- Project plans
- Project activities
- Legislation and standards
- Industry context
Last edited 21 Nov 2018
The thermal resistance of a material is a measure of how resistant it is to the transfer of heat across it. The thermal resistance of a material is measured by its R-value. The higher the R-value of a material, the more effective it is as an insulator.
Thermal resistance is the temperature difference between two defined material surfaces that induces a unit heat flow rate through a unit area. It is often described as the reciprocal of thermal transmittance, and can be derived from the thermal conductivity and the thickness of the materials.
R-values can be calculated by dividing the thickness of a material (in metres) by its thermal conductivity (k-value or lambda value (λ) in W/mK). 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.
R-values are not directly relatable to U-values however, as unlike U-values, R-values do not include surface heat transfers at the boundary of the element by convection and radiation, they are only a measure of the thermal resistance of the materials themselves.
 Related articles on Designing Buildings Wiki
Featured articles and news
Smart mapping approaches for building better infrastructure.
The importance of emergency planning.
Eight forms of resource optimisation.
CIOB responds to Chancellor Sunak's announcement on jobs and the economy.
Revised guide to competition rules available.
Brick slip soffit systems and intricate brick features.
An innovative engineering approach could have had tragic consequence for NYC.
Some secrets behind how canals work.
Breaking down possible steps of pre-contract management.
ICE event includes comments from Welsh Government Minister Julie James.
Designing Buildings Wiki becomes the world's first website to adopt the new knowledge standard.