Quote Originally Posted by blue matt View Post
as for the design,

I will lay down some DPM, on top of that lay either use 100mm or 75mm X 50mm " ( more often than not C16 CLS ) floor joists, with 50mm PIR board ( celotex / kingspan are 2 fairly common brands ) between the joists, then lay chipboard floorboards down
Ignoring the obvious (and acknowledged) BC and PP issues here, I can't quite get my head around this. What are these 75x50 CLS "joists" bearing on? Surely they cannot be spanning anything so presumably these are actually resting on an existing slab? If so - I can see the logic of the DPM but why bother with the joists at all? Why not just float the P5 on the PIR? And to be honest at 50mm for floor insulation- why bother at all? The heat saving through the floor will be negligible, but surely most of the buildings you are doing this to are limited to 2.5m height and so even a 63mm loss of headroom is sub-optimal?