I found a really interesting article...

https://www.sips.org/technical-information/r-values-in-the-real-world

Did you know R-value in Fiberglass drops when it gets cold?

and this is due to air-infiltration

Did you know R-value in EPS increases when it gets cold?

Only ever so slightly, but it does...

Also interesting that XPS is loosing R-value through time... while EPS doesn't have that issue. Basically trapped air is much more stable than trapped gas.