What is surface resistance?
Surface resistance is the thermal resistance of the thin, almost still layer of air directly against a building element. Heat has to pass from the air into the material, then through the material, and out again on the other side. The first and last steps are the surface resistances; the resistance of the material itself is the thermal resistance R.
There are two: Rsi at the internal surface (s = surface, i = interior) and Rse at the external surface.
Standard values
Under ISO 6946, building physics calculations use fixed design values, in m²·K/W:
| Situation | Rsi (internal) | Rse (external) |
|---|---|---|
| Horizontal heat flow (wall) | 0.13 | 0.04 |
| Upward heat flow (roof, ceiling) | 0.10 | 0.04 |
| Downward heat flow (floor) | 0.17 | 0.04 |
The internal value is always higher than the external one. Indoor air is almost still and therefore insulates in its own right, while outdoor air moves along the facade and carries the heat away immediately.
Why it matters
- The U-value. The thermal transmittance U is the reciprocal of the total resistance, including Rsi and Rse. Leave the surface resistances out and the U-value comes out too high.
- In well-insulated elements the effect is small. In a wall with R = 4.5 m²·K/W, the two surface resistances add 0.17 — a few per cent. In single glazing, with an R of roughly 0.004, they are nearly the whole resistance: that is why single glazing insulates at all, despite the high conductivity of glass.
- For thermal bridges and surface condensation a less favourable Rsi is used (0.25 m²·K/W in the corner of a room, where air movement is even lower), because that is where mould risk is assessed.
In practice
Furniture against a cold external wall, closed curtains in front of a window and thick carpet all raise the internal surface resistance locally. The surface behind them becomes colder — exactly the places where mould patches appear in older houses. A few centimetres of clearance behind a cupboard often solves it.
Related terms
- Thermal resistance
- U-value
- Thermal bridge
- Glazing
- Airtightness
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