Aluminum clad windows versus all fiberglass windows

by | Jan 24, 2021 | Windows and Doors | 0 comments

Last year we posted a blog explaining the difference between different Marvin windows and door types. However, we did leave some questions unanswered. After watching a video we posted on another site, one of our subscribers still had a few questions. Most of their questions were about the main differences between aluminum and fiberglass options.

What’s the difference between aluminum and fiberglass?

The Marvin Signature series is the window selection with Aluminum cladding as an option. The Marvin Essential and Elevate Series both have fiberglass as their core. Let’s compare these lines to each other based on each component.

Window Core

The Marvin Signature series’s core composition is wood, while the core of the Marvin Essential and Elevate Series is fiberglass. Some of our clients prefer a wood window, so we offer them the Marvin Signature (Ultimate). The wood core has some thermal resistance; however, the fiberglass core has a slight performance edge regarding energy efficiency. The difference is minimal, so most of our clients do not choose the core based on energy efficiency.

Window Glass

The performance of the glass is pretty much the same for all the windows, given the same specifications. If you choose a divided light option, the performance decreases slightly versus glass clear from edge to edge. The Marvin Signature uses aluminum cladding, slightly more conductive than fiberglass. However, both are applied with a tape that does provide a thermal break. The glass performance does matter. Ensure you design the glass based on your climate zone and where the windows are on the house.

Cladding options

Besides the core, the cladding is where you see the biggest difference between the wood and fiberglass core options. The only options you have for the exterior surface of the Marvin Signature windows and doors are “wood” and “aluminum cladding.” The wood is not a cladding for the Signature series. It is part of the sash and assembly, so it still needs a finish coat applied to it, or the elements will cause failure. The extruded aluminum has a very durable coating called Kynar. This finish is tested to be fade and failure resistant for several decades in areas with direct sunlight.

The fiberglass windows (Essential and Elevate) only have fiberglass as the exterior surface. The Elevate series is slightly different from the essential in that it has wood cladding on the inside of the window or door. This allows for a stain or custom paint to be added to it, not in the standard options for fiberglass.

The aluminum and fiberglass come in standard colors, so you don’t have to paint them. If you even did want the option to paint them, the fiberglass is much easier to apply a coating than aluminum. You can’t go to your local hardware store and Kynar. If you want to change the sash and frame color in the future, we recommend Marvin Essential or Elevate Series.


Both fiberglass and aluminum clad options are very durable. The warranty on the finish for the aluminum was 20 years, according to their site. This durability is much better than the oil-based coatings, which will show discoloration between 2-5 years.

Most Marvin showroom has a display showing a metal weight pulling down on a fiberglass sash. Compared to a composite or PVC window, there is minimal deflection in fiberglass. Marvin does not make an all-aluminum window since they are very inefficient without a thermal break. Fiberglass has a higher strength-to-weight ratio than aluminum. However, this is not a fair comparison since the cladding does not add strength to windows and doors.

Fiberglass may have an edge in coastal environments since it is resistant to the elements and potential corrosion from salt water.

Is the fiberglass option more expensive?

You would think that the Marvin Elevate series would be the most costly option due to its versatility. However, that is not the case. The Marvin Signature series costs more due to the extra manufacturing it takes to process and mill the wood. Aluminum is also slighting more expensive to work with. Aluminum needs the extra step of coating it after it is shaped. The fiberglass has added pigments that are the same color through the core.

Which one is easier to work with?

If you have a Marvin Signature Series and Marvin Essential placed side by side, you won’t see a significant difference. The installation is the same as well. They can both come with nail fins. They also install the same, and flashing details are the same for both. The main indicator they are different is that cladding is either aluminum or fiberglass. Both have a wood appearance on the interior.

What is a “thermal break”?

Highly conductive materials used on window and door construction, such as aluminum or steel, are inefficient in more circumstances. During the summer months, the sun’s heat will add to the cooling load of the structure in warm climate zones. Conversely, the heat you are paying for in the winter months will travel through these materials to the house’s exterior. To minimize heat loss or heat gain, a “thermal break” is needed. Thermal breaks need to be non-conductive, so the heat will not continue to transfer through it and will “break” at that point. The core of Marvin windows and doors are either wood or fiberglass, which both resist heat transfer, so an additional thermal break is not needed.

Which is best for your project?

Any Marvin product is going to work well in your home. We usually don’t go through each of these benefits with our clients and narrow them down based on the design. Each line comes in different shapes and sizes, which might be the limiting factor. Marvin Signature will be the best option if you want a custom window to replace an existing one in a 200-year-old church.

We typically start with Marvin Elevate, the fiberglass core with wood cladding on the interior, because it is the most versatile. We started getting pushback from some of our clients on the higher-end lines. Not due to the price. They wanted the option to be able to paint in 10 years. The Kynar finish is designed to last decades. Therefore, it should not be painted. However, the fiberglass can be painted, even though it was not designed to be or recommended.

Contact us today if you have additional questions about windows and doors and want a consultation for your home. We will set up a time to discuss the best solution for you.


Additional Resources

Other blog posts or discussions:

Marvin Online Reviews

ALL Windows and Doors






Pin It on Pinterest