The k-value, or heat transfer coefficient, is the measured value of the heat flow which is transferred through an area of 1 m² at a temperature difference of 1 K. The units of measure are watts per square meter per temperature difference (W/m²K). K-value = energy / (area x temperature difference x time).

This value is used to measure the Heat Insulation Efficiency of Buildings, and in the case of Passive Buildings, all components must have a k-value of at least 0.16 W/m²K to be considered energy efficient. In comparison, a wall constructed of typical honeycomb brick 36.5 cm thick has a k-value of 0.45 W/m²K, meaning in order to reach a k-value of 0.16 W/m²K consistent with a passive building, the wall would have to be 1.10 meters thick.

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