What is R-value?
R-value represents resistance to heat flow. The greater the R-value , the greater the resistance.
The R-value applies to all modes of heat transfer: conduction, convection, and radiation. It can be determined for a single material like an insulation or a combination of materials.
The resistance represented by R-value is valid for heat flow into a building and for heat flow out of a building.
Example Calculation of R-value
R-value has been calculated for Client at Bandar Serendah, Selangor for roof assembly (Diagram 1) using data and method from AS/NZ 4859 and ASTM STP 1116. The boundary conditions selected for the calculation are 40°C for the outside air temperature and 25°C for the interior air temperature. The roof slope is 5° from the horizontal thus requiring correlations for the heat-transfer coefficients as a function of the angle of inclination.
The thermal resistance values used to calculate R-value are listed in Table 1 along with the R-values obtained for the project considered.
External air film
AS/NZ 4859 APP.K (K5)
Manufacturer’s data for uncompressed wool
Envirotuff V Foil WF302B
Thermal resistance is neglected
Reflective air space (35mm)
ASTM STP 1116
Internal air film (Low Emittance)
AS/NZ 4859 APP.K (Table K1)
Total RSI (m²K/W)
Total U-value (W/m²K)
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