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Community and Q&A

Underlayment permeability?

nhbean | Posted in Green Products and Materials on

We are in the process of an energy-focused renovation (Climate Zone 4, Manhattan KS) and I am looking for advice in selecting an appropriate underlayment. Our planned roof stack-up is:

5″ closed-cell spray foam against the sheathing, encapsulating the 2×4 roof joists
3/4″ OSB
1×4 counter battens (laid flat and secured through OSB to the roof joists)
1×4 battens
Plastic barrel tile, fastened to the battens (Quarrix or Brava)

This stack-up gives us a ventilation space between the tile and underlayment, and we will have vents at the hips (this is a pyramidal hipped roof) and the eaves.

My primary question is what underlayment would be most appropriate, long lasting, and provide adequate drying for the OSB layer?

My thanks for any advice you can offer.


Nathan Bean

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  1. GBA Editor
    Martin Holladay | | #1

    Ordinary #15 asphalt felt is a so-called "smart" vapor retarder with variable vapor permeance. It will work.

    If you prefer a synthetic roofing underlayment, choose from the list below. These are all vapor-permeable. The list comes from my article, Synthetic Roofing Underlayments.

    The first column shows the manufacturer; the second column shows the product; the third column shows the vapor permeance. Click the image to enlarge it.


  2. nhbean | | #2


    Thanks for your help. Are there any guidelines on how permeable the underlayment should be in my case?



  3. GBA Editor
    Martin Holladay | | #3

    The only reason that your roof sheathing should ever get wet is due to a roof leak or flashing failure. The closed-cell spray foam that you plan to install on the underside of your roof sheathing is an air barrier as well as a vapor barrier, so no interior moisture will ever reach the sheathing.

    Under the circumstances, you don't really need to worry about outward drying. That's why I said that ordinary asphalt felt, or any of the synthetic roofing underlayments I listed, will work fine.

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