- 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.
 Related articles on Designing Buildings Wiki
Featured articles and news
HAB is a bridge design concept which incorporates an integrated hydraulic system in order to carry more weight.
ICE publish a discussion paper looking at the role of the engineer in creating inclusive cities.
A PQP describes the activities, standards, tools and processes necessary to achieve quality in a project's delivery.
How Lidl has been actively working to reinforce their brand through sustainability.
Association of British Insurers describe full-scale cladding tests as 'utterly inadequate'.
This article examines the changing policy commitments and evolving definitions of the zero carbon home.
Researchers believe they may have created a 'game-changing' new form of concrete using graphene.
Grouting refers to the injection of materials into a soil or rock formation to change its physical characteristics.
Part of Designing Buildings Wiki, BREEAM Wiki will advance knowledge sharing for the BRE family of sustainability tools.
From the decorative to the utilitarian, and from the photographed to the forgotten.
New BRE book considers the progression from project-based knowledge creation to whole-life urban knowledge management.
This CIOB article explores the concept of value in building design and construction.