Changing out all the windows is one of the most expensive renovations in a home. Yet this is what "green renovators" tell homeowners all the time. Sometimes it can be a waste of time, money and space at the landfill.
A Low e and argon window is a great choice for a new home. It has an r-value of 3.22 and an average-sized one costs about $700-$1000+ installed. But for renovations, even in a home with electric heat you probably aren't going to have the home as long as it takes to pay off this window in saved heat.
In a cold climate, if your windows are older wood or aluminum sliders and are in good shape one of the best insulators is the plastic sheet that you can attach on the inside.
Here's why. Plastic will not conduct cold in or heat out and it will seal any drafts from the outside. Also, any thermal convection between panes will not be transfered. In other words the currents between the glass panes that draw the heat from the home are stopped by the plastic sheet.
In addition, when the sheet is put on with 2-way tape and tightened with a hair dryer it takes a keen eye to tell that there is anything over the window.
The process takes about 6 hours in the fall to do and in the spring you take them down and roll them onto Christmas wrapping paper rolls for storage.