Temperatures of exterior surfaces are affected by various heat fluxes. Beside the heat fluxes due to heat conduction through the building component and heat convection from the surrounding air, radiation effects are important as well. Usually only the short-wave radiation heating up the component’s surface during the day is taken into account. The long-wave, thermal emission of the component’s surface is often neglected. Realistic temperature conditions on the component’s surface can not be computed very well in this way. It is shown how the balance of all relevant radiation components can be evaluated which lead to an increase or decrease of the surface temperature. Validation of the WUFI® results by comparison with measurements at the IBP test site demonstrates generally good agreement between measurement and calculation. Hygrothermal performance on exterior surfaces depending on radiation effects can now be investigated.