In screed underfloor heating is the traditional solution to incorporating underfloor heating in a home. If the floor is already being screeded it can be a relatively low-cost way of incorporating underfloor heating.
But arguments around thermal mass, made in its favour, deserve challenging.
High thermal mass does not automatically make underfloor heating more energy efficient. Thermal mass stores heat. It does not create heat.
If the heating is constant, there is no inherent advantage
Imagine two identical homes.
They have the same insulation, heat loss, floor area, flow temperature, heating efficiency and target room temperature.
One uses a responsive low-mass UFH system utilising conductive aluminium to quickly and evenly spread warmth.
The other uses pipe encapsulated in a large screed mass.
If both homes are meant to be kept continuously at 21°C, they ultimately have to replace the same heat leaving the room.
The screed stores more energy, but that does not reduce the room’s heat demand.
Under steady conditions, the two systems should therefore broadly tie.
If heating demand changes, responsiveness becomes an advantage
Now imagine you want a room warm from 5 pm until 8 pm.
A responsive UFH system can start delivering heat closer to 5 pm and reduce its output closer to 8 pm.
A high-mass screed floor has to be heated earlier and will continue releasing some stored heat after demand has reduced.
The issue is that it is less accurate and relies on prediction, the room temperature may overshoot, resulting in windows being opened. Or the room be warmer longer than the period desired – resulting in a less efficient use of energy.
So where heating demand is intermittent, a responsive system can match heat delivery more closely to the periods where comfort is actually required.
The important difference
A responsive system can always be run slowly. It can operate continuously and steadily if that is what the home requires.
A high-mass floor cannot suddenly become responsive.
Once heat has been stored in the screed, that energy will continue to enter the room.
That gives a conductive, low thermal mass UFH system an advantage where control and energy efficiency are concerned.
Better insulation improves the room, not the emitter
There is another point that is often confused with the screed-versus-retrofit debate. We often hear comments such as; ‘it is better to dig up the floor and put in insulation, then pipework and screed’. Yes that’s true, but only in part.
If an existing floor is dug up and additional insulation is installed beneath it, that can be a very worthwhile improvement. It improves the thermal performance of the room.
But that benefit comes from improving the building fabric.
It does not make the heating emitter itself more responsive or efficient at delivering heat when it is required.
If you are prepared to improve the floor construction by replacing the screed and adding more insulation, you can still install a responsive low-mass UFH below the floor finish.
You would then gain; better insulation, improved thermal performance and a responsive emitter, allowing heat delivery to follow demand more closely.
So improving insulation and choosing thermal mass are not the same decision.
It is important to explore all options available when it comes to further insulating the property.
What about utilising lower cost power and storing heat?
A large screed floor can be deliberately charged during cheaper energy periods and allowed to release that heat later.
But that is load shifting, not necessarily energy efficiency.
A well-insulated home already has substantial thermal storage in its structure, fabric and contents. A responsive heating system can also preheat the building when cheap energy is available without permanently introducing a large thermal store into the heating emitter itself.
Additional floor mass only becomes an advantage when additional heat storage is specifically required. E.g. In buildings like retirement or care housing where a steady, uninterrupted indoor temperature is needed throughout the day and night.
For most homeowners, the primary objective is simple: Make the rooms comfortable when they are being used, while using as little energy as possible. This is not to encourage ‘on / off zoning’ practises, rather recognising that utilising room comfort temperatures setback states are better practise for comfort and efficiency.
One advantage is that it is a cheaper add on when needing to pour screed.
If a new floor is already being constructed and screeded, placing UFH pipe into that screed can be a straightforward way of creating a large heating surface. That is a cost-cutting advantage.
It is different from saying that the thermal mass of the screed makes the heating inherently more energy efficient.
If everything is equal:
Continuous heating: responsive UFH broadly ties with screed.
Intermittent heating: responsive UFH has the advantage and leads to less wasted energy and more “real time control”.
And if the floor of the room or building itself needs better insulation, improve the insulation, then choose the emitter on its own merits.
Hopefully this explains why low thermal mass, responsive UFH systems should not be viewed as a compromise.
For the way many homes are used, responsiveness is simply the better characteristic.