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Community and Q&A

Insulation and poly (non-felt) roof underlayment

Shakennotstirred | Posted in Energy Efficiency and Durability on

I had a late start framing last fall and could not get an asphalt roof down on my deck before freezing and heavy snow. So, I laid a synthetic underlayment down on the deck for the winter. It will soon be time to put a roof down. However, the issue is that the roof is complex with one dead valley, and several rafter bays which will be nearly impossible to vent, as well as five gable ends with outlookers in cathedral applications on the rack ends which will block venting.

Manufacturers of synthetic underlayments state that they should not be used in unvented roof assemblies. Now this seems logical if there is any chance the roof sheathing would be green or wet as the foam would trap in the moisture and could thus be a problem. However, since the OSB sheathing has been allowed to fully dry and I could even leave it uninsulated until the heat of the summer, would I have a problem with an unvented assembly where necessary?

I recently read an article which did suggest great care was need for all flash and batt under OSB, so a refresher here might useful as well.

Thank you.

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  1. GBA Editor
    Martin Holladay | | #1

    Leaving the synthetic roofing underlayment in place will probably not cause any problems, as long as the roof sheathing is completely dry before you install insulation.

    However, it's worth noting that:

    1. This installation violates the manufacturer's instructions, and is therefore (technically) a code violation.

    2. If you install a type of roofing that can dry to the exterior -- for example, cedar shingles, concrete tile roofing, or ribbed metal roofing -- then your choice of underlament is unfortunate, because it will prevent the roof sheathing from drying to the exterior in the future.

  2. Expert Member
    ARMANDO COBO | | #2

    Another option: How about installing all rigid foam insulation on top of the roof sheathing. It maybe a hard roof to install, but you wont risk future moisture and condensation problems.

  3. Shakennotstirred | | #3

    Thank you Martin and Armando,

    Martin, my location has no code enforcement/inspections, but that does not mean I need to do something stupid. I understand the concern of not being able to dry to the exterior due to the synthetic underlayment, but I plan to use asphalt shingles anyway, And I understand using spray foam on the underside of the deck would then not allow for drying out in either direction. Thus betting farm on a no leak roof. One should had have leaks, but they can happen, so I would think as you suggest I "should be ok" if the sheathing is dry before foaming.

    Armando, foam over the sheathing is a no go, I already have 12 inch thick rafters with full 12 inch thick tails. Just too thick to add more.

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