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Still trying to understand unvented roof details

joenorm | Posted in General Questions on

I’ve been posting on here a lot, thanks for the feedback.

Couple of details I am still trying to understand for an unvented cathedral with rigid foam on top of sheathing and fluff in-between the rafters.

1) Is Ice and water shield OK to apply to the top of sheathing, with the rigid on top of that?

2) If I put down 1×4 straps over the Iso board, do I need any sort of barrier before I lay down my metal? If not, wouldn’t there be a lot of condensation underneath the metal dripping onto the Iso board? Maybe this is OK?

3) Where are the problem spots for air infiltration in these assemblies besides the ceiling? How are they typically addressed?

Thanks

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Replies

  1. richard_leblanc | | #1

    Hi Joe,

    I'm actually in the process of doing this exact same research for a spring build. Here are the answers I have found so far.

    1) Is Ice and water shield OK to apply to the top of sheathing, with the rigid on top of that? Yes. Waterproof, vapor impermeable peal and stick membrane is ok as your weather barrier, air barrier and vapor barrier will all be on the same layer. Rigid on top is OK as rigid foam can handle a bit of water and humidity quite well.

    2) If I put down 1×4 straps over the Iso board, do I need any sort of barrier before I lay down my metal? If not, wouldn’t there be a lot of condensation underneath the metal dripping onto the Iso board? Maybe this is OK? I would do horizontal, then vertical strapping to allow for any water accumulation to drain/dry out.

    3) Where are the problem spots for air infiltration in these assemblies besides the ceiling? How are they typically addressed? your air barrier layer is on the outside. What is important to ensure you have the proper insulation value on the exterior of the roof sheeting to control condensation on the inside layer of your OSB. It seems that 40 percent of the total amount of insulation should be on the exterior and the remainder of insulation should be either cellulose, rockwool or open cell sprayfoam. Alternatively, close cell sprayfoam can be applied to the underside of your roof sheeting, however it's not my first choice since the product is not good for the environment.

  2. GBA Editor
    Martin Holladay | | #2

    Joe,
    Have you read this GBA article? "How to Install Rigid Foam On Top of Roof Sheathing."

    1. joenorm | | #3

      Thanks Martin,
      I have read every word on the topic there is to read. I think I am just nervous about specific details and moisture issues. Hence all the questions.

  3. GBA Editor
    Martin Holladay | | #4

    Joe,
    Q. "Is Ice and water shield OK to apply to the top of sheathing, with the rigid on top of that?"

    A. Yes, unless you are worried about potential odor issues. (I wouldn't worry, but some people do.) You absolutely need a bulletproof air barrier above your sheathing. If you don't want to use Grace Ice & Water Shield because of odor worries, use one of the European air barrier membranes sold by 475 High Performance Building Supply or Small Planet Supply.

    Q. "If I put down 1×4 straps over the polyiso board, do I need any sort of barrier before I lay down my metal?"

    A. Yes. Building codes require you to install roofing underlayment (asphalt felt or synthetic roofing underlayment).

    Q. "Where are the problem spots for air infiltration in these assemblies besides the ceiling? How are they typically addressed?"

    A. The problem spots are the sheathing seam and all penetrations (for example, plumbing vent pipes). These areas are addressed as they are elsewhere in your house -- by sealing with high quality tapes or continuous membranes, supplemented by canned spray foam and caulk, and by verifying the work with a blower-door test.

  4. joenorm | | #5

    Is it interior warm air that is the true threat to moisture buildup?

    Are small exterior leaks less of a concern?

    1. Expert Member
      Dana Dorsett | | #6

      With fully adhered self healing membranes such as Ice & Water Shield exterior leaks are rare. The stuff seals itself around nail & screw penetrations quite well.

      But even with #30 felt up top the true primary moisture risk to the roof deck in cold climates is air leaks and vapor diffusion from the interior side.

  5. Expert Member
    Peter Yost | | #7

    HI Joe -

    Modified bitumen membranes and their adhesion is very temperature dependent. I don't trust these membranes to maintain their seals over time, especially on a roof going through really severe temperature swings. If you use a primer, that makes a lot of difference.

    And yes, in cold climates, it is warm moist air making its way through assemblies that leads to condensation. But bulk water management is no less important; the mantra is bulk water, air control, vapor control (directional dedicated drying potential), THEN thermal control layer (insulation).

    Peter

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