Replies

1. GBA Editor
   Martin Holladay | Dec 14, 2010 11:10am | #1

   Jon,
   You are correct; as the thickness of spray polyurethane foam increases, its permeance decreases.

   Six inches of Icynene MD-R-200 has a permeance of 0.65 perm.

2. 
   user-626934 | Dec 14, 2010 12:13pm | #2

   Jon,

   Just a suggestion - how about switching to 2x12 rafters and dense-pack cellulose?...or better yet, 12" TJI rafters?

3. 
   Riversong | Dec 14, 2010 12:17pm | #3

   I wouldn't expect MD-200 to perform differently from any other 2 pcf foam, except for its lower R-value, apparently because of the blowing agent.

4. 
   7CmVEdDP9e | Dec 14, 2010 03:26pm | #4

   John:

   Thanks for the suggestion. I should have noted that my roof is a full cross-gable, so I couldn't conceive of a way to get the equivalent of soffit venting to complement the ridge venting (ie, I have four valleys running down to each of the home's 4 corners). I assume you were suggesting an airspace between the deck and the cellulose, which I like as a solution for a simpler roof profile.

   Once I abandoned vented, I moved toward foam as the best means of providing air barrier and vapor retarder layers.

   Martin and Robert--thanks for the responses. I wonder whether the overall roof assembly makes sense to you both in my climate zone once I've gone the complicated roof route. I haven't observed any builders in Richmond use external roof insulation to manage sheathing temperature (and thus would not expect the local trades to do it well) so I am opting for "limit moisture access to the deck, allow to dry both directions--I think the permeance of thick MD-200 will allow this--and manage indoor winter humidity".

   Many thanks.

5. 
   Riversong | Dec 14, 2010 03:32pm | #5

   Jon,

   There will be negligible drying to the interior with any 2 pcf foam at 7.5". And don't expect to be able to see roof leaks.

   All PU foams are a mix of open and closed cells. The low-density (0.5 pcf) foams are mostly open and the 2 pcf foams are mostly closed.

6. 
   7CmVEdDP9e | Dec 14, 2010 05:12pm | #6

   Robert (and others):

   Thanks for the clarification on MD-200 as being essentially/predominantly a closed cell foam.

   BSC has a research report (BSD-149) on unvented roofs, suggesting that even high density/low permeance foams will dry to the inside (report excerpted below with ***) under the right conditions (high vapor drive).

   In the end, it seems that I am looking for a Goldilock's solution given my climate zone--enough of a vapor retarder to prevent interior vapor from getting to the cold deck in winter, but not so much so that it would prevent exterior vapor from getting to the inside to dry in summer. I suspect that one of these risks must be greater than the other?

   Thanks to all for the continuing education.
   -------
   Control Vapor Diffusion – Closed-cell spray foam acts as a throttle to control the rate of vapor diffusion. The foam insulation resists the diffusion of water vapor so that the amount of water vapor is reduced as it moves through the thickness of the foam. By the time the water vapor reaches the back of the roof sheathing, there is not enough left to cause condensation problems.

   In cold climates the ccSPF provides the first condensing plane. The ccSPF should be installed directly to the face (i.e. back) of the roof sheathing in a layer thick enough to prevent condensation from forming on the inside face of the foam.

   ***In hot climates up to 5 in.* (125 mm) of ccSPF can also be used. In this application the assembly can still dry to the inside during the daytime when roof temperatures are high (120-190°F or 50-90°C), the vapor pressure gradients are very large (5-35 kPa) and the foam is a vapor retarder and not a vapor barrier.***

   Controls Rain Leakage – Closed-cell spray foam has negligible water permeability, minimal water absorption, and excellent adhesion allowing it to act as a secondary rainwater barrier to limit damage when primary roof assembly rainwater control membranes leak. Rainwater migration is severely limited due to the low water transmission and high adhesion (“waterproofing”) characteristics of the foam and damage is limited to the area immediately adjacent the hole in the primary rainwater control membrane. This tends to contain the damage, making it easier to identify the source and preventing it from spreading throughout the assembly and to interior finishes which can be costly to repair.

7. 
   Riversong | Dec 14, 2010 05:56pm | #7

   "under the right conditions" is the critical determinant.

   If you have a dark colored roof and lots of direct sun, then water vapor will certainly be driven through even closed cell foam, albeit slowly. Water damage risk is a matter of time and temperature, so whether a problem will occur depends on how long the roof sheathing stays wet and whether it's warm enough to mold or rot.

   The Florida Solar Energy Center study on cathedral roof systems demonstrated that the least durable roofs, in either cold (Boston) or hot/humid (Miami) climates were those with closed cell foam underneath and peel-n-stick membranes on top (many of the new roofing underlayments are relatively impermeable as well).

   BSC, in my educated opinion, has bit of a bias toward plastic foams. Foam can do the job, but it's important to be aware of the potential liabilities. If you use ccSPF under the roof deck, then be sure to use a vapor-open roofing above (such as felt and shingles).

8. 
   7CmVEdDP9e | Dec 14, 2010 07:35pm | #8

   Robert, thanks once again. Clarifying...*asphalt* shingles laid in a standard lap configuration over the felt should give me the vapor open needed...