ᐅ Cement-based or anhydrite screed in new construction with underfloor heating?

Hello,

we are trying to decide between cement screed and the recently popular anhydrite screed.

We have an air-to-water heat pump combined with underfloor heating. There are varying opinions regarding cost, accuracy, and thermal conductivity. According to most sources, anhydrite screed transfers the heat from underfloor heating much more effectively. However, I saw the exact opposite in a YouTube video. What is correct?

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Best regards,
Almo

Hello,

I can only tell you what we used:
Bathrooms and utility room: cement-based heated screed, 60 mm (2.4 inches) thick
All other rooms: calcium sulfate-based heated screed, 60 mm (2.4 inches) thick
Tiles will be laid on top everywhere.

Almo85 schrieb:
We are trying to decide between cement screed and the recently popular anhydrite screed.

We have an air-to-water heat pump combined with underfloor heating. There are different opinions regarding cost, accuracy, and thermal conductivity. Most sources say that anhydrite screed transfers the heat from underfloor heating much better. However, I saw exactly the opposite in a YouTube video. So, what is correct?

You don’t need to worry about differences in thermal conductivity, physics will take care of that. If the heating response is a bit slower, you simply start the heating earlier in the morning and turn it off earlier in the evening. The heat isn’t lost; it remains stored in the screed, meaning in the room. Where else would it go?

That is not entirely correct. The heat needs to reach the surface. Liquid screed adapts better around the pipes than cement screed. This typically requires about 3 degrees less flow temperature.

Cement screed is used in basements and bathrooms. Otherwise, anhydrite screed is usually the preferred option.

Bookstar schrieb:
That is not entirely correct. The heat needs to reach the surface. Flowing screed conforms better around the pipes than cement screed. This allows for about 3 degrees less supply temperature to be required.

Cement in basements and bathrooms. Otherwise, anhydrite screed. This is usually the best approach.

Yes... it does conform better around the pipes, but in the video you can see that the cement screed supposedly heats up better and more evenly.

Short version: using anhydrite is obviously easier, and considering the small additional cost, it’s not worth cutting corners for a single-family house. I think we agree on that.

Regarding the physics, focusing solely on the thermal conductivity of the screed is an oversimplification. If you assume all heating parameters except the supply temperature are constant, it behaves exactly as you described. Alternatively, you can control a underfloor heating system based on the heat output and, for example, reduce the flow rate to increase heat extraction per room, even with a screed that has lower thermal conductivity. You just have to ensure that a certain number of kelvins are dropped between the supply and return. That exact amount of heat remains in the rooms and cannot go anywhere else.

@Almo85: the more even warming of the cement screed is precisely due to the lower thermal conductivity, which also causes the described thermal inertia. In reality, it doesn’t really matter how evenly your floor heats up. These are just measurement details...