Insulation in between Joist for Underfloor heating

6 Mar 2018
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United Kingdom

I have underfloor heating pipes installed in between joists for first floor.

Wonder what anyone has used in the past for insulation between joists? Rock wool or Celotex boards? or any other insulation.

Realise the celotex route will be more expensive.

Thanks in advice.
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Thanks for your answer woody.

Do you know what would be the best thickness for the Insulation board, ive read some bits online that suggest 100mm rock wool, would i need 100mm board too? or could i get away with a smaller size? or perhaps need thicker?
Any sort of quilt, such as rockwool or drytherm is crap for heating pipes as it wont reflect the convected and radiated heat, and it's only half as good as the equivalent thickness of quilt anyway. Use a foil faced PIR board and the thicker the better and a minimum of 60-70mm but 100mm would be better, and seal any gaps between the insualtion and joists, and between joists and walls at the bearings otherwise drafts will be blowing that heat away through the floor void
Thanks, this is really useful.

Last question, i think lol. Sealing the gaps, with expanding foam? or is there something else i can use.
Foam can be used but may be fiddly on smaller gaps. Slithers of the insualtion can be cut, or some tubes of caulk or sealant can be used.
mineral wool is very good for stuffing into gaps. It deforms easily to fill them, and is good at blocking draughts, which can be a problem especially within ground floors. Even upstairs, they are worst in the part of the floor adjacent to walls, and it is worth stuffing the perimeter for a foot or so even if you don't insulate the whole floor.

It will also reduce the risk of squeaking during pipe expansion, though hair felt is traditionally used for this.

I only use the brown mineral wool treated with Ecose, not the old yellow glassfibre that sheds irritant dust and fibres. it is made by Knauf but widely sold under own-brand labelling. You will find the "Ecose" label on the wrapper.
Or just use insulation tape? Or, any tape will stop drafts ...
yes insulation tape, I was thinking of maybe some bigger gaps that might need filling.

so perhaps I can use the pir boards for the majority(95%) and where there are some gaps where the boards might not go, i can put in some the mineral wool mentioned by John.

Think this would be sufficient?
convected warm air from the pipes will rise towards the floor you want to heat, it will not descend.

Why wouldn't mineral wool beneath the pipes prevent heat leakage to the ceiling below?
maybe a cheaper option would be mineral wool with a sheet of aluminium foil over it? or is that too much like a bodge job?
Why wouldn't mineral wool beneath the pipes prevent heat leakage to the ceiling below?
Yes. The plumber that installed first floor UFH told us to fit 150mm wool. We fitted 200mm The ceiling below was cool to the touch and had the added benefit of fully filling the gap.
Just to put my oar in I'd go for wool on the basis that heat rises anyway (hence the lower requirements for downstairs floor insulation) and downstairs is not outside in your case anyway so doesn't need huge r value.
If you do want to take advantage of the reflective properties of foil insulation make sure there's an air gap in front to refect the heat into.

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