For an alternative meaning, see kappa value.
A k-value (sometimes referred to as a k-factor or lambda value λ) is a measure of the thermal conductivity of a material, that is, how easily heat passes across it. It is a fundamental property, independent of the quantity of material. It represents the steady-state heat flow through a unit area of a material resulting from a temperature gradient perpendicular to that unit area. It is expressed in W/mK.
k-values can be used to compare the thermal conductivities of different materials. Typically this is important in assessing the potential for heat transfer between the inside and outside of a building.
The thermal resistance of a specific thickness of a material (its R-value) can be calculated by dividing the thickness of the material (in metres) by its k-value. R-values are therefore expressed in m2K/W (or ft2·°F·hr/Btu in the USA). The overall R-value of a multi-layered element can be calculated by adding the R-values of its component materials.
U-values (sometimes mistakenly thought to be the reciprocal of R-values) describe the thermal conductivity of an entire building element, including its internal and external surfaces. They can be calculated as 1 / (the sum of the resistances of the various layers of the element (its R-values) + the resistance of the internal and external surfaces of the element). U-values are measured in W/m2K.
NB Rather confusingly, in the Standard Assessment Procedure (SAP) and Simplified Building Energy Model (SBEM), used to demonstrate compliance with Part L of the building regulations, k-value (short for Kappa value) refers to the heat capacity per square metre of a material, measured in kJ/m2K. This is used to quantify the thermal mass of building elements such as walls and floors.
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