Although vinyl replacement windows are the most popular type of window, fiberglass replacement windows are becoming more common, experts say. Their cost is comparable to that of clad-wood windows. Like vinyl, fiberglass replacement windows are durable and need no maintenance. Unlike vinyl, fiberglass replacement windows can be painted. They're also resistant to temperature extremes and moisture, and they don't rust or splinter. Fiberglass is strong, making it especially suitable as a framing material for large openings and for triple-paned windows.
Fiberglass has the highest insulation value of any window frame material. In addition, several contractors posting to the forum ContractorTalk.com say that because fiberglass is 60 percent glass, it expands and contracts at the same rate as the windowpane. This means the seal between glass and frame is more likely to remain airtight than with other window materials. According to ConsumerReports.org, fiberglass and clad-wood windows are better than vinyl ones at preventing air leakage.
The main downside to fiberglass windows is that they are not available in as many sizes and shapes as are other types of replacement windows. Major manufacturers include Milgard, Marvin and Pella, as well as smaller firms like Fibertec and Fiberframe Windows.