Underfloor heating heating demand with at least 60 mm screed

Significantly more only with a considerably(!) thicker screed.

KlaRa is of course right that the discussion is against the background of binding standards for nuts.

The physical principle is the same as with the greenhouse effect: If I shift the point at which energy is transferred to another system (whether from the floor surface into the room or from the atmosphere into space, it does not matter) away from the heat source while keeping the input identical, the heat source must produce more to establish the steady state IN = OUT.

The reason is that a temperature gradient across the screed thickness is necessary to transport heat from the heating pipes to the surface. At 60 vs. 100 mm probably totally irrelevant. Therefore, irrelevant for practical application.

I always assumed that it really doesn't matter and that there is only a shift. I thought: same insulation, same energy input = same energy output into the house. I can understand that there is a different surface temperature. I was just under the impression that the energy "pushes on" longer (unfortunately I don't have a proper word for this in my vocabulary) and that the total energy input remains the same as a result. I also assume that your explanation is correct, I just don't understand it [emoji23]

You are already on the right track, but you must not ignore the losses through the envelope. If your "slow feed-in" is slower than the energy being "pushed" out of the house, you suddenly stop heating altogether. So you have to increase the flow temperature to compensate for the screed thickness (insulation). However, this proportionally increases the losses of your underfloor heating into the floor slab, etc., and you have a higher "consumption."

Or explained the other way around: if you slow down the heat transfer through insulation, the energy input is simply not the same, and you consume less. Consequently, your house logically becomes cooler. But since you want to keep the temperature constant, you have to increase the flow temperature to get back to the same speed, with the consequences described above.

Okay now I also understand why this is rather theoretical in nature. Then the screed would have to be so thick that it exceeds the insulation of the screed towards the base plate. At least if I understood it correctly.

No. Every centimeter counts. It just shifts the ratio and the slope in the screed. However, I don't know from when this becomes noticeable in practice.

77.willos explanation is correct.

What you mean by "pushing forward" would only be lossless if the loss downward (or in all other directions instead of the desired direction) were zero. Thanks to good insulation, it is small but still not zero.

Energy stored in the screed is not transported 100% into the room to be heated. Rather, only 100 - x %. And this x must always be replenished by the heating and increases with every additional screed thickness. The thicker the screed, the longer your "pushing forward" takes and the longer x heat flows away in an unwanted direction.

If the floor surface is to remain equally warm, the underfloor heating must provide different amounts of power with different build-up heights of the same material.

The principle equality to the greenhouse effect only holds under one condition: Our planet as a whole is naturally perfectly insulated downward, unlike a single-family house ;).