Installing a roof over unvented attic/cathedral roof?
ARMANDO COBO
| Posted in Green Building Techniques on
We are having a debate on best installation details for a nonvented roof assembly. CZ4b
1. One part is over a cathedral ceiling w/ 2×8 rafters and open cell foam and another part is attic space and open cell foam. Total insul: R38+
2. We agree on using 1″ of rigid foam over the roof sheathing for thermal bridging.
3. We disagree in installing a second sheathing on top of the rigid bd.
4. We disagree on roofing material: shingles vs. metal or tile.
Are there any shingle manufacturers that allows shingle roofing installation over an unvented roof assembly?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Unfortunately, many if not most manufacturers (I'm quite sure) now do - not because it has no impact on the durability and lifetime of the shingles but rather because of a fear of losing market share for a growing segment of the "green" housing market.
Thanks Robert. I should have mentioned my roof details: shingles, Triflex VB, 7/16 sheathing, 1" rigid insul. R7, 5/8 Sheathing, 7.5" open cell. Builder/Owner dosen't want to install 2nd sheathing and thinks he has to use metal or tile over nonvented roof assembly.
Sorry for typo: 1" insul is R10
What type of rigid insulation has an R-value of 10?
Armando, the thing that's questionable is having your sheathing sandwiched between impermeable or mostly impermeable layers, when we all know that (1) a certain amount of moisture will leak through the shingles, and (2) sheathing will decompose if it takes on water and can't dry.
Armando,
If you spray open-cell foam between your rafters, your roof assembly will be able to dry to the interior (as long, of course, and you don't install interior polyethylene) -- so I wouldn't worry too much about the "foam sandwich" problem in your case.
The more rigid foam you can afford on top of the roof sheathing, the better. One inch isn't much. And 1 inch of rigid foam certainly isn't R-10. If it's polyiso, it's about R-6.5.
You can use any roofing you want on top of the rigid foam. Metal roofing or tile might be easier, because you don't need a second layer of plywood on top of the rigid foam -- just 1x3 or 1x4 battens parallel to the ridge. If you install asphalt shingles, the best way is to install 2x4 nailers on top of the foam (with each 2x4 above a rafter, in order to allow for soffit-to-ridge ventilation), followed by plywood, asphalt felt, and shingles.
Thanks for all your answers. The shingles are a "slate" shingle to go with the architectural style of the house; real slate in NM is too expensive. The sandwich detail comes from the GBA.com and BSC. We’ve done this before with metal and tile roofing, but not with shingles. The open cell does dry to the interior, no worries there plus we have air supply to the attic as well. A good roof seal and good VB should take care of capillary draw. I did a roof assembly analysis and 1” of rigid foam is good for our climate.
By your answer Martin, can you install shingles directly over 1"+ rigid foam? Regular fasteners? That’s my only concern.
Armando,
No. You've got to have plywood, OSB, or board sheathing.
I agree with Martin's layering prescription (notice he recommends felt rather than polymeric underlayment), but disagree about his certainty that open-cell foam allows sufficient drying to the inside.
There is a great degree of variability between various foam products in the ratio of open to closed cellular structure, and of course it's the actual application that determines the ultimate qualities of the material.
Even BSC, which is a strong advocate of plastic foams, warns on their Permeability Table "While all spray foams are excellent for air sealing, they vary, often widely, in their density, R-value, blowing agent, water resistance, vapor permeability. These last two can have the greatest affect on just how you use spray foam in various building assemblies."
They list 3½" of Icynene as having a perm of 16 (the company claims 24), which would be about 7.5 perm for a 7½" installation. But BSC also notes that Icynene absorbs 34% of its volume in water when soaked. Once a hygroscopic matrix, like open-cell foam, becomes saturated, it loses its vapor permeance (which requires open lumens). In other words, it works like a sponge.
And do not forget that water vapor permeance is only one part of the equation - diffusion is an energy-driven process. The more thermally insulated an assembly, the less heat flux, the less vapor drive. And the salutary tendency toward roofings with a higher albedo to reflect solar radiation also dramatically reduces heat flux through a roof assembly.
Because of diffusion and felt not a 100% VB, that’s why we are using a better VB product, and yes, my moisture management details are good and I know the builder will do a good job with the installation. ASHRAE Fundamentals, 15-lb asphalt felt has a wet-cup permeance of 5.6 and 15-lb tar felt is 18.2 perms.
The rigid foam is a 2nd layer of moisture barrier, so I don’t expect any moisture getting to the soy-based open cell; plus I have ventilation in the attic and the house is in a dry climate.
In forest areas, vented roofing or cladding are highly discouraged; in some municipalities in the Southwest, they are not allowed.