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Community and Q&A

Attaching rigid foam below ceiling rafters

adamcohn | Posted in General Questions on

I’m building an insulated shed, with a ventilated flat shed roof. I’m planning to hang rigid foam below the 2×6 rafters in order to avoid the hassle to cutting them to fit in between. But how many inches of foam can I install so that I can still fasten plywood beneath, screwing directly through the foam?  I’m hoping for 3” of foam, but that would require very long screws.  

When installing the foam, should I use flat washers? Thanks all,

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  1. Expert Member
    BILL WICHERS | | #1

    The thicker the foam, the trickier it is to fasten through it. 3” is entirely doable, but will require more care with screws than 2” or 1” would be.

    I would use a few screws to hold the foam in place, probably just some drywall screws here. If you’re careful, you won’t need any washers. Once you hang the plywood, the plywood will do all the real work of holding the foam in place — there drywall screws are only there to keep the foam in place while you’re installing the plywood.

    I’d use some good GRK or Spax structural screws to secure the plywood to the rafters through the foam. Both those companies offer sufficiently long screws, and the structural screws tend to be higher quality than “regular” construction screws which is a plus in overhead applications.


  2. Expert Member


    If the shed roof is flat there is no point in ventilating it. Roof ventilation requires a change in height from eave to ridge to be effective.

    if you aren't ventilating the roof, why not add the insulation above the sheathing?

    1. adamcohn | | #3

      My bad, it is not flat, but a pitched shed roof. So I’m planning to ventilate from eave to eave. I’ve already completed the roof (it was necessary to do the roof quickly due to inclement weather here in the pacific NW). But yes, looking back it would have been much easier to work from above.

      1. Expert Member
        MALCOLM TAYLOR | | #6

        Your plan should work. I'd use cap-nails to secure the foam.

        For my own similar shed in the PNW, I sheathed the rafters, built a 2" curb around the perimeter, insulated with 2" EPS and put a second layer of sheathing over top. Luckily we had unusually sunny weather until the roof was complete.

  3. 730d | | #4

    You say a shed. I Minnesota a shed a a little building like a garden shed. In Wisconsin it can be most any outbuilding even bigger than a house. Is it attached or free standing ? Are you going to heat it and how often ?

    Two inches max for me would keep the cost of the screws less than the insulation.

    1. adamcohn | | #7

      Its an unattached 12'x16' (maximum size without needed a permit) pump house/workspace/food storage shed. The only heat will be what comes off the chest freezer (so I just enough insulation to keep the well head from freezing). Here on the Olympic Peninsula, the climate is temperate and doesn't drop too far below freezing for very long. While working, I'll just use a space heater.
      Thanks for the advice about the screws, they ARE expensive

  4. tommay | | #5 adhesive.

    1. Expert Member
      MALCOLM TAYLOR | | #8

      To hang foam or plywood from a ceiling? How would you do that?

      1. tommay | | #9

        It's a shed. Apply a liberal amount of adhesive to the bottom of the rafter and push the foam in place. Hold it for a few seconds and wala!

        1. Expert Member
          MALCOLM TAYLOR | | #11

          You want that foam loose so that the drywall screws will pull everything flat. I'd rather deal with a few cap-nails than risk blobs of adhesive.

    2. Expert Member
      Dana Dorsett | | #12

      Make that "foam board construction adhesive"

      The solvents used in standard construction adhesives attack polystyrene & polyisocyanurate.

  5. Expert Member
    Peter Engle | | #10

    There is enough room in the pictured rafters for a few inches of fluffy insulation and air channels. That's cheaper than more foam, and no screws needed.

    I agree with Tom's recommendation for adhesive to hold the foam until the plywood is installed. There are quick-grab adhesives that would easily do the trick.

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