Aren double (or triple) glazed windows more thermally efficient than old fashioned single glazed windows?
Well, maybe. You have to be careful that you are comparing apples to apples. A traditional single glazed movable sash has a lower R-value than double glazed insulated glass BUT that same traditional single glazed sash with a storm window has a greater thermal efficiency than the double glazed insulated glass. This is because the optimum spacing between sheets of glass is 3/4″ to 3″. A well made storm window over a single glazed window will reduce window heat losses by over 50%. To quote B.C.
