R-value (insulation) - Example (SI Units)

Example (SI Units)

To find the heat loss per square metre, simply divide the temperature difference by the R value.

If the interior of your home is at 20 °C, and the roof cavity is at 10 °C, the temperature difference is 10 °C (= 10 K difference). Assuming a ceiling insulated to R–2 (R = 2.0 m²K/W), energy will be lost at a rate of 10 K / 2 K·m²/W = 5 watts for every square metre of ceiling.