Ultrex vs. Fibrex Windows: What You NEED to Know Before Buying

If you’ve been researching replacement windows, you’ve probably come across two big names: Fibrex® from Andersen and Ultrex® fiberglass from Marvin’s Infinity line.

On paper, both sound impressive. Stronger than vinyl. More durable. Energy-efficient. Low maintenance. What else could we possibly want in our windows?!

But what’s the real difference between these two materials?
And more importantly...which one is actually right for your home?

At Southwest Exteriors, we’ve been helping San Antonio homeowners replace their windows for over 30 years. We install Infinity from Marvin, which means we sell windows made with Ultrex fiberglass. We don’t install Andersen Fibrex, but we’ve seen plenty of it in homes across Texas and we believe homeowners deserve to understand the pros and cons of both before making a decision. And so, because of that, in this article we’ll break down Fibrex vs. Ultrex as fairly and clearly as possible, so you can make the best choice for your home, your budget, and your peace of mind.

Jump to the FAQs and get quick answers about Ultrex vs. Fibrex, including cost, durability, warranties, and more.

What is Fibrex?

Fibrex is Andersen’s proprietary composite material, made of wood fibers and polymer (about 60% vinyl). Andersen markets Fibrex as:

• 2x stronger than vinyl.
• Resistant to warping, rotting, and corrosion.
• Low maintenance (no painting required).
• Eco-friendly (made from reclaimed wood fibers).

⚠️ The catch: Fibrex is only available through Andersen and Renewal by Andersen. That means your design and style options are tied to Andersen’s catalog. The upside is that Andersen offers a wide variety of window lines, colors, grille patterns, and hardware styles but if you don’t find something you like in their system, you can’t shop Fibrex anywhere else.

When it comes to cost, Fibrex sits in the premium category. Because it’s proprietary to Andersen, you’re often paying for that exclusivity. Prices vary by market and project, but Fibrex typically costs more than vinyl and can sometimes be priced similarly to fiberglass options.

So, while Fibrex isn’t a total departure from vinyl (vinyl is still a big part of its recipe), the added wood fibers make it stronger and more stable almost like vinyl’s tougher more reliable cousin who can handle a little extra stress.

👉 Want to see how Fibrex stacks up against fiberglass in general (not just Ultrex)? Check out our other article: Fiberglass vs. Fibrex: What You Really Need to Know.

What is Ultrex?

Ultrex is Marvin’s patented pultruded fiberglass, used across several of their product lines. You’ll find Ultrex in the Infinity line (all fiberglass, designed for premium replacement), as well as Marvin’s Elevate and Essential lines.

Marvin markets Ultrex as:

• 8x stronger than vinyl.
• 3x stronger than Fibrex in testing.
• Built to handle extreme temperature swings without warping or cracking.
• Finished with an acrylic cap that resists fading, chalking, and scratching.
• ENERGY STAR® certified for efficiency.

⚠️ The catch: Ultrex is exclusive to Marvin. That means you’ll choose from Marvin’s catalog of styles and finishes, depending on which line fits your project. Infinity offers the widest replacement-specific options, but Elevate and Essential also deliver Ultrex performance at different price tiers.

In terms of cost, Ultrex sits in the premium window category, just like Fibrex. Pricing can vary by dealer and project size, so it’s not always automatically more or less expensive but you should expect to pay more than vinyl.

Think of Ultrex as the marathon runner of window materials: lean, strong, and built to perform mile after mile without breaking down.

Side-by-Side Comparison: Fibrex vs. Ultrex

Who Each is Best For

• Fibrex (Andersen): Best for homeowners who want access to Andersen’s wide design catalog, a single-company warranty, and a low-maintenance composite upgrade over vinyl. Fibrex offers more strength and stability than standard vinyl and comes in a variety of finishes and styles within Andersen’s system.
• Ultrex (Marvin): Best for homeowners who want the long-term durability and strength of fiberglass, especially in climates with extreme heat swings like Texas. Ultrex is available in multiple Marvin lines, from Essential to Infinity, offering options for different budgets and design needs. Expect slimmer profiles, fade-resistant finishes, and strong warranty coverage from a company with more than 100 years in the business.

The Bottom Line

Both Fibrex and Ultrex are premium alternatives to vinyl windows, but they’re not the same. Fibrex is Andersen’s composite that blends vinyl with wood fibers, while Ultrex is Marvin’s proprietary pultruded fiberglass known for strength and stability.

Both Andersen and Marvin are highly reputable companies with long histories of quality and innovation. The difference comes down to the materials themselves and which brand’s catalog best fits your home.

• If you want Andersen’s composite material and design catalog, Fibrex is a solid choice.
• If you want fiberglass performance built to handle decades of Texas heat, Ultrex is tough to beat.

At Southwest Exteriors, we install Marvin Infinity windows, which means we sell Ultrex. But here’s the thing: our job isn’t to convince you one way or the other. It’s to give you the information you need so you can choose with confidence.

Think of us like a tour guide pointing out the landmarks, giving you the history, and letting you decide which path makes the most sense for your home.

📍 If you’re in the San Antonio area and ready to explore your options, schedule a commitment-free consultation today. We’ll walk you through everything, answer every question, and give you an honest price upfront, no pressure, just clarity.

Ultrex vs. Fibrex FAQs

Quick answers to the most common questions homeowners ask when comparing Marvin’s Ultrex fiberglass and Andersen’s Fibrex composite windows. 

Click on the question to see the answer.

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