DPM for UFH, build up question

[Digger1]

Question on the build up on top of an existing slab. 

 

I was thinking of the following construction:

 

Existing Slab

DMP sheet

PIR Insulation 100mm

DMP sheet

UFH

Liquid screed

 

Any opinion appreciated.

[Temp]

Can you afford more insulation? Say 120-150 mm.

 

If tiling beware that extra prep maybe required for Anhydrite Liquid screeds. They can form a weak layer/skin on top called laitance that is harder to bond to. This may have to be sanded off. 

[jayc89]

Looks sensible to me. Rather than focusing on insulation depth, my advice would be to focus on estimated u-values as that can alter dramatically based on your PA (perimeter area) ratio.

[Digger1]

Would a liquid DPM be better than a sheet on the existing slab?

 

The existing slab is uneven in places, i assume you can put a layer of sand/cement under the insulation so create a flat surface and compact it, just as you would on an oversight?

[Mr Punter]

Polythene DPM is probably cheaper and it is clean and easy to install.  If the slab is not flat you could use some sand to level it before you put down the DPM.