Why Fiberglass Windows?

When it comes to replacement windows, the frame material matters just as much as the glass. Ultrex® fiberglass—the proprietary material used in Infinity® by Marvin® windows—delivers performance that vinyl, wood, and composite materials simply cannot match. Discover why fiberglass is the smart choice for Quad Cities homes.

The Science Behind Superior Window Performance Infinity® by Marvin®

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3xThicker Finish
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Home Why Fiberglass Windows?

Materials Matter: The Power of Ultrex® Fiberglass

Infinity® by Marvin® windows aren't made with ordinary fiberglass—they're built with Ultrex®, a proprietary pultruded fiberglass material engineered specifically for window applications. This advanced material composition delivers exceptional strength, stability, and durability that sets Infinity windows apart from every competitor.

What Makes Ultrex® Different?

Content: Ultrex® fiberglass is created through a process called pultrusion, where millions of continuous glass fibers are saturated with carefully engineered resins and pulled through a heated die. This process creates a material that is:

Incredibly strong: able to withstand enormous pressure without flexing
Dimensionally stable: maintains its shape through extreme temperature changes
Impervious to moisture: won't absorb water, rot, or deteriorate
Finished for life: coated with a proprietary acrylic that resists fading, scratching, and chalking

The result is a window frame material that performs beautifully on day one—and continues performing for decades.

Ultrex® Fiberglass vs. Vinyl

Vinyl windows dominate the replacement market because they’re inexpensive to manufacture. But that low initial cost comes with significant compromises in performance, durability, and longevity. Here’s how Ultrex® fiberglass compares.

The Problem with Vinyl

Thermal Expansion Vinyl expands and contracts dramatically with temperature changes—a significant problem in the Midwest where temperatures can swing from below zero to 90°F+ within the same year. This constant movement:

Breaks down seals over time
Creates gaps that allow air infiltration
Causes windows to stick or become difficult to operate
Leads to premature seal failure and foggy glass

Structural Weakness Vinyl is inherently flexible. To provide adequate strength, vinyl window frames must be thick and bulky, which:

Reduces visible glass area
Limits your views and natural light
Creates a heavy, less refined appearance

Surface Degradation Over time, vinyl windows fade, chalk, and become brittle—especially in direct sunlight. Many homeowners find their “white” vinyl windows turning yellow or gray within 10-15 years.

The Ultrex® Advantage

Thermal Stability Ultrex® fiberglass expands and contracts 87% less than vinyl. This exceptional stability means:

Seals remain intact for the life of the window
Consistent, smooth operation in any weather
Superior energy efficiency year after year
No gaps, drafts, or air leakage

Exceptional Strength Ultrex® is 8x stronger than vinyl, allowing for:

Narrower frames that maximize glass area
More natural light and bigger views
Sleek, refined aesthetics
Frames that hold true without warping or bowing

Lasting Beauty Infinity’s proprietary acrylic finish is 3x thicker than competitive coatings, providing:

Superior resistance to fading, even in direct sunlight
Protection against scratching, denting, and chalking
A smooth, beautiful appearance for decades
No painting, scraping, or refinishing required

Visual Comparison:

Performance Factor	Ultrex® Fiberglass	Vinyl
Thermal Expansion	87% less	High — significant movement
Frame Strength	8x stronger	Weak — requires thick frames
Finish Thickness	3x thicker	Standard — prone to degradation
Expected Lifespan	30+ years	15–20 years
Maintenance Required	Virtually none	Periodic cleaning, eventual replacement

Ultrex® Fiberglass vs. Wood

Wood windows have a timeless beauty that many homeowners love. But traditional wood frames come with significant maintenance requirements and durability concerns. Ultrex® fiberglass offers an alternative that captures wood’s aesthetic appeal without the drawbacks.

The Challenge with Wood

Constant Maintenance

Wood windows demand regular attention:

Scraping and repainting every few years
Sealing and caulking to prevent moisture intrusion
Repairing rot, cracks, and deterioration
Replacing damaged components

For busy homeowners, this maintenance burden is time-consuming and expensive. Neglected wood windows deteriorate quickly, losing both performance and curb appeal.

Moisture Vulnerability

Wood absorbs moisture, leading to:

Swelling that makes windows stick
Rot that compromises structural integrity
Mold and mildew growth
Seal failures and energy loss

In the humid Midwest climate, wood windows face constant moisture challenges.

Cost

High-quality wood windows are expensive—often significantly more than fiberglass. When you add the ongoing maintenance costs, wood becomes one of the most expensive window options over time.

The Ultrex® Solution

Maintenance-Free

Performance Ultrex® fiberglass requires virtually no maintenance:

Never needs painting or refinishing
Won’t rot, warp, crack, or deteriorate
Impervious to moisture
Cleans easily with just soap and water

Wood-Like Beauty with EverWood®

Want the look of wood without the hassle? Infinity windows offer the EverWood® interior finish—a stainable, paintable option that captures the warmth of real wood with the durability of fiberglass.

Authentic wood grain appearance
Accepts stain for custom color matching
Pairs with any interior décor
Zero maintenance required

Long-Term Value

While quality wood windows may cost more than fiberglass initially, the maintenance savings and superior longevity make Ultrex® the better long-term value.

What is Fibrex®?

Fibrex® is a proprietary composite material made from wood fiber and PVC (vinyl). It’s marketed by Renewal by Andersen as a “wood composite” alternative. While Fibrex® performs better than pure vinyl, it’s not fiberglass—and the differences matter.

The Comparison

Factor	Ultrex® Fiberglass	Fibrex® Composite
Strength	8x stronger than vinyl	~3x stronger than vinyl
Ultrex® Advantage	3x stronger than Fibrex®	—
Thermal Expansion	87% less than vinyl	75% less than vinyl
Ultrex® Advantage	Still more stable	—
Material	Pultruded fiberglass	Wood fiber + PVC blend
Moisture Resistance	Impervious	Better than wood, not impervious
Lifetime Warranty	Yes	Limited

Why Ultrex® Wins

Superior Strength Ultrex® is 3x stronger than Fibrex®, enabling narrower profiles, more glass area, and better structural performance. This strength difference is measurable—Ultrex® withstands approximately 17 tons of pressure per square inch.

Better Stability While Fibrex® improves on vinyl’s expansion rate, Ultrex® is still more dimensionally stable. This means better seal integrity, smoother operation, and longer-lasting performance.

True Fiberglass Ultrex® is genuine pultruded fiberglass—the same material trusted in demanding applications from aerospace to marine construction. Fibrex® is a wood-plastic composite that, while improved over vinyl, doesn’t match the performance of true fiberglass.

Fibrex® is a registered trademark of Andersen Corporation.

Extreme Temperature Performance

Winter:

When temperatures plunge below zero, Ultrex® fiberglass maintains its shape and seal integrity. Unlike vinyl, which becomes brittle and contracts significantly in cold, Ultrex® stays stable:

No seal failures from contraction
Smooth operation even in frigid weather
Consistent insulation performance
No cold-weather brittleness or cracking

Summer:

When temperatures soar above 90°F, Ultrex® won’t soften or warp like vinyl:

Frames stay true without bowing
No heat-related seal failures
Windows operate smoothly
Maintains structural integrity in direct sunlight

Weather Resistance

Withstands 285°F

Ultrex® maintains its integrity at temperatures up to 285°F—far beyond any condition your windows will ever face. This heat resistance means:

No warping in direct sunlight
No softening on hot summer days
Consistent performance for decades

Moisture Impervious

Unlike wood (which absorbs moisture) and composites (which can allow some moisture penetration), Ultrex® is completely impervious to water:

Won’t swell, rot, or deteriorate
No mold or mildew growth
Maintains appearance in any humidity
Ideal for Quad Cities’ humid summers

How Ultrex® Fiberglass is Made

The exceptional performance of Ultrex® begins with a precise manufacturing process called pultrusion. Understanding how this material is made helps explain why it performs so much better than alternatives.

The Result: A window frame material that combines the strength of fiberglass, the stability of precision engineering, and the beauty of a premium finish—built to perform for decades in any climate.

Raw Fiberglass Strands

The process begins with millions of delicate yet incredibly strong glass fibers. These continuous filaments provide the foundational strength that makes Ultrex® so durable.

Resin Saturation

The glass fibers are saturated with carefully engineered thermosetting resins. These resins bond with the fibers to create a unified material that won't separate, delaminate, or break down over time.

Pultrusion

The resin-saturated strands are pulled through a heated die at precise temperature and speed. This pultrusion process:

Cures the resins into a rigid composite.
Aligns the fibers for maximum strength.
Creates a consistent, defect-free material.
Produces the exact profiles needed for window components.

Acrylic Finish

Finally, Ultrex® is coated with Infinity's proprietary acrylic finish:

3x thicker than competitive coatings.
Mechanically bonded to the fiberglass substrate.
Provides a smooth, itch-free surface.
Delivers lasting color and protection.

The Strength Test

In demonstrations, Ultrex® fiberglass window frames support extraordinary weight without flexing or breaking. While vinyl frames deform and fail under relatively light loads, Ultrex® maintains its shape under thousands of pounds of pressure.

Key Strength Stats:

17 tons per square inch: the pressure Ultrex® can withstand
8x stronger than vinyl: in direct comparison tests
3x stronger than Fibrex®:outperforming composites

What This Means for Your Windows

This exceptional strength translates to real-world benefits:

Narrower frames: more glass, bigger views, more light
Larger window sizes: fiberglass enables window configurations vinyl can't support
Lasting performance: frames won't warp, bow, or sag over time
Better operation: sashes glide smoothly without binding or sticking

Fiberglass vs. the Competition: Complete Comparison

Feature	Ultrex® Fiberglass	Vinyl	Wood	Fibrex® Composite
Strength	Best (8x vinyl)	Basic	Good	Better (3x vinyl)
Thermal Stability	Best (87% less than vinyl)	Poor	Good	Better
Maintenance	None	Low	High	Low
Moisture Resistance	Impervious	Good	Poor	Better
Heat Resistance	285°F	~165°F	N/A	~200°F
Frame Size	Narrowest	Thick	Medium	Medium
Glass Area	Maximum	Limited	Good	Good
Finish Durability	Best (3x thicker)	Fair	Variable	Good
Expected Lifespan	30+ years	15–20 years	20–30 years*	20–25 years
Lifetime Warranty	Yes	Varies	Varies	Limited
Initial Cost	Higher	Lowest	Highest	Medium-High
Long-Term Value	Best	Poor	Fair	Good

*With diligent maintenance

Frequently Asked Questions
We Answer the Top Questions Asked By Our Clients

Window shopping can be a time-consuming task, especially when there are several options and styles to choose from. Mainstream Home Improvement is the leading expert in window replacement options and installation services. Browse through some of our most frequently asked questions to see how we can help in your upcoming window replacement project. Can’t find your question here? View our full FAQ or contact our team in the Quad Cities area today.

View Full FAQ

Why do I have condensation on my windows?

Excessive humidity levels in your home is what causes condensation, not your windows. Condensation is proof of excessive moisture in the air, which can actually be fixed with a dehumidifier or the cleaning of your HVAC units by a professional technician. For more information on condensation, please visit AAMA’s website.

How much can I save by switching out my old windows with new energy efficient windows?

This answer varies depending on several factors: the characteristics of your old and new windows, the size of your home, your property’s location, and more. ENERGY STAR® has put together calculated estimates of the energy you are bound to save when replacing a single-pane or double-pane clear glass window. Ask our team for more specifics on how much money you could be saving with our windows.

Why won’t my window sash stay up?

If your window sash won’t stay up, it may be due to the wrong size window being installed or the balance of your window is off. Unfortunately, this fix requires help form the professionals as the repair is different for every window style and manufacturer. Get in touch with our team if you’re having issues with your window sash.

What window warranties do you offer?

As a window dealer of MI Windows and Doors, our window replacement products are all covered under a Limited Lifetime Customer Assurance Warranty. This allows for homeowners to order replacement parts at no additional charge when their MI windows are broken or not operating properly. Some of our windows also come with a Glass Breakage Limited Warranty where new glass can be purchased at no additional cost if your window breaks.

How can I make the window installation process go as smoothly as possible?

To have your window installation process go as smoothly as possible, make sure the opening to your windows has been cleaned before our contractor gets there. Once your custom windows are installed, make sure there isn’t any more work to be done before using your windows. Lastly, take care of your caulk and trim to make sure your windows last as long as possible.

Service Areas for Window Replacement & Installation

Looking for quality energy efficient windows in the Quad Cities Area. Click your city to find out more about our replacement window services in your area