Carpet on underfloor heating = thermal resistance?

Hello everyone,

It is about the fact that we are currently building and getting underfloor heating. My wife would like it to be nice and warm in winter, but she also wants cozy carpet flooring on the upper floor (except for the bathroom). I am increasingly keeping an eye on the costs.
Now I have researched all possible types of carpet flooring regarding durability, harmful substances, and price. In the end, I settled on polyamide short-pile, as high quality as possible and with a quality seal. Unfortunately, I only found ones with a poor thermal transmittance coefficient around 0.17 W/(m²·K). The seller of a larger floor studio told us that there are probably also carpet floors with 0.09. However, he did not have any of those in stock and also had difficulties finding the values for his carpets.

Now I have a few questions for you experts:
1. Do you know of a carpet floor that meets my criteria and has a reasonable thermal transmittance coefficient?
2. Can the underfloor heating be damaged by the thermal resistance at a flow temperature of only 33°?
3. Does the heat pump have trouble dealing with the fact that there are different return temperatures on the ground floor and also in the bathroom on the upper floor compared to the rest of the upper floor rooms?
4. Where exactly is the problem with the high thermal transmittance coefficient? If the carpet floor insulates too much, what happens then? The warm water of the underfloor heating cannot transfer heat to the floor as well/quickly and flows back warmer to the heat pump than it does with tiles, for example. Because of this, it takes longer for the room to get warm, but the energy is not lost at first and eventually the room also gets warm. The whole system just becomes more sluggish at first. Is the problem now that more heat escapes through the exterior walls (we have 16cm insulation) and windows than comes in through the now very sluggish underfloor heating and that one therefore needs a higher flow temperature to compensate? Could one then maybe tell the plumber to lay the heating pipes a bit closer together so that heat is emitted over a larger area? That would only be slightly higher acquisition costs.

Thanks and best regards

I quote

Flatwoven carpets, for example made of wool, jute or sisal, are the better choice compared to a high-pile carpet. Due to their flat weave, they are particularly permeable to heat, while high-pile carpets require more heating power and thus promote higher energy consumption.

I had Tretford carpet made of 80% cashmere and 20% virgin wool installed in the bedroom 11 years ago. Pleasant on bare feet, also ideal in bedrooms, as a natural material that can absorb and release moisture and according to technical data has a value of 0.11 or 0.12 thermal resistance. However, it might be that it does not meet your idea of price.

Do you know a carpet that meets my criteria and has a decent thermal transmittance coefficient?

Make sure that the carpet roll is suitable for underfloor heating and that the pile is not too thick.
We have carpet upstairs and have never used the underfloor heating up there. Okay, for children's rooms you should plan on heating, but I wouldn't really pay attention to that for some rooms.
Our carpet was quite affordable at €30/sqm. Except for a few traces of wear, it still looks good. Next time, I would choose goat hair carpet; we are still saving for that ;) (seriously: next time we would choose linoleum as the floor covering and decorate occasionally with large rugs.)

I would think about that carefully. We also have carpet in our bedroom and in the children's rooms. Newly laid 3.5 years ago. All of them look like crap. With stomach flu, the kids puked on it at 2 or 3 years old and the dog once pooped on it after I overslept and he didn't get out in time.

Better to choose an easy-to-care-for hard floor and then lay a nice carpet on top, that would be my tip :D

with a poor thermal transmittance coefficient of around 0.17 W/(m²·K)

but there are apparently also such carpets with 0.09.

Be careful you want a high value, the heat is supposed to pass through!

We moved in just over 8 years ago and have dark brown carpet in the hallway, bedroom, and guest/office room upstairs, and light-colored carpet in the children's room. Apart from a few scuff marks near the stairs in the hallway, it still looks good in my opinion, despite having a child and a cat.

We recently talked about maybe putting something different in during a renovation. However, in terms of feel underfoot and noise level, carpet flooring is unbeatable.