I thought my 12kW solar installation was overkill until I got an electric car 🤣 

Efficiency of underfloor heating varies depending on what covers it. A more insulating floor covering requires correspondingly higher water temperature for the desired heat power to pass through. Losses into the space below are also proportional to water temperature. So a more insulating floor covering increases losses, increases required water temperature, and reduces heat pump COP.

Concrete or anhydrite screed stuffed with pipes (lots of pipes) and covered with tile gives highest performance due to high thermal conductivity both of the slab material and the tiles. It also has high thermal mass, which can be a blessing or not, depending on what you want. For any other solution the thermal conductivity of the layers above the UFH pipes and diffusers must be considered carefully. It's a lot of work to install, may as well do it right. With too thick layers of semi-insulating material like wood, results could be disappointing...

Note in a well insulated home, underfloor heating does not need to radiate a lot of heat, so it doesn't feel warm under naked feet. It feels... well, not cold, so it's not bad lol.

There is also the radiant ceiling option, which sidesteps the problem as it is only covered by one layer of drywall which has decent heat conductivity. I don't like radiant walls because you never know if you're going to drill into a pipe when hanging a picture frame.

In summer, both can be used for "cooling" by circulating cold water from a reversible air to water heat pump, this is often quoted as a marketing argument but it really is quite limited. If the water is too cold, condensation will form on the pipes, on the floor, and inside the floor, which will lead to all sorts of nasty issues like rotting the house. Thus radiant ceilings or slabs can cool a little, but can't dehumidify the air, since they can't get rid of condensate water. Radiant floors covered with semi insulating material (wood...) are not usable for cooling. Radiant ceilings are better at cooling because heat rises, the ceiling gets hot, and radiates heat back on your face all day. Cooling the ceiling fixes that.

Actual cooling (as in air conditioning) requires dehumidifying the air, otherwise you get cool air with 100% relative humidity which feels like a cold swamp. This requires air handler units with extra pipes to get rid of the condensate water. You can install a separate split air/air heat pump, or you can also produce cold water (7°C) with an air to water heat pump and use fan coil units to blow cold dehumidified air. However every inch of cold pipe needs to be airtight and vapor tight insulated, otherwise they will condense water from the air and, same as above... rot the house.

However the most important bit for cooling is to not let the heat get into the house in the first place. If your roof is shadowed by solar panels, then great, the temperature of your roof should be at least 20°C cooler than if it was sun baked. This really matters. Next step is to install something on the windows to block the sun: blinds, shutter, louvres, etc, and that should be outside so when the sun heats it, the resulting heat does not end up inside.

I went with louvred shutters, roof shadowed by solar panels, 40cm blown in fiberglass in the attic, 20cm rockwool exterior insulation on the facade, the house is located in the south of France, here's the interior and exterior temps during last year's heat wave. I'm operating two awfully noisy mobile aircon units which are rated for 30m² each (the house is about 350m²).

 The most important factors are the shutters and night time ventilation. Ground floor has white steel perforated roller shutters, which block heat from the sun while allowing view and night time ventilation.

@bobflux 

Thanks Bob, that makes a lot of sense.

I'm looking at another 6kw of solar to go with the 11kw we have on the garage, we've 2 1/2 electric cars (Smart Fortwo EQ).

While we had gas the 16kw of batteries just about lasted the day, but now we've taken it out we'll need more electric.

I'm looking at a number of screen boards, this is one - Gypdeck Structural 18mm board, inc Vat its £4250 for my required 100sqm - I understand this type of product would be better than standard flooring, but over the last 60 years I've become attached to my kidneys 🙂

I'm going to put another post on here to get people views on this type of stuff. Without blowing hot air, I think the site and videos is a brilliant resource.

thanks for your input, Simon

This Gypdeck?

• Thermal conductivity (0.40W/mK)

OK so it's the same as Fermacell, twice as good as wood (about 0.2W/mK).

I just did the design for my radiant ceiling, and the materials are similar, except upside down. The insulating panels with slots to put in the pipes are pretty bad. Lookup "thermacome acosi+", it's the same method, but the panels have an aluminium diffuser plate 0.5mm thick and the pipe clips into it. Being aluminium, it spreads the heat much better than putting the pipe in a slot inside polystyrene, which insulates it on 3 sides, leaving only the top to make dubious contact with the underside of the board...

Price is ridiculous since it's basically fiber reinforced plasterboard and polystyrene...

Considering how expensive this thing is, if you're going to insulate from the bottom anyway, the slotted polystyrene panels provide dubious value, and it may even be cheaper to lay pipe on the floor and pour a self leveling screed... Checking prices in France... yeah it's cheaper. Concrete, liquid concrete and anhydrite are about 1.2-1.4 W/mK so 3x better than Gypdeck, but much more cumbersome to apply of course, being liquids. On the plus side, you don't have to do any cuts, the pump truck comes, they pour it, and all you have to do is watch it dry.

Also heavier, which raises the question of how much weight your structure can bear.

Yes, good points. I chose the aluminium / metal spreader plates where the pipe clicks in (there at 400mm centres) Under it there is 100mm Kingspan type pir. The cavity under the floor varies from about 200/300mm to about 1000mm.

thanks Simon.