- Project plans
- Project activities
- Legislation and standards
- Industry context
Last edited 16 Feb 2018
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.
 Find out more
 Related articles on Designing Buildings Wiki
Featured articles and news
When is there a right to light, and what happens if it is obstructed?
What would the nationalisation of economic infrastructure mean for GB?
A new guide to improving value by reducing design error.
We've reached 80,000 page views a day and 10,000 registered users. Why not join them?
A masterplan is a framework within which a location is encouraged to develop or change. Read our introductory article.
New consultation announced on a specialist Housing Court to settle landlord-tenant disputes.
ICE responds to a transport consultation advising the government to make decisions enabling more inclusive cities.
BRE and Loughborough University complete first phase refurbishment of demonstration home.
How the risk of collapse of fibrous plaster ceilings is being addressed in theatres.
If you’re a great writer and have practical experience of the construction industry, it could be you.
Frustrated by long documents or technical jargon? Put off by sign-up forms or costs? Take this 5 min survey to help improve construction knowledge.