I have double glazing (optimized for sound protection) on a 2m-by-2m window. What would be the effect (sound insulation and thermal insulation) of an additional layer of glazing about 16cm away from it? (That is: we are talking about secondary glazing on the other side of a windowsill created by insulation.)

The effect on thermal insulation should be easy to figure out, though I've yet to find a table that gives the R of a vertical air layer between transparent non-conducting surfaces for this thickness.