A light brown dog sleeps on a white couch in a living room with a white Infinity patio door in the background.

Standalone Strength

The power of Ultrex^® fiberglass

The foundation of Infinity by Marvin windows, our proprietary Ultrex material is made from millions of fiberglass strands, taking the ordinary and making it extraordinary.

What is Ultrex fiberglass?

Ultrex is a pultruded fiberglass material used in Infinity windows and doors, engineered for exceptional strength, durability, and long‑term performance. It resists warping, expanding, and contracting in extreme temperatures, making it more stable than vinyl or composite materials. Ultrex also delivers excellent energy efficiency and low maintenance, making it a reliable choice for replacement windows.

Ultrex is a custom reinforced fiberglass composite used to create windows and patio doors. Ultrex fiberglass has superior strength, stability, and durability to other window and patio door materials.

Infinity by Marvin fiberglass windows & patio doors

Engineered to outperform

A man standing on vinyl and Ultrex to compare.

Ultrex fiberglass vs. vinyl

Over time, vinyl warps and shifts, making it difficult to open and close your windows. The strength of fiberglass ensures they perform as smoothly on day 10,000 as the day they were installed.

A close up picture of window hardware for an Infinity window.

Ultrex fiberglass vs. wood

While classically beautiful, wood requires endless maintenance and care. Virtually maintenance-free Ultrex can be paired with stainable, paintable EverWood ^® interiors for a classic wood look without the upkeep.

Ultrex fiberglass benefits

Withstands 285°F

Weathers the extremes

The strength of Ultrex combined with our proprietary acrylic finish means your frames can resist warping, bending, flexing, and fading — even in the most extreme conditions.

3x thicker

Finished to last, beautifully

Dent, chip, scratch, chalk, and fade resistant, our proprietary acrylic finish is 3x thicker than others on the market, delivering a sleek, smooth look that stands up to the test of time.

87% less expansion

Maintains leak-proof fit

Expanding less than both vinyl and Fibrex^®, Ultrex frames stay tight and true to your glass panes, maintaining an energy-efficient seal year after year.

We are extremely pleased with our new windows. We had eight windows and one patio door replaced. The stained EverWood® matched our oak trim nicely. Windows seal well and are easy to operate. Workmanship was excellent. Attention to detail was evident. Job well done!”

Joe and Jean R.

Minnesota Homeowners

All Reviews

Fibrex® is a registered trademark of Andersen Corporation.

Power of numbers

From ordinary to extraordinary

Ultrex is 8x stronger than vinyl

The strength of fiberglass stands the test of time so you get more expansive views — built to last.

Ultrex fiberglass is 3x stronger than Fibrex®

Made using millions of fiberglass strands, Ultrex is stronger than the vinyl/wood composite used by competitors.

How our strength stacks up

Made with 100% fiberglass versus the blend of vinyl and wood fibers used in Fibrex^®, Ultrex protects the integrity of your home with superior bend and flex resistance. This strength is measured in KSI. For context, 1 KSI is equivalent to 1,000 PSI. The graph shows how these materials stack up under pressure.

Stunning, inside and out

Why Infinity

Expand your view

With more strength per square inch, Ultrex delivers expansive views with slimmer profiles.

Customize your style

A range of timeless finish options can also be painted to complement your home without compromising the protective, low-maintenance fiberglass material.

How we do it

Harnessing the strength of fiberglass

Raw fiberglass strands

Millions of thin but mighty glass cables are saturated with meticulously crafted resins.

Pultruded fiberglass

The saturated strands are pulled through a heated die to create the Ultrex material.

Smooth acrylic finish

Ultrex is coated with a proprietary acrylic that delivers an impossibly smooth, itch-free finish.

Frequently asked questions

What replacement windows are the most energy efficient?

Replacement windows with multiple panes of glass and a gas fill, along with Low-E glass coatings are considered the most energy efficient. The window frame material can play a role in energy efficiency with materials like fiberglass providing better insulation than other window materials. Window with two or more panes contain a gas fill, often an inert, non-toxic gas, that adds a layer of insulation. Low-E coatings help reflect solar heat to improve energy efficiency.

Are fiberglass windows better than vinyl?

Infinity by Marvin fiberglass windows have greater durability and strength than vinyl windows. Our Ultrex fiberglass has 8x the strength of vinyl to resist warping and seal failure. Ultrex fiberglass has more stability than other window materials and expands 87% less than vinyl. Plus, we offer a fade-resistant finish.

Your questions answered

Plan your replacement with confidence

Resource Hub

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Close up of a window corner with Ultrex fiberglass.

Why choose Ultrex fiberglass for your windows and doors?

Wood? Vinyl? Fiberglass? How do you choose the best material for your new windows?

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Exterior view of a double hung dormer window.

Vinyl vs. fiberglass replacement windows

When replacing your windows and doors, choosing the right material plays a key role in the process.

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Blue bathroom interior showing an opened Infinity awning window in EverWood finish.

Fiberglass vs. wood windows: What’s the difference?

One of the most important decisions when replacing your home’s windows is choosing the right material.

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White dining room with three white double hung windows and divided lites.

Best windows and doors

From materials to installation, it is important to do your research before making such an important purchase for your home.

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How to choose new windows for your home

When it comes to replacing windows, it's normal to feel a bit lost about where to begin.