Any problems with rigid foam on interior of roof rafters?
I am building a new home in the upstate of South Carolina which is climate zone 3a. It is a 1 1/2 story house (please no complaints… it is what it is), and I am planning on using (2) layers of 3″ thick poly-iso on the interior side of the rafters. The plan is to offset the joints and tape the seams at least on the 2nd layer of poly-iso. It does have to work around a few studs/ceiling joists and I plan on spraying foam around these intersections where there would be gaps because of not perfect fit up. Some of the reasoning I like this system is that it allows the wood roof sheathing to have air movement and breathe (dryout when needed) because there will be eave and ridge vents with open rafters. Also, it produces a pretty clean insulation line that is continuous and closes in the attics and mechanical.
My questions are this:
1) Is there a possibility of moisture problems with this?
2) To be more specific, is there any chance of dew settling on the outside of the insulation in the rafter cavity like how dew can set on a roof in the morning? The worry here is that the water could run down the sloped insulation on right on top of my wood exterior wall and have long term problems.
3) Does taping the seams of both layers of poly-iso do anything for me?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part