Fiberglass windows have several advantages over wood, clad-wood and vinyl windows, yet remain a niche product with only around 2.6 percent of the replacement window market, says Remodeling magazine's Jim Cory. Homeowner unfamiliarity is one reason, price another.
Cory reports that "contractors who install fiberglass say that cost alone makes it unlikely that fiberglass will rival wood and vinyl in market share." That price difference has grown quite small compared to clad-wood windows -- and sometimes is on a par or even less, especially with entry-level products. Compared to vinyl replacement windows, however, expect to pay a premium of as much as 30 percent to 50 percent, depending on window quality and options. Still, popularity is expected to increase and fiberglass windows to become more mainstream in the future.
In the case of a vintage home, where the aesthetics of wood are important, fiberglass will likely not be a satisfactory choice. But do the advantages of fiberglass justify their higher price tag compared to vinyl replacement windows? Most experts say that if your budget allows it, the answer is a definite yes.
Fiberglass replacement windows are durable and need no maintenance. They're resistant to temperature extremes and moisture, and they don't rust or splinter. Lee Wallender, home renovations expert at About.com, cites a 2007 case study that found that Fiberglass windows had an estimated 38 percent longer lifespan than vinyl.
Fiberglass windows do an excellent job in holding their structural integrity over time. Columnist and home-renovation and maintenance expert Jim Dulley notes that because fiberglass largely consists of spun glass fibers, it reacts to temperature changes much the same way as the glass panes in the window. "This makes the entire window system and weatherstripping more airtight," Dulley writes. "Internal stresses are reduced because all the components are expanding and contracting at about the same rate." That's borne out in testing; According to ConsumerReports.org, fiberglass (and clad-wood) windows are better than vinyl at preventing air leakage. Fiberglass's strength and structural stability make it especially suitable as a framing material for large openings and for triple-paned windows.
Foam-filled fiberglass has the highest insulation value of any window frame material. However, even hollow-core fiberglass frames insulate well enough in most locations. Foam-filled cores "may be a plus in extremely cold climates, but the type of glass you select is most important for energy savings and comfort," Dulley says.
Fiberglass is a paintable product. That's a good thing because factory color choices tend to be limited. Some manufacturers offer unpainted fiberglass windows. A stainable wood or faux-wood veneer on the inside for those who want the look of wood for their room is also available on some lines. Because the color sits on the surface and not below, scratches can show, however most say that fiberglass has good resistance to dents and scratches.
The American Architectural Manufacturers Association labels fiberglass as "an environmentally-friendly material that's built to last." Like wood replacement windows, fiberglass replacement windows are made from a sustainable resource -- in this case sand. Cynthia Phakos writing at the GreenHomeGuide.com notes that fiberglass has "low embodied energy" -- the energy used to create the product. She notes that the durable nature of fiberglass makes recycling a challenge, but that it is "safe to handle and dispose of."
You can generally get all of the same options as other replacement windows, including fancy grills and inserts, high-efficiency glass, and upscale window hardware -- at a price, of course. At one time, relatively few makers offered fiberglass windows, but now many well-known brands -- such as Milgard, Marvin, and Pella -- offer fiberglass as either a new construction or replacement product, and quite a few lesser-known window manufacturers are beginning to do the same. Windows are available in stock sizes, of course, and a few companies will custom make windows to fit any space.