Story at a glance:

  • Windows can account for up to 50% of the heating and cooling energy loss in a building.
  • Fiberglass windows can stand up to extreme temperatures and last more than 50 years.
  • Aluminum frame systems can help projects achieve LEED Gold or Platinum certifications.

Plentiful daylighting, thermal comfort, and views of nature—windows provide all this and more. These benefits are just part of why windows are often at the top of the list of decisions architects and designers need to make. But how do you choose the best windows? And what are the biggest differences between aluminum vs. fiberglass windows?

The USGBC says it has become very important for architects to understand how to choose a sustainable window and window frame system that will promote comfort, energy efficiency, durability, and longevity through quality construction. They offer a course on sustainable windows, exploring how choosing an ENERGY STAR–rated window can contribute toward the LEED for Homes credit.

The right windows not only let natural light in spaces, making for things like healthier offices and more sustainable homes, but they can also improve indoor air quality. The options, though, can be overwhelming. Here are the pros and cons of aluminum vs. fiberglass windows.

Fiberglass Windows

fiberglass windows

The Origin Apartments features high-performance fiberglass windows that helped the building win multiple awards for design and energy efficiency. Photo by Paul Grdina

Windows currently account for anywhere between 30 to 50% of the heating and cooling energy loss in a building, according to Cascadia Windows & Doors Technical Director Michael Bousfield. He previously told gb&d fiberglass windows improve building performance.

Fiberglass windows offer an avenue to deliver highly livable, long-lasting residential and commercial buildings, while improving energy efficiency and sustainability in general. Bousfield has called fiberglass an ideal structural material for window and door frames—specifically fiberglass with a high glass-fiber-to-resin formula.

“Nearly 10 times stronger than traditional vinyl, thermoset fiberglass is dimensionally stable, meaning it won’t creep and deflect over time. This stability and strength allow fiberglass frame windows to withstand higher wind load, resulting in larger possible windows—even on tall buildings with high wind loads,” he previously wrote for gb&d.

Fiberglass windows are impervious to decay, insect attack, and corrosion. They can also withstand extreme weather, including temperatures of -40°F through 350°F and higher, without becoming brittle or soft. Fiberglass windows can last 50 years or longer, more than twice the average lifespan of aluminum windows.

Bousfield has said that fiberglass also has an inherently low thermal conductivity, meaning that, without any additional thermal breaks or materials, fiberglass is 500 times less conductive than aluminum. That means a large-span double-glazed fiberglass window is more than 100% thermally efficient than a comparable aluminum window.

Pros: Impervious to decay, insect attack, and corrosion. Stand up extreme temperatures. Lifespan of 50+ years.

Cons: Fiberglass is harder on tools and requires significant personal protection equipment to ensure proper industrial hygiene.

Aluminum Windows


The AECOM team chose YKK AP products to stand up to the cold. Photo courtesy of YKK AP

The experts at YKK AP have thoughts on the topic, too, saying upgrading a building’s framing system with aluminum frames is a cost-effective option for improving your project’s sustainability or even achieving LEED Gold or Platinum.

YKK AP America’s Steve Schohan previously wrote about the benefits in an article for gb&d, exploring how aluminum building framing systems in particular have evolved.

Aluminum framing systems were known for their high thermal conductivity, or vulnerability to heat gain and loss, but in general Schohan says their thermal performance and resistance to condensation have evolved to become considerable strengths.

He says a framing system’s thermal performance is less effective when the aluminum that sits inside the conditioned space connects with the outside unconditioned material. “In cold climates the aluminum acts like an ice cube in your building, which requires more energy to control interior temperature. This makes the type of framing system, and the performance of that system, critical when considering the energy performance of a building,” he wrote for gb&d.

Schohan says thermal breaks in aluminum framing systems were first introduced to help solve the issue of high thermal conductivity as part of the response to the energy crisis in the 1970s. Today thermal break technology is even more advanced, as the outside of the aluminum frame is thermally isolated from the inside of the aluminum within the glazing system. This process delivers strong energy savings economically.

Schohan says products like YKK AP’s ThermaBond Plus and MegaTherm further save energy and reduce condensation, delivering proven performance over the life of the building.

Thermal enhancement technologies in aluminum framing systems significantly reduce heat loss during cold weather yielding warmer interior surface temperatures on the frames, which helps to mitigate condensation and thereby increasing a building’s thermal performance.

Pros: Condensation resistance, increased building thermal performance

Con: Traditional aluminum framed windows lose large amounts of heat.