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Rigid foam for roof, does backing material matter?

wzrdmatt | Posted in General Questions on


We’re building a home with a unvented attic space, r-38 open-cell sprayed at the roof deck, metal roof.  We have supply ducts going into the attic space to condition it. 

The metal roof is being laid directly on the Titanium PSU30 underlayment, and the underlayment is applied directly to the sheeting. We have no air channels under the metal roof due to local wildfire code (WUI) 

Our roofer is proposing to install 1/2 rigid foam board under the metal standing seam panels. Not for insulation value, but rather to smooth out the final finish of the metal.  R-2, helps a little, but big picture, the cost is trivial for a smooth roof. 

The panels he wants to use are standard home depot panels, called R-Tech from Henry.  They have a foil backing on one side, and a plastic coating on the other. 

Is there any concern using a product with zero perm rating with this type of detail?  It looks to be rated for roofing, but is there any chance I’m setting myself up for failure in some way using a product like this?

Really appreciate all the help!

– Matt

Ps. Zone 3 in the San Francisco Bay Area (Livermore, CA)

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  1. Expert Member


    The roof underlayment you are using is impermeable, so it doesn't make much difference if the foam above is or not. There is some concern that un-vented assemblies using open cell foam allow moist air to accumulate in the sheathing, and with those impermeable layers above it would have no way to dry. I don't know enough about spray foam roofs to comment usefully on whether it's a problem where you are.

    I've never seen a full foam underlayment used to prevent oil canning. The usual advice is a thin strip under the middle of the panels. Before going ahead, I'd confirm it was an effective solution.

    1. wzrdmatt | | #2

      Thank you Malcolm!

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