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Omitting Interior Air Barrier With Insulation Above Roof Deck

AlexWI | Posted in General Questions on

Hi there,

We are looking for more input. We have a tiny house (8×24), 1.5/12 single slope roof, we heat with wood, climate zone 6a.

Current roof design:
Interior ceiling is pine tongue and groove
2×6 filled with wool r-19
Exterior plywood taped at seems for sealing
7 inches polyiso R 30-35 (our calculations taking into consideration cold is R31) (possibly screwed into place)
fiber board (possibly fixed in place with liquid nails)
epdm 45 ml rubber (we now know it would have been better to go with 60 but too late)

The tiny house was originally constructed without air barriers. To add an interior air barrier in the ceiling is manageable. To add it to the walls means gutting the interior (counters, stairs, shelves, built in tables, etc). We cant do that at this time. We were thinking we should add an interior air vapor to the ceiling but not the walls (which likely makes it imperfect but we thought would at least help). We were given advice that we didnt need an interior air barrier (so just on the rafter side of the tongue and groove ceiling) because we have enough insulation above our roof deck that any vapor shouldnt condense and become an issue. We will have an erv system installed as well.
This advice makes sense to us but we want to get some other input. Any thoughts?

Another question: Do I need to screw down the polyiso and fiber board or can we get away with liquid nails? My thought was to screw down the polyiso and hope to sink the screws into the 2×6’s but had concern that if the screws missed and I didnt realize it then the screws could conduct cold and condense and possibly start some minor mold spots at the screw holes in the roof deck/plywood. We’ve seen screws condense and freeze on the interior of an uninsulated wall in a warm space. To help mitigate this my thought was to put the fiber board (which the epdm will be attached to) over the screws and not put the screws through the fiber board. I would then attach the fiber board with liquid nails around the edges (to make it easier to replace in the future).  Our consideration is also that we will need to replace the roof and want to make it as easy as possible with destroying all of the layers of polyiso.
Some questions:
1. Do we need to secure polyiso with screws or can we just use liquid nails?
2. If we secure them with screws, is it possible for the screws to conduct cold and condense or freeze (if they are sticking out into the rafter cavity meaning they missed the 2×6’s? Or is this not an issue? (we are going for no mold – mold sensitive person living in home)
3. If the screws could conduct the cold and be an issue for will this be mitigated by putting fiberboard on top of polysiso and screw heads, so fiber board will be secured to polyiso using liquid nails and is this sound?

Thanks for any thoughts
Alex

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Replies

  1. creativedestruction | | #1

    1. Personally I'm a fan of rock or paver/pedestal ballast on EPDM roofs but that's more of a commercial roof approach and the structural deck has to be thick enough for the weight. Easy to replace and keeps the roof membrane a bit cooler, lengthening the lifespan. But I've never seen ballast on 1 1/2":12 slope...
    2. Use epoxy coated screws if you go that route. Yes, they could form condensation but the amount is insignificant. The plywood roof deck can easily redistribute the moisture without mold concern as long as your tiny house doesn't include a 24-hour steam sauna.
    3. I'll refrain from speculating on the structural adhesion of your materials. Too many variables. Test the adhesives on scrap materials to ensure compatibility and make a judgment call.

  2. GBA Editor
    Kiley Jacques | | #2

    Hi Alex,

    Regarding your penetrating screws question, you might find this bit of info from Christine Williamson of Building Science Fight Club interesting: The Overdriven Fastener. It’s not your exact situation but it does speak to the fact that minor penetrations don’t matter if the assembly has good drying potential.

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