Underfloor heating only performs as well as the screed sitting on top of it. Choose the wrong type or the wrong depth and you end up with slow warm-up times, cold spots, cracking, or a floor covering that has to come back up. Here is a straight comparison of the options we install across Bolton, the North West and further afield, and how to work out which one suits your build.
Anhydrite or calcium sulphate liquid screed is what we recommend for the majority of underfloor heating jobs, and for good reason. It self-levels and flows completely around the pipes, so there are no voids or air pockets, which is the single biggest cause of cold spots in a heated floor. Thermal conductivity is typically around 2.0 W/mK against roughly 1.2 to 1.4 for sand and cement, so the same water temperature moves more heat into the room.
It also goes down thinner. A bonded or unbonded liquid screed usually needs 35 to 45mm of cover over the pipe, against 65 to 75mm for traditional. Less mass means the floor responds faster and your heat pump or boiler runs at a lower flow temperature, which matters a great deal if you are on a heat pump and chasing efficiency. Coverage rates are fast too, often 200 square metres or more in a day where access allows.
A 1:4 sand and cement screed with fibres is not obsolete. It is more forgiving where you have awkward falls, small or heavily cut-up areas, wet rooms needing a defined gradient, or a site where getting a pump lorry anywhere near the building is not realistic. Terraced streets around Bolton and Bury with rear-access-only yards are a good example of where a barrow-in traditional screed simply makes more sense.
The trade-off is drying time and responsiveness. The rule of thumb is one day per millimetre for the first 50mm, then slower beyond that, so a 75mm screed can take 90 to 100 days to reach the moisture level a timber or vinyl floor needs. It also has to be laid carefully around the pipe to avoid trapped air, and the extra depth means a sluggish floor.
If you are retrofitting underfloor heating into an existing house and cannot lose 60 or 70mm of head height, low-profile screeds and overlay boards are worth looking at. Modified cementitious low-profile screeds can go down at 15 to 25mm over a pipe or a routed panel system, and they cure quickly enough that some can take a floor covering within a week rather than a month.
They cost more per square metre than a standard liquid screed, and the reduced mass means the floor holds less heat, so it cools faster once the system switches off. For a well insulated extension or a single room that is usually fine. For a whole-house retrofit in a draughty older property, it is worth modelling the heat loss first rather than assuming the thin option will keep up.
The screed type matters less than the build-up underneath it. Insulation should be continuous with taped joints, perimeter edge strip should be fitted to every wall and upstand, and pipes must be properly clipped and pressure tested at around 6 bar and left under pressure while the screed goes down. If a pipe is nicked during the pour, you want to know immediately, not three weeks later.
Commissioning is where most problems are created. The heating should be brought on gently, starting at about 25 degrees flow and increasing by roughly 5 degrees a day up to the maximum design temperature, then brought back down the same way. Rushing this cracks screed. Before any floor covering goes down, get a moisture reading: 75 percent relative humidity for most coverings, and 65 percent for wood. With anhydrite screeds, the surface laitance needs sanding off and a suitable primer applying before tiling.
Wait a minimum of seven days after a liquid screed pour, and around 21 to 28 days for traditional sand and cement. Then ramp the temperature up gradually over a week or so rather than going straight to full output.
Yes, but not without preparation. The laitance layer must be mechanically sanded off, the screed needs to be dry to 75 percent relative humidity, and a suitable primer and a flexible cement-based adhesive should be used, as standard adhesives can react badly with calcium sulphate.
For thermal performance and speed, usually yes. But access, room size, falls to drains and your programme all feed into the decision, so it is worth having someone look at the actual site before committing.