Underfloor heating insulation

13 Feb 2009
Reaction score
United Kingdom
Hi all,

I'm having an extension built (6m x 7m) and installing wet UFH. The architect has specified the floor construction from ground up as

1, scalping oversite
2, DPM
3, 100mm celotex
4, 100mm concrete
5, 75mm screed

The UFH pipes will be clipped to the 100mm concrete prior to screed.

In the past I've normally installed the concrete, then celotex (pipes clipped to celotex) then the screed.

What is the preferred option? Obviously the floor will be stronger if installed as per architect, but will the UFH be less efficient as it will be heating the concrete slab as well as the screed?

If pipes are installed direct to celotex, the heat will be be reflected so no heat loss down, but is 75mm screed sufficient on top of 100mm celotex.

I want the best efficiency but also want a solid floor which wont crack.

So is it insulation - concrete - screed or concrete - insulation - screed?

Thanks for your help
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