Short answer: yes, screed depth changes underfloor heating output. At a 35/30 °C flow and return, 15 cm spacing and tiles, the floor gives 43.7 W/m² with 30 mm of screed over the pipe, 41.5 W/m² with 45 mm, 38.8 W/m² with 65 mm and 36.8 W/m² with 80 mm. An 80 mm screed instead of 45 mm takes 11% of the output - more than laying wood on a standard screed does.
"How long does it take to warm up" is what one heat pump owner asks on a UK forum, and the screed questions that follow are usually about strength, drying and that warm-up time. What an extra centimetre of screed over the pipe costs in heat output rarely comes up, yet it can be calculated.
Why screed over the pipe costs output
Heat from the pipe has to pass through the screed to the covering and on into the room. Every millimetre of screed is extra resistance on that path, just like the covering. The EN 1264-2:2021 characteristic treats both the same way: with the same water, a thicker layer over the pipe means fewer W/m² in the room. Cement screed conducts heat at λ = 1.2 W/(m·K), so 35 mm more of it adds a resistance of 0.03 m²K/W. That is less than the resistance tiles are designed with (0.10 m²K/W), but it sits on the same path, under the covering.
Floor output against screed over the pipe - 35/30 °C, tiles, 15 cm
wood on 45 mm: 38.6 W/m²
30 mm over the pipe43.7 W/m²
45 mm over the pipeHeatAlgo's default41.5 W/m²
65 mm over the pipe38.8 W/m²
80 mm over the pipe36.8 W/m²
Scale: floor output from 30 to 46 W/m².
What that means in practice
An 80 mm screed instead of 45 mm costs 4.7 W/m², or 11%. That is more than swapping tiles for wood on a normal screed, which costs 2.9 W/m². A thick screed usually goes down without a word about output; wood only after a long discussion.
| Option | Output [W/m²] |
|---|---|
| 45 mm, 15 cm, 35/30 °C | 41.5 |
| 80 mm, 15 cm, 35/30 °C | 36.8 |
| 80 mm, 10 cm, 35/30 °C | 40.3 |
| 80 mm, 15 cm, 36.5/31.5 °C | 41.4 |
Closer spacing does not win it all back: 10 cm with 80 mm gives 40.3 W/m², less than ordinary 15 cm with 45 mm. Only water about 1.5 K warmer does, and that means a heat pump running at a slightly higher temperature all season.
What this number does not show
The calculation is for steady running on a cold design day. It does not show how long a thicker screed takes to warm up and cool down - a different question, about thermal mass, which HeatAlgo does not calculate. It covers wet underfloor heating with the pipe embedded in a cement screed; an anhydrite (calcium sulphate) screed conducts heat differently, and dry systems are calculated by their manufacturer. How output depends on the covering and the spacing is in the guide underfloor heating flow temperature.
Screed thickness: what to check before the pour
- The depth over the pipe, not the whole screed. Designs and builders often quote the whole layer. What counts for output is the screed above the pipe: with a 16 mm pipe and 6 cm of screed that is about 44 mm.
- Where the extra depth comes from. Usually an uneven base or services run in the floor. Levelling with insulation rather than screed costs no output.
- Whether the design uses that depth. The designer should calculate the output for the agreed depth, not the default.