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Best way to insulate roof on a screened porch with exposed rafters and tongue-and-groove roof deck?

jeffster06 | Posted in General Questions on

I’m in central NC, zone 4A, and building a 12′ x 18′ screened porch with a gable roof.  The porch is being built atop a wood deck that is about 5′ off the ground. I’m concerned about trapping heat inside and have been pondering a few options to make it more comfortable inside.  I was thinking about using a built-up roof system as follows (top down list)
1. Asphalt shingles – HOA required…wanted to use metal 🙁
2. Tarpaper
3. OSB sheathing (with or w/o foil???)
4. Rigid foam insulation (with or w/o foil???)
5. 1x pine tongue & groove (ceiling/roof deck)
6. Joists (no attic space)

I’m looking for your advice on how much insulation would be needed to make a difference and how best to install it (air-gap with furring strips or just sandwiched together).  Where does the foil belong?  Does it matter? Or should I insulate it at all??

Also, I was thinking about doing a ridge vent.  Is it worth doing considering that the end triangle of the gable will be open (screened)?  I was thinking it would be good to have the ridge vented so the ceiling fans could pull cooler air up through the walls and floor and up out of the ridge. Or will the open gable be enough?

I’ve done a lot of googling on the subject and there are so many factors involved I’m just not sure how best to do it.

Thanks in advance for your insights!

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  1. GBA Editor
    Martin Holladay | | #1

    Will this porch be enclosed and conditioned (heated in winter and air conditioned in summer)?

    If it's not conditioned -- if it has insect screens and is open to breezes -- there really isn't any reason to insulate the roof assembly.

    To answer your questions:

    1. It makes no sense to include foil-faced OSB unless the foil is facing an air space. In your case, there is no air space.

    2. It makes no sense to include foil-faced rigid foam unless the foil is facing an air space. In your case, there is no air space. (That said, if you happen to see rigid foam for sale at a good price, and the rigid foam happens to have foil facing, the foil facing will do no harm.)

  2. jeffster06 | | #2

    Thanks! I'll do that.

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