Old suspended Fl; insulation, breather membrane, plywood between joists

27 Oct 2009
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United Kingdom
Sorry, this is a nested question..

Old suspended 1960s floor comprises:
3" void, 3" timber heights (sat on engineering bricks, levelled with cement), floorboards.
Joist spacing sees 440mm approx inbetween joists. Joists are 2.5" wide.

- insulate between timbers
- UFH: install over tray UFH (20mm Polypipe product), with thin (laminate etc) flooring on top.
- maximise resulting ceiling height.

- What choice of insulation? [the mix of cost-effectiveness, amount of labour, breathability - does it need to breath in to floor void, or can moisture escape via timbers?]
- How to minimise loss of ceiling height with the new UFH..?
..choice of new floor deck material, thickness, fixing/support..
T&G chipboard over top (loss of height)
T&G plywood over top (loss of height)
Plywood between joists - supported and fixed how?
Or, ply between joists and thin ply over joists..

Should there be a breather membrane somewhere? Why?

How would you go about this?
NB. I note that Gapotape fixed around PIR/Celotex board to give a push-fit airtight fit, improves insulation outcome by 20+% (and adds 15% cost to cost of PIR boards!) so maximising thickness of PIR is not as important for the outcome as optimising air-tightness of fit.
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Your ply deck will be 18mm so no noticeable loss of ceiling height. Unless there's a major issue with thresholds don't try anything fancy as the ply connects all the joists when screwed down

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