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Polyiso roof details

Steddy | Posted in Energy, Heating & Insulation on September 22, 2006 05:52am

I’m putting polyiso rigid foam insulation over the cardecking of a log cabin (over 30# felt), followed by 1/2″ ply and standing seam.
Based on costs and local availability, we are doing 2 layers of 2″ and 1 layer of 1/2″ to get to R-30 (okay, sticklers – 29.3, but that 1.5″ decking puts us over).

I certainly don’t want to tape the seams of all 3 layers of foam. Yes, the seams will be staggered vertically and horizontally (manufacture wants just 6″ btw). Assume a two layer application – I can’t find an answer on taping seams – both or one? I’ve found literature saying first, and some saying top. Input sought!

Also, any thoughts on an airspace under sheathing versus our plan of just sandwiching it like a SIP panel?

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  1. RayMoore2G | Sep 22, 2006 06:05am | #1

    Tape the bottom layer. Better yet, seal at the layer below the first layer. The object is to keep interior moisture from finding the cold outer surface with a convective loop. The fact is, the staggered laps alone will accomplish this. Taping the top layer is not effective at avoiding this issue.

    1. Steddy | Sep 22, 2006 06:35am | #2

      I would believe that the felt paper vaper barrier was accomplishing that, Ray. You were meaning to tape the 'underside' of the bottom layer, right?

      1. RayMoore2G | Sep 22, 2006 04:05pm | #3

        Felt paper is not adequate if you are in a cold or very cold climate. A vapor barrier on the other hand could be ideal if it is under the foam. I didn't know that you had one in your system. Check out http://www.buildingscience.com. I think you can find this roof system in their designs that work section.

        Again, just  the staggered laps will do the trick and the polyiso is a vapor barrier at that thickness. You are going to have a great roof. What are you using to fasten down the top layer of decking? Atlas has a website and some installation details for their ACFoam panels. There are different screw patterns for different wind loads and they suggest a solid 2x perimeter be let in to the foam. They sell 6" screws with 3" washers for holding down the roof deck through the foam.

        1. Steddy | Sep 28, 2006 06:14pm | #4

          We feel pretty good about the roof system, too. A good challenge we have is straightening the peak and getting the gable ends of the main house to plane in with the middle of the house (homeowner-built log cabin with gables at a different pitch than the center), but that's conversation for another thread. I don't know if you're meaning that tape is not necessary with 2 layers, and Atlas' web didn't feel like working at this time. The building site is in the Seattle area - certainly not a 'cold' climate. As for fastening, we have tried 6" screws 1' on edge and in the field with moderate success - not wanting to pierce the underside of the cardecking (finished ceiling), with longer screws planned for runs down the log rafters 4' on center. I'm not surprised it's been a challenge with the imperfect plane of the cardecking, sandwiching all of the layers together, and not being able to find 6 1/4" or 6 1/2" screws (yet?).

          1. RayMoore2G | Sep 29, 2006 02:24am | #5

            6.5 and 7" screws are available. I'm glad to hear you are doing this type of system. I believe in it. Good luck.

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