The R value is a measure of thermal resistance of a material. In the building and construction industry all insulation materials are rated for their performance in restricting heat transfer which is expressed as the R value. The R value represents the scale of efficiency of insulation materials - the higher the R value, the greater the insulating effect.

Under uniform conditions, R value (thermal resistance) is the ratio of the temperature difference across an insulation of a specified thickness and the heat flow per unit area through it. Increasing the thickness of an insulating layer increases the thermal resistance. For example, doubling the thickness of fibreglass batting will double it’s R-value. Products which have the same R value will provide exactly the same insulation, provided they are correctly installed.

Units and Measures

Following are the units for measuring thermal mass:

  • SI Units: square-metre kelvins per watt (m²•K/W) or square-metre degree Celsius per watt (m²•°C/W)
  • Imperial Units: Square feet degree Fahrenheit hour per British thermal unit (ft²•°F•h/Btu)

R-value is usually measured by using the Heat Flow Meter Apparatus and following the process called Steady-State Thermal Transmission Properties. In this test method a specimen placed between a hot plate and a cold plate to measure it’s thermal resistance.

Example Values

Following are metric R-values for common building materials:

Material R-Value
Fiberglass batts R3.1
Rockwool batts R2.5
Polyethylene foam R3.0
Gypsum plasterboard R0.06
Clay roof tiles R0.02
Autoclaved Aerated Concrete Blocks (200mm 350kg/m3) R2.0


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