Insulation for roof rafters under a metal roof (not-vented)
I’m the homeowner, so I’m not 100% informed on the ins an outs of metal roofing yet. I have a metal roof going on a renovation project and from what I understand the roof is not vented. We have a mixture of depths in the rafters, some are the original 2×6 (i think those are 6″) and then a bunch of new TGIs. It’s a 2-story wood frame house.
I haven’t meet with the contractor and his insulation sub-contractor yet, but I’d like to go in a bit more informed. What is the best approach or insulation type for the rafters where there is a metal roof that isn’t vented?
The architect is recommending spray foam. If that’s the best option do I ask for ‘closed’ or ‘open’ cell when it comes to a metal roof?
I’m in the midwest and we can get some hot summers and cold winters.
Thanks,
Dave
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Replies
David,
If you want your insulation to follow the roof line, start by reading this article: How to Build an Insulated Cathedral Ceiling.
And by the way, the insulation details will usually be the same, regardless of what type of roofing you choose to install.
David,
It doesn't really matter that you are getting a metal roof, your needs would be no different with shingles. You are limited to a non-vented system and that means spray foam insulation. What type of foam you can use depends on where in the mid-west you live. My company is in Ohio (climate zone 5) and we have to install foam with low vapor permeability. That means that half-pound open cell foam isn't an option but we get to less than 1 perm with a one-pound density foam, which is satisfactory. We could achieve slightly lower permeability with a two-pound closed cell foam but cost goes way up with only marginal improvements in r-value. Find out from your building department how much r-value you need to satisfy them. It is rare that we install closed cell foam in anything but the 2x4 rafters we sometimes find in old farm houses.
On the matter of foam terminology, I find it useful to think of density rather than cell structure. Our industry defines foam as closed cell if it has less than 20 percent open cells so obviously there is some arbitrariness to the term. Use the weight of the foam, which is always quoted per cubic foot of cured material, and check the vapor permeability and r-value at your installed thickness.
Good luck with your project.
David,
While Torsten advises you to install spray polyurethane foam -- the type of foam that his company sells -- it's not the only option. Most green builders would advise that it's better to install one or more layers of rigid foam above your roof sheathing than to install spray foam under your roof sheathing.
Thanks guys!
Great article Martin. I'm starting to have a much better understanding of the different approaches. I found a JLC article that was also very good. We already have the roof, water and vapor shield on the roof sheathing, so inside is my only option going forward. I've seen zone 5 and 6 for Chicago, I guess things are changing.
I was reading some forums talking about how good Nu-Wool is. It can be used in cathedral ceilings with a layer of 'closed cell' spray foam or rigid insulation.
Thanks again!
"I'm in the midwest " isn't a good indicator of climate, since it covers ever climate zone from 3-7, and it makes a difference:
The roof deck can be made moisture safe in any midwestern location with only 2" of closed cell foam, which is the maximum that can be safely applied in a single pass. But depending on your actual location you could need up to 60% of the center-cavity R to be closed cell foam or as little as 15% to avoid wintertime moisture accumulation at the foam/fiber boundary.
Closed cell foam isn't really very green due to both the high polymer content per R, and the HFC245fa blowing agent used by the vast majority of vendors. So when you're pretty much painted into that corner it's generally better to go with the minimum necessary for dew point control at the foam/fiber boundary, and use cellulose, which will buffer and redistribute the moisture burden far better than fiberglass or rock wool.
The percentage needed for dew point control by midwestern climate zone is:
Zone 3 :15%
Zone 4: 30%
Zone 5: 40%
Zone 6: 50%
Zone 7: 60%
This is independent of the total R or the code minimums in the area. Say you're in zone 3, and R38 is your local code min. You need 15%, an inch of closed cell foam and 8.5" / R32 of cellulose gets you there, but if the cavity is deeper than that you'll need more foam to keep the average temp at the foam/fiber boundary the same.
If you're in zone 7 and R49 is code min you'll need 5" of closed cell foam, which would have to be installed in 3 lifts of no more than 2" per, with a curing/cooling period between passes, and 5.5" of cellulose.