Replies

1. GBA Editor
   Martin Holladay | Jan 06, 2012 04:51am | #1

   Ron,
   I don't know the vapor permeance of the spray foam in question, but I imagine that it is intermediate between the vapor permeance of open-cell foam and that of closed-cell foam -- in other words, it's not very permeable.

   In general, I advise people that they should choose to install foam on one side of the wall sheathing, not both, so that the wall sheathing can dry in at least one direction. You are proposing a foam sandwich with plywood in the middle, and that's not a great idea.

   If you want foam, choose where you want it most: between the studs or outside of your sheathing. But don't put foam on both sides.

2. 
   Foamer | Jan 06, 2012 08:47am | #2

   Ron,
   Do you know who makes the foam you are describing? I would like to research it. We spray a lot of 1 pound foam, which is 50/50 open cell/closed cel, and it works extremely well. Martin is right, though. Your foam will be very close to a true closed cell in it's lack of permeability. You can use open cell foam with the EXP on the outside but be careful that you don't get too much r-value on the inside of your vapor barrier. With 1.5" EXP, your cavity insulation should be no more than r-17 to be safe. Another option would be to skip the OSB and use diagonal bracing for racking strength. The EXP becomes the sheathing and you can spray high density foam in the cavities, which will increase the total r-value.

3. 
   oLCfiRpjVP | Jan 06, 2012 08:51am | #3

   Martin

   Thanks! You gave me the information that I needed--I have read enough of your posts to be concerned about that foam sandwich issue. I will go with the XPS on the outside to help with Thermal Bridging and do either fiberglass batts or blown in cellulose on the inside. I think I can get pretty close to R 30 that way and for this cabin that will only be used a few days a month I think I will be in good shape.

   From previous posts, I think you would find it acceptable to do the closed cell spray foam in the rafters above the ceiling. Am I correct on that?

   Thanks again for your help. This web site has really helped educate me on insulation and I hope improve the efficiency of my cabin.

   Ron

4. GBA Editor
   Martin Holladay | Jan 06, 2012 11:22am | #4

   Ron,
   If you want to use closed-cell spray foam on the underside of your roof sheathing, there is no reason not to. Just be sure that the R-value of the foam is at least equal to the minimum code requirements, and be sure to provide a thermal barrier for fire safety. I recommend that you cover the cured foam with gypsum drywall.

5. 
   oLCfiRpjVP | Jan 06, 2012 11:42pm | #5

   Tortsen

   Thanks for your help. The company that manufactures the hybrid foam is called ThermoSeal. I found their website by just doing a google search with that name.

   Martin--thanks for the followup. I think I am ready to go on this.

6. 
   jklingel | Jan 07, 2012 12:33am | #6

   Ron: If you go with foam on the outside, Martin wrote a blog here that gives guidance to the minimum thickness. That would be well worth reading.