Who Makes the Best Fiberglass Replacement Windows in 2026?

Infinity® by Marvin® Fiberglass Windows

Infinity® by Marvin® replacement windows are made with Ultrex® fiberglass on both the interior and exterior of the frame. Ultrex® is a pultruded fiberglass material designed to provide high structural strength, dimensional stability, and low-maintenance performance.

One of Infinity's strengths is that it combines fiberglass construction with relatively narrow frame profiles and a range of finish options. Homeowners can choose from multiple exterior and interior colors, including EverWood®, a stainable engineered wood grain interior finish designed to provide a warmer appearance without using a traditional wood window frame.

Where Infinity performs well: Infinity is particularly well suited for homeowners who want a premium replacement window with fiberglass construction, low maintenance, refined sightlines, and options that work with both traditional and contemporary homes.

A real limitation: Infinity® by Marvin® is a premium product and typically has a higher upfront cost than many vinyl replacement windows. Homeowners whose primary goal is completing the largest number of window openings at the lowest initial cost may find that a quality vinyl window is a better fit for their budget.

Infinity also offers a Limited Lifetime Warranty for eligible owner-occupants who purchase through an authorized Infinity retailer. Warranty coverage and installation workmanship coverage are separate considerations, so homeowners should review both before choosing an installer.

Gravina's Window Center of Littleton® has installed and serviced Infinity® by Marvin® windows in Colorado homes for more than two decades. Our experience with the product is one reason we sell Infinity, but it is not the right window for every homeowner or every budget.

Pella® Impervia® Fiberglass Windows

Pella® Impervia® is a fiberglass window line from Pella. Its Duracast® material is described by Pella as a five-layer pultruded fiberglass composite reinforced with a patented interlocking mat.

Pella® Impervia® is available in several window styles and uses a powder-coated finish on the fiberglass surfaces. Depending on the configuration, Pella also offers dual-pane and triple-pane insulating glass options.

Where Pella® Impervia® performs well: Pella® Impervia® is a strong option for homeowners who want a fiberglass window from a large national manufacturer with multiple configurations, color options, and established sales and service channels.

A real limitation: Pella® Impervia® interior finish options are more limited than window lines that offer real wood interiors or stainable wood-look finishes. Homeowners seeking the appearance and warmth of traditional wood may prefer a different product category.

Pella publishes specific performance data, size limitations, glass options, and warranty information for Pella® Impervia® products. Because available configurations can vary by window style and market, homeowners should compare the exact product specifications for the window they are considering rather than relying on general brand claims.

Milgard® C700 Fiberglass Series Windows

Milgard® C700 Fiberglass Series is Milgard's current fiberglass window and patio door line, replacing the previous C650 Ultra™ Series. Milgard describes its fiberglass frames as glass fibers and resin bonded together to create a long-lasting, weather-resistant material that resists swelling, rotting, warping, and corrosion.

The Milgard® C700 Fiberglass Series is available in several window styles and offers six exterior colors: white, black, bronze, bark, adobe, and steel gray, with a matching range of interior color options. Milgard also notes that its fiberglass is paintable if a homeowner wants to change colors later.

Where the Milgard® C700 Fiberglass Series performs well: The Milgard® C700 Fiberglass Series can be a good fit for homeowners who want fiberglass construction, low thermal conductivity, low-maintenance factory finishes, and a broader color palette including darker frame options.

A real limitation: The Milgard® C700 Fiberglass Series is a recently introduced product line. Homeowners buying a window they expect to last decades have less long-term field history to evaluate than they would with a series that has been installed and serviced for many years. Homeowners who want a real wood or stainable wood-look interior will also need to consider a different product category.

Milgard publishes configuration-specific information for glass, energy performance, hardware, grids, finishes, and warranty coverage. As with any window line, the exact performance depends on the window style, size, glass package, and configuration selected.

Renewal by Andersen® Fibrex® Composite Windows

Renewal by Andersen® is frequently encountered during fiberglass window research, but its Acclaim® replacement windows are not made from fiberglass. They are made from Fibrex®, Andersen's proprietary composite material made from reclaimed wood fiber and thermoplastic polymer.

Andersen describes Fibrex® as combining the strength and stability of wood with the low-maintenance features of vinyl, and states that the material is approximately 40 percent reclaimed wood fiber. Andersen also states that Fibrex® resists rot and temperature-related expansion, and that its color process supports dark exterior finishes.

Where Renewal by Andersen® performs well: Renewal by Andersen® can be a strong fit for homeowners who want a single-company replacement process that covers consultation, custom manufacturing, installation, and warranty under one roof.

A real limitation: Renewal by Andersen® offers one replacement window line. Homeowners who want to compare multiple materials, series, or configurations within a single conversation, or who want to cross-shop the same product between installers, will find that harder with a single-source model than with lines sold through independent dealers.

Renewal by Andersen® publishes its own specifications, glass options, color options, and warranty terms for Acclaim® products. Because the product is custom manufactured to fit each opening, homeowners should compare the exact specifications and warranty language for their configuration rather than relying on general brand claims.

Marvin Elevate® Fiberglass and Wood Windows

Marvin Elevate® windows pair an Ultrex® fiberglass exterior with a natural wood interior. That is a different construction from Infinity® by Marvin®, which uses Ultrex® fiberglass on both the interior and exterior.

Marvin describes the Marvin Elevate® collection as featuring Pine interiors that can be stained to match a room or factory-finished with a clear coat, Painted Designer Black, or Painted White. The Ultrex® exterior is offered in six colors, and dual-pane and triple-pane glass are available on select products.

Where Marvin Elevate® performs well: Marvin Elevate® can be an excellent fit for homeowners who want the appearance of a real wood interior but prefer a low-maintenance fiberglass exterior instead of maintaining wood on both sides of the window.

A real limitation: A wood interior is not a maintenance-free interior. Depending on sun exposure, humidity, and the finish selected, wood interiors can need periodic refinishing over the life of the window, and bare Pine requires finishing before or shortly after installation.

Marvin Elevate® is also built for a broader range of applications than a dedicated replacement window system, and may be specified for replacement, remodeling, or new construction depending on the project. Marvin publishes configuration-specific information for glass, finishes, sizes, and warranty coverage, so homeowners should compare the exact installation method and product configuration being proposed for their home.

What Is Ultrex® Fiberglass?

Ultrex® is the proprietary pultruded fiberglass material used in Infinity® by Marvin® windows and doors. During pultrusion, fiberglass strands are pulled through resin and formed into strong, consistent window frame components.

According to Infinity® by Marvin®, Ultrex® fiberglass is 8 times stronger than vinyl and expands 87 percent less than vinyl.

That dimensional stability matters in Colorado, where window frames are exposed to intense sunlight, cold weather, and large temperature swings. A frame that expands and contracts less can help maintain its shape, fit, and operation over time.

Strength alone does not determine how well a replacement window will perform. Glass selection, manufacturing quality, installation, air and water management, and ongoing service all matter too. Ultrex® provides the structural foundation, but the entire window system and the installation should be evaluated together.

What Affects the Cost of Fiberglass Replacement Windows?

Fiberglass replacement windows generally cost more upfront than entry-level vinyl windows. Beyond that, there is no single price that accurately represents every project, and any source quoting one figure for a fiberglass window is describing a specific configuration rather than a market rate.

The final cost changes based on several factors:

• Window size and style: Casement, double-hung, sliding, picture, specialty shape, bay, and bow windows can have very different costs.
• Number of windows: The size and scope of the overall project affects the final investment.
• Installation method: An insert replacement involves a different scope of work than a full-frame installation requiring additional interior or exterior finish work.
• Glass package: Low-E coatings, insulating glass configurations, tempered glass, specialty glass, and other performance upgrades affect cost.
• Colors and finishes: Interior finishes, exterior colors, hardware, and grille patterns can change the final price.
• Condition of the existing openings: Rot, water damage, structural problems, or other repairs discovered during replacement can increase the scope of work.

Why Two Quotes for the Same House Can Differ by Thousands

Those factors are also why two proposals for the same home can arrive at very different numbers. One quote may assume insert replacement while another assumes full-frame. One may include interior trim and exterior finish work while another treats it as an add-on. One may include a better glass package. The difference is often scope, not price.

Gravina's Window Center of Littleton® does not publish a single per-window price for fiberglass replacement windows, because a number that ignores installation method, opening condition, and glass package would misrepresent what a Colorado homeowner actually pays.

When comparing proposals, confirm that each one covers the same number of openings, the same window configurations, the same glass options, the same installation method, the same interior and exterior finish work, and the same warranty and service commitments. Once those are matched, the price comparison becomes meaningful.

When Fiberglass Windows May Not Be the Right Choice

Fiberglass can be an excellent replacement window material, but it is not automatically the best choice for every homeowner or every project.

Your Budget Is the Primary Concern

If your main goal is replacing the greatest number of failing windows for the lowest initial cost, a quality vinyl window may make more sense. In some situations, replacing every problem opening with a good vinyl product is more practical than replacing only part of the home with a more expensive fiberglass product.

You Plan to Move Soon

If you expect to sell the home within the next few years, you may not personally benefit from enough of fiberglass's long-term durability and reduced maintenance to justify the additional upfront investment.

Your Home Requires a Specific Historic or Architectural Look

Some historic homes, architectural designs, and HOA requirements call for profiles, materials, finishes, or detailing that a fiberglass replacement window cannot reproduce. Homes in Denver's designated historic districts are subject to Landmark Preservation review, and several Front Range municipalities have comparable processes that can specify window materials, profiles, and configurations. In those situations, wood, clad wood, or another specialty product may be the better fit, and confirming the applicable requirements before ordering anything is worth the time.

You Are Replacing Windows in a Rental or Investment Property

For some rental properties, the economics of a premium fiberglass window may not match the owner's investment strategy. A durable, appropriately specified lower-cost product can provide a better balance between initial cost and expected ownership period.

Your Home May Not Need Whole-House Replacement

Not every window problem calls for replacing every window. If the existing frames are structurally sound and the real issue is a failed seal, damaged hardware, or two or three specific openings, addressing those directly may serve the home better than a full project. A homeowner should understand the condition of the existing openings before committing to the scope, and any company proposing whole-house replacement should be able to explain why each opening needs it.

The right replacement window is not simply the most expensive product or the strongest material. It is the product that best fits the home, the budget, the installation requirements, and the homeowner's long-term plans.

Frequently Asked Questions About Fiberglass Replacement Windows

Is Renewal by Andersen® fiberglass?

No. Renewal by Andersen® replacement windows are made with Fibrex®, a proprietary composite material made from reclaimed wood fiber and polymer. Fibrex® is commonly compared with fiberglass because both are alternatives to traditional vinyl and wood window frames, but they are different materials.

Which is better, fiberglass or vinyl windows?

Neither material is automatically better for every homeowner. Fiberglass generally provides greater structural stability and less expansion and contraction than vinyl, while vinyl typically offers a lower initial price. The better choice depends on your budget, climate, expected ownership period, product quality, and installation.

How long do fiberglass windows last?

There is no universal lifespan for every fiberglass window. Longevity depends on the manufacturer, frame construction, glass system, installation quality, exposure, maintenance, and climate. Fiberglass is commonly chosen for long-term replacement projects because it resists many of the expansion, contraction, rot, and maintenance issues associated with some other frame materials.

Are fiberglass replacement windows worth the extra cost?

They can be for homeowners who value structural stability, low maintenance, durability, and long-term ownership. They may be less compelling when the lowest upfront price is the priority or when the homeowner expects to move within a relatively short period.

Is it Infinity® by Marvin® or Infinity from Marvin?

The current brand name is Infinity® by Marvin®. The older name "Infinity from Marvin" still appears in older websites, articles, and online references, but Infinity® by Marvin® is the correct current brand name.

So, Who Makes the Best Fiberglass Replacement Windows?

There is no single fiberglass replacement window that is best for every home.

Infinity® by Marvin®, Pella® Impervia®, and Milgard® offer legitimate fiberglass options with different strengths, finishes, distribution models, and price points. Marvin Elevate® combines a fiberglass exterior with a real wood interior, while Renewal by Andersen® uses Fibrex® composite rather than fiberglass.

For homeowners looking specifically for a premium fiberglass replacement window with low-maintenance fiberglass construction inside and out, Infinity® by Marvin® deserves serious consideration. Its Ultrex® fiberglass provides exceptional structural stability, narrow frame profiles, and long-term durability.

We also recognize that Infinity is not the right choice for every homeowner. Budget, architectural requirements, installation method, how long you plan to own the home, and the level of service you expect should all be part of the decision.

Need Help Comparing Fiberglass Windows in Colorado?

Gravina's Window Center of Littleton® has helped Colorado homeowners with replacement windows since 1973 and has worked with Infinity® by Marvin® for more than two decades.

We can show you how Infinity® by Marvin® is constructed, explain where fiberglass performs well, discuss its limitations, and help you determine whether it makes sense for your home. Our goal is not simply to sell you a fiberglass window. It is to help you understand what you are buying and why.

Schedule a free window consultation or visit one of our Colorado showrooms to see Infinity® by Marvin® windows in person.