If you are researching replacement windows, you have probably seen several frame materials, including vinyl, wood, composites, and fiberglass. The material used to build the frame matters because it affects the window's strength, stability, appearance, maintenance, and long-term performance.
Ultrex® fiberglass is the proprietary fiberglass material used in Infinity® by Marvin® windows and doors. It is engineered to remain strong and stable through changing temperatures while allowing for narrow frames and more visible glass.
For Colorado homeowners, where windows can be exposed to intense sunlight, freezing temperatures, wind, snow, and large temperature swings, those material characteristics can make a meaningful difference.
Here are five important benefits of Ultrex® fiberglass and what they can mean for your home.
What Is Ultrex® Fiberglass?
Ultrex® is a pultruded fiberglass material developed by Marvin for use in Infinity® by Marvin® windows and doors.
Fiberglass strands are combined with resin and formed into rigid window components. Unlike vinyl, which is a thermoplastic material, Ultrex® is a thermoset material reinforced with glass fiber.
That difference in construction gives Ultrex® high strength and rigidity while helping it resist excessive movement as outdoor temperatures change.
According to current Infinity® performance data, Ultrex® has a flexural modulus of 3,000 ksi compared with 385 ksi for vinyl, as well as a tensile strength of 41.6 ksi compared with 6.2 ksi for vinyl.
How Is Ultrex® Fiberglass Made?
Ultrex® begins with fine strands of fiberglass that are saturated with specially formulated resins. Those strands are then pulled through a heated die in a process called pultrusion, which forms the rigid window and door components used in Infinity® by Marvin® products.
After the fiberglass profile is formed, it is finished with Infinity's proprietary acrylic coating.
The continuous fiberglass strands help give Ultrex® its strength, while the pultrusion process helps create a consistent, stable material designed for long-term window and door performance.
5 Benefits of Ultrex® Fiberglass Windows
1. Exceptional Strength and Rigidity
One of the biggest advantages of Ultrex® fiberglass is its strength.
Infinity® reports a flexural modulus of 3,000 ksi for Ultrex®, compared with 385 ksi for vinyl. It also reports a tensile strength of 41.6 ksi for Ultrex®, compared with 6.2 ksi for vinyl.
In practical terms, Infinity® says Ultrex® fiberglass is 8 times more resistant to bending than vinyl.
What that means for homeowners: a stronger frame can stay rigid, hold its shape, and support narrower profiles with more visible glass.
That can mean cleaner sightlines, more natural light, and a less bulky-looking window.
2. Less Expansion and Contraction
Window frames repeatedly heat up and cool down as outdoor temperatures change. When a material expands and contracts significantly, that movement can place additional stress on the window assembly over time.
Infinity® reports a coefficient of thermal expansion of 0.6 for Ultrex® fiberglass, compared with 3.5 for vinyl and 1.6 for a vinyl/wood composite.
One of the most useful comparisons is glass itself. Infinity® lists both Ultrex® fiberglass and glass at a coefficient of thermal expansion of 0.6. That means the fiberglass frame and the glass expand and contract at nearly the same minimal rate as temperatures change.
That dimensional stability can help the window stay tight and true over time while reducing stress on seals and other parts of the window assembly.
This is especially relevant in Colorado, where a window may be exposed to strong sunshine during the day and much colder temperatures at night.
3. Greater Stability in Temperature Extremes
Colorado windows have to deal with more than cold weather. Strong sunlight and high summer temperatures can also heat exterior building materials considerably.
Infinity® reports a heat deflection temperature of 285°F for Ultrex® fiberglass, compared with 166°F for vinyl and 173°F for a vinyl/wood composite.
This does not mean your window frame will reach those temperatures. Heat deflection temperature is a material-performance measurement used to show how well a material maintains its form when exposed to elevated temperatures under load.
Material stability can also affect how a window operates. A frame that bends, swells, warps, or changes shape can make a window harder to open and close. Ultrex® is engineered to resist that type of movement, helping windows remain square and operate properly over time.
4. A Durable, AAMA-Verified Acrylic Finish
Ultrex® fiberglass uses a proprietary acrylic finish that is verified to the AAMA 624 specification.
According to Infinity®, this verification includes testing designed to simulate harsh conditions a window or door may encounter over its service life.
The finish is designed to resist scratching, chalking, and fading, including in darker exterior colors. Infinity® also states that the finish is three times thicker than competitive painted options.
For homeowners, that means less exterior maintenance and greater confidence in long-term appearance.
5. Slimmer Frames and More Visible Glass
Strength does more than help a window frame remain stable. It can also influence how the window is designed.
Because Ultrex® fiberglass is exceptionally rigid, Infinity® can create relatively narrow frame profiles while maintaining structural strength.
For many homeowners, that means a cleaner appearance and a larger visible glass area.
More glass can provide:
- More natural light
- Less visual obstruction
- Cleaner sightlines
- A more open view to the outdoors
This is especially noticeable when replacing older windows with thick or bulky frames.
How Does Ultrex® Fiberglass Compare With Vinyl and Fibrex®?
Homeowners often compare fiberglass and vinyl because both can provide low-maintenance alternatives to traditional wood windows. They are not, however, the same type of material.
| Material Characteristic | Ultrex® Fiberglass | Vinyl | Vinyl/Wood Composite (Fibrex®) |
|---|---|---|---|
| Material Type | Thermoset | Thermoplastic | Thermoplastic |
| Reinforcement | Glass Fiber | None | Wood Fiber |
| Heat Deflection Temperature | 285°F | 166°F | 173°F |
| Flexural Modulus | 3,000 ksi | 385 ksi | 808 ksi |
| Tensile Strength | 41.6 ksi | 6.2 ksi | 5.5 ksi |
| Coefficient of Thermal Expansion (x10-5 in/in/°F) | 0.6 | 3.5 | 1.6 |
| Surface Finish | Acrylic, AAMA Verified | PVC | PVC, Polyester Urethane, Acrylics |
| Thermal Conductivity1 | 0.15 | 0.10 | 0.12 |
1 Highest rate of thermal conductivity shown. Published test results range from 0.09 to 0.12 for Ultrex® and 0.10 to 0.13 for vinyl/wood composite (Fibrex®).
Source: Infinity® Ultrex® Performance Guide, ©2025 Marvin Lumber and Cedar Co., LLC, republished with permission. Vinyl/wood composite figures reflect published data from Andersen® Fibrex® Manual (2007) and Renewal by Andersen® Fibrex® Material (2008), as cited in that guide, and may not reflect current product formulations. Fibrex® is a registered trademark of Andersen Corporation. Material specifications are subject to change. Comparison data should be considered alongside window design, glass performance, installation quality, warranty coverage, and the needs of your home.
What Do These Window Material Ratings Mean?
Flexural Modulus
A measure of how resistant a material is to bending. A higher number means greater resistance to flexing under load.
Tensile Strength
A measure of how much force a material can withstand when being stretched or pulled before it breaks.
Coefficient of Thermal Expansion
A measure of how much a material expands or contracts as temperature changes. Lower movement can help a window remain more dimensionally stable.
Heat Deflection Temperature
The temperature at which a material may begin to deform when exposed to heat and stress.
Is Ultrex® Fiberglass Better Than Vinyl?
Fiberglass and vinyl can both be used successfully in replacement windows, but they serve different priorities.
Vinyl windows are popular partly because they can offer a relatively affordable way to replace older windows. For a homeowner focused primarily on the lowest initial purchase price, vinyl may be worth considering.
Ultrex® fiberglass is designed for homeowners who place greater emphasis on characteristics such as strength, dimensional stability, narrow frame profiles, low maintenance, and long-term performance.
Instead of simply asking whether fiberglass is "better," we recommend asking a more useful question:
Which window material gives you the combination of performance, appearance, maintenance, price, and longevity that makes sense for your home?
A good replacement-window company should be able to explain those differences without pretending that one material is automatically right for every homeowner.
Why Ultrex® Fiberglass Makes Sense for Colorado Homes
Colorado can be demanding on windows and doors.
Homes along the Front Range regularly experience intense sunlight, freezing winter temperatures, warm summer days, snow, wind, and rapid temperature changes.
That makes dimensional stability an important consideration when choosing a window-frame material.
Because Ultrex® fiberglass experiences relatively little expansion and contraction compared with vinyl, it is well suited to environments where temperatures can change significantly.
Its strength and durable exterior finish are additional reasons many Colorado homeowners consider fiberglass when replacing their windows.
However, frame material is only one part of long-term window performance. Proper measurement, flashing, sealing, insulation, and installation are also critical.
The window matters. The install is the difference.
Why Installation Matters With Any Replacement Window
You can purchase an excellent window and still be disappointed if it is installed incorrectly.
Replacement windows have to fit the opening properly and integrate correctly with the home's existing structure, weather barrier, insulation, trim, and exterior conditions.
Problems such as air leakage, water intrusion, difficult operation, and poor finishing are not always caused by the window itself. Installation can be just as important as the product.
Gravina’s Window Center of Littleton® has served Colorado homeowners since 1973. Our Infinity® by Marvin® windows are installed by our own installation teams, and our workmanship is backed by a lifetime workmanship warranty.
That allows homeowners to evaluate both sides of the replacement-window decision: what window they are buying and who will be responsible for installing it.
Can Infinity® Fiberglass Windows Look Like Wood?
Yes. Homeowners who like the appearance of traditional wood interiors can choose Infinity® by Marvin® windows with EverWood® interior finishes.
EverWood® is designed to provide the appearance and texture of wood while being used with an Ultrex® fiberglass window frame.
It can be stained or painted to coordinate with interior trim, cabinetry, flooring, and other finishes in the home.
This can be an attractive option for homeowners who want a warmer interior appearance without choosing a traditional exposed-wood window frame.
What Are the Downsides of Ultrex® Fiberglass Windows?
No window material is right for every home or every budget.
The biggest consideration for many homeowners is cost. Fiberglass replacement windows will often have a higher initial price than many entry-level vinyl windows.
Other factors worth considering include:
- How long you expect to remain in the home
- Your overall project budget
- The architectural style of your house
- Your preferred frame thickness and sightlines
- Your desired interior and exterior finishes
- Glass and energy-efficiency options
- Installation quality
- Manufacturer and workmanship warranties
If your primary objective is simply finding the lowest-cost replacement window available, Infinity® by Marvin® may not be the right fit.
If you are comparing windows based on long-term strength, stability, appearance, maintenance, and installation quality, Ultrex® fiberglass deserves consideration.
Are Ultrex® Fiberglass Windows Worth It?
Whether Ultrex® fiberglass is worth the investment depends on what you expect from your replacement windows.
The value is not based on one feature alone. It comes from the combination of:
- High strength and rigidity
- Low expansion and contraction
- Temperature stability
- Durable exterior finishes
- Narrow frame profiles
- Low maintenance
For homeowners planning to remain in their house for many years, those characteristics may justify spending more upfront than they would for some lower-cost window materials.
The best way to decide is to compare the actual products, understand the differences, and look beyond the initial price alone.
See Infinity® by Marvin® Fiberglass Windows in Person
Specifications are useful, but replacement windows make much more sense when you can see and operate them yourself.
At Gravina’s Window Center of Littleton®, you can compare Infinity® by Marvin® window styles, frame profiles, finishes, hardware, glass options, and interior choices before making a decision.
We have served Colorado homeowners since 1973 and are one of the country's longest-standing authorized Infinity® by Marvin® dealers.
Our team can help you understand not only which window may work for your home, but how the window will be measured, installed, sealed, finished, and supported after installation.
Ready to compare replacement-window options? Contact Gravina’s Window Center of Littleton® to schedule a free consultation.
Frequently Asked Questions About Ultrex® Fiberglass
What is Ultrex® fiberglass made from?
Ultrex® is a thermoset fiberglass material reinforced with glass fibers. The material is formed through a manufacturing process called pultrusion and is used in Infinity® by Marvin® windows and doors.
Is Ultrex® stronger than vinyl?
Yes, based on the material-performance figures published by Infinity®. Ultrex® has a reported flexural modulus of 3,000 ksi compared with 385 ksi for vinyl, along with a tensile strength of 41.6 ksi compared with 6.2 ksi for vinyl.
Does Ultrex® expand and contract?
All building materials experience some movement as temperatures change. However, Infinity® reports a coefficient of thermal expansion of 0.6 for Ultrex® compared with 3.5 for vinyl, meaning Ultrex® experiences considerably less thermal movement.
Are fiberglass windows good for Colorado?
Fiberglass can be a strong option for Colorado homes because of its dimensional stability, strength, and resistance to temperature changes. Those characteristics can be beneficial in a climate with intense sunlight, cold winters, and frequent temperature swings.
Do Ultrex® fiberglass windows require maintenance?
Ultrex® is designed to be a low-maintenance exterior window material. Normal cleaning and manufacturer-recommended care are still appropriate, but the fiberglass exterior does not require the same routine scraping, staining, or refinishing associated with exposed exterior wood.
Are Ultrex® fiberglass windows more expensive than vinyl?
They can be. Fiberglass replacement windows are generally positioned differently from lower-cost vinyl windows. The final price depends on window size, style, glass package, configuration, installation requirements, finishes, and other project-specific factors.
Can Infinity® fiberglass windows have a wood-look interior?
Yes. Infinity® by Marvin® offers EverWood® interior finishes designed to provide the appearance and texture of wood while being used with an Ultrex® fiberglass window system.
What matters more, the window or the installation?
Both matter. A well-engineered window can still perform poorly if it is measured or installed incorrectly. Product quality and installation quality should be evaluated together when choosing a replacement-window company.
Talk With a Colorado Infinity® by Marvin® Window Expert
If you are considering replacement windows and want to know whether Ultrex® fiberglass is the right fit for your home, we can help you compare your options without the guesswork.
Gravina’s Window Center of Littleton® has served Colorado homeowners since 1973 and is one of the country’s longest-standing authorized Infinity® by Marvin® dealers.
Our team can help you evaluate:
- Ultrex® fiberglass window styles and configurations
- Frame profiles and visible glass area
- Interior and exterior finish options
- EverWood® interior finishes
- Glass and energy-efficiency options
- Installation requirements for your home
- Warranty and long-term service considerations
We also use our own installation teams and back our workmanship with a lifetime workmanship warranty.
The window matters. The install is the difference.
Gravina’s Window Center of Littleton®
209 W. Littleton Blvd.
Littleton, CO 80120
Phone: 303-794-0490
Originally published May 2015. Updated September 30, 2026 to reflect current Infinity® by Marvin® Ultrex® performance information and our experience serving Colorado homeowners.