Modern coated glass needs to be simulated spectrally

Almost all new window glass is metal coated, and these coatings have very different properties in different wavelengths. For example, they transmit visible wavelengths much better than infrared wavelengths. If the window/curtain wall element has more than one pane it is very important to calculate the optical properties for each wavelength, otherwise the energy transmittance will be underestimated. The reason for this is that each pane will reduce wavelengths independent of each other and the same light (mostly infrared) will be reduced more than once.

The good news is that most glass manufacturers can provide fully spectral measurements and that there are a number of simulation tools that can use these measurements. Two examples of such tools are IDA ICE and ESBO. Both tools use fully spectral data, not just for reference calculations according to ISO 52022-3 and 15099, but dynamically in each timestep of the simulation. If you are interested in evaluating modern coated glass you can do this for free using #ESBO that can be downloaded from #ES-SO, the European Solar-Shading Organization.

In the image above I have simulated the same glass combination fully spectral and non-spectral properties according to ISO 52022-3. As you can see the difference in direct solar transmittance is huge. Other properties like U-value and light transmittance are hardly affected at all. The example is a highly spectral glass.