Rania,
Someone with some more knowledge might correct me here but I am pretty sure that the simple glass material in Radiance does not distinguish between reflected and absorbed portions of light (I believe the assumption is that all light that does not pass through is reflected). The Trans Material component should help you if you need to account for tansmissivity and reflectivity values that do not add to 1. Something like this might fit your situation:
I would check what the material looks like in a radiance rendering just in case. Also, if you are only looking at glass just from an outdoor condition, I usually use a simple mirror material to capture the general specular properties of the glass that I am interested in since there’s no need to go through the extra work of simulating light on the interior of the building.
I hope that this helps,
-Chris