Posted By jonr on 27 Jun 2014 10:16 PM
Not sure why shrinkage of rigid foams was brought up, but according to numerous sources, EPS effectively stops shrinking after 42 days at normal temperatures.
I like the implied precision of "42", but it's a number that simply has to be B.S. without the fine print. Or did they mean six weeks? The shrinkage over time is probably initially logarithmic, so while the vast majority of the shrinkage happens in the first few weeks, the "effectively" in "effectively stops" isn't an absolute, and the absolute shrinkage over decades still matters.
After 42 days I'm sure more than 99.5% of the pentane blowing agent has left (unless it's a very thick block rather than sheet goods, or it has facers.) For a lot of applications 0.1% shrinkage over the next two decades may be meaningless, but 0.1% is about 1/10" along the 8' axis of a 4x8 sheet. That could create about 0.2" gap between abutting sheets on the 8' axis, or 0.1" between adjacent sheets on the 4' axis. Either is enough to be of air leakage concern if not properly planned for. (Tapes are better than goops here.)