I'm doing a cut and cobble insulation job in the roof of a 12'x16' Tuff Shed. The rafters are 2x4 and the spacing of each bay varies because of installation plus warping/twisting. I'm using 2.5" felt-faced polyiso insulation to allow a 1" air gap between the radiant barrier roof decking and the insulation (I've added appropriate soffit and ridge vents to ventilate that space).
The climate is classified as predominantly warm and dry (climate zone 3B), though this doesn't tell the full story because the temperature and humidity vary widely in the West Texas panhandle not only seasonally, but also daily and weekly depending on the intersection of regional weather patterns.
My question is about sealing the inevitable gaps between the polyiso pieces and the rafters. I know spray foam is often used for this, but not only is it difficult to apply upward, it will eventually crack and fail in this climate. I would use Gapotape but it is expensive and not yet easily available in the US.
As an alternative, I'm thinking to use rolls of sill gasket foam (e.g., Propink ComfortSeal) and just staple that up to help friction fit and seal around the edges of the polyiso. If I use a 5.5" wide sill gasket folded in half, it will fit nicely inside the rafters and nominally give me up to 3/8" of compressible foam on either side of the rigid foam board.
Do you foresee any problems with such an approach? If so, what other options are there?
Here is a photo of what I have in mind (2.5" rigid foam board will then go in flush with the bottom of the rafters leaving a 1" ventilated gap along the bottom of the reflective roof material):
