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Rigid foam above roof sheathing: can I use a synthetic underlayment?

claythonmclaw | Posted in Energy Efficiency and Durability on


I have read warnings about using a vapor-impermeable synthetic underlayment (e.g. Grace Tri-Flex) over an unvented space (which I do not assume to mean a roof with absolutely no ventilation channels).

However, in the “How to install Rigid Foam On Top of Roof Sheathing” article, a roof system is described where taped foil faced polyiso is installed above the roof sheathing and below the ventilation channel–despite the fact that foil faced polyiso is also vapor impermeable.

Why is it that this is permissible with such a roof? And in such a case would it therefore be OK to use vapor-impermeable synthetic underlayment overtop the foil faced polyiso?

Thank you

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  1. GBA Editor
    Martin Holladay | | #1

    There are several issues here.

    First, you need to comply with the installation instructions of your chosen roofing underlayment. (Following manufacturers' installation instructions is a code requirement.) Many, but not all, manufacturers of synthetic roofing underlayments forbid the use of their products over unvented assemblies. A recent web search appears to show that Grace Tri-Flex can be used over unvented roof assemblies -- there don't appear to be any restrictions on its use in that department.

    Manufacturers may be coming around to the idea that most roof assemblies don't dry to the exterior, a fact which makes the old restrictions on the use of synthetic underlayments seem a little silly. For more information on this issue, see VAPOR PERMEABILITY PROVIDES NO PERFORMANCE BENEFIT FOR ROOFING UNDERLAYMENTS IN VENTILATED ATTICS.

    Second, vapor-impermeable materials that have an R-value (like foil-faced polyiso) behave differently in some types of building assemblies than vapor-impermeable materials that have no R-value (like synthetic roofing underlayment). In general, R-value is protective -- because it raises the temperature of the materials on the interior side of the insulation during the winter. Materials that are warmer are protected from condensation and moisture accumulation. Materials without R-value can't perform this function.

    Q. "Would it therefore be OK to use air impermeable synthetic underlayment overtop the foil faced polyiso?"

    A. Yes, as long as the polyiso is thick enough to keep the materials on the interior side of the polyiso above the dew point during the winter. The foil-faced polyiso is vapor-impermeable, so one more vapor-impermeable layer does no harm.

  2. claythonmclaw | | #2

    Thank you for the quick response. This was very helpful.

    Also, I mistyped something in my original post but I don't see an option to edit it. I meant to type "vapor impermeable synthetic underlayment"--not "air impermeable synthetic underlayment"

  3. GBA Editor
    Martin Holladay | | #3

    I have corrected your typo.

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