Hong Life CalculatorsUnderstanding the U-value calculator — concept and principle
The U-value (thermal transmittance) tells you how many watts of heat pass through one square metre of a wall, roof or floor for every kelvin of temperature difference between inside and outside. This calculator builds the U-value of a multi-layer construction from each layer's thickness and conductivity to check insulation and compare build-ups.
How it is calculated
U = 1 / R_total, with R_total = R_si + Σ(dᵢ / kᵢ) + R_se.
- Layer thicknesses (mm, comma-separated) are converted to metres.
- Conductivities kᵢ (W/m·K) are listed in the same order; the number of entries must match the thicknesses.
- R_si and R_se are the inside and outside surface resistances (m²K/W). The defaults 0.13 and 0.04 are the ISO 6946 conventional values for horizontal heat flow through walls.
Each layer contributes R = d/k, and the result is shown both as U (W/m²K) and as the total R.
The principle
Heat leaving a room must pass in series through the still air film on the inside surface, every solid layer, and the outside air film. In series, thermal resistances add just like electrical resistances, so the total resistance is the simple sum. The surface resistances represent combined convection and radiation at each face; the outside value is smaller because wind removes heat more effectively.
Worked example
- Layers: 100 mm at 0.77 W/m·K (brick), 150 mm at 0.035 W/m·K (insulation), 12.5 mm at 0.21 W/m·K (plasterboard); R_si = 0.13, R_se = 0.04.
- Layer resistances: 0.1/0.77 ≈ 0.12987, 0.15/0.035 ≈ 4.28571, 0.0125/0.21 ≈ 0.05952 m²K/W.
- R_total = 0.13 + 0.12987 + 4.28571 + 0.05952 + 0.04 ≈ 4.6451 m²K/W.
- U = 1 / 4.6451 ≈ 0.2153 W/m²K.
The insulation provides over 90% of the total resistance.
| Inside surface resistance | U-value (W/m²K) |
|---|---|
| 0.0325 | 0.2198958113 |
| 0.065 | 0.2183354521 |
| 0.0975 | 0.2167970812 |
| 0.13 | 0.2152802371 |
| 0.163 | 0.213784471 |
| 0.195 | 0.2123093467 |
| 0.26 | 0.2094193373 |
How to read the result
Lower U means less heat loss. Heat flow through the element is Q = U × A × ΔT, so 20 m² of this wall with a 20 K difference loses about 0.2153 × 20 × 20 ≈ 86 W. An uninsulated solid masonry wall is often around 1.5–2 W/m²K, while modern insulated walls in cold climates are usually well below 0.3 W/m²K. Always compare against the limit set by your own national or regional code.
Common mistakes
- Listing thicknesses and conductivities in different orders or with a different count.
- Entering metres instead of millimetres.
- Forgetting the surface resistances, or using wall values for roofs and floors (heat flow direction changes R_si).
- Ignoring studs, ties and other thermal bridges, which raise the real U-value.
Frequently asked questions
Why does R_si change for roofs and floors?
Convection depends on heat flow direction. ISO 6946 uses 0.10 for upward and 0.17 for downward heat flow, compared with 0.13 for horizontal flow.
Is U-value the same as R-value?
They are reciprocals: U = 1/R. Note that US R-values are often given in imperial units (ft²·°F·h/BTU), which differ from SI by a factor of about 5.68.
Sources: ISO 6946 — Building components and building elements: Thermal resistance and thermal transmittance · CIBSE Guide A — Environmental Design