Thermal Resistance is the property of a material that indicates its ability to resist heat flow through it.

The heat flow through the building envelop depends on the conductivity and the thickness of the materials used. It also depends on the temperature difference as an external factor.

The thermal resistance is proportional to the thickness of a layer of the construction and inversely proportional to its conductivity. A greater thickness means less heat flow and lower conductivity. Materials with high thermal resistance are used as insulation in construction and building industry.

## Units and Measures

Thermal resistance is measured as R-value which is the ratio of the temperature difference across a material of a specified thickness and the heat flow per unit area through it.

Following are the units for measuring thermal resistance:

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

The reciprocal of R-value is U-value.

## Useful References

- http://www.learn.londonmet.ac.uk/packages/clear/thermal/buildings/building_fabric/properties/resistance.html
- http://en.wikipedia.org/wiki/Thermal_conductivity

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