Foam sheathed roof
In an article in Fine Homebuilding (“Rigid Foam on the Roof and Walls”), Martin Holladay discusses omitting wood-based sheathing products and sheathing walls with foam boards in conjunction with an alternative wall bracing method.
Could the same be done for a roof assembly? I wasn’t able to find any examples for such an assembly.
If I were to use this method for a wall, I would sandwich a redundant WRB between two layers of foam boards, such as Tyvek or polyethylene sheets. I believe Mr Holladay once referred to this method as a “belt-and-suspenders approach”, to overcome the potential of foam shrinking.
For a roof assembly, maybe the same can be done with a fully adhered membrane?
Thanks,
Ben
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
On a roof you need something to attach the roofing to and, depending on the roofing material, something to span between the rafters and support the weight of the roof.
With cedar shakes or metal you could put 1x strapping over the foam I guess. Can't see this working with asphalt shingles.
Thanks for the reply; I was indeed thinking of metal roofing over 1x strapping that is attached back to the roof trusses.
However, this detail shouldn't limit the scope of the question. Since the roof will have an exterior foam layer, some roofing systems might require wood-based panels to the exterior of the foam (as can be seen in the drawing in the article I referenced).
The question is, can the wood-panel sheathing be omitted to the interior of the foam layer, with the assumption that the foam layer is detailed such that it serves as an air barrier and wrb.
You can't do foam only roof as there would be no way to walk on it. No roofer would sign up to install this. You need at least 1 layer of plywood somewhere.