GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

New roof: add rigid foam?

TonyTexas| Posted inEnergy Efficiency and Durabilityon

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.
Tony
210.825.2092

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  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.

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |