Replies

1. 
   user-2310254 | Jul 22, 2016 10:57pm | #1

   Sandra. This article and its embedded links will answer most of your questions. https://www.greenbuildingadvisor.com/blogs/dept/musings/how-install-rigid-foam-sheathing

2. 
   user-6030522 | Jul 22, 2016 11:12pm | #2

   Thanks you, that article is very helpful but I am not sure of the vapour barrier placement between the studs and the polyiso in addition to the tyvek is ok.

3. GBA Editor
   Martin Holladay | Jul 23, 2016 04:33am | #3

   Sandra,
   To rephrase your description of your house in more conventional terms: You are planning to build a house with PERSIST walls, with all of the insulation on the exterior side of the studs. (For more information on PERSIST, see Getting Insulation Out of Your Walls and Ceilings.)

   Obviously, if you are building walls without conventional sheathing, you'll need to come up with a plan to brace your walls. (For more information on this issue, see Four Options for Shear Bracing Foam-Sheathed Walls.)

   You wrote that you are planning to install a "vapor barrier between the polyiso and the framing," and then you asked, "ls this OK or will we have problems?"

   You didn't tell us what you plan to use as a vapor barrier. In the traditional PERSIST approach, this vapor barrier consists of peel-and-stick membrane over OSB or plywood. But you don't want to install OSB or plywood, so there is nothing to attach your peel-and-stick membrane too.

   If you don't want to install OSB or plywood, you could choose a type of rigid foam that is also a vapor barrier. The easiest way to proceed would be to choose foil-faced polyiso -- a type of rigid foam with a very low vapor permeance, qualifying as a vapor barrier. That should satisfy your building inspector.

   There are a couple of other hurdles you need to face in addition to the vapor-barrier question. Here's a big one: How will you make your walls airtight? If you follow the conventional PERSIST approach, you've got an excellent air barrier (the peel-and-stick membrane attached to the OSB or plywood sheathing). In your case, you'll need to detail the polyiso in an airtight manner. That can be challenging, but it's possible.

4. 
   user-2310254 | Jul 23, 2016 07:52am | #4

   Martin. Would Sandra want to propose MemBrain to address code enforcement's desire for an interior "vapor barrier?"

5. Expert Member
   MALCOLM TAYLOR | Jul 23, 2016 10:57am | #5

   Sandra,
   Intermittent sheathing poses all sorts of problems. Depending on your exact location on Southern Vancouver Island, the amount of shear wall required will be either 25% or 40% of your exterior. Why not sheath the rest?

6. Expert Member
   MALCOLM TAYLOR | Jul 23, 2016 10:59am | #6

   Sandra is fine with the placement of her VB as long as it is on the warm side of her insulation. There is no requirement for an 'interior" VB in Canadian building codes.
   https://www.greenbuildingadvisor.com/community/forum/building-code-questions/59781/poly-and-canadian-building-codes
   Given that there will be no vapour movement through the foam there isn't any real advantage to substituting a smart membrane for poly.

7. 
   user-6030522 | Jul 23, 2016 11:55am | #7

   Thank you very much for the clarification and yes I see that sheathing the would be a much better solution and we will use 6 mil polyethylene over the sheathing if that is ok.

8. GBA Editor
   Martin Holladay | Jul 24, 2016 06:03am | #8

   Sandra,
   I'm glad you have decided to install OSB or plywood sheathing on your walls. The traditional PERSIST detail calls for the installation of peel-and-stick membrane on the exterior side of the wall sheathing, but as I noted in my article on the PERSIST technique, "Alaskan builders are experimenting with changes to the PERSIST system. To save money, some have switched from peel-and-stick membrane to 6-mil poly."

   From a building science standpoint, there is nothing wrong with using polyethylene in this location on a PERSIST wall. The only problem is that polyethylene doesn't make as good of an air barrier as the peel-and-stick membrane -- so if you use poly, pay attention to airtightness by sealing the OSB seams or the polyiso seams.

9. 
   user-6030522 | Jul 24, 2016 11:50am | #9

   Thank you very much Martin all is very clear now. Cost is a factor so we sill use the 6 mil poly and we will be very careful to seal all seams well.