Ive been looking at different foam boards because Ive read they have the highest r-values per inch. 
Im now reading that manufacturers are overstating the R-values for polyiso by 6%, and 10-25% for XPS, according to a study by Sachchida N. Singh and Paul D. Coleman. And then the article “Reason Foam Fails #3 – Degrading Thermal Insulation Values”  goes on to say foam degrades with time, but doesn’t give any numbers..   
What’s the bottom line on this? If XPS is overstated by 25%, why would we believe manufacture tests on ageing?  Because it wouldn’t take allot of degradation for foam to equal or have less r-value of insulation’s that dont degrade..

Thanks