Hi all, just hoping for some advice on wet underfloor heating. I live in a 60s bungalow and the install would be in a flat-roof single story 'annexe' which is connected to the main property. My home is very poorly insulated and heated by a gas boiler and radiators but I'm currently speaking to architects with the aim of extending the property and bringing my energy usage right down (a new well-insulated roof and wrap around external insulation finished with render). The wet UFH in the annexe is a test to see if I would like it throughout the rest of the property.

How deep will I need to dig down to end up with a new floor at the same level as the current concrete floor?

After digging down I understand the layer order will be: 300mm insulation - DPM - concrete - 60mm wet UFH in screed - 15mm engineered wood floor. Is this in the correct order? Am I missing any layers? What thickness will the concrete be?

In the short-term the wet UFH will be powered by a gas combi-boiler but I may end up with an air source heat pump and/or solar water heating in the future. Is there anything in terms of plumbing/thermostat set-ups I should do now to ensure the UFH is compatible with my future energy plans?

Hope all that makes sense, any advice much appreciated, thanks

300 EPS; DPM; 125 or 150 concrete (whichever your SE will agree to); a193 mesh in the concrete onto which your UFH pipes are cable tied

That saves the cost/time of pouring the screed. Depending on the duration of the build, and thus drying out time of the conc slab, you may need to roller-on a liquid DPM over the slab, to retard the vapour upward passage which could cause issues with the finish layer.

Have done that lots of times over the years. Works a treat, plus you've a finished level floor right away, not monkeying around with allowances for the screed, on every measurement.

Skinning cats

Thin screeds are difficult to keep the pipes contained in and they crack more easily, sometimes breaking over where the pipes are too near the surface. Have also see door threshold carper screws drilled into pipes in doorways.

In our case we poured the slab first, then mesh & pipes, then screed and brick floor laid with the screed.

The mesh sits on little upstands, so it is approx mid concrete. That means pipes on top of the mesh, which then gives about 50mm top cover.
I use the 193 mesh as it's heavier than the usual 142 (?) as walking on the thinner mesh makes it jump around and bend.
There are plastic "stools" to set the mesh on, though my guys tend to prefer the steel "toblerones" (don't know the correct name - steel mesh supports should give you some google hits).
From memory, conc slabs dry out / cure about a mm per day (of course that varies). If you pour the slab early, which I do before the kit is erected, then you may have enough time for curing, and no need for the liquid DPM. It's not a massive cost for the DPM, and easy enough to roller on.
You need to do a bit of floating, to give a reasonable surface of the concrete, but not polished, but certainly not tamped ridges.
If you find the odd low area (ponding) a little latex self level does the trick.

Don't forget edge insulation.

Also beware your levels if putting in inward opening doors. Check with the door manufacturer how high the bottom of the door is from the level the door frame sits on (yes I have that T-Shirt).

maxsm - yes, laid loose ontop of the DPM.

Heat transfer, my guess is better in concrete than a screed, but check the heat transfer rating. It's prob not a lot in it, so wouldn't chose one way over another based on that.

Posted By: borpinAlso beware your levels if putting in inward opening doors. Check with the door manufacturer how high the bottom of the door is from the level the door frame sits on (yes I have that T-Shirt).

We have a mat well just inside the front door that allows the door to open without having to go over the top of the bamboo flooring (purely coincidental you understand :) Our other doors are outward-opening, except the patio door that we set the FFL from. (Oops!)

Mmmm, maybe I've misunderstood the clay tile system (??), but there needs to be a structural conc slab at some point. My preference is conc slab only, being the top layer, not conc slab plus screed. Saves carbon, labour, material costs and 70mm floor height (plus no chance of floor level errors, as the finished floor level is there from the start).