Single pane vs Double pane geometry


Since I started using radiance I have always modeled a single plane to
represent a window pane (single or double). The RAD material of course would
give the plane the correct VLT.

However, when I model two planes the light is decreased because the first
pane lets in X% (vlt) and then the second pane further reduces it by X%

I would like a material that takes into account that there are two planes
physically modeled. Reason being I am importing a significant amount of
windows from a revit model consuming of many family objects all containing
windows with physically two planes of geometry glazing.

I don't know all the tricks to writing material definitions, so I ask is
there a material function that will take into account the extra plane of
geometry and still give me the correct VLT? Or is it just a simple
mathematical formula that will equal the VLT I need. (e.g. 85% + 85% =

Thank you in advance.