What thin insulation under wet underfloor heating?

9 Aug 2022
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United Kingdom
Hi All,
I'm currently halfway through a kitchen extension, and in the newfloor we have 150mm of insulation over the concrete slab and will put in underfloor heating on top. The plan was to take up the existing kitchen floor and do the same, however we've just discovered the kitchen slab is part of a raft foundation, so we can't touch it. This means we have no room for adding thick insulation and a screed - we only have about 40-50mm depth which will include the flooring. We want LVT or SRC tiles which are around 6mm deep.
I've been looking into low profile UFH and it seems we can still have a wet system, so I'm just trying to work out the best way of insulating, given the depth restrictions. I believe our pipework (in a liquid screed) can be as little as 20mm, so we have about 20mm for possible insulation.
Nu-heat recommend a system that is 22mm deep in total with no insulation underneath and they say we'd lose very little heat downwards, but I can't see how this is right. Can't find any R values for their system online.
I've also been recommended just a 6mm superfoil insulation under the liquid screed, which would be better than nothing... but I'm wondering if I can also include a 10mm XPS insulation board, or 10mm Jackoboard or something? So the build up from bottom to top would be concrete slab > 10mm XPS > 6mm Superfoil > 20mm UFH pipework in liquid screed > 6mm vinyl tiles.
Has anyone done similar? Does anyone know if combining the XPS and superfoil like this is a good/bad idea?
Thanks for the help!
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We have done a fair bit of underfloor (polypipe reg installers)
I would personally send a detailed drawing to one of manufacturers.
Let them design it install exactly as specified taking lots of pics as you go.
Any issues you will have a factory backed warranty.
Not sure I follow - I'm not expecting issues I just want as much insulation as possible. One manufacturer says you don't need any insulation underneath (nu heat) and another says use superfoil. I'm sure neither of these will have issues, but they won't be as efficient as I want.
I want to put more insulation in than they suggest.
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I always thought that superfoil needs an airgap either side to work best.
But it seems that SuperFOIL claims it works without an airgap either side:
Try out their guide:

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