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Frame material guide

All frame materials

Pultruded fiberglass

Glass-fiber reinforced walls expand and contract at nearly the same rate as window glass, keeping seals tight in extreme heat and cold.

3D window model

The window with a frame of each material

Pultruded fiberglass

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Switch between vinyl, fiberglass and wood to see the window with that frame; drag or use ← / → to rotate it.

What it is and how it is built

Glass fibers soaked in resin are pulled through a heated die, which gives a thin but very rigid composite shell. The cavities inside can be filled with insulation, the finish is applied at the factory, and weatherstripping seals the sash against the frame.

Why you'd want it

Fiberglass is the frame for hard conditions: strong sun on south and west walls, dark exterior colors and large swings between winter and summer. Because it is partly made of glass, it moves with temperature much like the glass it holds, so the seals stay tight.

Vinyl covers the common case for less money, and wood is chosen for a real wood interior; fiberglass is the choice when shape stability and a dark finish matter more than the lowest price.

Pros and cons

Pros

  • Holds its shape in strong sun, even with a dark exterior color.
  • Moves with heat and cold less than vinyl, which helps the seals of the glass unit.
  • A rigid profile allows slimmer frames and more visible glass, and the frame can be repainted later.

Cons

  • Costs more than a vinyl frame of the same size.
  • Fewer models, colors and installers in some regions than for vinyl.
  • Its warmth depends on the insulation inside the profile, so an unfilled frame gains little over good vinyl.

It's for you if…

  • You want black, bronze or other dark frames on south- or west-facing walls in strong afternoon sun.
  • Your climate swings from freezing winters to hot summers and you want minimal frame movement.
  • You prefer slimmer frames that leave more visible glass in each opening.

Don't overpay if…

  • You are ordering white or almond frames in a mild climate, where quality multi-chamber vinyl does the job.
  • You are told fiberglass alone stops street noise: quiet comes mostly from the glass unit and the air tightness.
  • A salesperson claims only fiberglass prevents condensation, when indoor humidity and the glass spacer matter most.

How it affects the price

Fiberglass usually costs more than vinyl and is often close to clad wood: pultrusion is slower than extruding vinyl, the material is more expensive, and the finish is applied at the factory. The exact gap depends on the size of the window and the finish.

Within fiberglass the price moves with the insulation inside the profile, a dark or custom color, the size of the opening and the number of sashes. The market reference for each material is on the page of all frame materials.

All materials and their price references →

Lifespan and care

Fiberglass needs no regular painting: wash the frame with mild detergent and water once or twice a year, keep the tracks and weep holes clear and check the weatherstripping around moving sashes each season. If you change the color of the facade, the frame can be repainted with a paint suited to fiberglass.

Figures with sources

Shape stability
Dimensionally stableSource:Types of Windows — U.S. Department of Energy, Building Science Education Solution Center
Thermal performance with insulated cavities
With the air cavities filled with insulation, superior to wood or uninsulated vinylSource:Types of Windows — U.S. Department of Energy, Building Science Education Solution Center
Expansion with temperature compared with vinyl
Vinyl frames expand and contract more than fiberglassSource:Fiberglass windows come of age — The Construction Specifier (March 2022)

What it pairs with

A fiberglass frame pairs well with a double-pane glass unit and a coating that suits a sunny wall: the frame keeps its shape in the heat, and the coating keeps the sun's heat out of the room. A careful installation keeps the seal around the frame as tight as the frame itself.

Double-pane glass for a fiberglass frame →A glass coating for a sunny wall →Compare installation methods →

Compared with the other frames

Published market range

Your estimate shows prices for your windows and installation.

Frequently asked questions

Can a fiberglass frame be repainted?

Yes. The factory finish holds its color for a long time, but if you change the color of the facade, the frame can be repainted with a paint suited to fiberglass; check the manufacturer's instructions first.

Why does fiberglass cost more than vinyl?

The material is more expensive, pultrusion is slower than extruding vinyl, and the finish is applied at the factory. You pay for shape stability and a dark finish that holds up in strong sun.

Is fiberglass warmer than vinyl?

It depends on the construction and the insulation inside the profile. A fiberglass frame with insulated cavities performs well, but compare the whole window by the U-factor on its NFRC label.

Is it suitable for dark colors and south-facing walls?

Yes, that is where it makes the most sense: it holds its shape in strong sun and with a dark finish, where a budget vinyl profile can soften and bow.

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