Warmdeck Roof Insulation - Layering

[Glanmor]

I remeber reading somewhere that, although polyisocyanurate PIR insulation look good on specs but performs poorly in low temperatures.

If laying 150mm warmdeck roof, is it better to layer it as follows:

(1) Layer1 - 50mm PIR (warm side)
(2) Layer2 - 50mm XPS/EPS Polystyrene (to prevent cold-bridging with 3rd layer cold side)
(3) Layer3 - 50mm PIR (cold side) but foil-faced for accepting adhesive roof covering.

I've stuffed 100mm PIR in the floor slab thinking it was best insulation but looking back I think it was a mistake. EPS/XPS was more ideal for floor slabs, and cheaper. Only positive in that is that at least the floor isnt exposed to very low Winter temperatures.

Any expert comments please?

Thanks for looking.

Cheers

 

[noseall]

Have you been reading a sales pitch from a polystyrene supplier? Ignore any slanted or prejudicial nonsense documents supporting one product over another.