PIR degradation has been discussed for years but I'm not aware of any definitive, independent info on it. The basis is that PIR is foamed with a gas (pentane) which, over many years, leaks out and is replaced with air. Air has a lesser thermal property so the insulation becomes less efficient. What UK manufacturers say is that this is accounted for in the Lambda of the material. In other words they are saying the actual Lambda is better than the quoted figure when you buy it new and reduces to the quoted figure over x number of years. I think they usually quote 25 years? Either way, unless you want to invest in Aerogel, or have the space to fit EPS, it's all we've got at the moment and I think still the best option.