Window Library U-factor and SHGC Results

Last Updated: 
Thursday, September 14, 2017

Software/Versions Affected:

Algorithm Changes between WINDOW 4.1 and 5.0

WINDOW5’s calculations for U-values, SHGCs, and visible transmittances will be a little different than those from WINDOW 4.1. These small differences are due to the fact that WINDOW5’s algorithms have been updated to reflect the best known practices for determining window thermal and optical properties using computer simulations. These improvements also make WINDOW5 consistent with the latest ISO and ASHRAE standards (currently in draft form) on this subject. These standards are the results of many years of work by the international community and incorporate several improvements to WINDOW 4.1’s algorithms developed a decade ago.

Specific algorithms which have changed which influence U-factor calculations:

  • gas properties and gas mixtures have been updated
  • an improved gap heat transfer correlation is used
  • an improved interior film coefficient results in noticeable changes for single glazed windows with changes becoming less significant the more insulating the glazing system

SHGC and Visible Transmittance calculations are impacted by the above changes. They can also be significantly impacted by the use of a different (global not direct) solar spectral irradiance function, as recently approved by ISO TC160. This means the sun is represented by direct and diffuse solar radiation not just direct radiation. For glazings which transmit evenly across the solar spectrum (clear glass) this will not lead to a noticeable change. For glazings which transmit selectively in the solar spectrum (such as green glass or some low-e coatings), solar and/or visible properties may change by up to 0.04 or so.