Short answer: yes and it is one of the better floor finishes you can put over underfloor heating. At around three millimetres there is almost nothing between the heat source and the room, so response time is quick and the system runs efficiently. The long answer is about commissioning, because that is where these floors go wrong - and when they go wrong it is almost always in the first cold month, months after anyone was on site.
Why microcement suits UFH
Thermal resistance is the enemy of underfloor heating. Every millimetre of covering slows the response and raises the flow temperature needed to reach the same room temperature. A thick engineered board or a deep tile bed is working against the system; three millimetres of microcement is barely there. It is also continuous, so there are no joints for differential movement to concentrate at.
The commissioning cycle - the part that matters
A heated screed does two things people forget. It carries residual construction moisture that has to leave and it expands and contracts as it heats and cools for the first time. Both of those must happen before a rigid 3 mm finish is bonded to the top, not after.
So the sequence is fixed:
- Screed cures naturally. No heat. Typically a week per 25 mm for sand-cement, longer in poor drying conditions - but this is a guide, not a substitute for a reading.
- Commissioning cycle begins. Flow temperature raised gradually, usually about 5°C per day, from ambient up to maximum design temperature.
- Hold at maximum. Several days at full design flow temperature. This is the stage that drives out residual moisture and lets the screed do its thermal moving.
- Ramp back down at the same gradual rate, to ambient.
- Moisture readings taken once cool. Not assumed from the calendar - measured.
- Application, with the heating off and the slab at stable room temperature.
- Heating stays off through application and the full cure period, then is ramped back up gradually rather than switched on.
Compressing this to save a week on programme is the single most common cause of a cracked microcement floor. The crack does not appear during the works - it appears when the heating comes on properly for the first time, which is often in November, long after everyone has been paid and left.
Moisture limits
The specific figures depend on the screed type and the system manufacturer and we work to whatever the technical data sheet for the specified product states rather than to a rule of thumb. What is not negotiable is that a reading is taken, by instrument and recorded. Anhydrite screeds in particular are far more sensitive to residual moisture than sand-cement and need their laitance mechanically removed as well - that combination is behind a large share of the failures we are asked to inspect.
Movement joints
Heated screeds are laid in bays with movement joints between them and those joints exist because the slab genuinely moves. Microcement at 3 mm has no capacity to bridge that movement, so every joint in the substrate must be carried through to the finished surface.
This is worth setting out at drawing stage rather than discovering on site. A joint that has to appear across the middle of an open-plan kitchen is much better placed deliberately - under an island, at a change of use, at a threshold line - than wherever the screeding contractor happened to stop. We are happy to mark them up with the architect before the screed is poured.
Decoupling
On some substrates a decoupling membrane between the screed and the finish is the right call - it allows a degree of independent movement and can be specified over anhydrite or where the slab history is uncertain. It adds height and cost, so it is a judgement rather than a default and we would rather make that judgement having seen the floor.
Running the floor afterwards
- Ramp, do not switch. At the start of each heating season, bring the temperature up over several days rather than in one step.
- Observe the maximum surface temperature the seal is rated for. Most polyurethane seals are comfortable at normal UFH surface temperatures, but an over-driven system running hot to compensate for poor insulation elsewhere is a different matter.
- Rugs and mats. A thick rug over a heated floor creates a hot spot beneath it. This is true of every floor finish and is worth knowing.
Electric mat systems
The same principles apply with one addition: electric mats heat and cool much faster than a wet system, so thermal cycling is more aggressive. We want the mat properly encapsulated in levelling compound with adequate cover rather than sitting close to the surface and we still want a commissioning cycle before application.
What we ask for before quoting
Screed type and depth, date laid, whether the system has been commissioned and when and whether a commissioning certificate exists. If the answer to the last one is no, that is not a problem - it just means the cycle needs running before we can start and that has to be in the programme rather than discovered halfway through it.
Our seamless floor build-up is here and if you are at drawing stage we would much rather look at the screed specification now than the crack later. Send us the drawings.