Foil backed rigid insulation??
A friend has left over rigid backed foam insulation and asked me my thoughts on using it to completely fill the rafter space (no air space) for a portion of a roof on small houseboat. He want to use two layers of the foam (both foil backed). Orientation of foil? Bad idea altogether?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
You can do it, as long as each piece of foam is installed with air sealing in mind. In most cases, that means that the perimeter of each piece of foam needs to be sealed with canned spray foam.
The orientation of the foil is irrelevant, since it won't be facing an air space. The foil doesn't hurt and it doesn't help. It can face any way you want.
Thanks Martin - to clarify: Do you mean that the entire face of all four edges need to be foamed (buttered) or merely the seams between the foam boards once set in place?
Just the perimeter of each piece. Not the faces.
Awesome - thanks for the quick turnaround - really appreciate the help.