GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Am I creating a double vapor barrier roofing disaster?

Nick H | Posted in Green Products and Materials on

Hello everyone, I’ve been researching about green building and building science and trying to learn the lingo for a couple of months now. I’m working on a renovation to a ground floor addition on the north side of our house.I live in a cold climate in south eastern ontario according to the hygrothermal map at building science corp: ( The IECC climate zone map is for the USA, but I believe I’d fall into Climate zone 5 and not 6 (

The project goal is to reroof and reinsulate a ground floor addition on the back (north side) of our two story house. We started the project because the roof has had problems leaking since day one (20+ years ago) due to an improperly installed skylight and lousy roofers (why bother with flashing against the house? just butt it up and tar the sucker!). The roof is an insulated unvented shed roof which slopes down from the second story to the north side and on the interior it’s a cathedral ceiling (half of a cathedral? term unknown). The addition was initially insulated with pink fiber glass batts in both the ceiling and walls, vapor barriered and finished with drywall.

The existing steel roof, the large skylight, and the shingles underneath the steel roof are going to be removed so that rotten sheathing can be replaced, the skylight area sheathed over and a new steel roof installed. My plan for the inside was to insulate with closed cell spf (BASF WAllTite Eco, tech specs: to a depth of 6″ if we can afford it. My understanding is that spraying ccSPF to this depth means that it will act as a class II vapor retarder (between 0.1 and 1.0 perms).

My main concern is with the installation of the synthetic roofing membrane under the steel roof in combination with the ccSPF. The steel roof materials are applied in this order: 5/8″ sheathing > synthetic roofing membrane > 1×4 strapping (a ventilated space correct? don’t want ice dams!) > 29 or 30 gauge steel roofing

The synthetic membrane proposed is a product called Resistor (Tech specs: which has a Perm rating of 0.5 Perms. This in effect is also a class II vapor retarder. My understanding is that I would effectively be placing a class II vapor retarder on both sides of my 5/8″ sheathing which is BAD. From what I’ve read from Lsitburek and Straube and comments on here again and again is that the assembly needs to dry in at least one direction. The sheathing in this scenario can technically dry in both directions but in a very limited way. Now I’ve also read (from those same fine folks at building science corp) that unvented roof assemblies with ccSPF on the underside of the roof deck, paired with a perfectly installed vapor semi-impermeable roofing membrane work well – no problemo! The crux of the problem is the workmanship needs to be PERFECT, and the membrane mustn’t fail and the roof sheathing needs to essentially never get wet since it has a very limited ability to “breathe”. For me this perfect storm of excellence seems unlikely.

Also, I cannot insulate on the roof deck because of clearance issues with the windows on the second floor of the house. If I raise the roof deck with insulation, the roof is going to hit the window sills or worse, the glass. My insulation needs to be in between the rafters.

My gut instinct is that I need to install a vapor semi-permeable class III vapor retarder as my roofing membrane (between 1.0 and 10 perms) OR a vapor permeable Class IV vapor retarder (this option seems very strange to me). Am I nuts? Do I need to hang my head in shame because I’ve missed something so blindingly obvious? Should I bulldoze the addition into the adjacent field and return to my regularly scheduled life? I apologize the explanation is so lengthy, I just hope I’ve covered all the relevant information.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Aj Builder, Upstate NY Zone 6a | | #1

    Choices that I say would work;

    1-plywood sheathing covered in ice and water shield, vented and this time proper flashing with venting where the roof meets the wall.
    2- upgrade to pressure treated plywood and your assembly is bullet proof with any water barrier such as the one you list or 30# felt half lapped or ice and water shield.
    3- Zip sheathing for the sheathing

    I would NOT use standard OSB. It becomes garden mulch if any moisture issues arise.

    Lastly, personally venting a rigid foam roof with failable taped seams is mandatory for me. As to sprayfoam, it is the one roof system that can work unvented, but venting is worth doing.

  2. Nick H | | #2

    Thanks for your reply AJ. Fortunately the roof is already sheathed in plywood not OSB. For sheathing replacement where it is rotted and where the skylight will be filled in I'll double check if the roofer is going to use plywood or OSB. As I understand it from this article plywood is better at expelling water than OSB (see Figure A at the bottom of page). For discussions sake, since I will have a ventilation space thanks to the strapping then installing some OSB should not be as problematic?

    Thanks for the suggestion for I&W shield, I'm researching different manufacturers now. From looking at the tech specs for Gracie I&W shield it has a permeance of 0.05 which would class it as a vapor barrier.
    Am I wrong about how I'm looking at this roof? If the roof membrane is a vapor barrier and the closed cell spray foam on the underside of the sheathing is a vapor barrier then if the sheathing gets wet in any way (improper installation or a failure anywhere) it will basically be unable to dry?

    The closest question to mine on this site I could find was this one: Durable, yet breathable roofing underlayment. The big difference is he is asking about open cell SPF whereas I'm hoping to use closed cell SPF. My thinking is in line with response #14 by Riversong - that there should really only be one material acting as a vapor barrier in my cold climate when building an unvented roof assembly using ccSPF on the underside of the roof sheathing, In my case this means I would have to use a vapor semi-permeable or permeable membrane (where perms >1)?

    Edit: okay after further research I found this report from Building Science. My scenario is on page 7, Figure 10. In the diagram they use good ol' fashion Roofing Paper. Specs for 15# IKO roofing paper show it has a vapor permeance between 3 (brand new, perfect condition) up to 51 (after aging) (tech data here. Would anyone recommend AGAINST using roofing paper?

  3. GBA Editor
    Martin Holladay | | #3

    Unlike A.J., I'm not a fan of installing a peel-and-stick membrane like Ice & Water Shield over your entire roof. I also prefer good old fashioned asphalt felt underlayment rather than synthetic roofing underlayment. The reason is that you can get some drying outward with asphalt felt, especially if you have metal roofing with a few ribs, or metal roofing installed on purlins.

    Almost all low-perm synthetic roofing underlayments forbid the use of their products over unvented roof assemblies like the one you are contemplating. Surprisingly, your "RESISTOR sous-couche de toiture synth├ętique" does not limit its application in this way. For more information, see Synthetic Roofing Underlayments.

    Your plan to install closed-cell spray foam on the underside of your roof sheathing is a good plan, in light of the fact that you can't install rigid foam above your roof sheathing. Closed-cell spray foam is less risky that open-cell spray foam in this location, especially in your climate zone.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |