I found out when checking the IDF file of a simulation that the EPGlassMat component automatically sets the Back Side Solar Reflectance at Normal Incidence and the Back Side Visible Reflectance at Normal Incidence to 0.
Is there a particular reason for that ?
I’m comparing a base case simulation between a honeybee model and a Design builder model, and after having eliminated most of the potential sources of differences, I came across this difference in the idf files. It seems that Design Builder glass materials are almost always with a back side relectance equals or rougly equals to the front side reflectance. That doesn’t make DB’s way of doing things the right way necesseraly but I was wondering why it was otherwise in HB…
Also, I ran a test simulation for which I modified these backside reflectance values (0.06 and 0.08 for the inner/outer glazing of my double glazing system) and the result was kind of counter intuitive :
When applying a non-null value for these back side reflectance, over all, it was chiller (by 0.1 degree in average) in the test room (unconditionned) than when the values were 0. I was expecting the opposite since having reflectance towards the inside of the zone should tend to increase the solar gains…
The definition of these relfectance from energy plus is pretty poor and I still don’t quite understand there true meaning : https://energyplus.net/sites/all/modules/custom/nrel_custom/pdfs/pdfs_v8.7.0/InputOutputReference.pdf page 158