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Community and Q&A

Spray foam under roofs & moisture

jberks | Posted in Energy Efficiency and Durability on

Hey GBA,

I am planning to get my spray foamer in soon for apecidoc areas that need air sealing and higher R value. But I started to think twice about the spots under some of the roofs.

So my main roof has 6″ of polyiso above the deck, so I will have 9.5″ of Rockwool insulation in the joist bays to let the deck breath donward to the interior.

However, the rest of the roofs are very small decks, where there isn’t insulation on top, just the polyurethane roof membrane on top of the plywood decking. Thus I want to use 2lb spray foam to get the highest R value possible for both efficiency and hvac/comfort for the occupied spaces below.

My worry is if I sprayfoam the underside, within the 9.5″ joist bays, based on discussions here, it would create a rot sandwhich for the decking. But then I’m wonder how severe the moisture /rot would actually be, given that they’re relatively small areas.

These roofs are 112sqft, 38sqft & 18sqft.

Does anyone have any experience to offer? Should I not worry, or is there an alternative solution I’m not thinking of?

Any advice is always appreciated.



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  1. user-2310254 | | #1


    Remind us where you are located.

  2. GBA Editor
    Martin Holladay | | #2

    It's routine to install closed-cell spray foam on the underside of roof sheathing, even though there is no ability for the roof sheathing to dry outward or inward. As long as the sheathing is dry on the day that the spray foam is installed, this approach works.

    If you had planned ahead, you could have included the same 6 inches of polyiso above all of your roofs. But it sounds like it's too late for that now.

  3. jberks | | #3

    Hi Steve,

    I'm in zone 6 (Toronto)

    Martin, these particular roofs in question I designed with keeping the top at floor, for instance one is a 3rd floor walk out patio, the other is an at grade roof with skylights down into the basement, the other an awning with a bay window area underneath so only partial interior space. So outsulation wasn't really an option.

    But I'll see if I can get my hands on a wood moisture meter before I spray foam it. Is there a specific moisture context you'd recommend?



  4. Expert Member
    Dana Dorsett | | #4

    A 20% moisture content would be the maximum, less than 10% would be great. Wouldn't sweat it if it were in the 15-20% range. Over 20% dry-rot risk increases pretty rapidly.

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