UFH with celotex under consrete slab???

[tolly]

Hi, ive got a job coming up where the builder has put the insulation underneath the concrete slab rather than ontop of it. We have got 75mm to play with for ufh pipes and screed. Can i just have my loops straight on the slab with 75mm screed on top, or do i still need further insulation above the slab even if there is 100mm of celotex underneath it.

Ive never done it this way before so was concerned of unesesarily heating the slab aswell and having a massive thermal mass to heat up. Even though it will be set to run continously at 21deg with a night setback of 16deg.

I could potentailly put 12mm celotex down still but i wont if i dont to?

Any opinions on this will be appreciated.
Thanks

 

[joni os]

100mm celotex beneath slab will provide sufficient insulation and slab will act as thermal store. Response times will be slower than if insulation was on top but still acceptable. I regard 75mm as a minimum to UFH pipes and would consider including fibre reinforcement in screed. 25mm insulation should be continued at perimeter. Ceramic tiles or similar will aid efficiency. Wood flooring is not best option with UFH but is still popular with clients, in which case fixing battens are best incorporated in screed.

 

[tolly]

Cheers joni, yes the floor covering is porcelin tiles. I thought there was a chance i would have to get them to lay more insulation down and then switch to a liquid anhydrite screed due to less depth. I presume i wont need any polythene sheeting then if theres no celotex aswell. I can live with a slower response time, also it will be on heatmiser controls so will factor in the response times by itself.

 

[Chalked]

Builders lie !
I would all 25mm of kingspan to detach from the bottom screed. As it will have no edge insulation, it will not be able to expand and will lose heat into the walls.
have they put in a vapour barrier? If not the drying action will force vapour up u dr the the porcelain and de laminate the tiles.

 

[joni os]

Assuming normal building practice and time scale the 100mm floor slab will have been laid 16 weeks prior to tiling and thus comply with the rule of thumb recommendation of 1 month per inch for fully cured slab. It will be necessary to allow screed to cure but this can be hastened after 4 weeks by use of UFH. Thin screeds have tendency to curl and this propensity increases when totally separated from base. You can confirm insulation exists at perimeter, which, with insulation below slab is necessary for compliance with Building Regs. for anything except large AREA/PERIMETER ratios. Cleanliness of slab, removal of plaster droppings etc will aid stability of screed, and, together with the use of suitable tile adhesives, have proved effective with UFH laid 20 years without delamination. During same time period, curled hollow screeds have needed removal and replacement at great cost and disruption to household. I can only speak from practical experience and accept others people's theory's may differ.

 

[Chalked]

Good reply.
As this is probably just an oversite screed. I would just be worried about no edg expansion or vapour barrier.

 

[tolly]

The replys were much appreciated. Laid the loops on saturday. There was 25mm kingspan edge insulation and there was also a vapour barrier installed aswell. Has been screeded today. Thanks for the help.