Fiberglass Replacement Windows
Fiberglass Replacement windows are relatively new and not yet widely available. They have the highest R-values of all frames; thus, they are excellent for insulating and will not warp, shrink, swell, rot, or corrode. Fiberglass replacement window frames can be made in a variety of colors and can hold large expanses of glass. Some fiberglass window frames are hollow; others are filled with various forms of insulation.
Fiberglass replacement windows can also be painted which provides some flexibility if you wish to change the color of your new replacement windows in the future.
Windows made of fiberglass however will be more expensive than replacement windows made of aluminum or vinyl. In addition, as they are not yet widely available on the market the array of sizes will be limited.