When I lived at home, even nearer Heathrow that I do now (!!) we had secondary glazing put in, paid for by the airport when they opened up a new terminal. It's easily got 9" between the glass but it was a complete 'inner' set of windows rather than double glazing. It was brilliantly effective. Looks very dated now but dad and most of the people in the area don't want it replaced by double glazing because the secondary glazing is so much better.