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.
 Related articles on Designing Buildings Wiki
Featured articles and news
In the week of the momentous Heathrow decision, we look back at the development and design of T5.
BSRIA’s flagship event will address performance and wellbeing beyond compliance.
Young Architects and Developers Alliance launched to build the relationship between the two disciplines.
BS 8536-2:2016 Design and construction: Code of practice for asset management (Linear and geographical infrastructure).
Paying for off-site goods or materials can be useful, but it puts the client at risk.
People power can be transformative if properly informed and inspired.
ZHA win competition to build an Urban Heritage Administration Centre in Saudi Arabia.
Leaps, not steps, are needed to avoid a ticking time bomb, say BRE in response to Farmer Review.
A multi-purpose hall in France covered in a translucent orange membrane.
Winning designs revealed for a rock formation-influenced residential complex in Rennes.
An article explaining the techniques, regulations and environmental impacts of carbon capture and storage.
Watch one of the first documentaries by the acclaimed Adam Curtis, examining the substandard system building of the 1960s.