My sister had a bunch of work done on her house a couple of weeks ago, and one of the things she did was to have the weights to her old sash windows cut, something called “sash springs” installed, and the cavities filled with blown insulation. Having just filled our kitchen window cavities with foam and insulation, I can testify to the results of this operation, and also guess that I’m losing hundreds of dollars in heating bills through our windows.
“The window things are called sash springs; they’re essentially thin pieces of sheet metal bent at about a 45-degree or so angle; the two ends have screw holes and get mounted to the sash slot, and the angle hits the window sash. The friction/pressure is what keeps the window up. The thing I’ve found is that it makes the windows kinda hard to operate–I have to gunk them up with silicone spray when I do the storms this winter.”
This might be them: They’re advertised as spring holders, and they list for $1.99 each.
I found this link to the primary manufacturer of the spring balances, Pullman, and it appears I can order them direct from the factory for $23. I think I’m going to order a pair after weighing one of my sashes and see how it works.