Do we need a thermal break?

[eandg]

We have a precast insulated floorslab (325mm EPS topped with 50mm structural concrete) on which we plan to lay our UFH covered by 50mm screed. UFH supplier says we'll need a thermal break otherwise we'll lose our heat to below. Is that an issue we should be concerned about - wouldn't that just give a greater thermal mass so the issue would be slower/steadier release of heat and slightly less responsive times rather than any great heat loss per se?

[Nickfromwales]

UFH installer is just unfamiliar with anything low energy, it’s a common issue tbh with most not knowing what they’re quoting for.

There will be next to zilch downwards heat loss over 325mm of EPS!! And the 50mm plus 50mm will mimic the 100mm concrete slab which most have in their passive raft foundations. 

Carry on as you are, it’s completely fine.

How do you intend to fit the UFH onto the 50mm of concrete, and what exactly is making that ‘structural’?

Edited April 24, 2022 by Nickfromwales

[eandg]

6 hours ago, Nickfromwales said:

UFH installer is just unfamiliar with anything low energy, it’s a common issue tbh with most not knowing what they’re quoting for.

There will be next to zilch downwards heat loss over 325mm of EPS!! And the 50mm plus 50mm will mimic the 100mm concrete slab which most have in their passive raft foundations. 

Carry on as you are, it’s completely fine.

How do you intend to fit the UFH onto the 50mm of concrete, and what exactly is making that ‘structural’?

Thanks very much for the very informative reply.

 

I think we'd be getting castellated trays if going direct onto the slab. There's steel mesh in the concrete but the structural element comes from the end bearings on each slab (typically 1800mm wide) which extends to 140mm and has proper steel fixings. There's only one very small section where the concrete extends all the way to the found to transfer the load where there's a steel post (not sure if we'll feel that and have cold bridging without a little bit of additional insulation there but looking at £800+ plus losing a bit of volume if adding it). 

[PeterW]

23 minutes ago, eandg said:

 

I think we'd be getting castellated trays if going direct onto the slab.

They won’t be suitable for what you want as it will basically create a 50mm floating slab over the concrete that will only be 10/15mm  thick in places. It will crack. 
 

I would potentially use bonded rails and pipe and a liquid screed or even up the size of the slab and put the pipe in the slab wired to the reinforcing and forego the screed altogether. 

[eandg]

20 minutes ago, PeterW said:

They won’t be suitable for what you want as it will basically create a 50mm floating slab over the concrete that will only be 10/15mm  thick in places. It will crack. 
 

I would potentially use bonded rails and pipe and a liquid screed or even up the size of the slab and put the pipe in the slab wired to the reinforcing and forego the screed altogether. 

The slab is in already. Need to investigate options and whether insulation and stapling pipe to it is then the best option in any case!

[joth]

Rather adding redundant insulation can't you increase the screed thickness instead?

 

[eandg]

1 hour ago, joth said:

Rather adding redundant insulation can't you increase the screed thickness instead?

 

That wouldn't solve the issue of securing the pipes, unless I'm picking you up wrong?

[joth]

1 hour ago, eandg said:

That wouldn't solve the issue of securing the pipes, unless I'm picking you up wrong?

I was thinking if combined with the "bonded rails" approach as per PeterW but I'm no expert. 

[Nick Laslett]

Here is a similar discussion with some ideas. 
 

https://forum.buildhub.org.uk/topic/17519-ufh-pipes-on-to-concrete-slabs/
 

Here is a thread at another forum:

 

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6598

 

One suggestion I liked was to use thin reinforcing mesh, and tie the Pipes to that. Another was to screw the clip tracks to the concrete. A third suggestion was hammer clips, which need a pre-drilled hole. 
 

https://floorheating-direct.co.uk/blogs/underfloor-heating/different-installation-methods-water-underfloor-heating-systems

 

https://ufhsupplies.co.uk/product/hammer-clips/

[Thorfun]

I had a liquid screed in the basement on top of our insulated slab and decided it was actually cheaper to lay 25mm PIR and staple the UFH pipes to that rather than pay for a deeper screed and a.n.other form of fixing the UFH pipes down.