Vinyl vs. Fiberglass Windows: A Comparison

Installing New Construction Window
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When shopping for new-construction or replacement windows, you have a choice of several different materials used in the window frames. Wood windows are still available, but windows framed with aluminum, vinyl, fiberglass (or fiberglass composite) are also available.

Increasingly, the choice comes down to vinyl or some variation of fiberglass or fiberglass composite. So how do these materials compare when used for window frames?

Vinyl Windows

Vinyl windows are made from extruded polyvinyl chloride (PVC) as the base material. They may include metal as an internal structural element to stiffen the frames. Vinyl windows first appeared in meaningful numbers in the 1970s, as small window fabricators began offering vinyl-framed windows to meet custom sizes that large window manufacturers could not offer. It was not until the 1990s, however, that vinyl windows really became a popular option. Large companies quickly saw the merit, and today several major window manufacturers, including Owens Corning and CertainTeed offer full lines of vinyl-framed windows.

The principle advantage of vinyl-framed windows is that they are very affordable, costing only about half what wood-framed windows cost. They are also quite energy efficient, due to the fact that they are fabricated using a honeycomb structure that traps air to improve R-value. They can be fabricated to fit any opening size. Vinyl windows never need to be painted, and are available in a limited range of colors.

Fiberglass Windows

Fiberglass is created when polyester resins are activated by a catalyst and then pultruded, or pulled, through a heated die. Strands of glass or glass mats are impregnated with resins. The finished product is called a lineal and it is machinable and can be shaped. Fiberglass has long been used to create ultra-strong, lightweight materials for skis, surfboards, and canoes. 

Fiberglass (and fiberglass-composite windows) began to gain meaningful popularity around 2000. They were originally developed to address certain limitations to vinyl windows. As vinyl ages, it loses its resiliency, a drawback fiberglass does not have. Fiberglass is also stronger than vinyl. Fiberglass windows also can accomplish the same energy-efficiency with smaller frames with a lower profile, making fiberglass frames a more attractive option. And fiberglass can be fabricated in a manner that closely resembles wood—right down to being able to paint it different colors.

More recent fiberglass windows are more accurately described as fiberglass composites since they are fabricated with a mixture of fiberglass and polyester resins. Stronger than wood, and occupying a price range between vinyl and wood, fiberglass-framed windows are an increasingly popular choice.

Side-by-Side Comparison of Features

Comparing vinyl and fiberglass is best done by looking at individual features so you can choose based on the elements that are most important to you.

Cost

Both vinyl and fiberglass windows are cheaper than wood, but of the two, vinyl is the less expensive alternative. Vinyl is roughly 10 to 30 percent cheaper to buy and install. In one analysis, the cost of installing a single vinyl window averaged $450, compared to $600 for one fiberglass window. A six-window replacement job using vinyl windows averages $3,156; using fiberglass windows, $4,398.

  • Advantage: Vinyl windows

DIY Suitable?

Here, vinyl windows have a definite advantage, as they are fairly easy for DIYers to install themselves. Fiberglass windows, on the other hand, must be installed with great precision because the material is very rigid, requiring close tolerances. Professional installation is best with any windows, but with fiberglass, it is virtually mandatory. Some fiberglass window manufacturers may be reluctant to sell windows directly to homeowners, and may not warranty them if they are not installed by approved contractors. Vinyl windows are often stocked at home improvement centers for easy purchase, while fiberglass windows must be special ordered.

  • Advantage: Vinyl windows

Strength and Durability

Vinyl and fiberglass windows are both made from inert substances that do not rot and are not tempting to insects like termites or carpenter ants. In that sense, vinyl and fiberglass windows rank close together for durability, especially when compared to wood windows—made from organic materials that bugs love to eat and which can rot.

Between vinyl and fiberglass, fiberglass windows will be more durable structurally. Fiberglass windows are estimated to have a 38 percent longer lifespan than vinyl, according to a 2007 study. Also, because fiberglass is sourced from glass—the same material as the window-glass—both frames and glass panes expand and contract at the same rate, leading to less chance of seal failure. Seal failure in the IGU (insulated glass unit) is a major cause of fogging and condensation. Vinyl windows have welded corner seams, which is where seal failure usually occurs.

Fiberglass makes for a stronger, more rigid frame than either vinyl or wood. These windows do not warp, ever. They are very low maintenance—other than painting them, if you so choose. Fiberglass can be as much as nine times stronger than vinyl.

It's important to remember, though, that windows themselves provide no structural support. So the sheer ability to support heavy weights (an advertising claim of some fiberglass window manufacturers) is helpful, but not as meaningful as it seems on the surface. If the window opening is framed properly, vinyl windows do perform well. 

  • Advantage: Fiberglass windows

Maintenance

Both windows are equally low maintenance. Mold, mildew, moss, and dirt can be hosed off and then the frames cleaned by scrubbing with a sponge and warm water.

  • Advantage: Tie

Eco-Friendliness

Fiberglass gets the nod here, mostly because the fiberglass used in the window frames consists of about 60 percent recycled glass. Vinyl windows are made from PVC plastic, a material that is difficult to recycle at the end of its life.

  • Advantage: Fiberglass windows

Colors and Finishes

Fiberglass does offer more options for color and finish, but it's not an enormous advantage. Vinyl windows come in a number of "baked-in" colors, as do fiberglass windows. But fiberglass has the additional advantage of being paintable, while vinyl cannot be painted.

Some fiberglass windows, like Milgard's WoodClad series, are constructed with a fiberglass core but have a wood veneer covering the outside (only on the interior side of the window).

  • Advantage: Fiberglass windows (slightly)

Energy Efficiency

The glass fibers in fiberglass do not expand and contract under heat and cold, and thus they are a slightly better insulator than vinyl. Fiberglass is rated as roughly 15 percent higher in R-value than vinyl-framed windows. Both types, however, offer very good insulating value, since they are constructed with hollow cavities that do a good job of slowing the transmission of thermal energy.

Along with better energy efficiency, fiberglass windows are better at providing insulation from noise transmission.

  • Advantage: Fiberglass windows

Real Estate Value

Fiberglass windows tend to reap higher resale values than vinyl windows. Windows, in general, are not a large factor in determining the sale price of a home, but where the windows are considered fiberglass is considered a more desirable option.

Appearance

Fiberglass windows provide more glass space than vinyl windows. Since vinyl is not as strong as fiberglass, more material is needed for the frames. This means that vinyl window frames are slightly thicker than fiberglass, impinging on glass space. Fiberglass windows contain more glass "real estate" than vinyl—and more glass means more natural light.

Fiberglass can also be textured to closely resemble the texture of natural wood.

  • Advantage: Fiberglass windows

Bottom Line

Both vinyl-framed and fiberglass-framed windows are good products that may be a better choice than wood when it comes time to choose new-construction or replacement windows. Vinyl has the advantage when cost is the main factor, and where you want to do the work yourself. But for best appearance, strength, durability, and energy efficiency, fiberglass is the preferred option.