Windows — Treepublic Solar


If you are looking for a window product with longevity and durability, look no further than fiberglass.

But don't take our word for it. Do your own research. Fiberglass adopters have been those who like to thoroughly research important home improvement products. We predict that the more you research your window options, the more you will like fiberglass.

Below is a compilation of facts and figures on fiberglass which we hope you will find informative:



Fiberglass is made by a process called pultrusion, in which thousands of glass fibers are pulled through a die. In contrast, aluminum and vinyl are extruded, which means pushing a material through a die to shape it.

Fiberglass composite consists of glass fibers and the resin that bind it together. Glass fiber has a high tensile strength, and will not break, similar to a reinforcing bar in a concrete mix. The resin acts like concrete, because it performs well in compression. Together, the glass fibers and resin are good in both compression and tension, creating a strong material. Since the window is, in effect, all glass, it comes without the frame expansion and contraction that causes vinyl windows to lose their dimensional integrity. Fiberglass can be used in hot climates and can be painted dark colors, even in high sun exposure applications. Fiberglass, nine times as strong as vinyl, is made using thermoset resin. Vinyl windows are made from PVC, a thermoplastic resin, which often results in what is called “vinyl smile”, when the head of a large under intense heat. (“Fiberglass Fenestration: A Durable, Sustainable, and Economic Alternative for Windows and Doors”, Barbara A. Nadel, Architectural Record)


Technical Information:

According to the American Composites Manufacturers Association (ACMA), Pultrusion Industry Council, the American Architectural Manufacturers Association (AAMA), and the Fiberglass Material Council, fiberglass has the following properties:

  • Fiberglass’ thermal conductivity is 800 times less than aluminum, making it the optimum material for super-insulating windows and preserving overall performance for the life of a window.
  • Hot and cold performance: Fiberglass composite rates excellent because it withstands heat up to 200 degrees Fahrenheit, and cold to -40 degrees Fahrenheit. Aluminum also rates excellent, but vinyl rates fair, because it warps, sags, and softens in extreme heat; and becomes brittle in extreme cold. As evidenced by the widespread use of fiberglass boats, fiberglass composite resists the corrosive effects of salt water, hot and cold temperature extremes, growth of mold, rot, and fungus, and can withstand long-term exposure to water, humidity, wind, and moisture
  • Thermally stable: Fiberglass composite has a very low coefficient of expansion. As a result, it moves very little as the climate changes, resulting in less stress on the installation, seals, and glazing of the window or door. Low thermal expansion maintains the windows’ structural integrity and minimizes warping or leakage, making fiberglass the most durable framing material that will not compromise the performance of the full window over time. Fiberglass composite has a similar thermal efficiency to wood and is ideally suited for windows and doors in any climate, and almost all building types. Fiberglass composite's low expansion and contraction rate (like glass) ensures a consistent bond between sash and glass and a weathertight fit between the sash and frame. It has three times less thermal expansion than aluminum frames, in testing performed in accordance with ASTM testing standards. Vinyl is considered only fair, because its high expansion and contraction rates (which differ substantially from the glass they hold) may compromise the air and water seal between glass and frame. Because the fiberglass frame expands at the same rate as the glass; no glazing gaskets to maintain because of expansion and shrinkage
  • Efficient: Comparing U-factor of materials, fiberglass offers 89% better insulation than aluminum (fiberglass U-factor is 0.2 – 0.3 compared to aluminum thermally broken of U-factor 1.0). Fiberglass composites rank high because of their excellent insulating properties, condensation resistance, and low U-values. (U-values are the inverse of R-values, with lower numbers indicating better insulating properties.) Aluminum rates fair, because it is a natural conductor, and a poor insulator; i.e. people seated near windows may be uncomfortable. Vinyl rates an excellent in this category.
  • Durability and Strength: Superior strength to weight ratios making fiberglass window ideal for large window openings. Fiberglass is the strongest, most durable material available in windows and doors. In ASTM testing, fiberglass composite material has excellent ratings, twice as strong as aluminum, and eight times stronger than vinyl. In addition to the inherent structural qualities, certain factory-applied finishes render fiberglass composite virtually indestructible, for an excellent rating. Aluminum rates a little lower at very good, because of the potential for thermal break failure, and it is prone to corrosion on seacoast environments. Vinyl rates good, because it becomes brittle over time and cannot be repaired easily if damaged.
  • Impact Resistance: Fiberglass is resistant to the dings and dents that frequently occur on the job site or in applications subject to daily abuse. The pultruded material distributes impact load to prevent surface damage even in sub-zero temperatures. Aluminum and vinyl are rated good and fair, respectively, as aluminum dents and vinyl shatters quickly at low temperatures.
  • Sustainable: Fiberglass composite is energy efficient, with a U-value 2.4 times better than thermally- broken aluminum. It provides superior thermal comfort to occupants seated near windows, and is a sustainable material that consumes low embodied energy to produce compared to aluminum and vinyl.
  • Unlike vinyl, fiberglass provides a great base for paint. Since fiberglass composite is very heat tolerant, it can be painted dark colors without concern for heat deformation.
  • Fiberglass reduces sound transmission from the outdoors to indoor spaces.
  • Installation, operation and maintenance: Fiberglass composite units typically arrive on-site pre-assembled and pre-finished, which means better quality control, reduced field installation time, and reliance on contractor’s own workforce rather than outside specialists. . Unlike wood, fiberglass windows are virtually maintenance-free.
  • Fiberglass windows are an economically competitive alternative to vinyl, aluminum, and wood window products.

In summary, fiberglass combines the best and most desirable qualities from other materials, such as durable and low maintenance aluminum, energy efficient vinyl, and wood, with its aesthetic, high performance, and environmentally friendly properties. The net result is that fiberglass composite offers a uniquely balanced, best-of-all-worlds window offering that is gaining widespread appeal from homeowners and contractors alike.

(Sources: Architectural Record, Remodeling Magazine, National Fenestration Rating Council)


Technical Information:


Summary Charts


Aluminum with
Thermal Break







Eight times stronger than vinyl; twice as strong as aluminum.

Very Good

Becomes brittle over time

Hot/Cold Performance

Withstands heat up to 200° F and cold to -40°F


Warps, sags and softens in extreme heat; becomes brittle in extreme cold


Low expansion and contraction rate assures consistent, weathertight seal between sash and frame

High expansion and contraction rate may break seal between glass and frame.

High expansion and contraction rate may break seal between glass and frame.


Withstands impact without deformation





Very Good
Potential for thermal break failure; prone to corrosion in seacoast environments.

Cannot be repaired easily if damaged.


Superior insulating properties, condensation resistance and U-values

Natural conductor, poor insulator. People seated by windows may be uncomfortable.


**** Excellent

*** Very Good

** Good

* Fair

*In testing performed in accordance with ASTM testing standards, fiberglass composite has displayed superior performance in strength, ability to withstand extreme heat and cold, and resistance to dents and scratches.


Fiberglass vs. Aluminum vs. Wood Comparison:




Corrosion Resistance

Superior weatherability and resistance to a broad range of chemicals.

Can cause galvanic corrosion.

Can warp, rot and decay from exposure to moisture, water and chemicals.
Coatings or preservatives required to increase corrosion or rot resistance can create hazardous waste and/or high maintenance.


Very lightweight - about 70% the weight of aluminum on a density basis. Specific gravity = 1.7
Pultruded fiberglass has significantly higher strength-to-weight ratio compared to both wood and aluminum

Lightweight - about 1/3 that of copper or steel.

Specific gravity = .51 (oven dried).

Electric Conductivity

Non-conductive - high dielectric capability.

Conducts electricity - grounding potential.

Can be conductive when it is wet.

Thermal Conductivity

Insulates - low thermal conductivity.
0.17 BTU/h-ft-F 5.0 x 10-6 in/in/°F
100 in. sample at a temperature change of 100°F = 0.05

Heat conductor - high thermal conductivity.
Pure Aluminum – 136.9 BTU/h-ft-F Alloyed Aluminum – 92.4 BTU/h-ft-F Thermal coefficient of expansion 11-13 (IN/IN/Fo)106.

Insulates - low thermal conductivity.
Pine/Spruce/Fir/Larch/Mahogany (softwoods) – 0.08 BTU/h-ft-F Oak/Maple (hardwoods) – 0.09 BTU/h-ft-F Low thermal coefficient of expansion 2.5 (IN/IN/Fo)106.


Ultimate flexural strength (Fu)
LW = 30 ksi
CW = 10 ksi.
Compression strength is 30,000 psi.
Tensile Strength (KSI) 60.0
Modulus of Elasticity (PSI x 106) 3.0
Pultruded fiberglass has 86% of the yield strength of aluminum and, pound-for-pound's, stronger than aluminum in the lengthwise direction.
Stiffness: Pultruded fiberglass is approximately 1-1/2 times as rigid as wood. Modulus of elasticity LW = 2.5 x 106 psi, CW = .8 x 106 psi.

Flexural strength (Fu) 35 ksi.

Extreme fiber bending = up to 2800 psi.*
Compression parallel to grain = up to 1800 psi.*
Stiffness: Modulus of elasticity = up to 1.8 x 106 psi.*
Deteriorates with age


Special colors available. Composite design can be customized for required finishes.

Silver color. Other colors require pre-finishes, anodic coatings and paints.

Must be primed and painted for colors. To maintain color, repainting and refinishing may be required.

Impact Resistance

Glass mat in pultruded fiberglass distributes impact load to prevent surface damage even in sub-zero temperatures. Will not permanently deform under impact.

Easily deforms under impact.

Permanently deforms or breaks under impact

Life expectancy

80+ years without any maintenance.

30 to 40 years less if exposed to a corrosive environment such as coastal environments and acidic rain.

15 to 20 years if properly maintained, sanded, sealed and refinished every year. Less if not properly maintained

(Source: Pultrusion Industry Council)

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