Should I put poly behind tongue-and-groove paneling on exterior walls?
RichardMH
| Posted in General Questions on
Location is in NE Ohio. 2×4 wall construction, fiberglass insulation, OSB and 3/4″ polyiso exterior foam and finally housewrap. Some AC for one to two months a year.
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Replies
Richard,
Q. "In Ohio, should I put poly behind tongue-and-groove paneling on exterior walls?"
A. No. The wall will perform better without the poly (especially since the house will be air conditioned). However, your wall will fail miserably without a very tight air barrier on the interior. What you need to do is install interior gypsum wallboard, paying close attention to airtightness at all electrical boxes and other penetrations. The drywall seams must all be taped. Then you can install your tongue-and-groove boards over the drywall.
Martin,
Thank you for the quick reply. In the interest of speed and labor savings, is XPS with taped seams an alternative to drywall?
Richard,
Q. "Is XPS with taped seams an alternative to drywall?"
Q. Yes, as long as you use a high quality tape.
For more information on taping XPS, see Backyard Tape Test.
For more information on worries about XPS shrinkage, see Using Rigid Foam As a Water-Resistive Barrier. (Scroll down to the section that begins, "Do rigid foam panels shrink?" and read the paragraphs that follow.)
Rich: First, I am not a pro, and I know what the general feeling in the building science community is regarding the longevity and ubiquitous use of tape. However, are you willing to bet your house on tape sticking to foam over the long haul? Are you saving that much by not sheetrocking first, then paneling? Two air barriers beat one. Just something to think about.
John: I think that the right tape would likely stick for the lifetime of the application. In my day job we use industrial grade adhesives that are nothing short of remarkable. In addition to the time savings, I'm thinking that XPS would reduce thermal bridging and add a little R-value. That being said, no, I'm not 100% confident that XPS and tape is a better solution than drywall.
+1 on avoiding the use XPS as an interior air-barrier, unless dual layers are used.
In NE OH with 3/4" iso on the exterior one could get away with using dense-packed cellulose and no interior air barrier under the t & g wood, not that it's ideal. The hygric buffering of 3.2lb or higher density cellulose would usually be sufficient to handle the very modest air-transported moisture in that climate. Using a "smart" vapor retarder such as MemBrain, detailed as an air barrier and any blown insulation would be better though (but still cellulose offers more resilience than fiberglass or stone wool.)
The vapor permeance of 1x t&g with a polyurethane, acrylic, or latex finish is ~1-1.5 perms (about the same as 1" of XPS) but the air leakage is high. The more air-retardent the cavity fill, the less moisture get's convected into the cavity. If fiberglass, go dense-packed to 1.8lbs minimum, using a new-school blowing wool like L77, Optima, or Spider. If cellulose, 3.2lbs min.
I'm leaning toward covering walls with CertainTeen Membrain instead of XPS or drywall. Any reason this is a bad idea?
Richard,
If you're depending on MemBrain to be your air barrier, make sure that the MemBrain seams are taped and the perimeter of the MemBrain is carefully sealed. You will also need airtight electrical boxes, and you'll need to seal the seams between the electrical boxes and the MemBrain.
My plan is to use 3m Flashing Tape for the seams and silicone caulk for edges along top and bottom plates and to the face of the electrical box mud-rings (extensions). Thanks for pointing out the electrical boxes. Didn't consider that as a possible leak point. I've already put up kraft faced fiber glass insulation. Would you suggest slitting the paper facing or leaving it intact?
Slitting the paper facers does not affect their vapor retardency, only their air tightness. While kraft facers are ~0.5 perms when dry, should the humidity in the cavity rise they too will become much more permeable, so they're something of a smart vapor retarder by themselves.
But kraft facers are downright LOUSY air-barriers (even without slits), with too many seams/leak-points to seal reasonable, which is why you need to detail the MemBrain (or something else) as an air barrier.
Bottom line, leave them in place, don't slit them, and put the MemBrain (detailed as an air barrier) over it.
Thank you very much for the feedback. I'll certainly follow the advice I've received.