This paper investigates the heating and cooling energy impact of low thermal emittance values for architectural glazings. The importance of low emittance values and the sensitivity for small changes in the emittance is investigated for three very different climates and for two different types of buildings. Our results imply that minor changes in the parameter values lead to very small changes in the energy performance and are mostly insignificant for the end user. Furthermore, the relative importance of the solar transmitting properties is recognized. For instance, if the thermal emittance is reduced by 2-3% and at the same time the total solar energy transmittance is also reduced by 2-3%, the energy performance of the window is worse if it is used in a south facing position in a residential building in a heating dominated location (Stockholm).