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Community and Q&A

Foil Faced Polyiso Rigid Foam

Adam Croft | Posted in Energy Efficiency and Durability on

I am planning on using this type of rigid foam on the exterior of my house, specifically RMax Rmatte Plus 3. It says that when using under cement siding which is what I will be doing, that the white (non-reflective) side face-out. This will be applied to OSB/Plywood without housewrap. I know there is some controversy here, but I don’t feel housewrap will perform properly in my remodel since I am doing it sort of piece by piece, so overlapping some areas would be difficult.

If I am going to use fur strapping to create a rainscreen, does this still apply. Does it make more sense to put the foil side out on southern facing walls.

I am planning on using two layers each 1/2″ thick. This is probably a stupid thought but would it work to put foil face of first layer towards interior to keep heat in and put foil face of second layer towards the outside to keep heat out. I have a feeling this will create problems since I havent found anything about doing this.

I am located in Georgia, so mild winters, humid hot summers.

Thanks for the help.


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  1. User avatar GBA Editor
    Martin Holladay | | #1

    You will not gain any thermal benefit from the foil facing unless the foil faces an air space. So, for your bottom layer of polyiso, it doesn't make any difference which direction the foil faces.

    For the top layer, you guessed correctly: the foil facing should face the rainscreen gap. This will not cause any problems with your siding installation; in fact, the use of furring strips is much better than trying to nail the siding through the foam.

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