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Fiberglass Windows

We ask a lot of our windows. We expect them to usher in light and fresh air when open, and to keep out the wet, the cold, and the heat when closed. And we expect them to function well for decades. Wood has been doing the job for centuries, but even with a low-maintenance exterior cladding, it swells and shrinks with temperature swings, undermining longevity. Vinyl windows do away with upkeep issues, but the floppy material must be made into chunky profiles that reduce the amount of glass, and it loses resiliency as it gets older, not a hallmark of durability.

Fiberglass doesn’t have these shortcomings. It’s stiffer and lighter than wood, as low-maintenance as vinyl, and unaffected by water or temperature fluctuations. While early fiberglass models had some limitations, improvements in manufacturing have resolved the issues of the past.

“Fifteen years ago, fiberglass windows were available only in limited sizes, turned chalky from sun exposure, and came in just one color—white,” says Matt Risinger, a builder based in Austin, Texas, with more than 20 years’ experience. Today you get custom sizes, durable UV-blocking coatings, a wider color selection, and even the option of a wood interior. “With all these windows have to offer, at such a reasonable price, they’re hard to beat,” Risinger says.

Shown: These traditional-looking fiberglass windows feature wood interiors that have been painted to match the rest of this white kitchen.

Integrity Wood-Ultrex Double Hungs, Marvin

Continue for a look at how fiberglass windows are made and the ways they can light up a house, inside and out.


How much do they cost? On average, fiberglass windows run about 25 percent more than vinyl and about half the price of aluminum-clad wood. Expect to pay about $300 for a basic 3-by-5-foot fiberglass unit.

DIY or hire a pro? They’re available for new construction as well as replacements. Some manufacturers sell only through approved installers. Others will sell directly to homeowners or through big-box stores. Hiring a pro is recommended to ensure long-term, leak-free performance.

How long do they last? Warranties against construction defects range from 10 years to “lifetime”- or for as long as you own your house. Insulating glass typically carries a 20-year warranty against seal failure.

How much care? Not much, other than cleaning the glass and replacing the weatherstripping every decade or so. If the factory-applied coating gets scratched, you can touch it up with a 100 percent acrylic paint.

How They’re Made

Illustration by Ian Worpole

Most parts for a fiberglass window are fabricated by pultrusion. In this automated process, lengths of fiberglass roving and strand mat are bathed in a resin, covered with a fiberglass veil, and pulled into a heated die that hardens the resin. (Separate dies are used for each window part.) The smooth, rigid lineal that emerges from the die is then typically cut to length, coated, and fitted with hidden nylon-reinforced corner blocks. When screwed and glued together, lineals and blocks form tight, clean, nearly indestructible joints.

Are Fiberglass Windows Right for You?

The answer is yes, if you’re looking for a window that offers high performance and exceptional durability at an affordable price


  • Not bothered by water Because fiberglass is inert to moisture, there’s no concern about rot, corrosion, mold, or shrinking and swelling.
  • Temperature stable Neither heat nor cold will induce fiberglass to flex, sag, or change dimension. That reduces the chance of leaks around window perimeters.
  • More glass, less frame The stiffness and strength of fiber-reinforced resins means that window frames and sashes can be narrower and less bulky without compromising their ability to resist high winds.


  • Fewer options Compared with wood, fiberglass is more difficult to customize into unique shapes and profiles, and has fewer color and hardware options to choose from.
  • Harder to find Only a handful of companies make these windows, and not all of them distribute nationwide.
  • Sensitive to UV Fiberglass resin becomes chalky when exposed to UV rays. A long-lasting, factory-applied coating on the exterior is a must; the thicker, the better.

Shown: These insert windows are installed from the inside, so the existing wood trim can remain in place.

Fiberglass Three Ways: All Fiberglass

Choose the look that works best with your decor and budget

They resemble painted wood inside and out. For about 10 percent more, Pella can apply different colors on the interior and the exterior. Marvin’s optional Everwood interior finish (shown) which looks like wood when stained, adds 15 to 20 percent to the price.

Fiberglass Three Ways: Wood Interior

The sills, sashes, and frames are fiberglass, but the interior surfaces are covered with either wood veneer or solid wood (shown). Compared with all-fiberglass models, the ones with solid-wood interiors command about a 15 percent premium.

Fiberglass Three Ways: Hybrid

Only the sash of the Andersen A-series window (shown) is fiberglass, which moves as little as glass. The interior parts are wood, while the exterior frame is a composite of ground wood and vinyl. Its price is comparable to that of a wood window clad in aluminum.

Improving the View

No longer plain-Jane vinyl look-alikes, fiberglass windows have undergone big changes in key areas.

Coatings Factory-applied finishes are much tougher and more UV-resistant than before, whether as a thick acrylic layer applied during the pultrusion process or as a spray-on paint or powder coating added before window assembly.

Colors Once available only in white, fiberglass windows now come in a range of colors, both on the interior and the exterior (right). And if you want a different hue later on, fiberglass can be painted as easily as wood.

Curves The pultrusion process makes only straight pieces, but manufacturers can now mold fiberglass into classic arch-tops (left) and curves.

Combinations Windows no longer have to be grouped together on-site to put together bays, bows, and other combined sets; they can now be delivered as integrated, ready-to-install units.

Energy performance By itself, fiberglass does a fairly good job of keeping out the cold. Filling the frame with foam and adding triple-pane glazing improves performance even more.

The Competition: Vinyl

When compared with other window materials, fiberglass comes up strong.

This extruded plastic makes the least expensive and most commonly installed windows, but it turns brittle with age and exposure to cold. The frames and sashes have thin, flexible walls, can’t be repaired, and don’t take paint well. Joints are heat-welded together, leaving obvious bumps at corners.

The Competition: Clad Wood

These high-end wood windows have an exterior layer of vinyl or metal—usually aluminum—that reduces maintenance and improves longevity. They are a close match to fiberglass in performance, but have a much higher price point.

The Competition: Aluminum

This lightweight, extruded metal is about as rigid and low-maintenance as fiberglass. But it’s a terrible insulator, readily carrying heat to the outside in winter and to the inside in summer. That’s why this type of window is suited only for mild climates.

Do the Numbers

High-performing fiberglass is rigid, stable, and slows heat flow.

Pick Your Style: Wide Open

In addition to their structural virtues, fiberglass windows hold their own in terms of aesthetics and versatility.

These wood-lined casements and transoms demonstrate how narrow frames and sashes can be when they’re made of fiberglass. One more benefit: The sashes can’t warp, as wood ones are prone to do.

Shown: Essence Series; Milgard Windows & Doors

Pick Your Style: Seeing Green

Gray, black, forest green—colored windows are more popular than ever, but dark colors are especially vulnerable to fading. The powder-coat finish on these custom casements and double-hung windows is virtually immune to UV rays.

Shown: Essence Series; Milgard Windows & Doors

Pick Your Style: Wall of Glass

Big windows open up big views, but then they have to resist big wind loads, too. These flanking casements are strong enough to defy winds up to 142 mph, while the picture windows and transoms can withstand 190-mph gusts.

Shown: 300 Series; Fibertec

Pick Your Style: Bathing Beauty

These solid-fiberglass picture windows and transom are immune to the humid air and occasional splashes that come with a bathroom environment. They nicely frame a tub-side view, too.

Shown: Impervia; Pella

Pick Your Style: Throw a Curve

Bow windows add space to a room and funnel in lots of natural light. These five fiberglass casements are assembled into a single faceted curve to form a traditional bow unit.

Shown: Infinity All Ultrex; Marvin

Pick Your Style: Historic Look

Three double-hungs fit perfectly into the original gable openings of this 1897 Queen Anne. The center arch-top was custom-made using a proprietary molding process.

Shown: Integrity Wood-Ultrex; Marvin