Replies

1. GBA Editor
   Martin Holladay|Apr 06, 2012 08:38am|#1

   [For readers who don't know about the brand name that Michael is talking about, "Tuff-R" is a brand of polyisocyanurate rigid foam insulation.]

   Michael,
   If you have rigid foam on the exterior of your walls, it's best to choose a vapor-permeable insulation for your stud cavities. That's why open-cell spray foam would, indeed, be a better choice than closed-cell spray foam in this application.

2. 
   Michael Strid|Apr 06, 2012 11:17am|#2

   Thanks for the answer. Just wanted to double check. So if I want a higher r-value I would be better with no rigid on the outside on the outside and the closed-cell in the cavities.

3. GBA Editor
   Martin Holladay|Apr 06, 2012 11:31am|#3

   Michael,
   The trouble with putting all of your insulation between your studs is that you still have a thermal penalty due to the thermal bridging through the studs.

   My advice: if you want your wall to have a higher R-value, install a thicker layer of rigid foam insulation on the exterior of your wall.

4. 
   Michael Strid|Apr 06, 2012 01:26pm|#4

   So 3 layers of 1/2in tuff-r staggered with taped seams on all layers over outside wall and open cell foam would be a good choice. It would would put the condensation point outside the sheathing. Use 1x4 over foam as a rain screen for fiber cement siding. I live in Connecticut so the 1 1/2" should be enough.

5. GBA Editor
   Martin Holladay|Apr 06, 2012 01:33pm|#5

   Michael,
   Yes, that would work. Or you could save yourself some labor and just install 2 layers: one 1/2-inch layer and one 1-inch layer.

6. 
   Michael Strid|Apr 06, 2012 01:47pm|#6

   Thanks for the help. Now I have to review my window details to work with thats system. When I work with commercial systems we usually put 2x blocking around the frame of the window so the glaziers have a place to attach the window.