Doubling up PIR board - lay back to back?

[JP_]

I ordered some 50mm PIR board from the local building merchant, but due to stock shortage, they are giving me 25mm instead - double the number of boards so still 50mm total.

I am using this for wooden suspended underfloor insulation - would I be best to lay it with the foil on the outside top and bottom, or with foil on top on both layers?

 

[Notch7]

I ordered some 50mm PIR board from the local building merchant, but due to stock shortage, they are giving me 25mm instead - double the number of boards so still 50mm total.

I am using this for wooden suspended underfloor insulation - would I be best to lay it with the foil on the outside top and bottom, or with foil on top on both layers?

I thought PIR always had foil both sides?

 

[JP_]

oh, maybe it does!
Yes, looks like it will. Well, that's also double the vapour barrier then!