Insulating under suspended wooden floor.

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As part of refurbishing my dad's bungalow I must lift the dining room floorboards to stabilise the joists (bouncy floor) and modify some water/heating pipework. The existing boards are at the end of their life, so I am going to re-floor with 8'x2' (21mm) chipboard sheets. I am going to insulate between the 6" joists with 100mm foil faced Celotex supported from below with roofing battens screwed either to the side or underside of the joists.

The question is a simple one. Should the foil face be hard up against the underside of the new chipboard flooring? Some I have asked say 'yes', some say to leave a 1" gap so air can flow, but surely that defeats the object of trying to trap the heat in the room above. Each is convinced they're right, but what is the verdict of the skilled DIYnot contributors, AND WHY.

Regards, MM
 
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No gap for PIR.

I'd use rock wool too, it will do the same job, and be cheaper.
 
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Most of the heat loss from a floor is due to draughts. Mineral wool is easy to stuff tightly between the joists to block draughts. No precision cutting is needed for a close fit, as with PIR.

Unsurprisingly, kingspan want you to use kingspan.
 
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In conclusion then, the opinion of Kingspan is that the foil faced PIR should be tight up against the underside of the floorboard, and that's the way I'll install it. Thanks for all your contributions.
As for the cost of PIR; it's only installed once, so I'll spend the money now and reap the benefits year on year, in that way the benefits are there long after the cost of installation are forgotten.
Many years ago I installed Rockwool under the floor of an extension, supported underneath with staple-fixed chicken wire. Many years after that (and after selling the house) I returned to repair a washing machine leak, to find that the water had soaked and compressed the Rockwool, so it was neither fluffed up against the floor nor doing it's job well. PIR insulation shouldn't suffer that fault.

MM
 

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