Thermal mass and underfloor heating: does a heavy screed save energy?

Underfloor heating marketing pulls in two directions. Some say a deep screed is more efficient because it "stores" heat. Others say a thin, fast overlay is more efficient because it only heats when you need it. Both contain some truth, and both miss the main point: the energy a home uses is set by how warm you keep it, for how long, and how fast it loses heat.

This guide explains what thermal mass does in an underfloor heating floor, when it helps and when it costs you, how it interacts with heat pumps and boilers, and how to control a heavy floor so it works well. It is written for homeowners choosing a floor system and installers explaining the trade-offs.

What is thermal mass in an underfloor heating floor?

It is the material around and above the pipes that has to warm up before the room does. A screed floor puts the pipes inside a solid layer of sand and cement or calcium sulphate. A low-profile overlay or routed board puts them in a thin board just under the floor finish.

Floor system	Material over the pipes	How it responds
Screed over pipes	At least 35mm of screed above the pipes, often a 65mm or deeper screed	Slow: hours to warm and cool
Castellated panel with liquid screed	Thinner flowing screed	Faster than a deep screed, still hours
Overlay or routed board	Floor finish only, often over a foil or cement face	Faster
Joisted floor with spreader plates	Aluminium plates and timber deck	Faster than screed
Electric mat on insulation board	Tile adhesive and tile	Fastest

A thick screed holds a lot of heat. That makes the floor slow to heat and slow to cool, and it needs pre-heating well before you want the room warm. Thin systems react faster but need tighter control to avoid swings. For screed depths, see our guide to screed depths and drying times.

Does a heavy screed save energy?

No, not on its own. Heat flows out of a house in proportion to how much warmer it is inside than outside, and for how long. Thermal mass does not change that. Every unit of heat you put into a screed comes back out, into the room above or, if the insulation is poor, into the ground below.

What thermal mass changes is timing. A heavy floor keeps releasing heat after the thermostat is satisfied and keeps the room warm after the heating switches off. That can be useful, but it has three costs:

• Setbacks save less. If you turn the heating down for the night or while you are out, a heavy floor barely cools, so the house stays warm and keeps losing heat at nearly the full rate.
• Recovering takes a lot of energy. If a heavy floor does cool right down, it takes a long time and a lot of input to bring it back. One heating manufacturer's advice is that a night setback of more than 2°C needs so much energy in a short time that it can cancel the saving.
• Sunny days overshoot. When the sun comes through big windows, a heavy floor already full of heat cannot stop giving it out, so the room can overheat.

So a screed floor run all day at a steady temperature uses about what the house's heat loss says it should. A screed floor run on and off, like a radiator system, does not save as much as the switching suggests.

Does insulation matter more than thermal mass?

Yes. Insulation under the pipes matters more than the mass over them. Heat that goes downwards is lost, and a heavy screed on thin insulation loses more because it holds heat against the cold slab for longer. BS EN 1264 sets minimum insulation resistances of 0.75m²K/W above a heated room, 1.25m²K/W above the ground or a room of different use, and 2.00m²K/W above outside air.

Any floor, heavy or light, needs at least that. Our guide to insulation under underfloor heating explains the options.

When does thermal mass help?

When you want steady heat for long hours, and especially with a heat pump.

• Heat pumps like steady running. They are most efficient making low temperature water for long, quiet periods. The Energy Saving Trust's installer guidance says weather compensation lets a heat pump run steadily instead of cycling on and off, which protects efficiency and product life. A heavy floor smooths out demand and supports that.
• Lower water temperatures. A screed floor with close pipe spacing can deliver its heat at a low flow temperature. One study found the flow temperature needed for the same room rose from 36.6°C at 100mm pipe spacing to 47.5°C at 400mm. One heat pump maker says each 1°C above the ideal flow temperature adds about 2 to 2.5% to electricity use.
• Fewer starts. Frequent starts wear a heat pump and lower its efficiency. Water and screed in the floor act as a buffer between the heat pump and the rooms.
• Cheaper electricity hours. With a dynamic or time-of-use tariff, a heat pump can use cheaper hours to charge the floor within comfort limits, then coast through the expensive ones.

In other words, thermal mass does not reduce the heat a house needs, but it can let a heat pump make that heat more efficiently and more cheaply. For more on design, see underfloor heating with a heat pump.

When is a low-mass floor the better choice?

When rooms are used in short, predictable bursts, or the heat comes on and off with your day.

Situation	Better choice	Why
Heat pump, whole house, occupied all day	Screed	Steady low-temperature running
Time-of-use electricity tariff	Screed	Floor stores heat from cheap hours
Bathroom used morning and evening	Overlay or electric	Warms quickly when needed
Home office or guest room	Overlay	Heat only when in use
Retrofit over an existing floor	Overlay	Low height, no screed drying time
Big south-facing glazing	Lower mass, or good controls	Less overshoot on sunny days

Low-mass boards such as MaxHeatPro foil-faced panels rely on the foil to spread heat between pipe runs. See our guide to low-profile overlay underfloor heating for heights and finishes.

How should I control a heavy screed floor?

Gently. The aim is to keep the floor at a steady temperature and let it work, rather than chasing the room temperature up and down.

1. Use weather compensation on the heat pump or boiler, so the water temperature follows the weather.
2. Keep setbacks small. About 2°C overnight. BEAMA recommends automatic setback, rather than switching off, for screeds thicker than 65mm.
3. Avoid switching off for short periods. Some manufacturers advise that screed systems run best when not switched on and off often.
4. Use optimum start. The Heatmiser neoStat can start heating up to 5 hours early to reach the set temperature on time.
5. Change settings slowly. Raise or lower setpoints by about 1°C a day, not in big jumps.

Does thermal mass affect the first heat-up?

Yes. A new screed must cure before it is heated, then be brought up slowly. One screed maker gives 7 days for cement screeds and 24 days for anhydrite screeds before heating starts, then raises the temperature by no more than 2°C every 24 hours. The maximum is held for at least a day per centimetre of screed, so a 6cm screed needs at least 6 days, before stepping back down. Overlay systems avoid this wait. See commissioning underfloor heating.

Frequently asked questions

Is screed or overlay underfloor heating more efficient?

Neither is more efficient in itself. Screed suits steady, all-day heating and heat pumps. Overlay suits rooms heated for short periods. Insulation and controls matter more than the floor type.

Does thermal mass store free heat?

No. A screed only gives back the heat you put in. It changes when the heat reaches the room, not how much the house needs.

Is thermal mass good with a heat pump?

Usually, yes. It helps a heat pump run steadily at low flow temperatures with fewer starts, and lets it use cheaper electricity hours on a time-of-use tariff.

Should I switch a screed floor off at night?

No. Use a small setback of about 2°C. A heavy floor that cools right down takes a lot of energy and time to recover.

Why does my screed floor overheat on sunny days?

The screed is already full of heat and cannot stop releasing it when the sun warms the room. Weather compensation and small setbacks help.

How deep is the screed over underfloor heating pipes?

One screed maker sets a minimum of 35mm above the pipes. BEAMA's controls advice refers to screeds thicker than 65mm as needing automatic setback.

Sources: BEAMA guidance on controls for water UFH, Energy Saving Trust weather compensation guidance, Göbel et al. 2021, underfloor heating system model, Heatmiser neoStat datasheet