It's simple, but often ignored advice: Fix your drafty windows. You're paying for heat. Do you want to see your money drift out the window? I don't think so. I'll explain the easiest and cheapest ways to stop drafts. I'm a carpenter and I fix windows all the time. Trust me, it's easy. Read on.
Fixing Basic Air Leaks
When a window is installed, the window opening needs to be a bit larger than the window itself. This allows the carpenter to simply place the window in the "RO" (rough opening) and secure it plumb and level with shims. In many older homes trim was then installed with this air gap unsealed. Later, as people became more energy conscious, insulation was stuffed into these cracks and openings. While more effective than nothing, complete air sealing was never achieved. In modern building (and as a method to seal older windows) expanding spray foam is now widely used to seal these gaps. I would have to put "Great Stuff" (spray foam insulation) on the top ten list of awesome home improvement technology.
The most common trick to sealing older window openings is to pop off the interior trim and inject the spray foam into the gaps between the window jam and the framing. This is easy with the handy spray tube that comes with this miracle product. In addition to sealing this gap, caulking the exterior and interior trim provides even more resistance to air loss. In the case of exterior caulking, added protection from water intrusion is also achieved.
The second type of air leaks that windows have is due to the seals themselves in the operable parts of the window. Here older windows often had simple wood components and the fit was never very tight. Adding foam weather strip here is pretty easy. Storm windows, adhesive weather striping and the heat shrink window films also help. These are normally installed on a seasonal basis and provide an additional barrier to the elements.
Dealing with Heat Conduction and Convection
Other thermal loses occur via non-leakage channels. Heat literally can get conducted out of the house. In the case of windows, this is often via the window's frame. One of the most conductive types is an aluminum-framed window. These were very common in the 60's and 70's and I replaced a number of these in my own home when I performed a remodel some years back. My older aluminum windows were also single pane, which allowed the glass itself to be a conductive channel. Wood and vinyl are much better at preventing these conductive heat losses.
Modern windows are usually double or even triple paned with an insulating chamber of argon nicely sealed between the panes. This property of modern windows contributes to a window's U-value - a measurement of heat loss and area. It is a bit different than that of R-values used for insulation. With U-values, a low number is better.
Sitting next to a large cool window you can actually feel the "chill" as the colder air descends past you. That's convection. Common fixes for convective losses also involve heat shrink films, and thermal blinds. These remedies tend to keep these air movements form occurring.
Replacing old drafty windows is one way to improve the heating and cooling budget of your home. But this improvement often comes at a high cost. Air sealing with spray foam and caulk, and insulating with shrink film or storm windows, can also improve your situation at a much lower cost.