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?
Replies
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.
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?
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.
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?).
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.