To calculate thermal bridging in purlins and rafters, use the following formula to calculate thermal resistance of the metal section:
Where:
Rs = thermal resistance of purlin or girt (m2.K/W)
D= flange depth (m)
B= flange width (m)
t= flange thickness (m)
km= thermal conductivity of metal, 47.5 W/m.k in case of steel
R1, R2= contact resistances between the steel element and the upper and lower side, 0 m2.K/W in case of full contact, 0.03 m2.K/W in case of loose contact
R3= thermal resistance of air gap (if available) on the upper side or lower side of the steel element, 0.17 m2.K/W in case of 20mm non-reflective air gap
Knowing the thermal resistance of the insulation (Ri) that is added between purlins or rafters, and the fraction of insulation (fi) and metal section (fs), Total R-value of the roof (Rtotal) could be calculated using the formula below:
The negative effect of thermal bridging on the added insulation value can be calculated by subtracting the Total R-value from the insulation R-value.