What thin insulation under wet underfloor heating?

[Mattsville]

Hi All,
I'm currently halfway through a kitchen extension, and in the new floor we have 150mm of insulation over the concrete slab and will put in underfloor heating in 75mm screed 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 screed - we only have about 40-50mm depth which will include the flooring. We want LVT or SRC tiles which are around 6mm deep for the flooring.
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 to use 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?

I realise this isn't ideal and we wanted much more insulation, however at this point there's not a lot we can do. 
Thanks for the help!
Matt

[JohnMo]

Is a step an option?

 

With that little insulation in the floor, you should really reconsider if UFH is the right thing to do for that room.  The downwards heat loss will be big.  Your downwards heat loss, will be the delta of the water flow temperature (could be around 35 to 40 degrees and the ground temperature, around 7 degrees, multiplied by the u value and area.

 

Superfoil requires an air space either side of it to get advertised performance.  So don't go there. Your need to use PIR or better still PUR.

[Mattsville]

A step isn't an option. The new extension is approx. 30m2 and the existing kitchen is about 18m2. I would consider not having any heating in the old kitchen part but there is a barrier at high level (steel supporting previous back wall) and low level (kitchen island/peninsula) so I don't think the air would circulate much into the kitchen part. I think I'll zone the UFH so worst case we can not use the UFH in the kitchen area and install a rad somewhere, but I'd still like to put the best insulation I can under the UFH. Even 10mm of XPS alone will be a lot better than nothing. Tthe kitchen is already designed and paid for and there isn't a brilliant place for a rad, so I don't mind spending money on this. I considered some vacuum insulation board but can't find any prices or stock.

Matt

[Mattsville]

PIR isn't an option as there's no room for a deep screed on top. So my options are 12mm XPS + superfoil, 20mm XPS or something else that is around 20mm deep and doesn't require a deep screed on top.

Seems odd that some of the big companies recommend and advertise superfoil for under floor insulation. Surely that shouldn't be allowed if it's useless? I realise it'll never get near their claimed values, but if it's completely useless that's another thing.

Matt

[JohnMo]

20mm PIR would be 30 to 40% better performing than your suggestion.  The other option is aerogel (not cheap), 30% better performing than PIR.

 

I have a single piece of UFH pipe in the kitchen, between the wall kitchen units and the island.  Not sure it actually contributes that much.  Kitchen is always warm.

 

The single pipe at the bottom of the image by the red thing, that is the kitchen, the bit to the right is the dining area.

 

[Mattsville]

I can't use PIR because it would need a deep screed on top of it as it's not structural boards. Or so I've been told? 

I'm also now being told that nu heat boards need to be fixed directly to a concrete floor and can't be put on top of insulation, and that XPS boards with pipework on top would need a 50mm liquid or a layer of plywood on top so that's not ideal either.

 

[JohnMo]

Strange my structural drawings show PIR with 75mm concrete screed.  Many on here have also used the same, other not but more to do with cost, choosing thicker EPS/XPS at a lower cost for same U Value.

[Mattsville]

 

Yes that will be right - 75mm is what I'm calling a deep screed (could use 50mm self levelling, but I have space for neither).  Turns out XPS cement coated boards only need a few mm of fibre reinforced self levelling screed so I've gone with 10mm XPS cement coated backer boards (Jackoboard) underneath 18mm XPS cement coated routed boards for 12mm pipe. It's far from ideal but I'm reasonably happy having 25mm odd insulation for this small area. Much better than the nothing I had under electric UFH before.

[Onoff]

A sobering thought I always quote is @Jeremy Harris's calc that even with 300mm of EPS under his 100mm slab, he lost 8% of his heat through the floor. 

[Ferdinand]

Can you add peripheral skirt insulation round it inside or outside? That increases the heat escape conduction path, and over time heats up the patch of ground.

 

F

[Lemna gibba]

I can't help with this, but am very interested as I think we may be in a similar situation. How did you find out your floor was a raft foundation? 

[Temp]

On 24/08/2022 at 23:39, Mattsville said:

we only have about 40-50mm depth which will include the flooring. We want LVT or SRC tiles which are around 6mm deep for the flooring.

 

I would fit big rads. Possible UFH in the walls? 

 

Aerogel is the best insulation known to man but is only about twice as good as PIR. We have 80mm PIR under UFH and wish we had more.

 

If you really must have UFH perhaps look at a system that doesn't need screed. Even liquid screeds typically need to be 40mm thick because the UFH pipe reduces the thickness locally.

 

This 18mm or 25mm system claims it can be tiled over without screed..

 

https://www.continal.co.uk/system/tilefix

 

I think good floor tiles are going to be 10mm thick plus 5mm adhesive. Thin tiles might not be strong enough for point loads over these type of UFH system?

 

15+25=40mm

 

It might be possible to find a 10mm insulation you can put under the UFH panels.

 

 

 

 

 

 

[Temp]

3 minutes ago, Temp said:

 

I would fit big rads. Possible UFH in the walls? 

 

Aerogel is the best insulation known to man but is only about twice as good as PIR. We have 80mm PIR under UFH and wish we had more.

 

If you really must have UFH perhaps look at a system that doesn't need screed. Even liquid screeds typically need to be 40mm thick because the UFH pipe reduces the thickness locally.

 

This 18mm or 25mm system claims it can be tiled over without screed..

 

https://www.continal.co.uk/system/tilefix

 

I think good floor tiles are going to be 10mm thick plus 5mm adhesive. Thin tiles might not be strong enough for point loads over these type of UFH system?

 

15+25=40mm

 

It might be possible to find a 10mm insulation you can put under the UFH panels.

 

The more I think about it the less I like the idea.

 

Can it possibly be rigid enough to tile over reliably?