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Exposed beams and deck with rigid insulation above

user-7255161 | Posted in GBA Pro Help on

I’m going to have a cathedral ceiling with 2 x 6 tongue and groove decking that is exposed from below, so all the insulation must be above. We are going to have a metal roof. The pitch is only 2 and 1/2 + 12 so there will be an ice and water shield. Unless you have another recommendation. We need to meet r38 but need to minimize the thickness as much as possible.

The climate is Pensacola Florida which is hot and humid most of the time. 

Whatwould you recommend?

The rafters will be 4 by 8 space at about 4 feet on Center and they will come out and become exposed so that may add an additional layer of complication to the roof wall interface detail. Please advise.

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  1. Expert Member
    Dana Dorsett | | #1

    Hitting IRC code min on a U-factor means ducking under U0.030, which is anything greater than R33.3 "whole assembly", that includes the R-values of the roof decking, interior & exterior air films, roofing, etc.

    The 1.5" of 2x6 t & g is good for about R1.8, and for a low-pitch roof the interior + exterior air films add up to about 1.17 in summer, 0.78 in winter, call it R1-ish average. so you're up to about R2.8 which means you can hit code min with as little as R30.5 foam above the roof deck. If a nailer deck is applied above the exterior insulation the deck itself is good for the 0.5, so it's really R30 of actual insulation. That can be done with 5.5" of 2lb roofing polyiso (recommended) or 5" of foil faced polyiso. With either you'd be looking at a 1x8 for a roof edge facia board above the rafter beams.

    Reclaimed roofing polyiso is fairly widely traded at roughly 1/3 or less the cost of virgin stock foam. Repurposed Materials Inc in Atlanta shows up in this search as one possible source:

    Insulation Depot in Monroe LA might be just as close:

    1. user-7255161 | | #3

      Thank you. I didn't know about the used polyiso. That's a big help.

    2. user-7255161 | | #4

      Florida Building Code says R 38 forzone 2A. Is there some exception that I don't know about? There heard there is prescriptive method and a calculated method, but I have no experience with this. We also think we need to use 1x4 sleepers for the metal roof. We'd really like to keep the fascia 1x8 if possible.

  2. BrianPontolilo | | #2

    It sounds like you are well into the design phase. How do you plan to insulate and finish the space between the exterior wall's top plate and the bottom of the roof deck?

    I recently worked on a project where the homeowners wanted exposed eaves in a super efficient home. The architect and builder came up with a pretty slick solution. It's a different situation, but some of the thinking may be helpful. Here is a link:

    1. user-7255161 | | #5

      Thanks for the article. Very nice work by shoreline builders.

  3. GBA Editor
    Martin Holladay | | #6

    It would be helpful to know your name. (I'm Martin.)

    You may want to read this article: "How to Install Rigid Foam On Top of Roof Sheathing."

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