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Thermal resistance calculation

Thermal resistance of 60×160 timber rafters at 660 mm centres

Build-up: rafters with insulation between them, plasterboard below and roof boarding above

λ_timber = 0.17 W/[mK]

R_timber = 0.16 m/0.17 {W/[mK]} = 0.94 m²K/W

the internal surface resistance (indoor air to material) = 0.13 m²K/W

the external surface resistance (outdoor air to material) = 0,04 m²K/W

R _total =                                                                                                      1,11 m²K/W

For 160 mm of glass wool (if you use a better insulation, please state which insulation, e.g. λ_{plastic foam} = 0.025 W/[mK]) the following applies

R_{glass wool} = 0.16 m/0.04 {W/[mK]} = 4 m²K/W

the internal surface resistance (indoor air to material) = 0.13 m²K/W

the external surface resistance (outdoor air to material) = 0,04 m²K/W

R _total =                                                                                                      4,17 m²K/W

For the construction, 1/R _{total, c(onstruction)} = 0,9/4,17 +0,1/1,11} = 1/3,27 => R_c = 3,27 m²K/W

For plastic foam,

R_plastic = 0.16 m/0.025 {W/[mK]} = 6.4 m²K/W

the internal surface resistance (indoor air to material) = 0.13 m²K/W

the external surface resistance (outdoor air to material) = 0,04 m²K/W

R _total =                                                                                                       6,57 m²K/W

For the construction, 1/R _{total, c} = 0,9/6,57 +0,1/1,11} = 1/4,4 =>                     R_c = 4,4 m²K/W

At the corners these values are even lower. The plasterboard and roof boarding increase them only marginally. This construction therefore does not comply, since

since 2015 the following Rc values have applied to new-build in the Netherlands: – walls Rc >= 4.5 m²K/W – roof Rc >= 6.0 m²K/W.