Short answer: yes. At 95/86 °F supply and return water, 15 cm (5.9 in, the usual 6-inch) tubing spacing and tile, the floor gives 13.9 Btu/h·ft² with 30 mm (1.2 in) of concrete over the tubing, 13.2 Btu/h·ft² with 45 mm (1.8 in), 12.3 Btu/h·ft² with 65 mm (2.6 in) and 11.7 Btu/h·ft² with 80 mm (3.1 in). 3.1 in of cover instead of 1.8 in takes 11% of the output, more than laying wood instead of tile on the standard depth does.
A homeowner on HeatingHelp asked whether it matters if the PEX sits at the bottom of a 4-inch slab or in the middle. Most replies on slab thickness are about warm-up time and thermal lag. What an extra inch of concrete over the tubing costs in steady output 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). Staple-up, joist-space, plate and panel systems under a wood subfloor are not calculated by HeatAlgo.
Why concrete over the tubing costs output
Heat from the tubing has to pass through the concrete to the floor covering and on into the room. Every bit of concrete above the tube is extra resistance on that path, just like the covering. The EN 1264-2:2021 characteristic treats both the same way: with the same water, more cover means fewer Btu/h per square foot in the room.
The calculation uses a cement-based mix that conducts heat at λ = 1.2 W/(m·K). At that conductivity, 35 mm (1.4 in) more of it adds a resistance of 0.03 m²K/W. That is less than the 0.10 m²K/W tile is designed with, but it sits on the same path, under the covering.
| Concrete over the tubing | Floor output, tile, 95/86 °F, 6 in |
|---|---|
| 1.2 in (30 mm) | 13.9 Btu/h·ft² |
| 1.8 in (45 mm), HeatAlgo's default | 13.2 Btu/h·ft² |
| 2.6 in (65 mm) | 12.3 Btu/h·ft² |
| 3.1 in (80 mm) | 11.7 Btu/h·ft² |
| For comparison: wood on 1.8 in | 12.2 Btu/h·ft² |
Calculated with 16 mm (5/8 in OD, the size of 1/2-inch PEX) tubing in a 68 °F room.
What that means in practice
3.1 in of cover instead of 1.8 in costs 1.5 Btu/h·ft², or 11%. Swapping tile for wood on the standard depth costs 0.9 Btu/h·ft² (2.9 W/m²; the table rounds each row, so it reads as 1.0). Extra concrete usually goes in without a word about output; a wood floor only after a long discussion.
| Option, tile, 68 °F room | Output |
|---|---|
| 1.8 in cover, 6 in on center, 95/86 °F | 13.2 Btu/h·ft² |
| 3.1 in cover, 6 in on center, 95/86 °F | 11.7 Btu/h·ft² |
| 3.1 in cover, 4 in on center, 95/86 °F | 12.8 Btu/h·ft² |
| 3.1 in cover, 6 in on center, 97.7/88.7 °F | 13.1 Btu/h·ft² |
Tighter spacing does not win it all back: 10 cm (3.9 in, so 4 in on center) under 3.1 in gives 12.8 Btu/h·ft², less than ordinary 6-inch spacing under 1.8 in. Only water about 2.7 °F warmer does, and with a heat pump that means a slightly higher water temperature all season.
For the bottom-versus-middle question, the same logic applies: tubing tied up in the middle of a slab sits under less concrete than tubing lying on the insulation at the bottom, and the table shows the direction and the size of the effect per inch of cover.
What this number does not show
The calculation is steady output on a cold design day. It does not show how long a thicker slab takes to warm up and cool down. That is a question about thermal mass, which HeatAlgo does not calculate. A gypsum-based thin slab (gypcrete) conducts heat differently from the cement-based mix used here, so these figures do not apply to it; panel systems come with their manufacturer's output. How output depends on the covering and the spacing is in the guide on radiant floor heating water temperature.
What to check before the pour
- The depth over the tubing, not the whole slab. Drawings and crews often quote the full pour. What counts for output is the concrete above the tube: with 5/8 in OD tubing and a 2.4 in (6 cm) pour, that is about 1.7 in (44 mm).
- Where extra depth comes from. Usually an uneven base or services run in the floor. Making up height with insulation rather than concrete costs no output.
- Whether the design uses that depth. The floor output should be calculated for the agreed cover, not a default, and compared with each room's heat loss from the contractor's room-by-room load calculation.
A HeatAlgo report follows EN 12831-1 and EN 1264, not ACCA Manual J, and is not accepted for permits or rebates.