Replies

1. Expert Member
   MALCOLM TAYLOR | Mar 24, 2019 04:28pm | #1

   Spencer,

   How far you go to avoiding interior p0ly may be governed by your appetite for conflict with your local building inspector, and the complexity of meeting code by approved alternatives. The Ontario building code allows other materials to be used as a vapour barrier, but those alternatives aren't included in your proposed double wall. The one easy fix is to use a variable perm membrane in place of the poly. Something your local trades would be familiar with installing.

   Before deciding that poly is a inappropriate in your location, it's probably worth noting that houses in Ontario don't usually face problems associated with its use - as long as the rest of the wall assembly has good drying potential to the exterior - and any double wall assembly should include a vented rain-screen.

   Whether you use poly or a membrane, I'd air-seal your new exterior sheathing, and not worry about detailing the drywall for air-tightness.

   Sounds like a lot of work ahead. Good luck!

1. 
   spencer_true_story | Mar 25, 2019 02:07pm | #4

   Malcolm, Thanks for such a swift reply! I have used MemBrain before and was leaning towards that method, and I appreciate the encouragement. Outward drying should not be a problem as far as I can tell. I would also note the building inspectors here seem to be pretty friendly, and I have included them in every stage of this project so far, mostly as general council, which they seem enjoy. Thanks!

2. GBA Editor
   Martin Holladay | Mar 25, 2019 05:24am | #2

   Spencer,
   First of all, if you are insulating your cathedral ceiling with closed-cell spray foam, then you already have a vapor barrier. So that assembly certainly doesn't need interior polyethylene. (Hopefully, even a Canadian building inspector knows that much.)

   I sympathize with on on the uncertainties surrounding your walls. While I think it is highly unlikely that interior polyethylene will cause any problems for your walls, even if you install air conditioning, my advice is to install a smart vapor retarder like MemBrain instead of interior poly. The small upcharge is worth it -- if only to lessen your worries.

1. 
   spencer_true_story | Mar 25, 2019 02:10pm | #5

   Thank you Martin. I have used MemBrain before and have a few rolls left over, so that may be the way to go. Would you apply it like poly in colder climates, with acoustical sealant at all the joints? My understanding is that it would be my primary air barrier and as such such be installed as neatly as possible.

1. GBA Editor
   Martin Holladay | Mar 25, 2019 02:20pm | #7

   Spencer,
   There are really three separate issues here. Many homeowners and builders (especially Canadian homeowners and builders) tend to confuse these three issues.

   Issue 1. What type of vapor retarder do I need on my wall?

   Issue 2. How do I satisfy my building inspector?

   Issue 3. What type of air barrier is best for my wall?

   With any luck, you can kill two birds (Issues #1 and #2) with one stone by using MemBrain.

   That said, I don't advise builders to try to use polyethylene or MemBrain as an air barrier. For walls, I favor taped exterior sheathing.

3. 
   Peter Yost | Mar 25, 2019 09:37am | #3

   Hi Spencer -

   There is a long history and tradition of using 6-mil poly on the interior of building assemblies in Canada, with the 6-mil poly detailed as both a vapor retarder and air control layer.

   There is nearly an equally long history of using 6-mil poly on the interior of building assemblies in cold climate US and NOT doing a good job of detailing as both vapor retarder and air control layer. As a builder/remodeler in NH in the 80s, I was as guilty of this as anyone. It simply was HARD to detail the 6-mil poly as an air barrier, at least the ways that I was doing it.

   I think a lot of the uncertainty around 6-mil poly is related to this issue.

   Peter

1. 
   spencer_true_story | Mar 25, 2019 02:13pm | #6

   Thanks for the reply. This makes sense, poly can be incredibly frustrating to work with through all the geometry of a house. The detailed approaches using acoustical sealant at laps seems like the best approach for air control. Since I may be using MemBrain as my air barrier, would you recommend this approach?