- Project plans
- Project activities
- Legislation and standards
- Industry context
Last edited 19 Feb 2019
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 sq. m of a material, measured in kJ/m2K. This is used to quantify the thermal mass of building elements such as walls and floors.
 Related articles on Designing Buildings Wiki
Featured articles and news
Common difficulties when improving the management of building services.
England’s railway heritage from the air. Book review.
Identifying interim heritage areas for a neighbourhood plan.
Inspecting and reporting on moisture-related problems.
Will Norway build the world's first floating tunnel?
Domestic Retrofit training course.
Preparing to sell a commercial property.
Local Plan Route Mapper and toolkit.
Thermal mass in buildings.
CIAT's AT Academy.
The UK's most dangerous industries to work in.