Replies

1. Expert Member
   MALCOLM TAYLOR | Sep 11, 2023 11:11am | #1

   orange_cat,

   When choosing building assemblies, the first place to go is your code to see what is allowed. Canadian building codes require an interior vapour-barrier. So if you omit the poly, you need to replace it with some other material to act in that capacity.

   You do not have the recommended amount of foam for your climate zone to keep the sheathing warm enough to prevent moisture issues. The way to compensate for that is to include an interior vapour-retarder.

   See these two articles:

   https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing

   https://www.greenbuildingadvisor.com/article/rethinking-the-rules-on-minimum-foam-thickness

2. 
   orange_cat | Sep 11, 2023 12:52pm | #2

   Thank you. Should I ask for https://www.certainteed.com/building-insulation/products/membrain/
   instead of https://csrbuilding.com/products/regency-6-mil-vapour-barrier-poly

   Does it make a meaningful difference (have to pick my battles).

1. Expert Member
   MALCOLM TAYLOR | Sep 11, 2023 01:14pm | #3

   orange-cat,

   I'm hoping Akos will weigh in. He builds in your climate, and understands the nuances of exterior foam use in Ontario.

3. Expert Member
   Akos | Sep 12, 2023 09:01pm | #4

   The OBC does allow you to skip the interior vapor retarder provided you follow the ratios in Table 9.25.5.2 .

   So for an R20 wall that would be only 20*0.2=R4, which seems too low based on the info here:

   https://buildingscience.com/documents/building-science-insights-newsletters/bsi-100-hybrid-assemblies#t02

   So based on that technically your wall will pass code with no VB, but I think it is a bit too close.

   You'll also have much better luck with a building inspector if they see 6mil poly (difference between a guaranteed pass VS an argument over code), so based on that I would include a vapor barrier. The smart vapor retarders are silly priced up here in the great white north, hard to justify the cost plus dealing with trades using an unfamiliar product. Plus a vapor barrier makes for a great secondary air barrier.

   The reality is that these walls, R5 rigid+2x6+poly have been built up here for a long time and they hold up just fine. I while back I opened up a 40year old wall that was built with similar construction and the inside was pristine, like the day it was built.

1. 
   orange_cat | Sep 13, 2023 11:18am | #6

   Thank you - I very much appreciate your response. It helps a lot to hear this.

4. 
   AC200 | Sep 13, 2023 12:25am | #5

   I'm also struggling with the use of a smart vapor barrier in Zone 5. As Akos said, poly is a standard code item that inspectors look for and has been used since I can remember.

   A smart barrier like Intello is like 10x the cost of poly and I don't relish the thought of asking the installers to minimize the waste during the install so I will probably end up with a $500 waste of an additional roll.

   I read Martin's article and other like it and it seems like I could be border line OK with poly on the above grade floors but safer with a smart barrier. Not crazy about how easy membrain tears so substituting intello or siga for poly will probably be a last minute decision.

1. Expert Member
   BILL WICHERS | Sep 13, 2023 01:10pm | #7

   I've never found Membrain to tear all that easily. It doesn't have the stretchiness of polyethylene vapor barriers, and it doesn't have the physicaly strength of some of the beefy (and expensive) smart vapor retadrers like Intello, but it holds up OK if you're reasonably careful with it. The weak point is tears starting at sharp inside corners of cuts, and staple pull-throughs. I avoid the issue with staples by tacking it up carefully and avoiding letting it "roll back" on itself at the last staple. For cuts, I try to avoid interior cutouts, or reinforce the area needing the cuts with some flashing tape prior to cutting with a SHARP (sharp like new really helps) knife. With those few precautions, I've had no problems using Membrain, and I use it preferentially over the more expensive ones because, well, Membrain is cheaper :-) Since I always put up a decent amount of exterior rigid foam -- normally enough to not need an interior vapor retarder at all -- the Membrain is just there for extra insurance, and I don't need top level performance.

   Bill

1. 
   jen_fh | Sep 13, 2023 01:29pm | #8

   Hi Bill! Thanks for this info... I'm also in Canada considering the same issue re choice/cost of smart vapour barriers. Do you use just acoustical sealant to attach the Membrain to the framing (in addition to the staples, of course), as is standard here for poly? As far as I can tell, it doesn't come with its own tapes unlike e.g. Intello.

1. 
   AC200 | Sep 13, 2023 05:09pm | #10

   I'm not an expert but from what I have read, I don't see the benefit of the special tapes justifying the cost. Our code is acoustical sealant where framing members are butted together. I plan on staples and blue tuck tape, unless someone has a good reason not to.

   I'm also loathe to insist on a special installation technique unless it is really necessary. It's one thing to explain to my builder and the owner or foreman of the trade. It's another thing to have it well communicated and executed by the workers since many just go into auto pilot mode when working.

2. Expert Member
   BILL WICHERS | Sep 13, 2023 05:34pm | #11

   I don’t think acoustical sealant would be able to hold anything in place, since it can flow. I use polyurethane sealants from loctite’s PL line. I put down a bead of sealant around the perimeter of the wall, then put up the MemBrain by stapling it over the bead of sealant. Lastly, I roll the areas where the sealant is to flatten it out and make sure there aren’t any voids. It sounds complicated, but it goes quickly in practice.

   I don’t use any fancy tapes either. I usually just use a pretty plain 3M flashing tape for seams and reinforcement where I need to cut out for an electrical box or other reason. I do try to land seams between sheets at studs though, which makes it easier to tape the seam without exerting too much pressure on the MemBrain material.

   Bill

3. 
   jen_fh | Sep 14, 2023 09:14am | #13

   Thanks, Bill and AC -- very helpful to hear your thoughts.

2. 
   AC200 | Sep 13, 2023 05:03pm | #9

   Thanks for the input Bill. I may have to look at membrain again. It's only 2.5x poly versus 10x for Siga and Intello. I don't mind patching up any tearing with tape when the installers are done.

   We have vented attics here, so I don't see any benefit to use a smart barrier on the ceilings and will stick with poly there. In fact I think poly as air barrier is better.

1. Expert Member
   BILL WICHERS | Sep 13, 2023 05:36pm | #12

   With a vented attic, you don’t need a vapor barrier or retarder on the ceiling. I think Canadian code requires one though (while I do have family in Ontario; I don’t actually work over there). I just detail the ceiling drywall as the air barrier, but that might not satisfy Canadian building code requirements. I would defer to Malcolm and Akos here, both of whom work in Canada and are far more familiar with the intricacies of Canadian building codes than I am.

   Bill