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New roof: add rigid foam?

TonyTexas | Posted in Energy Efficiency and Durability on

I enjoy your articles.
We are putting new decking and metal standing seam roof on a 1970 home. The rafters are 2×4 and there is also a cathedral ceiling.

When we remove the existing decking, can I put 1″ rigid foam panel directly on top of the rafters, then add the decking, then the synthetic layer , then the metal roof?

Should I use radiant barrier 1″ decking? and which way doe the shiny side go, up to the sky or facing downward?

Thank you.

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  1. Expert Member
    Akos | | #1

    Foam always goes over structural decking, so leave the deck in place install the foam on top followed by either a layer of OSB/CDX or strapping before the metal roof goes on.

    The radiant doesn't do much for most well insulated assemblies, but if you go with one, the shiny side should face an air gap. Since you'll need an underlayment under the metal, there is no easy way to create one without adding more strapping. Either way, you'll get more energy savings by bumping up your foam or going for a cool roof colour.

    You are in warm enough climate that 1" foam over 2x4+batts is sufficient for condensation control but it is not a very high performance assembly. Still much better than just the 2x4 with batts.

    Bumping that up to 2.5" would almost double the assembly R value, which would halve the heat gain through the roof. The extra cost might be worth it just by letting you downsize to a smaller AC unit.

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