Replies

1. GBA Editor
   Martin Holladay | Aug 04, 2011 06:57am | #1

   Neil,
   Q. "Can I install the polyiso panels and leave the current insulation in place?"

   A. Yes. Remember, two layers of 2-inch insulation (installed with staggered seams) are better than one layer of 4-inch insulation.

   Q. "Can a use a 'rubber' underlayment on top of the new roof deck beneath the new shingles? Would this create a problematic double vapor barrier?"

   A. Yes, you can do it, but I don't recommend it except at eaves and valleys. If you are worried about ice dams, I think a better approach is to install 2x4 vent channels (eaves to ridge) on top of the top layer of plywood or OSB.

2. 
   user-939142 | Aug 04, 2011 10:46pm | #2

   "Yes. Remember, two layers of 2-inch insulation (installed with staggered seams) are better than one layer of 4-inch insulation."

   why is introducing more failure points better?
   i understand you should stagger if you have seams, like say using 6 or 10" of XPS and using 2" sheets, but why stagger if you don't need to?

3. GBA Editor
   Martin Holladay | Aug 05, 2011 05:34am | #3

   Bob,
   If you use a single layer of foam, you will get convective air currents through the seams, even if you foam and tape the seams. (Rigid foam shrinks.) The heat loss at the seams will telegraph through the roofing -- it will be visible on cold days when the roofing has a layer of frost.

4. 
   user-939142 | Aug 05, 2011 03:28pm | #4

   that makes sense

   while you double the number of seams and weak points, the additional layer(s) remediate that