Existing Concrete Slab Help needed UFH

[renovator]

Hi All

 

So I've posted a few topics on the extension I'm having regarding UFH. So far, I've originally specced a Wunda EPS400 SYSTEM covering 52SQM across both the original part of the house and the new extension. This was on the assumption that beneath the current floor (15mm engineered oak), there was a thin ply board on top of the existing slab. 

 

The builder has removed the flooring to find there's the following:

 

1. 20mm plyboard 
2. 50MM of polystyrene insulation which sits on the existing slab (picture attached)
3. Approximately 90MM of depth to line up with new extension slab, although this can go to 120MM potentially.

 

There are two options or more if you advise:

 

1. Remove the rest of it to expose slab, add 50mm celotex xr4000, membrane then Wunda EPS400 
2. Remove existing insulation, install 50mm celotex, membrane then UFH screed.

 

I'm sure there will be better suggestions and thanks for your input in advance. Ultimately the outcome is to have an UFH system which now it seems the existing slab may not need to be dug out due to the level of the new slab which is yet to go in.

Edited November 9, 2023 by renovator

[JohnMo]

Really would not bother with UFH with so little insulation. Buy some nice radiators and have way lower heating bills.

[Andehh]

50mm isn't enough for UFH, it'd probably work but wouldn't be efficient. You need at least double that.

 

Can the slab be dug out? Only a couple days work, tool hire and a skip for the builder?

 

Not all insulation is the same, PIR is the better kind so ensure you use that not the generic polystyrene you've got there.

[Conor]

Dig the slab out and get a build up that has a min of 100mm insualtion (ideally 150-200mm). Now's the time to do it.

 

What's the build up of the extension?

[renovator]

On 10/11/2023 at 06:48, Conor said:

Dig the slab out and get a build up that has a min of 100mm insulation (ideally 150-200mm). Now's the time to do it.

 

What's the build up of the extension?

 

Thanks Conor, I've attached the drawing for the extension which is a full width (7.2M (5.8M floor space) x 3.5M). This part of the extension will have 150MM whereas the original part of the house (before the nibs) which has the 50MM polystyrene, 20MM chipboard and 15MM flooring accounts for roughly 31 SQM (including Utility, WC and Hallway). So total 'material' depth above current slab is around 90MM.