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Community and Q&A

Roof ventilation

easimonsen | Posted in Energy Efficiency and Durability on

I have a low pitch (1/12) single slope shed roof on my house. (Marine Zone 4c near the coast with very little snow and ice) The framing is 2×8 filled with insulation. Drywall on bottom and skip sheathing on top- no ventilation. The existing roofing is built-up asphalt.
I plan to remove the built-up roofing and install 2″ of polyiso, then 1 and 1/2 inches of EPS, 5/8″ of plywood, then a standing seam metal roof with mechanically seamed joints.

Should I Install 2×4 strapping between the foam and the plywood to leave a ventilation gap? Ice dams are not a problem here, so I’m not sure I need it. Might it help with cooling in the summer? I’m not sure this is a problem either, as the house never heated up beyond my comfort level last summer without any AC.

If it is not necessary to ventilate this assembly, could I install 2×4 strapping over the polyiso and then put the 1 1/2″ of eps between the 2×4’s? I realize this will decrease the thermal performance somewhat, but I think it would allow easier installation of the plywood and provide some room for adjustment in order to keep the roof deck in plane.

Finally, should I install ice and water shield over the old roof sheathing? I’m hesitant about trapping moisture, but it seems to me the foam will be fairly vapor impermeable anyway so I might as well seal it up good.

Any insights are appreciated! Thanks.

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  1. Expert Member
    Michael Maines | | #1

    Easimonsen, low-slope roofs can be hard to deal with properly; I would start by reading this and the related articles listed:

    Have you checked with your roofer that they will warrant standing seam metal on a slope that low?

  2. easimonsen | | #2

    Thanks for the response Michael. We have a local manufacturer here in Washington (Nu-Ray metals) that has a standing seam roof good for a pitch down to 1/4:12.

    I have read Mr. Holliday's article, and the related articles previously, and again this morning based on your recommendation. I guess it all boils down to this: I'm using enough foam on top of my existing sheathing that I shouldn't need any ventilation. I think my hesitation to build it unvented stems from years of advice from older builders I have worked with. eg: "That wall has got to breathe!"

    My main sticking point that I am trying to resolve is the use of ice and water shield on top of my existing roof deck before adding foam. The recommendation in: is to install an air barrier, and seems to imply it should be vapor permeable?
    I'd like to use ice and water shield so I can keep my roof water tight during construction. I'm lucky to get 36 hours in a row that are rain free here... It seems to me that the foil faced polyiso is vapor impermeable anyway so having a vapor permeable air barrier would offer no advantage. Am I missing something?

  3. Expert Member
    Michael Maines | | #3

    Easimonsen, builders have good reason to be conservative when it comes to new approaches or materials. In my opinion an assembly that can dry in both directions is more resilient than one that can't dry, so I prefer vented assemblies to unvented assemblies when possible, which contradicts the standard GBA advice to wrap everything with foam. Even in a hot roof, ideally any materials that can absorb moisture would be able to dry easily in at least one direction. But if your rooftop foam is thick enough, and you have one or more good airtight layers, and your roofing is waterproof, I don't see why the airtight layer over your existing roof deck would need to be vapor permeable. As you say, the foil facing on the polyiso is going to block drying to the exterior anyway.

  4. GBA Editor
    Martin Holladay | | #4

    The Ice & Water Shield is fine in my book. It provides a good air barrier, and vapor permeance isn't an issue in light of the fact that you are installing two layers of rigid foam.

    You can vent above the top layer of rigid foam if you like, but the vent channels are optional. If you are worried about future roof leaks, they will allow for a drying path for small leaks.

    You can also install 2x4s above the first layer of rigid foam, and then install rigid foam between the 2x4s if you want. That will provide a small decrease in total R-value, but not enough to worry about.

  5. easimonsen | | #5

    Thanks for the input! Looks like i'm going to get a break in the weather soon, and I'll be able to build with confidence.

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