Underfloor heating
Thermal resistance of a tiled floor over underfloor heating
- where
- thermal resistance of the build-up (m²K/W)
- tile thickness (mm)
- tile thermal conductivity (W/mK)
- adhesive bed thickness (mm)
- adhesive thermal conductivity (W/mK)
- thermal resistance of other layers (m²K/W)
- temperature drop across the covering (K)
- design heat output (W/m²)
Explain this equation
Adds up how much the tiles, adhesive and any boards above underfloor heating resist the flow of heat. Use it to check a floor build-up will not hold back the heating output.
Each layer’s resistance is its thickness divided by its conductivity λ: thin, dense materials such as tile (λ about 1.3) and stone (about 3.5) barely resist heat. Rx covers membranes or boards whose makers declare an R value. tog is the same resistance in the units carpet makers use. ΔTs is how much hotter the heating plane runs than the tile face.
Layers in series add up like resistors in a circuit. The 1000 turns mm into metres. The temperature drop follows the heat-flow equivalent of Ohm’s law: flow times resistance.
Sense check: 10 mm porcelain on 5 mm adhesive is about 0.013 m²K/W, or 0.13 tog, so at 100 W/m² the surface is only about 1.3 K cooler than the heater. Electric systems usually allow up to 1.5 tog.
Watch out: an insulated backer board above the heater can add more resistance than all the tiles. Voids in the adhesive bed also reduce heat transfer.
Adds up the thermal resistance of the tile, adhesive and any boards or membranes above the heating (R = Σ t/λ) so you can check the covering will not choke the heat output.
Enter your values to see the result.
Good to know
Typical conductivities: ceramic tile about 1.3 W/mK; granite and marble about 3.5; sandstone about 2.3; hard limestone about 1.7; cement mortar about 0.9. Insulating tile backer boards and uncoupling membranes have much higher resistance: use the maker’s declared R. 1 tog = 0.1 m²K/W. A tiled finish is normally well under 0.05 m²K/W, which is why tile and stone suit underfloor heating. Electric mat makers commonly cap the covering at 1.5 tog (0.15 m²K/W); BEAMA quotes 2.5 tog for carpet plus underlay on water systems. The temperature drop is how much warmer the heating plane must be than the tile surface at the design output.
Common mistake
Laying an insulated backer board over the heating cable instead of under it. Boards belong below the heater; above it they add most of the resistance.
Limits
One-dimensional steady-state conduction through uniform layers. It ignores contact resistance, voids in the adhesive bed and lateral spreading between pipes or cables. Surface temperature limits (29 °C occupied, 33 °C bathrooms) still apply.
Sources
- BEAMA Underfloor Heating: Choosing your floor covering (1 tog = 0.1 m²K/W; 15 mm stone λ 3.5 W/mK)
- Warmup: floor covering thermal resistance (tog = mm ÷ (100 × λ); 1.5 tog maximum for electric UFH)
- SEAI DEAP guide: thermal conductivity of materials (CIBSE Guide A, EN ISO 10456): ceramic tiles 1.3, granite/marble 3.5, mortar 0.88 W/mK
- The Tile Association: Tiling to heated floors (2013/2014) – bay sizes, movement joints, thermal expansion coefficients, screed drying, floor loads
Results are calculations from the published methods and manufacturer figures shown, and depend on the data you enter. Always follow the tile, adhesive and grout manufacturers’ instructions, BS 5385 and the Tile Association guidance. Coverage rates vary by product and substrate. Check prices and VAT rules with your own records and HMRC.