Floor insulation - AIRTEC foil gap

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Hi,

I need to insulate under my suspended floor and thinking of doing the following:
  • Breathable membrane stapled to underside of joists to prevent wind wash
  • 100mm PIR board. Gaps sealed with foam/tape
  • Over the joists/PIR, YBS AIRTEC REFLECTIVE double foil as (admittedly expensive) vapour control layer
  • Floor boards, underlay, carpet
YBS will help insulate if there is an air gap. For this reason I was thinking rather than having the PIR boards flush with the top of the joists, dropping them slightly to leave a 15mm gap between them and the foil. Has anyone done this, does this sound sensible?
 
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There’s a lot of differing opinions on using membranes on here:cautious:

I used rock wool, full depth of joist. With a building paper on top, then floorboards.

If I was using foam boards, I’d leave no gap, and bring to the top of the joists.
 
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Thanks. I think keeping it flush we the the joints is best. I have some spare rock wool which I might fill the 20mm gap between the PIR board and the breathable membrane. Probably a bit pointless, but every little helps I guess.
 

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