Short answer: insulation under underfloor heating is specified as a thermal resistance, not in millimetres. EN 1264-4:2021 asks for at least R = 1.25 m²K/W under the pipes over the ground (about 4.4 cm at λ = 0.035 W/(m·K)), and with only that minimum the water sends an extra 18.4% of the heat the floor gives the room downwards. On a floor insulated to U = 0.30 W/(m²K) it is 7.4%, at U = 0.20 4.8% (tiles, 45 mm of screed over the pipe).
"100 mm is fine" against "150 mm is needed" - the thread on how much insulation goes under underfloor heating runs on DIY and self-build forums, often with a reply that the insulation belongs under the slab, not below the loops. What each option actually sends downwards rarely gets a number, yet it can be calculated.
Why underfloor heating also heats downwards
A pipe in a screed gives off heat both ways: up through the screed and the floor covering into the room, down through the insulation into the ground or the ceiling below. The split depends on two resistances: Ro above the pipe and Ru below it. That is why EN 1264-3:2021 (Formula 13) has the water carry the room's heat times (1 + Ro/Ru). The Ro/Ru part is heat the water sends down on top of what the room gets.
Above the pipe, with tiles and 45 mm of cement screed, Ro = 0.230 m²K/W (Formula 14: 0.0926 at the surface, the covering designed at 0.10 and 45 mm of screed at λ = 1.2 W/(m·K)). Neither the pipe spacing nor the water temperature changes the share. Only what lies under the pipe does: the bigger Ru, the less heat goes down.
Extra heat downwards as a % of the room's heat - tiles, 45 mm screed
Minimum R 1.25EN 1264-4, ground18.4%
Floor U 0.307.4%
Floor U 0.204.8%
Floor U 0.153.6%
Floor U 0.102.4%
Scale: extra heat downwards from 0 to 20% of the room's heat.
Minimum insulation under underfloor heating: the standard's table
EN 1264-4:2021 (Table 1) gives the least resistance under the pipes, by what is under the floor. It gives a resistance, not a thickness: thickness is the resistance times the board's λ. The table shows it for an example board at λ = 0.035 W/(m·K).
| What is under the floor | R min [m²K/W] | Thickness at λ 0.035 |
|---|---|---|
| A heated room | 0.75 | 2.6 cm |
| An unheated room or the ground | 1.25 | 4.4 cm |
| Outside air, 0 °C and above | 1.25 | 4.4 cm |
| Outside air, 0 to -5 °C | 1.50 | 5.3 cm |
| Outside air, -5 to -15 °C | 2.00 | 7.0 cm |
That is the minimum for the underfloor heating alone. A ground floor is usually set by the building regulations' U-value instead. In our Polish example house the limit is U ≤ 0.30 W/(m²K), which with tiles leaves Ru = 3.10 m²K/W under the pipe - nearly two and a half times the standard's minimum.
What that means in watts
For every 1,000 W the floor gives the room, the water sends down, on top:
- 184 W with only the R 1.25 minimum under the pipes over the ground,
- 74 W on a U 0.30 floor,
- 61 W at U 0.25,
- 48 W at U 0.20,
- 36 W at U 0.15,
- 24 W at U 0.10.
So "100 or 150 mm" is an argument about a few per cent: between U 0.30 and U 0.20 the difference is 2.6 percentage points of the room's heat. The big gap is between a floor insulated to the regulations and one with only the standard's minimum: 18.4% against 7.4%. Every watt pushed down, the heat pump or boiler still has to make.
Over a heated room the minimum is R 0.75. With the ceiling's surface resistance that gives Ru = 0.92 m²K/W, and the extra share rises to 25%. That heat stays in the house, but it warms the room below through its ceiling whether that room needs it or not.
This extra is not the floor's heat loss to the ground. The room's heat loss calculation counts that separately, and the underfloor heating design adds Ro/Ru on top. They are two different numbers; do not merge them.
How to check your own floor
Settle the insulation before the screed goes on - afterwards you cannot change it.
- Find out what is under the floor: the ground, an unheated space, a heated room or outside air. That picks the row of the table.
- Take λ from the product's data sheet. Thickness = R × λ. For R 1.25 and λ 0.035 it is 1.25 × 0.035 = 0.044 m, so 4.4 cm. A board with a lower λ, such as PIR, gives the same resistance in a thinner layer.
- On a ground floor, check the U-value in the design. Work out Ru = 1/U - Ro and the extra Ro/Ru. At U 0.30 with tiles it is 7.4%.
- Ask about the edges. Edge strip and perimeter insulation are a separate item. What else to settle before a quote is in the underfloor heating planning checklist.
What this number does not show
The standard's formula compares the resistances above and below the pipe. The real heat flow downwards also depends on the temperature of the ground or the space underneath. Thermal bridges at the walls and edge insulation are separate. The figures are for tiles and 45 mm of cement screed over the pipe: another covering or a thicker screed changes Ro, as the answer on whether a thicker screed reduces underfloor heating output shows. Always work out thickness from the declared λ of the actual product. The example's -20 °C Warsaw design day is colder than a UK one, but the shares above do not depend on the outdoor temperature.
Check your floor