Replies

1. 
   davidmeiland | Dec 26, 2013 05:58pm | #1

   My casual opinion is that well-installed poly behind T&G functions reasonably well as an air barrier. This is based on many blower door tests in houses with wood ceilings and sometimes wood walls, using IR and other methods to find the leaks. Poly is a lot better than nothing. I would still install taped drywall or plywood over the studs before T&G.

2. 
   user-1140531 | Dec 26, 2013 10:15pm | #2

   I have heard many people conclude that poly is worthless because of all the fastener piercings, but I don’t believe that causes an actual problem with the performance. When a nail or screw pierces poly, it stretches the poly to open up to the fastener diameter. This stretched condition leaves the poly tightly conforming to the nail in a way that is airtight for practical purposes.

   Adding still more sealing performance is the fact that the area surrounding the nail piercing through the poly will be clamped between the drywall of other finish material and the stud face, which will further reinforce the poly-to-fastener contact in the piercing. It will also insure that the poly will not be flexing due to air movement in the area where the fastener penetrates the poly. This will prevent flexing from causing the fastener to tear the poly or open up around the pierce point. I would use relatively thick, heavy duty poly and make any seams fall on the framing so the seam can be clamped physically in addition to tape or sealant.

   If a fastener misses a stud and then is removed, that can leave an open hole or even a tear in the poly. If that happens midway through fastening a sheet of drywall, for instance, one may not want to remove the sheet just to repair a little hole in the poly. So it pays to be careful to avoid missing the framing with a fastener.

   I put in a 1 X 6 pine T&G ceiling over poly without any drywall. I put up the poly on the bottom chord of the scissors trusses, and then added 2 X 2 furring strips over the poly. I attached the furring with screws. So the poly was fully in place and clamped off very securely before adding the T&G. When I nailed the T&G, the nails only went into the 2 X 2s, and never reached the poly. The screws through the battens went through the poly, but they were less frequent than the nails for the T&G.

3. GBA Editor
   Martin Holladay | Dec 27, 2013 05:45am | #3

   Richard,
   The standard advice given to anyone who wants to use tongue-and-groove boards as an interior finish material for walls or ceilings is to first install drywall as an air barrier.

   I agree with Ron and David that carefully installed polyethylene is much better than nothing. But polyethylene isn't recommended very often any more as an air barrier material, because it can sometimes cause problems if the home is air conditioned during the summer.

4. 
   passivhausfan | Dec 27, 2013 09:28am | #4

   I didn't fully understand the question. Has the cottage already been built? If not, you could build a REMOTE wall. Then the interior wall surface wouldn't matter.

5. 
   slugboy6000 | Dec 28, 2013 12:32pm | #5

   David's idea of using plywood got me thinking about using it as the interior finish. I could seal the seams over the framing somehow, and hide them with battens, or I could fur out the plywood over poly as Ron suggested allowing the plywood to accept fasteners without damaging it's ability to air seal.

   Thanks for the advice and the warnings, I have lots to think about.

   (F W : the exterior of the cottage is complete (stick frame with rainscreen cedar clapboards).