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Silex Windows and Doors have the Fiberglass Advantage

fiberglass frame

Fiberglass Frame

Factory-assembled fiberglass windows with outward-opening sash or tilt in opening sash or full piece fiberglass installed in frame.  3 1/4” Pultruded Fiberglass Closed Back Frame completely filled with laser die cut Polystyrene, PBT corner key reinforced Mechanical joint Corners, sealed with silicone sealant and are independently tested to AAMA 101/I.S.2, CSA 101/I.S.2 /A440-05 standards.

  1. Fiberglass Profile Frame
  2. Fiberglass Sash
  3. Insulated Frame
  4. Exterior Brickmould or Panning
  5. Insulating Glass Unit
  6. Interior Jamb Extension
  7. Foam Insulated Weather Stripping
  8. Glazing Bead

Fiberglass Advantages

High Dimensional Stability

The coefficient of expansion/contraction of pultruded fiberglass material is extremely low and almost identical to that of float glass. The low thermal expansion rate means tighter seals against weather and longer lasting. Vinyl expands at over 7x this rate.

Low Thermal Conductivity / Energy Efficient

Pultruded Fiberglass profile insulates better than any of the traditional window and door materials. The thermal conductibility ratio is 500x less than aluminum. Insulating the frame and sash components reduces the conductivity even further, allowing for the creation of some of the most energy efficient window and door systems.

Environmentally Friendly

Fiberglass Strings are made mostly from silica sand (sustainable natural resource), the lineal manufacture process consume the least embodied energy when compared to other common window frame materials such as Aluminum and PVC.

Low Maintenance

Fiberglass is the most stable material which in virtually unlimited colors and an interior wood veneers. The maintenance-free frames stay true to form due to strength of materials. No need for sanding, painting, or staining.

Structural Integrity

Fiberglass is used to replace steel in many applications due to its high strength to weight ratio.

  • Fiberglass is 3x stronger than wood
  • Fiberglass is 3.5x stronger than vinyl/wood composites
  • Fiberglass is 8x stronger than vinyl

Fiberglass replacement windows are efficient and maintenance-free

In my opinion, fiberglass windows are worth the extra initial expense. You can get some relatively inexpensive vinyl and aluminum windows. If you compare the fiberglass windows to top-quality, name-brand windows using other frames materials though, often the cost is not much higher.

Fiberglass windows are grabbing a bigger share of the replacement window market every year. A decade ago, only a handful of northern and Canadian companies offered fiberglass windows because of their high energy efficiency. Today, some of the major name-brand manufacturers are offering them along with their existing line of standard replacement windows.

Fiberglass is the strongest window frame material available other than metal which is not a natural insulator. The frames are made of about 75 percent glass fibers and 25 percent resin. The glass fibers are made from silica sand which is melted and spun. Silica sand is in abundant supply.

Another advantage of a fiberglass frame is it reacts to outdoor temperature changes much like glass panes do. This makes the entire window system and weatherstripping more airtight. Internal stresses are reduced because all the components are expanding and contracting at about the same rate.

The pultrusion process, by which most fiberglass windows frames are made, is different than for making boat hulls. For the frames, long glass fibers are pulled through a die (similar to a cookie cutter) and coated with resin. It is a thermoset resin so once it sets up, heat cannot weaken it. For dark colored frames exposed to the hot southern sun, this is an advantage over vinyl.

Fiberglass is maintenance-free and will never crack, split or warp, even in humid climates. If you prefer the indoor look of natural wood, you can order fiberglass frames with a real wood veneer of hemlock, oak, cherry, etc. The veneer is permanently bonded to the indoor surface of the frame.

Fiberglass frames are painted at the factory. There are usually three to five standard colors available with custom colors at a slight surcharge. Spilt colors (different indoors and outdoors) are also available. Fiberglass is easy to paint yourself, so you can change the color anytime.

The hollow fiberglass frame is an effective natural insulator, but you can select ones with additional foam insulation inside the frames. Since you are considering high-quality windows, pay a little extra for the most efficient glass options such as low-e argon, triple-pane or Heat Mirror.

Fiberglass Properties Compared to Traditional Materials


Window Material Properties


Modulus Numbers
These are the modulus numbers for common window frame materials:

  • Aluminum 11,000,000
  • Fiberglass 6,500,000
  • Wood 1,450,000
  • Vinyl 410,000

The modulus numbers tell us the stiffness of a material. This is an important number to know when selecting a window material.

Using a 1’ long piece of 1″ hollow square tubing with a 0.90″ wall thickness, and a 40 lb. load on the end of the tube. * Wood is solid.

modulus numbers


Yield Strength
These are the yield strength numbers for common window frame materials:

  • Fiberglass 60,000
  • Aluminum 35,000
  • Vinyl 8,000
  • Wood 800

Yield strength and yield stress are the same. Yield strength indicates the margin of safety built into a window.

Using the same tubing we can indicate how much weight the material can withstand before breaking.

yield strength


Yield Strength
These are the expansion numbers for common window frame materials:

  • Fiberglass .000004
  • Aluminum .000012
  • Vinyl .000029
  • Wood depends on how much moisture is absorbed

Building a sash, we heat it up to 120 °F and watch what happens.

fiberglass sash

Fiberglass Sash
Nothing happens to the glass and sash. They both expanded at the same amount.
0.007″ – about the same thickness as 1.5 sheets of paper

aluminum sash

Aluminum Sash
The sash expanded more than the glass.
0.015″ – about the same thickness as 4 sheets of paper

vinyl sash

Vinyl Sash
The sash expanded more than the glass.
0.045” – this can cause leaking seals


Yield Strength
These are the thermodynamic properties for common window frame materials:

  • Fiberglass 0.11 at 280°F heat deflection
  • Aluminum 0.11 at 140°F heat deflection
  • Vinyl 0.20
  • Wood 140

Heat loss thru different window frame materials excluding the glass package


Glass Performance Data

Glass Performance Data

* SHGC – Solar Heat Gain Coefficient

Shading Coefficient = SHGC / 0.87

*LSG – Light to Solar Heat Gain Ratio

Performance values are based on 3 mm Glass COG


Fiberglass Structural Integrity

High Dimensional Stability

Fiberglass expansion rate is nearly identical to the rate of glass. This low thermal expansion rate translates into virtually zero seal failure. Which makes the entire window system and weather stripping more airtight. Internal stresses are reduced because all the components are expanding and contracting at about the same rate.

Fiberglass Structural Integrity

Material Expansion Rates

Expansion of various framing materials compared with glass

Material Expansion Rates
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