Counterintuitive Test Results From Sound Engineers. In the chart below notice that if you installed insulated glass which is made up of two layers of 1/8" thick glass with a 1/4" air space, this would not cut out the sound nearly as well as one layer of 1/4" glass.
Also notice that insulated that is 1-1/2" over all thickness with two layers of 1/4" glass is no better than one layer of 3/8" thick laminated glass, even though two layers of 1/4" glass adds up to 1/2" of glass overall.