Heat loss and underfloor heating: will UFH heat my room?

The single most common question about underfloor heating is "will it be warm enough?" The honest answer is that it depends on the room, not the heating. A floor can only give out so much heat per square metre before the surface gets uncomfortably hot. If the room loses heat faster than that, no pipe layout or thermostat will make up the difference.

This guide shows how to estimate a room's heat loss, how to compare it with what underfloor heating can deliver, and what to do if a room comes out short. It uses the same method as BS EN 12831, the standard behind the MCS heat load calculator, with two worked examples. For how pipe spacing and water temperature change the output, see our guide to underfloor heating heat output.

How much heat can underfloor heating give per m²?

About 100W/m² across the main floor area, and up to about 175W/m² in a perimeter band by cold walls. The limit is set by the floor surface temperature, not by the pipe.

One manufacturer's underfloor heating handbook gives the BS EN 1264 surface limits and a heat transfer coefficient of 11.1W/m²K between the floor surface and the room. Multiply that by how much warmer the floor is than the room and you get the output.

Zone	Maximum floor surface	Approximate maximum output
Occupied area	29°C	About 100W/m² (9K above a 20°C room)
Bathroom	Room temperature + 9K (33°C at 24°C)	About 100W/m²
Perimeter zone, up to 1m from wall	35°C	About 175W/m² (15K above room)

The surface limit is a fixed temperature, so a warmer room has less headroom. At a 21°C design temperature the 29°C limit leaves 8K, which is about 89W/m². One manufacturer's own worked example shows a 27°C floor in a 20°C room giving 77.7W/m².

The floor finish does not change this ceiling, but it changes how hot the water must be to reach it. One manufacturer's pre-calculations assume a covering resistance of 0.10m²K/W in living rooms, with up to 0.15m²K/W by agreement. Thick carpet and underlay push you towards that limit. Our guide to carpet tog ratings covers this.

How do I work out a room's heat loss?

Add the fabric heat loss through every outside surface to the ventilation heat loss from air leaking in and out. Both are worked out at the coldest design day.

The Open University's building energy course sets out the two formulas:

• Fabric loss (W) = sum of (U-value × area) for each element × temperature difference. U-value is in W/m²K, area in m², temperature difference in K (the same number as °C).
• Ventilation loss (W) = 0.33 × air changes per hour × room volume in m³ × temperature difference.

The MCS heat load calculator describes the same steps: fabric loss for each wall, floor, ceiling, window and door from its U-value, area and temperature difference, and ventilation loss for each room from its air volume and air change rate.

1. Measure the room. Floor area, ceiling height, and the area of each outside wall, window, door and roof. Our guide to measuring a room for underfloor heating shows how.
2. Find the U-values. Use the build specification for new work, or typical values for the age and type of construction.
3. Pick the temperatures. A design indoor temperature for the room and a design outdoor temperature for your area (see the next section).
4. Work out each loss. Multiply U × area × temperature difference for every element, then add ventilation.
5. Divide by the heated floor area. Leave out floor under fitted units, baths and other fixed items.

What temperatures should I design for?

Use the indoor temperature you want on the coldest day and the local design outdoor temperature. These set the temperature difference used in every sum.

The MCS heat load calculator takes its indoor temperatures from the CIBSE Domestic Heating Design Guide. For older homes it uses 21°C for living and dining rooms, 22°C for bathrooms and 18°C for bedrooms, halls, kitchens and landings. For newer, well-insulated homes it uses 21°C throughout, with 22°C in bathrooms.

Location (MCS reference)	Design outdoor temperature (99.6%)
London, Heathrow	-3.0°C
Edinburgh, Gogarbank	-5.4°C

MCS picks the nearest of 14 reference weather stations to your postcode and lowers the figure by 0.6°C for every whole 100m your home sits above that station. For floors on the ground, MCS guidance suggests a ground temperature of about 10°C rather than the outdoor air temperature.

Worked example: will UFH heat a new extension?

Yes, easily. Take a 5m × 4m single-storey extension used as a living room, 2.4m ceiling, three outside walls, 8m² of glazing, a flat roof, built to the 2022 Part L values for extensions, in London.

Element	Area × U-value × temperature difference	Heat loss
Walls (net of glazing)	23.2m² × 0.18 × 24K	100W
Glazing	8m² × 1.4 × 24K	269W
Flat roof	20m² × 0.15 × 24K	72W
Floor to ground (10°C)	20m² × 0.18 × 11K	40W
Ventilation, 0.5 air changes	0.33 × 0.5 × 48m³ × 24K	190W
Total		About 670W

The temperature difference is 21°C inside less -3°C outside, which is 24K. The U-values are those in a council Building Regulations booklet for the June 2022 changes: 0.18 for new walls and floors, 0.15 for roofs and 1.4 for windows. The 0.5 air changes per hour is the MCS default minimum for most habitable rooms.

670W over 20m² is about 34W/m². Divide by 11.1 and the floor only needs to sit about 3K above the room, around 24°C. That is well inside the limit and suits a low flow temperature from a heat pump. Our guide to underfloor heating with a heat pump explains why that matters.

Worked example: will UFH heat an older room?

Maybe, and it depends heavily on the draught assumption. Take a 4m × 4m ground floor living room in an older house: two solid stone outside walls, a 3m² window, a heated bedroom above, and the same London temperatures.

Element	Area × U-value × temperature difference	Heat loss
Solid walls (net of window)	16.2m² × 2.23 × 24K	867W
Window	3m² × 2.0 × 24K	144W
Floor to ground (10°C)	16m² × 0.25 × 11K	44W
Ventilation at 1.5 air changes	0.33 × 1.5 × 38.4m³ × 24K	456W
Ventilation at 0.5 air changes	0.33 × 0.5 × 38.4m³ × 24K	152W

The 2.23W/m²K wall value is a standard book figure used in an OpenEnergyMonitor worked example, which also notes measured values for thick stone walls can be nearer 1.5. The window and floor values (2.0 and 0.25) are those used in the Open University worked example. The 1.5 air changes per hour is the old CIBSE default for living rooms; the newer EN 12831 approach used by MCS often gives a much lower figure.

• At 1.5 air changes: about 1,510W, or 94W/m². In a 21°C room the 29°C limit gives about 89W/m², so the floor alone falls just short.
• At 0.5 air changes: about 1,210W, or 75W/m². The floor copes, with a little in hand.

The lesson is that a heat loss figure is only as good as its assumptions. The OpenEnergyMonitor example shows a design load rising from about 3.3kW to about 7.5kW as cautious assumptions stack up, and that the over-sized figure would have pushed the design flow temperature from 35°C to 50°C.

What if a room loses more heat than the floor can give?

Close the gap from both ends: cut the heat loss, or raise the output in the places that need it. One manufacturer's handbook says that where the room heat load exceeds the underfloor output, additional heating for the shortfall should be considered.

• Add a perimeter zone. A band of closer pipe up to 1m from cold glazing can run up to 35°C at the surface. See our guide to pipe centres.
• Reduce the heat loss. Draught-proofing, better glazing or wall insulation often help more than any heating change.
• Pick a thinner floor finish. It does not raise the ceiling, but it lets you reach it at a lower water temperature.
• Add a radiator or towel rail for the shortfall on the coldest days.
• Check the downward loss. One manufacturer's guide says insulation under the heating must keep downward heat loss to 10W/m² or less, as BS EN 1264 requires. Heat lost downwards is heat the room never sees.

Large glazed rooms are the usual problem. Our guide to open-plan rooms and glazing covers them.

Do I need a professional heat loss calculation?

For a heat pump, yes. MCS installers size systems with a room-by-room calculation to BS EN 12831-1:2017, and the MCS heat load calculator checks it against the MCS Heat Pump Standard. For a boiler-fed system in a modern, well-insulated home, the method above is a good check that each room will be warm enough.

OpenEnergyMonitor's heat loss guidance suggests that an accurate heat loss lets you design for a flow temperature of 40°C or lower, which is where heat pumps run most efficiently.

Frequently asked questions

How many watts per m² does underfloor heating give?

Up to about 100W/m² in the main floor area at the 29°C surface limit in a 20°C room, and up to about 175W/m² in a perimeter band. Most well-insulated rooms need far less.

What is a typical heat loss per m² for a room?

It varies widely. In our examples, a new extension built to 2022 Part L values came out at about 34W/m², and an older room with solid stone walls at 75 to 94W/m² depending on draughts. Work it out for your own room.

Is underfloor heating enough on its own in an old house?

Often, but not always. Check the worst rooms first. If heat loss per m² comes out above about 90W/m², plan for better insulation, a perimeter zone or a small radiator alongside.

What outdoor temperature should I use?

Use the MCS design temperature for your nearest reference station, for example -3.0°C for London Heathrow or -5.4°C for Edinburgh, lowered by 0.6°C per 100m of extra altitude.

Does furniture reduce underfloor heating output?

Fitted units, baths and other fixed items cover floor that cannot give heat, so leave them out of the heated area. Loose furniture and rugs slow heat output where they sit.

Why does air change rate matter so much?

Ventilation loss scales directly with it. In the older room example, moving from 0.5 to 1.5 air changes per hour adds about 300W, enough to tip the room from comfortable to short.

Can I use the same heat loss for radiators and underfloor heating?

Yes. Heat loss belongs to the room, not the heater. What changes is whether the emitter can deliver it at your chosen flow temperature.

Sources: Open University: Energy in buildings, heat loss calculations, MCS Heat Load Calculator: understanding heat loss, MCS Heat Load Calculator: design conditions, MCS Heat Load Calculator: ventilation rates, OpenEnergyMonitor: heat loss calculation, Extensions and alterations Building Regulations booklet 2022