Liquid screed and underfloor heating
Liquid screed and underfloor heating suit each other: the screed surrounds the pipes with no air gaps and can be 25 mm over them. With a 16 mm pipe that's a 41 mm calcium sulphate floor, against 65 mm of sand and cement.
The heating also dries the screed. From day 7 on calcium sulphate; from day 14 on cement-based, at no more than 45°C.
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How deep over the pipes
| Screed | Cover over the pipes | Total with a 16 mm pipe |
|---|---|---|
| Calcium sulphate | 25 mm minimum, 30 mm nominal | 41 to 46 mm |
| A thin calcium sulphate screed+ | More than 20 mm | Over 36 mm, and not under the 35 mm floating minimum |
| Cement-based liquid screed (cement) | 25 mm in houses | 45 mm (its floating minimum is higher than 16 + 25) |
| Cement-based liquid screed (cement) | 25 mm above the pipes | 41 mm |
Why thinner matters
Calcium sulphate screed conducts heat at about 2.2 W/mK, against 1.1 for sand and cement. Less screed over the pipe, and screed that passes heat better, means the room warms up sooner and the system can run cooler. That's the point of it with a heat pump, which is most efficient at low flow temperatures.
Getting the pipes ready
- Pipes laid to the maker's spacing and clipped at least every 400 mm, so they can't float up in the screed.
- Pressure tested before the screed, preferably with water.
- A 10 mm perimeter strip where there's heating, against 8 mm without.
- Joints between separately controlled heating zones and at door thresholds.
- Heated bays no bigger than 300 m² or 40 m² (cement-based liquid screed).
Commissioning before the floor goes down
The heating must be commissioned and run before any floor covering, however dry the screed seems. The cycle: start at about 15°C for a day, go up by no more than 5°C a day to the running temperature (50°C at most), hold it for 7 days, come back down 5°C a day, then switch off for 48 hours before the moisture test. It typically takes 21 days. Force drying, step by step.
Liquid screed poured by the contractor who takes your call
Calls on this site ring JAM Floor Screeding: James Mulhall, based in Portlaoise, 25 years laying floors. Lines open Mon to Sat, 7am to 6pm. These are his jobs.
“JAM were punctual, professional and efficient and did an outstanding job on pouring our new floor as part of a renovation. Very highly recommend!”
“Excellent work, laser level finish, neat and tidy worker, highly recommend.”
“Top class work by james and his crew, highly recommend”
Quoted word for word from JAM Floor Screeding's Google reviews. Read them on Google
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Underfloor heating questions
How much screed goes over underfloor heating pipes?
At least 25 mm with liquid screed. 30 mm is the normal cover.
When can I turn on the underfloor heating after liquid screed?
From day 7 on calcium sulphate and day 14 on cement-based liquid screed, at no more than 45°C. Bring it up slowly.
Is liquid screed good with a heat pump?
It suits one: less screed over the pipes and better conductivity let the floor run at a lower flow temperature, which is where a heat pump is most efficient.
Do the pipes need to be pressure tested first?
Yes. Pressure test them, ideally with water, before the screed goes down.
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