Will open-cell foam work for a hot roof (unvented)?
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| Posted in Energy Efficiency and Durability on
I need to pull out fiberglass bats in a cathedral ceiling and re-insulate. There is a new roof on top, so I’ll pull out the FG through 2′ wide openings every 4 to 6 ft. perpendicular to the 2X10 rafters. Would 9″ of open-cell foam be an adequate vapor barrier as well as air barrier at this depth? Could closed-cell foam be injected into these 4′ to 6′ bays between openings?
I’d prefer taking off and replacing the roof after adding 4″ of polyiso and dense packing the bays, but the owner doesn’t want to tear off a new roof. In Zone 5 – bad ice dams last winter!
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Replies
James,
Open-cell spray foam is an air barrier, but it is not a very effective vapor retarder. It is vapor-open.
For example, 5 inches of open-cell Icynene has a permeance of 10 perms. That is relatively high.
In Zone 5, the 2006 IRC requires ceiling insulation to have a minimum R-value of R-38. Your proposed installation of 9 inches of open-cell foam would have an R-value of only R-33 -- less than the code minimum.
It's difficult to install any type of spray foam -- open-cell or closed-cell -- in a closed cavity.
To insulate this ceiling, your best options are:
1. To install all of your insulation on top of the existing roof sheathing and to install new roofing.
2. To remove all of the finish ceiling (drywall, perhaps?) and then install closed-cell spray polyurethane foam to a minimum thickness of 6 inches.
3. To leave the existing ceiling in place and install at least 6 inches of rigid polyisocyanurate insulation on the interior side of the ceiling, followed by strapping and new drywall.
James,
Your best bet may be to locate an installer of 1 pound density, semi-closed cell foam. It will give you the r-value you need (40+) and it expands in a manner suitable for your application. Space your 2 foot openings 8 feet on center and the foam application should be straight forward in a 2x10 cavity.