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Community and Q&A

Would like to know if applying rigid foam under metal roof is acceptable.

vZNXFmZfGh | Posted in Green Building Techniques on

I will be replacing the old corrugated metal roof on my house with a new Standing Seam style metal roof.
I am doing this in preparation for installing Solar panels, and I will be doing rainwater harvesting.
I will be using a high reflective coating on the metal roof to reduce inductive heating of the attic and thereby the house.
I would like to construct my roof in the following manner.
Plywood sheathing, then roofing felt, than 1.5” rigid foam, Metal roofing.
Will this work?
I know that most roofs have ventilation under them to prevent overheating of the roofing material. Will that be a concern with Metal roofing?

I have a 90 year old stone cottage in Portland, Oregon. The attic floor is now insulated with 8” of blown in insulation. There are no soffit vents and a continuous ridge vent. My rafters are old growth true dim. 2×4’s. My south facing roof has perfect placement for Solar panels which also means I get a lot of heat in my attic. Due to the smallness of my house I use and need my attic for storage.
I will be adding some sort of soffit vents, I am not exactly sure what kind. The stone work extends up between the rafters. If I use the ridged foam this should allow me to create a space for soffit venting. I will also be using ridge vents and probably adding a Cupola with a whole house fan built in.

These old growth 2×4’s with about 30 growth rings would probably blow away a new 2×6 with 5 or 6 growth rings. I have put a floor over my blown in insulation. I am just thinking that the ridged foam would stop a lot of heat from conducting thru into the attic. This would protect the things that I have stored there. Less of a build up of heat in my attic, less transmission thru to the people space below.
I guess the question i am trying to answer is, will having foam under my metal roofing cause any problems? Why do I need to ventilate the underside of metal roofing? With asphalt there are heat build up issues and they would probably melt. But not metal.


  1. GBA Editor
    Martin Holladay | | #1

    You'll need to install strapping (1x4s or 2x4s) on top of the foam, parallel to the eave, 24 inches on center. The strapping should be screwed through the foam to the rafters. Then your metal roofing can be fastened to the strapping.

  2. gusfhb | | #2

    2 things jump out:

    2x4 rafters, solar panels umm, I dunno...

    and foam on the roof, blown in on the floor, seems not the most effective strategy

  3. wjrobinson | | #3

    In an air conditioning climate, I think it might help keep your attic cooler. Otherwise we need to know more about your home, location, roof assembly and existing insulation and attic venting.

    My update after your update.... go ahead and try your plan, seems your attic would have to be cooler, its just not a standard idea to split insulation with your venting layer.

  4. Envirocon | | #4

    I live in a 100 year old adobe in Northern Utah and I installed a system of exactly what you describe except I put the roofing felt over the poly-iso insulation, and the metal roof is a modified standing seam from Metal Sales that has small corrugations that let a small amount of air to flow under the panel. This has worked extremely well for me for the last 15 years. You could then cc spray foam the interior of the attic and make it a habitable space/storage area.

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