Triple glazing as sound insulation

 

No wonder it was so hot in the hotel room. It turned that as well as the traditional double glazing there was an inner sliding pane. Since the hotel was on the motorway, it must really have been about sound insulation. At the risk of seeming to promote a commercial interest, I did a search and this was one of the first to come up. It suggests that at least 100mm air gap is needed. Well, that would be the gap to the secondary glazing here. It also suggests that panes of different mass would be best. I'd need to look another time. However I've also found a paper on the topic to go away and read which might give an academic account.