Hi all.. I'm doing double-stud walls with plywood sheathing in a cold climate. Need ideas as to best or cost-effective ways of getting a tight shell. I was planning on using EPDM P-gaskets under the outer edges of the sheathing, with 3M tape over the seams. Are there any better ways? Caulks? What kind? Can I use acoustical sealnts? Or are they just for sealing drywall? Thank-you for any ideas/products....... -michael

Caulks and acoustic sealants are more likely to get squeezed/left out of a gap as materials expand. Then the material contracts and you are left with an air leak. Spray foam is even worse (it's not very flexible in small gaps).

Consider using a thermal camera during construction to find leaks.

Two air barriers are better than one.

Tape and high-quality sealants work best. Belt & suspender approach. EPDM gaskets are great also but I haven't seen any that could be used between the sheathing without doing a custom job which would be very time consuming and expensive. Is the EPDM cut on-site to fit the gaps or do they come in pre-cut sizes?

Jonr.. I do plan on trying for an airtight drywall layer as well as tight sheathing. What do you recommend for joints, like between the 2 top plates?? Thanks.

Lbear..Can you recommend any high-quality sealants? Exterior or interior? I plan on using some of the products from this company:

http://www.conservationtechnology.com/building_gaskets.html
Thanks.

With double stud walls and air permeable insulation your air barrier must be on the inside, as in air tight drywall.  You need to keep the warm interior air inside the insulation to avoid moisture issues.  A proven compromise is to use sheathing on the outer face of the inner wall as the air barrier, however this requires a "modified" construction plan where the outer wall is erected unsheathed with it's , optional, sheathing installed after insulation.   With sheathing mid wall tapping the seams and acoustic caulk in corners and top and bottom gives good reslts with modest effort.

Liebler.. I've seen that method of sealing. I guess I like my sheathing on the outside. Better for hanging siding, drain wraps etc. Or are you doing 2 layers of sheathing? Are you insulating the interior bays? or is it just for wiring and plumbing? I'm not sure it makes sense. If you're going to use airtight drywall (only) then does it matter what's behind it?
Most people say you should get BOTH layers as tight as possible......
Thanks.

My "boss" insists on a "brick" exterior.  So what I'm planning is, from the outside face of the outer studs is:  Tyvec, Greenguard DC14 (a  i/4" thick fan fold XPS drainage plane),Ambrico EZwall  a steel substrate for "thin brick) , 1/2" thin brick.  I see no reason for sheathing with this combination.   The Tyvec will be taped but it is not the primary air barrier.  I will insulate the inner cavity as well.  My insulation choice is 3 layers of 3 1/2" mineral wool.

your air barrier must be on the inside, as in air tight drywall.  You need to keep the warm interior air inside the insulation to avoid moisture issues.

Of course in the summer, the warm air is on the outside and you don't want it flowing through the insulation either. So an air barrier on both sides is best for moisture. And it provides less through wall air flow (all other things being equal).


I haven't seen long term comparisons of air leakage in houses built with various air sealing methods. Until then, I'd use gaskets wherever possible and good tape elsewhere.

you should air seal the exterior anyway, even if you use the ADA. Keep the cold air out of the wall. Tape the sheathing - using ZIP sheathing and it's tape is the easiest way, but remember that the top and bottom of the sheathing do not get taped so the sheathing must be caulked to the framing. Urethane caulks are the best, but we are having trouble finding anything locally available. We are going to a mechanical chase to keep the wiring and holes out of the insulation layer, but have gotten down to .6ACH50 with the standard double wall. We've also gone to an Intello air barrier on the bottom of the trusses, with strapping below.
EPS is the best product for under the slab - use a minimum of 6" in your climate - it's your only chance and small money. Foundation walls should be insulated on the interior with sheets of urethane foam and if exposed, th interior layer needs a fire retardant rating.

Hi Bob..

Instead of Zip, which is OSB, I'm going with plywood and 3M tape. On tops and bottoms, I was going to use rubber p-gaskets (unless I can find a decent caulk). I'd like to use more foam under slab, but not sure if the concrete guys know how to do that much. I hope I can get things sealed tight enough without using a chase system. Is it necessary to use a smart vapor retarder in the ceiling? Why not just poly?

Thanks
-m

foam is pretty easy to do yourself, but i agree that concrete installers might not be trained in foam application. I've used the 3M 8067, but not as much as the others; I don't think it is applicable in as many applications. Poly vs smart: depends on your location for one, but it helps to remind you that the issue is air leakage. Poly is almost never sealed, and it can be difficult to tape. A membrane will be treated as unusual and the installers will pay more attention, and you can easily see where the seams and staples are taped.

I've had good luck with I guess what you would call a standard ceiling poly job. Taped seams, but fast and cheap then have them hang the lids as usual. THEN, before attic insulation, I light up the attic and air seal from the attic side. All the errors and omissions are very easy and obvious to all but the simplest of grunts. Have them tape and foam it up, then the old master can go up and put the finishing touches on an airtight ceiling plane.
Its blower door and infrared approved.

* there will still be an area or two that needs special attention from the inside, don't get me wrong and vaults, and 4/12 with no energy heels, and I'm making an assumption of decent workmanship. There is no silver bullet for crappy framing and wads of poly in corners.

See my post in the RIM thread about how I feel about caulks for air sealing joints...if you care.