What is the Rc value?
The Rc value is the thermal resistance of an entire construction expressed in m²K/W: the sum of all material layers plus air layers, finishes and structural members, corrected for cold bridging among other things. The higher the Rc value, the better the construction insulates. The term is Dutch, and you will meet it on product data sheets and drawings coming out of the Netherlands and Belgium.
Rd, Rc and U compared
These three figures get mixed up constantly, but they measure different things.
| Quantity | Unit | What it describes | Direction |
|---|---|---|---|
| Rd | m²K/W | Thermal resistance of a single material layer: Rd = d / lambda, thickness divided by thermal conductivity. This is the figure printed on the insulation packaging. | Higher is better |
| Rc | m²K/W | Thermal resistance of the whole construction, all layers together and corrected. | Higher is better |
| U | W/m²K | Thermal transmittance including surface resistances: U = 1 / (Rsi + Rc + Rse). Used for windows, doors and frames. | Lower is better |
So a board rated Rd 3,5 never by itself produces a wall at Rc 3,5. The rest of the build-up and the cold bridges count too.
How this maps onto UK practice
In the UK the same physics goes by the plain name R-value, and a whole build-up is described by its total R-value. The difference is regulatory rather than physical: the Building Regulations set targets in terms of U-values, not in terms of a minimum whole-construction R-value. That makes the U-value the number a UK designer quotes and the number a building control body checks.
Practically, if you receive a Dutch specification quoting an Rc value:
- Read it as a total R-value for the build-up, not as the rating of one insulation layer.
- Convert to a U-value before comparing with a UK target, using U = 1 / (Rsi + Rc + Rse). You need the surface resistances, which depend on the element and the direction of heat flow.
- Check what the correction covers. A Dutch Rc value already accounts for cold bridging through fixings and structural members, so it is not the same as adding up layer resistances on paper.
The Dutch requirements as an illustration
For context, the Dutch new build requirements sit in the Besluit bouwwerken leefomgeving (Bbl), article 4.152 with table 4.148B, and are determined to NTA 8800. These apply in the Netherlands only.
| Element | Average Rc |
|---|---|
| External wall | 4,7 m²K/W |
| Roof | 6,3 m²K/W |
| Floor over crawl space, on ground or on water | 3,7 m²K/W |
A minimum of 2,6 m²K/W applies to each individual part of a separating construction on top of that. Windows, doors and frames are governed by U-values instead (Bbl article 4.153): 2,2 W/m²K at most individually, and 1,65 W/m²K at most as an area weighted average. These are the values in force as of September 2026.
Related terms
- R-value
- U-value
- Lambda value
- Cold bridge
- Thermal transmittance
- Insulation
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