Fiberglass Replacement Windows
Fiberglass windows are gaining market share and becoming more accepted by the buying public. Initial concerns over strength, resistance to extreme temperatures, and frame construction have gradually been overcome. These windows are constructed out of fiberglass cloth and epoxy resins, similar to modern boat construction techniques. This results in a smooth, strong finish that can be painted if the home owner eventually decides they wish to do so. With vinyl, generally speaking, the windows should not be painted and once installed your stuck with that color.
There are a few models available that is essentially a wood window that is clad with fiberglass, which could be a viable option for someone who really wanted the properties of fiberglass but wanted to retain a tradition wood look on the interior.
Another factor to consider is fiberglass will lend itself to larger openings due to greater strength and stiffness properties than vinyl. For standard sized opening this will not be a factor but if you have a particular situation where the opening is a good deal larger, fiberglass could provide a better application. Vinyl has good strength, and again with standard sizes will provide more than you need but as the size of a vinyl window grows eventually you do encounter concerns over stiffness.