Plywood and polyiso sandwiched between vapor barriers?
![](https://secure.gravatar.com/avatar/6b68f259454b3dc6352345690dbf3c64?s=52&d=https%3A%2F%2Fimages.greenbuildingadvisor.com%2Fapp%2Favatars%2F200x200%2Fi.png&r=g)
Hi all, I have one final detail question before I can begin my exterior insulation roof install that I’m really stumped on. From the endless articles and threads I’ve read this seems to be okay but I’d like to get some confirmation.
My proposed buildup is as follows from bottom to top (CZ 3B):
1. 1/2″ plywood structural roof sheathing
2. Ice and water air and vapor barrier on the full roof and turned down the wall 8″ to ensure full air barrier (home has no overhangs)
3. 1.5″ polyiso insulation, canned spray foam between any gaps such as where the polyiso meets at the ridge
4. 1/2″ plywood sheathing screwed to structural roof deck using SIP screws (20 per full panel)
5. Sharkskin peel and stick air and vapor barrier as roofing underlayment (per roof manufacturer recommendation) on the full roof and turned down the wall 8″ to encapsulate the edges of the foam
6. Striated standing seam metal roof
I’m trying to understand if it’s really okay to have 2 vapor barriers as specified? I may be overthinking this process but won’t any moisture that’s in the polyiso or the top layer of sheathing be completely encapsulated at this point between the I&W above the structural roof sheathing and the Sharkskin above the plywood sheathing?
What if the fresh plywood has some extra moisture during install, won’t it get completely sealed off at this point and not have a chance to dry out?
Any other final comments about this assembly before I begin?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
![](https://www.greenbuildingadvisor.com/app/themes/greenbuildingadvisor/dist/img/modules/detail-library/thumb1.png)
![](https://www.greenbuildingadvisor.com/app/themes/greenbuildingadvisor/dist/img/modules/detail-library/thumb2.png)
Replies
This is a pretty typical assembly, and if installed dry shouldn’t have issues. That being said, you could install a vapor diffusion vent on outer plywood layer that could allow drying of the polyiso and plywood. This would consist of ripping back the plywood at peak a couple inches and installing a high perm peel stick, over 30 I would suggest, over gap, then using a ridge vent. I would probably do this if it were my roof, though I’m not sure it would be needed, just a little extra peace of mind.
freyr_design,
The trade off in adding a diffusion port is that while it may provide an outlet for excess moisture that gets into the foam, it also makes that more likely to happen by interrupting both the underlayment and roofing.
My feeling is: in for a penny, in for a pound. It's either an un-vented roof with the weaknesses and benefits that brings, or a vented one.
I disagree. There is a lot of evidence that this works very well. This has been used to fix sip roof ridge rot failures as well dehumidify entire attic spaces. It is not really like interrupting your underlayment as you will still have a membrane that will shed bulk water. This provides insurance while negating the downsides of simply having a ridge vent. Especially if the climate of the op, I don’t think there is really a downside.
freyr-design,
I miss-read the stack up and thought the I&WS was directly under the roofing, which I didn't see the point of interrupting. You are right, there seems little downside to adding the vapour-diffusion port.
For similar roofs with exterior insulation and metal roofing, I've used permeable underlayment for the top layer. This allows for a bit of drying capacity in case anything gets into the foam layer. The cost premium of permeable roofing underlayment is pretty minimal. There is not much point for the peel and stick on the top layer except where your local codes require it.
Freyer_design, I think this is good peace of mind, especially since I do have a metal ridge beam, so hopefully any condensation that may occur due to the steel beam will diffuse out.
To implement the diffusion port I will do the following:
1. Keep a 2 inch opening (approx 1/150 of 1900sqft roof) where the plywood meets at the ridge on both the structural sheathing and the upper layer plywood sheathing
2. Install vaproshield slopesheild SA approx 24 inches wide over the ridge and over the I&W underlayment on both the structural sheathing and upper layer plywood sheathing. Overlap by 4"
I still plan to run the insulation tight at the ridge and spray foam any gaps where the insulation meets at the ridge, but the polyiso states it has permeability of <1 perm (polyiso with fiber facer) and no matter how well you spray foam the gaps at the ridge, I'm sure there will be some inevitable gaps.
Does this seem like a good idea or should I just put the vapor diffusion port at the upper layer of plywood sheathing and keep the vapor barrier at the structural roofdeck?
Akos, I agree but I would just be concerned about having warranty issues down the line and having the roof manufacturer blame my choice of underlayment vs their preferred brand. Although I'm sure at the ridge I should have no issues if I run the vapor diffusion port.
I would not do a diffuse port on your structural sheathing layer, this will introduce interior moisture into your assembly with a poor drying path if you continue your polyiso over it. Just do it on exterior layer sheathing and keep your ice and water continuous on the lower.
Interestingly enough, when I submitted this buildup to my building department, they balked at the "vapor barrier sandwich encapsulating the polyiso".
This doesn't really make sense to me, because aren't insulated flat roofs typically detailed the same way?
I don't think the building department has really seen this buildup before since it's a rare occurrence to install exterior rigid foam in my area on a non flat roof.
To appease the building department, I can either install a vapor open peel and stick under the polyiso or under the metal roof.
Under the polyiso will be cheaper, since I can use Adhero 3000 ($1/sqft).
Under the metal roof I would rather use a high temp SA such as Vaproshield SA ($2/sqft).
Any thoughts on preference of the vapor open air barrier location or should I just go with the cheapest route?
Look at my earlier post #5. Solves the vapor barrier sandwich and cost.
Standing seam metal roofs are liquid tight, there is no benefit to having peel and stick directly under it.
Thank you Akos and freyr_design! Great suggestions I'm very thankful for your input.