R-Value Calculation

### 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.

Table 1: Calculated R-value
No
Item
RSI (m²K/W)
Source
1
External air film
0.040
AS/NZ 4859 APP.K (K5)
2
Metal Deck
0.000
Manufacturer’s data
3
Mineral Wool
1.389
Manufacturer’s data for uncompressed wool
4
Envirotuff V Foil WF302B
0.000
Thermal resistance is neglected
5
Reflective air space (35mm)
0.994
ASTM STP 1116
6
Underliner
0.000
Manufacturer’s data
7
Internal air film (Low Emittance)
0.160
AS/NZ 4859 APP.K (Table K1)
Total RSI (m²K/W)
2.583
Total U-value (W/m²K)
0.387

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