Posted By Birdman on 21 Aug 2012 09:31 AM
I think the roof is in fact a "site built" SIP ie they laid thick EPS on the sheathing then applied another layer of OSB and screwed through the whole sheebang. When the EPS shrank, because it was only one layer it opened up a continuous gap from warm to cold. Very similar to the issue with SIP's but not exactly. I suppose this could be easily mitigated by a.) using XPS that doesn't seem to shrink and b.) building the insulation up in layers so it doesn't have continuous joints through the whole thickness and c.) taping or sealing the joints.
Peel and stick needs to be continuously supported - this is a standard issue and one I hit in commercial work all the time. Even trying to span a gap of 1/4" will result in sagging an eventual failure. In most cases it's usually easy to span the gap with a bit of thin sheet metal then apply the peel and stick over that.
In hearing Joe tell stories I think they had thermal leakage telegraphing to the roof from the very beginning. This showed in the winter with snow melt at the seams. They very quickly came to the conclusion that you need to install foam in layers.