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Community and Q&A

Exterior Roof Insulation

Ron Inget | Posted in Energy Efficiency and Durability on

I live in north west Louisiana. I’ve been searching for a way to insulate my ceiling/roof without covering the togune and groove cedar ceiling. The house was built int he late 70’s as a camp so I guess they decided not to put an attic. The roof consist of sheathing and shingles on the exterior and the cedar planks on the interior. I like the existing cedar and don’t want to cover it up.
Since there is no insulation in the summer months we can feel the heat in the upstairs in the morning and quickly begins to work it’s way done in the late morning. We need to insulate the roof but I want to do it from the outside.
What would be the best way to do this.
Is there a direct spray roof I could use on a residental application?
The roof pitch is a gentle slope at about a 12″X4″ going fore and aft starting at about the center of the house which is 28′ plus overhangs.
I guess the other option would be to remove the existing shingles, build the roof up using 2″X4″s standing on end. Spray a close cell foam down and then place more sheathing over it then install a new metal roof.
Doing this I would worry about roof strength. It would seem 2″X4″s standing on end would reduce the overhead strength. The ceiling joist are 2″X6″s doubled up extending fromt the center fore and aft on 24″ centers.
I intend to go with a metal roof so this is the second question.
If I do install the 2″X4″s on end and spray the closed cell foam can I install a metal roof directly with no sheathing or would I have to place the sheathing? I don’t want to put too much weight on the roof but want to insure the insulation and roof last. I’ve seen the SIP roof panels out there but I got to believe I can build my own.
Any suggestions would be appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    The easiest way to insulate your roof is with one or two layers of rigid foam insulation installed above the roofing. If you are planning to install metal roofing, then you would install asphalt felt above the rigid foam, followed by 1x4 or 2x4 purlins, 24 inches on center -- then the metal roofing. This system works well for through-fastened metal roofing. If you plan to install standing-seam roofing, talk to the roofer to be sure that the purlins will work before proceeding.

    Instead of rigid foam, you can also choose nailbase (rigid foam with OSB on one side) or SIPs.

    Spray foam is usually more expensive. And if you install spray foam between new 2x4s, you end up with thermal bridging through the 2x4s.

    If you have any doubts about whether your existing rafters are strong enough, you should hire an engineer to evaluate the roof framing.

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