions due to the price of lumber remaining extremely high.
My "want" is that I want some redundancy in air sealing. My builder is very confident that air sealing the drywall, detailing the recessed lighting and all penetrations and seams, etc with sealants, and recessed lighting boxes (fire rating) will do the trick.
My builder suggested the Tyvek as a possible solution for a 2nd barrier, but expressed concerns about moisture build-up. I'm concerned with future moisture/mold issues as well, and would prefer to just not do a wrap if mold will be an issue long term.
Tyvek is not a vapor barrier, it is vapor permeable. There is no issue with using Tyvek here, it's not the same as polyethylene sheet (which IS a vapor barrier).
I completely understand trying to save money in today's world of crazy high building materials prices! I've been having to update bids on jobs lately due to copper costs (most of my work involves electrical systems). I had a store manager just yesterday tell me they were having a hard time keeping all the prices on their items updated because they keep getting new prices. Crazy stuff. Luckily, drywall seems to not be going crazy so let's hope it stays that way.
I really don't think you need a second air barrier on the ceiling. Ceilings are usually less of an issue than walls, so I'd put your extra detailing work into the walls instead. The only potential thing I would consider if you want extra air sealing on your ceiling is to put up a layer of 1/2" polyiso first with seams taped, then drywall. The foil faced polyiso is an excellent air barrier, a vapor barrier (which is OK on the ceiling since the other side is open), and it's easy to tape and seal. You also get a little extra R value this way. It isn't going to save you any money though -- it's going to increase your build costs.
I would do the ceiling in the usual way, but I'd be sure to use 5/8" drywall. I would air seal it in the usual way (canned foam and caulk), then put in blown cellulose above (assuming this is a normal vented attic). Cellulose will help a little with air sealing too to limit the leaks from anything you may have missed.
Put your efforts into doing a good job on your exterior walls and rim joist areas where there is more to be gained from redundant air barriers and extra air sealing detail work. Ceilings are usually fairly simple by comparison.
Bill