Posted 13 February 2016 - 12:29 PM
As heat loss is going to be slightly greater through a roof light than through a vertical window, then I'd go for better glazing than the standard 4-16-4-16-4 arrangement. All our windows are 4-20-4-20-4, argon filled, with double low e coating, one of the inner face of the outer pane, one on the inner face of the middle pane (the normal spec). Probably also worth looking at using glass with a lower overall IR transmission, too, to allow light in but limit solar gain.
Unless the glass is likely to be subjected to any unusual impact loads I can't see the need to go for 6mm, or toughened glass, unless it's in a location where the regs require it. PV panels seem to do just fine with a single layer of non-toughened low iron 4mm glass.