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Roofing membranes in an unvented roof

Tinybuilds4U | Posted in General Questions on

Hoping to get some suggestions for my unvented roof. Is it recommended to use vapor permeable membranes in unvented roof assemblies? Bellow as well as above rigid foam board, Or is it best to use vapor impermeable membranes? 

Thank you,

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  1. Expert Member


    What type of un-vented roof are you building? Foam in the rafter bays? Foam board over the sheathing? Batts under the foam?

    This article shows some of the most common types:

    1. Tinybuilds4U | | #3

      Mineral wool insulation in rafter bays. Rigid foam board over the sheathing. Then another layer of plywood on top of foam board and either rolled roofing or metal roof.


  2. jamesboris | | #2

    What type of roofing are you using? If it's vapor-impermeable (I think most roof claddings are), then I'd use a nice peel and stick like Carlisle WIP 300 HT on top. Unvented roofs are unforgiving if something gets through the cladding, so go for a premium underlayment in my opinion. WIP is a thick, sticky asphalt membrane that's easy to walk on.

    The last one I did like this, I used Solitex Adhero below the rigid foam. It's vapor permeable and a good air-seal. But I had board sheathing. If you have plywood, I'd just tape the seams with good tape like Zip, Vana, 8067, etc. If you can get the rigid foam on quickly, you don't really need a fancy barrier like my Adhero below the foam. To protect from weather exposure, any decent underlayment is fine.

  3. Tinybuilds4U | | #4

    Is it okay to use synthetic membranes above and bellow rigid foam?

    The roof is going to be low slope 1.5/12 maybe 2/12

    1. jamesboris | | #5

      On a roof like that I would 100% use a high quality peel and stick above the plywood-atop-the-rigid-foam. As for under the rigid foam, see above for my view. Unless you're using a thin layer of EPS, the rigid foam is impermeable, and even with the EPS, you're not going to get significant drying-out through that rigid foam.

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