Short answer: insulation under radiant floor heating is a resistance, and the less of it there is, the more of the water's heat goes down instead of up. With only the minimum EN 1264-4:2021 sets for a slab on the ground (7.1 h·ft²·°F/Btu), the water sends an extra 18.4% of the heat the floor gives the room into the ground. On a better insulated floor, with an overall U-factor of 0.053 Btu/h·ft²·°F, it is 7.4%, and at 0.026 Btu/h·ft²·°F it is 3.6% (tile, 1.8 in of concrete over the tubing).
A homeowner on HeatingHelp asked whether a contractor skipping the insulation under the PEX is standard practice. Replies on radiant slab insulation tend to argue products and thicknesses. How much heat the missing insulation sends into the ground rarely gets a number, yet it can be calculated.
These figures are for hydronic tubing embedded in concrete: a slab or a thin slab poured over the tubing (the EN 1264 "type A" floor). HeatAlgo does not calculate staple-up, joist-space, plate or panel systems under a wood subfloor.
Why a radiant slab also heats downward
The tubing gives off heat both ways: up through the concrete and the floor covering into the room, down through the insulation into the ground. The split depends on two resistances: Ro above the tubing 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 tubing, with tile and 1.8 in of concrete, Ro is 1.3 h·ft²·°F/Btu: the surface, the covering designed at 0.6 h·ft²·°F/Btu and the concrete. Neither the tubing spacing nor the water temperature changes the share. Only what lies under the tubing does: the higher the R-value under the radiant floor, the less heat goes down.
| What is under the tubing | Extra heat sent down, as a share of the room's heat |
|---|---|
| Only the EN 1264-4 minimum over the ground, 7.1 h·ft²·°F/Btu | 18.4% |
| A floor with a U-factor of 0.053 Btu/h·ft²·°F | 7.4% |
| U-factor 0.035 Btu/h·ft²·°F | 4.8% |
| U-factor 0.026 Btu/h·ft²·°F | 3.6% |
| U-factor 0.018 Btu/h·ft²·°F | 2.4% |
Tile and 1.8 in of concrete over the tubing; Ru is 1/U minus Ro.
Minimum R-value under radiant floor heating
EN 1264-4:2021 (Table 1) gives the least resistance under the tubing, by what is under the floor. It sets a resistance, not a thickness: the thickness is the R-value divided by the board's R-value per inch.
| What is under the floor | Minimum | Thickness at 0.24 Btu·in/h·ft²·°F |
|---|---|---|
| A heated room | 4.3 h·ft²·°F/Btu | 1 in |
| An unheated space or the ground | 7.1 h·ft²·°F/Btu | 1.7 in |
| Outside air, design day 32 °F and above | 7.1 h·ft²·°F/Btu | 1.7 in |
| Outside air, 32 to 23 °F | 8.5 h·ft²·°F/Btu | 2.1 in |
| Outside air, 23 to 5 °F | 11.4 h·ft²·°F/Btu | 2.8 in |
That is the floor for the radiant heat alone, not a target. In the Polish example the numbers come from, the building rules ask for a floor U-factor of 0.053 Btu/h·ft²·°F or better, which with tile leaves 17.6 h·ft²·°F/Btu under the tubing: nearly two and a half times the standard's minimum.
What that means in Btu/h
For every 1,000 Btu/h the floor gives the room, the water sends down, on top:
- 184 Btu/h with only the minimum under a slab on the ground,
- 74 Btu/h with a floor U-factor of 0.053 Btu/h·ft²·°F,
- 48 Btu/h at 0.035 Btu/h·ft²·°F,
- 36 Btu/h at 0.026 Btu/h·ft²·°F.
Skipping the insulation entirely leaves even less resistance than the minimum, so the share goes higher still. Every Btu pushed into the ground, the boiler or heat pump still has to make.
Over a heated room the minimum is 4.3 h·ft²·°F/Btu. With the ceiling's surface that gives Ru of 5.2 h·ft²·°F/Btu, 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 own heat loss to the ground, which a room-by-room heat loss calculation counts separately. They are two different numbers; do not add them twice.
How to check your own slab
Settle the insulation before the pour - 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.
- Read the R-value on the board. Rigid foam boards are labeled with their R-value; compare it with the row's minimum.
- Ask for the floor's U-factor. Ru is 1/U minus Ro, and the extra share is Ro/Ru.
- Ask about the edges. Slab-edge insulation is a separate item. What else to settle before a quote is in the radiant floor heating quote checklist.
What this number does not show
The standard's formula compares the resistances above and below the tubing. The real heat flow down also depends on the ground temperature under the slab. Thermal bridges at the walls and slab-edge insulation are separate. The figures are for tile and 1.8 in of concrete over the tubing: another covering or deeper concrete changes Ro, as the answer on radiant slab thickness and floor output shows. The shares hold for any climate; the house behind the example does not, and this is not a sizing of a US house.
Check your slab for freeA HeatAlgo report follows EN 12831-1 and EN 1264, not ACCA Manual J, and is not accepted for permits or rebates.