Looking at an insulation chart, it says that 12 inches of that stuff would produce an R value of 78. Is that correct?
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Looking at an insulation chart, it says that 12 inches of that stuff would produce an R value of 78. Is that correct?
Member
Closed cell foam is about r-5 per inch.
It could be correct. It will depend upon the density of the actual product you use. Remember if you spray it on the underside of the roof sheathing, you still need to leave air paths, soffet and ridge vents for roof ventilation between the roof sheathing and the insulation.
Lee Schierer
USNA '71
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My understanding is that part of the benefit of closed cell foam is that you don't have to allow for ventilation if you have enough thickness. Not an expert, but that's what my architect is telling me.
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Something like that...closed cell foam is close to R7 per inch when properly installed. In our addition, they certainly didn't fill the 12" rafters as that would have been over-kill and raise the cost, but we still have at least an R50 going on and the closed cell foam also is a great vapor and wind barrier as well as adds significant structural stability. We effectively ended up with "conditioned" space, too, which is nice because the HVAC system for the addition is up there as well as a communications distribution rack and a bunch of storage.
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I'm confused, If sprayed straight against the sheathing, you still need to have vents for the space between the insulation and sheathing... ??Originally Posted by Lee Schierer
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The last four houses i built we used closed cell foam. No gap is left in the rafters or arround the sauffit for air flow. Here in New England it is the ice dam cure!
It seemed strange to me at first also. Times change though and so does everything else. FYI we put cedar shake roofs on all four of those homes. Under the cedar shakes a layer of what is called cedar breather is put down. Almost like a giant honeycomb brillo pad type material that allows for air flow bellow the shaked and atop the sheathing. Utop the sheathing the roof is covered completely by Grace ice and water shield. All peaks and valleys get lead coated copper or copper.
We did do one asphalt job and no channels for air flow where left. It would had defeated the purpose. The sole purpose of the project was to stop a major ice dam problem. We gutted all the exterior walls in the home, we then spayed or had the exterior walls sprayed along with the whole attic that was no finished.
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Well we have a 1925 house that we recently renovated, the attic was encapsulated with open cell spray foam in the rafter bays sprayed right against the roof decking and spray foam in the stud bays of the gable ends, as part of this process all of the attic venting were either closed off or removed.
As previously mentioned, on a 95 degree day, one can go into the attic and the interior temperature of the attic is like 75 degrees. Needless to say this makes the HVAC units job in the attic much easier and the 2nd floor more comfortable..
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Closed cell foam can be applied directly to the roof sheathing with no ill effects due to it's nature. There are some jurisdictions that still don't allow that and require baffles because they are less familiar with the material. My township was that way when we were building our addition. (they also required a bunch of other redundant things that considerably raised the cost of construction...you could park a (loaded) cement truck on top of our addition and it would likely hold it up!) My understanding is that they have relaxed this requirement at this point.
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The most expensive tool is the one you buy "cheaply" and often...
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I had spray foam installed on the floor of the attic in my house. It was only about an inch of spray foam and then fiberglass blown in on top. The spray foam was done because there was no proper vapor barrier. My attic is not usable space and is not conditioned. I have no HVAC up there as it is all in the basement.
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Moving the conditioned envelope up to the rafters and fully sealing the attic is generally a huge win. Last winter everyone around us had monster ice dams-- many folks had $100-200K in damage, there were several teardowns as a result of ice dam damage (lots of contractors with new pickup trucks this fall!). We didn't have so much as a single icicle.
It will tend to make the roof shingles hotter in the summer and could affect their lifetime. For an ideal installation you'd put down rigid foam on the outside of the roof to block thermal bridging through the rafters, sleepers to provide soffit to ridge vent ventilation then a second roof deck for your shingles. In most places the differences in shingle lifespan would never pay back for this.
Moderator
To vent or not to vent...lots of interesting research on this topic regarding roofs.
If you can't foam seal the attic deck/floor, then I'd do the rafters. Closed foam is about R7/inch and open cell is about half that. Learn about both.
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