in energy savings(that from this locsl biz).http://www.evergreenyourhome.com/news/2014/07/03/skinny-windows

More than admirable(which is that too), saving old windows can be the greener alternative, and very thrifty.

But every case is different. Your rotted windows sound newer than your house in Onset. If so, 30 years is great service for a 'new' window. I wonder if they replaced the better originals? There have been great advances in coatings but energy performance of windows hasn't rocketed to what marketers will have you believe.

Double hung windows with weights are tougher to make energy efficient. The above house as well as mine, below, were quite cheaply built in the 1850's, and have single hung-no sash weights, easier to make efficient.

My trick is to make the seal on the outside with a storm window(weatherstripped 4 sides-air lock). Other than that, good old oil based window glazing-one application can last as long as new windows. Other pluses are, with a larger air space-they're quieter than new double glazed. A nice feature if you're next to traffic(as we do). And old windows have a 'depth' feeling from within a home, that new windows can't compare to.

My old house is two houses;1850 forward, 2000 aft(new design/construction). Forward are all original windows refit with storms - aft all new windows. I'll probably live long enough to need to replace the aft ones, forward, another 100?. Crazy!

For background, my old house was an abandoned 4 family apartment house when I bought it-1999. The previous house above(and across the street-short commute!) that I'm restoring, has been uninhabited for 30 years. Now reinvented and state of the art energy efficient.

http://www.evergreenyourhome.com/news/2014/07/03/skinny-windows