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The Ultrex Fiberglass Advantage

More than 20 years ago, Marvin® started building fiberglass replacement windows and doors. We created the replacement industry’s strongest frame material with our fiberglass. Ultrex® fiberglass—our proprietary fiberglass solution offers unsurpassed durability and timeless style. Learn more about the advantages of Ultrex fiberglass. 

What is Ultrex fiberglass?

Ultrex is a custom reinforced fiberglass composite used to create windows and patio doors. Ultrex has superior strength, stability, and durability to other window and patio door materials. Ultrex won’t melt or lose its shape, even when exposed to extreme temperatures of up 285° F. 

    1. Raw Fiberglass Strands:

      We start with thin strands of strong glass cables and saturate them with specifically compounded resins.

    2. Pultruded Fiberglass:

      We pull the strands through a heated die and cut them with diamond-edged blades to form Ultrex.

    3. Proprietary Acrylic Finish:

      We top Ultrex off with our proprietary acrylic finish that is smoother and 3x thicker than competitive options.

Marvin Replacement Double Hung Window with a close up of Ultrex fiberglass

Ultrex Fiberglass Benefits

Ultrex fiberglass has unmatched strength, stability, and outstanding fade-resistance.

It's an easy choice for windows and patio doors you can enjoy for years to come.  

Diagram of a barbell


Ultrex fiberglass is built tough to take the hits—like wind, rain, hail, and branches.

Diagram of a level


Ultrex's low expansion means our windows and patio doors stay tight and true.

Sun Icon


Not even 48,000 hours of sunlight can fade the thick, superior acrylic finish of Ultrex fiberglass.*

*Average sunlight exposure per 10 years 

Ultrex Fiberglass: Proven Strength for Peace of Mind 

Compared to other materials, Ultrex is the strongest window framing material in the replacement industry.

Ultrex is 8x stronger than vinyl and 3x stronger than Fibrex

Ultrex offers superior bending and flexing resistance to protect the integrity of your home. 

Wood/Vinyl Composite (Fibrex®)
Flexural Modulus (KSI) — ASTM D790

Ultrex Fiberglass: Leak-Resistant + More

Our Ultrex windows and patio doors are resistant to leaks, seal failures, and stress cracks. 

Ultrex expands 87% less than vinyl and 75% less than Fibrex

Vinyl expands at a much higher rate than glass, which may compromise your window’s performance.

Expansion and Contraction x10-5 in/in/F — ASTM D696

Fade-Resistant Finish

Our proprietary acrylic finish is 3x thicker than competitive options to resist scratches, fading, chalking, chipping, denting, and peeling. Best of all, it’s virtually maintenance-free. 

Ultrex Texture Example


Smooth, thick finish

Competitor Finish


Rough, thinner finish

Bigger Views with Ultrex Fiberglass

Ultrex's incredible strength lets us build narrower frames. You get more incredible views for your money. Thinner frames allow you to focus on what’s outside and enjoy a better view.  

Ultrex Window Component


Specifically designed to resist warping and weathering in any conditions for long-lasting durability.

Vinyl vs Window Component


Can expand and contract under everyday conditions— potentially compromising your home’s integrity.

An Investment That Can Save You Money on Energy Bills

A white Marvin Replacement Double Hung window along a staircase

Ultrex is 5,000x less conductive than aluminum, so homes retain more heat in the winter and stay cooler in the summer. When combined with energy efficient glass options, energy bills can decrease.*  Your home will feel comfortable with Marvin Replacement Ultrex windows and doors. 


*Savings reflects installing ENERGY STAR® certified products compared to non-certified when replacing single pane windows based on the average savings among homes in modeled cities. Actual savings will vary by product type, location, method of installation, individual home characteristics, local climate and conditions, utility rates and other factors.

Outline of USA and states

Why Choose Marvin Replacement?

Marvin Replacement provides superior window and patio door material. Plus, we strive to provide exceptional experiences with our seamless installation process. Expect our team to keep you informed every step of the way so you can look forward to your new view.


Schedule Your Free Consultation

Interior view of a Marvin Replacement Bi-fold patio door. opened in the middle.

If you’re ready to get started, discuss your replacement window and door options with a design consultant. We’re here to answer any questions you may have, and help make your project easy.

Schedule your consultation