UFH 1970s bungalow - insulation beneath concrete

[bradR]

Hi, I need some advice on what wet UFH system to get for our renovation of a 1970s 3 bed bungalow. The house had subsidence so in the process of fixing that the floors were dug up and replaced with the following construction: hardcore base, membrane, 150mm insulation, concrete finish. Unfortunately we had no say in this setup so we're stuck with it now. We're left with a 40-50mm gap between the concrete and the base of the front door, which obviously limits what we can do with UFH.

 

There are of course low-profile retrofit UFH options and we'd have liked to have gone with the Nu-Heat LoPro Max with self levelling compound but that is way too expensive for our budget (double what we want to pay). What are our other options, especially considering our 40-50mm gap, and the 150mm insulation beneath the concrete? I've seen the pre-cut boards that pipes are laid in which have no compound/screed on top, would these work well enough with our floor construction? Or should we go down the normal route with pipes in screed but only have 40mm thickness?

 

Insulation below a concrete slab means the concrete becomes a part of the thermal mass, so because it has a slow response time would it make sense for the UFH to be run continuously? We'll be installing an air-source heat pump together with insulation above the ceiling and on the outside of the house with a render and/or cladding, so would this help with the response time and/or be sufficient to run continuously or intermittently?

 

Any help would be really appreciated!

[tonyshouse]

What are your intended u-values?

 

in my book slow response =comfort, fast response = discomfort 

[PeterW]

Ok so @bradR first question is how thick is the concrete ..? 100mm..?? 200mm..?? 50mm..? Depending on this you could just shot fire standard clip rails to the concrete then use a liquid screed over the top but you’ll be limited on your floor finishes as that will be a minimum of 40mm I expect. 

[HughF]

11 hours ago, PeterW said:

Ok so @bradR first question is how thick is the concrete ..? 100mm..?? 200mm..?? 50mm..? Depending on this you could just shot fire standard clip rails to the concrete then use a liquid screed over the top but you’ll be limited on your floor finishes as that will be a minimum of 40mm I expect. 

This sounds like a nice plan, and is probably what I'd do. Clip rails down with a hilti gun, liquid screed.

[AliG]

Your floor finish will have a big impact on this. If you have tiles or wooden floor planned then you need to allow almost 20mm which means you don't have enough depth for screed. You would need to use probably some kind of vinyl flooring.

 

It is important to consider your u-values as @PeterW says. We put UFH in the kitchen of our 1990s timber frame house and had to run it at 60C to keep the room warm in the winter. If you don't have enough insulation you may struggle to keep it warm and need to run the ASHP at a very high temperature ruining the efficiency.