Replies

1. GBA Editor
   Martin Holladay | Sep 14, 2017 05:07am | #1

   Gary,
   The short answer is no -- you don't want to install a vapor barrier on the interior of this assembly.

   The justification for interior vapor barriers -- a justification which has little scientific basis -- is that an interior vapor barrier keeps sheathing dry by preventing outward vapor diffusion during the winter. In the type of roof assembly you are describing, the exterior rigid foam keeps the sheathing warm and dry all winter long. (If the sheathing is warm, condensation and moisture accumulation are impossible.) So you don't need interior polyethylene.

   (Interior polyethylene can cause problems during the summer, since the polyethylene prevents the roof assembly from drying toward the interior.)

   Your roof assembly has a total R-value of about R-47 -- perhaps a little less -- with about 57% of the R-value provided by the exterior rigid foam. That is a robust roof assembly for all climate zones except Zone 8, so you should be fine unless you live in a very cold location.

   For more information on this type of roof assembly, see How to Install Rigid Foam On Top of Roof Sheathing.

   Two more points:

   1. I assume that your stack-up includes a layer of plywood or OSB roof sheathing between the bottom layer of rigid foam and the 2x12 rafters. Is that correct?

   2. When you install the Roxul mineral wool in the rafter bays, remember that the mineral wool batts have to be in direct contact with the roof sheathing. One way to do this is with steel wires designed for the purpose.

2. 
   garyov | Sep 14, 2017 08:01am | #2

   martin i,m not sure you under stand my assembly but the rigid foam is under neath the 1/2 inch sheathing.I have 1 5/8 vented air space then the rigid foam in the rafter bays and then the batts tight agianst the foam

3. 
   user-2310254 | Sep 14, 2017 08:13am | #3

   Gary:

   Where are you located.

4. Expert Member
   Dana Dorsett | Sep 14, 2017 08:28am | #4

   The vented air space doesn't affect Martin's answer. The vent space provides a path for the roof deck to dry, and to purge any minor air leaks in your cut'n'cobbled foam. A vapor barrier on the interior isn't necessary to keep the fiber dry- the foam-R/fiber -ratio is sufficient to keep wintertime moisture accumulation to acceptable levels.

   Adding a vapor barrier on the interior would prevent the fiber insulation from drying, reducing rather than enhancing the resilience of the assembly,

5. 
   garyov | Sep 14, 2017 09:12am | #5

   i am in northern michigan

6. GBA Editor
   Martin Holladay | Sep 14, 2017 09:21am | #6

   Gary,
   I guess I did misunderstand. I assumed that the rigid foam layers were continuous layers that covered the entire roof. Instead, it sounds like you cut the rigid foam into narrow rectangles and inserted the narrow rectangles between the rafters (the cut-and-cobble approach).

   Installing the rigid foam in continuous layers would have been much better. If you are still at the design stage -- if you haven't built this roof yet -- you might reconsider your plan.

   If the roof assembly is already built and insulated, it will work -- even though you'll have thermal bridging through the rafters.

   As Dana Dorsett noted, my answer on the polyethylene is unchanged.