Slab-on-ground U-value calculator
For the Romanian house without a basement: a concrete slab on the ground, with polystyrene under the slab or on top of it, under the screed. The calculation gives two values: the floor U-value, which Mc 001-2022 and the Passive House Institute compare, and the U-value including the ground under the house (EN ISO 13370), which gives the heat lost in a year. Drag the insulation in the drawing or move the sliders.
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Drag the orange marker along the scale: the insulation thickness adjusts to reach the U you pick.
PHI: Passive House Institute, cool-temperate zone, element in contact with the ground (0.15 / fPHI 0.6) · nZEB: Mc 001-2022, new residential buildings, slabs on ground (R′ 5.00) · renovation: Mc 001-2022, major renovation (R′ 4.50) · Order 2017: Order 2641/2017 (R′ 4.50). Recommended values, shown for comparison.
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The U-value with the ground is calculated in steady state, following EN ISO 13370; the thermal mass of the ground shifts losses between months, but the yearly total stays close. The thermal bridge at the wall–slab junction is not included and is calculated separately. Ground water and moisture under the slab are not calculated. It is not a design calculation.
The slab on ground, with and without insulation
The slab without insulation has U 3.487 W/(m²K); with the ground under the house, 0.491, and a 120 m² house loses about 3,644 kWh through it in a winter. With 10 cm of polystyrene on the slab the loss falls to 1,422 kWh, and with 15 cm to 1,108 kWh. Click a floor to open it in the calculator.
| Floor | Floor U W/(m²K) | U with the ground W/(m²K) | Loss per winter |
|---|---|---|---|
| 3.487 | 0.491 | 3,644 kWh | |
| 0.569 | 0.276 | 2,047 kWh | |
| 0.310 | 0.192 | 1,422 kWh | |
| 0.213 | 0.149 | 1,108 kWh | |
| 0.318 | 0.195 | 1,448 kWh | |
| 0.167 | 0.125 | 928 kWh | |
| 0.213 | 0.136 | 1,008 kWh |
10 × 12 m house (120 m², 44 m perimeter), 45 cm walls, clay ground (λ 1.5), ceramic tiles, 5 cm screed, 12 cm slab, damp-proof membrane. PVGIS typical meteorological year for Bucharest.
How much insulation for the reference values
The thickness, rounded up to the centimetre, at which the floor U-value reaches 0.25 (Passive House Institute, element in contact with the ground), 0.22 (Mc 001-2022 for renovation and Order 2641/2017, R′ 4.50) and 0.20 W/(m²K) (Mc 001-2022 for new residential nZEB buildings, R′ 5.00).
| Insulation | U 0.25 | U 0.22 | U 0.20 |
|---|---|---|---|
| EPS A150 · Austrotherm expanded polystyrene λ 0.034, on the slab | 13 cm | 15 cm | 17 cm |
| EPS A100 · Austrotherm expanded polystyrene λ 0.036, on the slab | 14 cm | 16 cm | 17 cm |
| XPS TOP 30 · Austrotherm extruded polystyrene λ 0.035, under the slab | 13 cm | 15 cm | 17 cm |
With ceramic tiles, 5 cm screed and a 12 cm slab. The reference values are recommendations, not requirements.
The ground insulates more under a large house
Heat leaves the slab mostly at the edges, where the way through the ground to the outside air is short. In a large house the middle of the slab loses very little, so the U-value with the ground falls as the ratio of area to perimeter grows.
| House | Area · perimeter | U with the ground, no insulation | U with the ground, 15 cm EPS |
|---|---|---|---|
| 8 × 10 m | 80 m² · 36 m | 0.556 | 0.156 |
| 10 × 12 m | 120 m² · 44 m | 0.491 | 0.149 |
| 12 × 15 m | 180 m² · 54 m | 0.431 | 0.141 |
| 15 × 20 m | 300 m² · 70 m | 0.365 | 0.132 |
The slab on ground and its insulation
How much insulation does a slab on ground need for nZEB?
Mc 001-2022 recommends R′ ≥ 5.00 m²K/W for the slabs on ground of new residential nZEB buildings, i.e. U ≤ 0.20 W/(m²K). With ceramic tiles, a 5 cm screed and a 12 cm slab, that means at least 17 cm of EPS A150 on the slab or 17 cm of XPS under the slab. For renovation (R′ 4.50, U 0.22) 1–2 cm less is enough.
Polystyrene under the slab or on top of it?
For the same thermal resistance the U-value is practically the same: 10 cm of EPS A150 on the slab gives U 0.310, and 10 cm of XPS under the slab 0.318. The difference is in the build: under the slab goes XPS, which withstands moisture and the weight of the slab, and the slab stays inside the heated space and adds thermal mass to the house. On the slab, under the screed, EPS A150 is used, which is cheaper; only the screed then needs heating, which helps underfloor heating.
Why is the U-value with the ground lower than the floor U-value?
Because the ground under the house insulates too. Mc 001-2022 and the Passive House Institute compare the floor U-value without the ground (0.213 for 15 cm of EPS), while the heat lost is calculated with the U-value including the ground per EN ISO 13370 (0.149 for a 10 × 12 m house on clay). The larger the house, the lower the U-value with the ground, because the middle of the slab is far from the outside air.
Is plinth insulation worth it?
Yes, especially for slabs with little insulation. With 15 cm of EPS on the slab, 10 cm of XPS on the plinth and foundation lowers the U-value with the ground from 0.149 to 0.142 when it reaches 0.5 m deep and to 0.136 at 1 m. Plinth insulation also shortens the thermal bridge at the foot of the wall, which this calculation does not include.
How much heat does a house floor lose in a year?
For a 10 × 12 m house in Bucharest (2005–2023 typical meteorological year, 172 days averaging 12 °C or less, 2,579 degree-days), the uninsulated slab loses about 3,644 kWh in the cold season, with 10 cm of polystyrene 1,422 kWh, with 15 cm 1,108 kWh, and with 15 cm and the plinth insulated down to 1 m 1,008 kWh.
Does the ground under the house matter?
Yes, because soils conduct heat differently. EN ISO 13370 gives λ 1.5 for clay and silt, 2.0 for sand and gravel and 3.5 for rock. For the uninsulated slab of a 10 × 12 m house, the U-value with the ground is 0.491 on clay, 0.615 on sand and 0.921 on rock; with 15 cm of EPS the differences shrink: 0.149, 0.161 and 0.179.
How the calculation works
The floor is calculated two ways. The floor U-value, as for any element (EN ISO 6946), is compared with Mc 001-2022 and the Passive House Institute. The U-value with the ground, per EN ISO 13370, accounts for the soil and the size of the house and gives the heat lost.
Floor U-value
Each layer's resistance is its thickness divided by λ. Rsi 0.17 is added (heat flows down); towards the ground there is no outer surface.
Source: EN ISO 6946:2017
Characteristic dimension
B′ = A / (0.5·P), from the slab area and the exposed perimeter. A 10 × 12 m house has B′ ≈ 5.5 m.
Source: EN ISO 13370:2017
Equivalent thickness
dt = w + λsoil·(Rsi + Rf + Rse), with the wall thickness w, the floor resistance Rf and Rse 0.04.
Source: EN ISO 13370:2017
U-value with the ground
If dt < B′: U = 2λ/(πB′ + dt)·ln(πB′/dt + 1); otherwise U = λ/(0.457B′ + dt).
Source: EN ISO 13370:2017
Plinth insulation
Vertical insulation of depth D adds Δψ = −(λ/π)·[ln(2D/dt + 1) − ln(2D/(dt + d′) + 1)], and U′ = U + 2Δψ/B′.
Source: EN ISO 13370:2017, annex B
The whole year
The U-value with the ground times the area and the degree-days of the city's PVGIS typical meteorological year (days averaging 12 °C or less, heated to 20 °C).
Source: PVGIS
Checks
| Check | Reference | Calculator |
|---|---|---|
| EN ISO 13370, uninsulated slab, B′ 4 m, clay, w 0.30 m (hand calculation with the standard's formula) | 0.698 | 0.698 |
Limitations
- Steady-state calculation; the thermal mass of the ground shifts losses between months.
- The thermal bridge at the wall–slab junction is not included.
- Houses with a basement or a suspended floor over a ventilated void use other formulas in EN ISO 13370.
- λ is the declared dry value; ground water is not calculated.
- Mc 001-2022 and Passive House values are shown as references, not as a verdict on the floor.
Sources
- EN ISO 13370:2017, heat transfer via the ground
- EN ISO 6946:2017, thermal resistance
- Passive House Institute, criteria for opaque construction systems (fPHI 0.6 for ground contact)
- Mc 001-2022, minimum thermal resistances (summary, Romanian)
- Order 2641/2017, table 1 (Romanian)
- Austrotherm Romania, EPS data (Romanian)
- PVGIS, climate of the cities
Calculation and text: SEMPER SUPRA INITIATIVE S.R.L. · Climate data: PVGIS © European Union, 2001–2026.