What Exactly is a Thermoset Fiberglass Composite?

You have a lot of choices when it comes to new construction and replacement windows and doors. Some composites are often less expensive, but it’s not always about the immediate cost.

Choosing doors and windows made of composites is a great solution for a wide range of construction applications, but not every composite window is made from the same materials, nor do they have the same properties. Take, for example, wood chip/thermoplastic composites and fiberglass composites—two very different materials which are not only created using different processes, but which also behave differently over their lifespans.

What is Thermoset Fiberglass Composite Material?

For many general contractors, it may be unclear exactly what makes thermoset fiberglass composites different from thermoplastic composites, and thus worth the additional cost. To answer that question, it’s vital to understand what these composites are and how they’re created.

Most plastics and vinyls are made using thermoplastic methods, which melt the material, pour it into a form and allow it to harden. When thermoplastic materials are then re-heated, they begin to re-melt. When it comes to products like windows and doors, this can mean that the product—especially in hot climates—begins to misfit the opening, making it hard to open and close, or that the seals become compromised in a relatively short period of time.

On the other hand, thermoset materials are created through a chemical process that changes the product at the molecular level. They are fundamentally more than the sum of their parts by the end of the manufacturing process. Thermoset fiberglass composites, for example, are made by saturating cables of fiberglass with resins, heating them until the two materials become one and then allowing the new material to cure. Because of the process in which it’s made, a thermoset fiberglass composite won’t soften or melt when exposed to environmental temperatures.

What to Expect with Thermoset Fiberglass Material 

Superior acrylic finishes. 
Dark finishes are becoming popular again, but many composites simply can’t hold a dark finish like black or bronze if the building gets a lot of sun exposure. Even darker Marvin Ultrex® fiberglass composite windows can withstand increased heat loads, degradation by UV light or fading after years of use. These composites also resist cracks, dents, chips and peeling as they age.


Retains its shape, even in the hottest climates. 
Vinyl windows have a terrible reputation for losing their shape, breaking seals and becoming more difficult to open and close over time. That’s not an issue with thermoset fiberglass composites. Since the frame material has a very similar expansion rate to the glass it encases, seal failures and stress cracks are not a worry.


Unsurpassed strength.
When stress-tested for tensile strength, thermoset fiberglass composite material has been found to be as strong as steel—significantly outperforming both vinyl and vinyl/wood composite materials. It’s more than three times stronger than vinyl/wood composites in tensile strength and nearly eight times stronger than vinyl alone.


Longer life expectancy. 
Case studies by the University of British Columbia found that windows made of thermoset fiberglass composite materials last longer than those windows made of vinyl. In fact, windows made of fiberglass lasted 38 percent longer—meaning more years of trouble-free use for future owners and better resale or leasing prospects as a structure ages.

Choosing Composite Materials Isn’t Always Comparing Apples to Apples

Naturally, your project has to be budget-conscious, but not all composites are made equally. Instead of choosing your next set of windows or doors based solely on price, dig a little deeper into the types of materials that go into that composite. If it’s vinyl and wood chips, or other vinyl composites made with thermoplastic manufacturing processes, you may find that investing a little bit more gains you a significant boost to your reputation and can make your structures more attractive to your customers.

In fact, the Construction Specification Institute’s MasterFormat, who had traditionally grouped all types of composite windows and doors together, recently acknowledged the significant difference in composite materials and has ensured that fiberglass thermoset composite materials are now in a separate category.

Homeowners, business owners and investors can all appreciate the peace of mind that thermoset fiberglass, like Ultrex, can provide. Imagine having worry-free windows for decades! If only all parts of homes you build could be guaranteed so easily.