The k-value is the thermal transmittance of a building element such as a wall, roof or window. It states how much heat passes through the element per square metre and per degree of temperature difference, expressed in W/m²K. The lower the k-value, the less heat is lost and the better the element insulates. In many countries the term k-value has been replaced by the internationally used U-value; it is the same quantity with the same unit.
What it means
The k-value (now U-value) describes the whole element, including all its layers. Single glazing has a high value and insulates poorly; double glazing and especially low-e glass have a much lower value. The k-value is related to the thermal resistance R: where R describes the insulating effect of one layer (higher is better), the k- or U-value describes the heat leaking through the whole build-up (lower is better).
Application
The k- or U-value is used to check whether windows, walls and roofs meet energy requirements and to compare insulation measures. When choosing glass, frames or a wall build-up you aim for the lowest possible value.
Points to note
- Where old documents mention “k-value”, read it as today’s U-value
- A low U-value only pays off if the whole element, including joints, is insulated well
Related terms
- Insulation value
- Cavity wall
- Facade
- Building physics
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