Terminology

• Pultrusion: continuous process pulling glass rovings and mats through a resin bath and heated die
• Composite window: broad term; may mean fiberglass, wood-fiber composite, or blends
• Foam-filled frame: insulated hollow chambers for lower U-factors
• Coefficient of thermal expansion: fiberglass is close to glass, so seals stay stable
• Factory finish: baked or powder-coated paint; paintable in the field
• Wood interior option: fiberglass exterior with wood veneer or wood interior trim
• Triple-glazed: three-pane IGUs common in high-performance fiberglass lines

Standards

Ratings

• NAFS class: R, LC, and CW lines; some fixed units higher
• Thermal: frames insulate well; with triple glazing, whole-window U-factors suitable for passive house projects
• Dimensional stability: much lower movement than vinyl; dark colors stable

Test standards

• AAMA/WDMA/CSA 101/I.S.2/A440 (NAFS-22)
• AAMA 305: fiber-reinforced thermoset profiles
• NFRC 100, 200, and 400

Materials

• Pultruded fiberglass: frames and sashes
• Finishes: factory paint or powder coat; field repaintable
• Interiors: painted fiberglass or wood veneer
• Glazing: double or triple low-E IGUs; laminated where required

Detailing

• Flash and install like other nail-fin or block-frame windows; sill pans with back dams
• Use frame stiffness to reduce mullion size in large mulled assemblies
• Specify triple glazing and warm-edge spacers where condensation and comfort matter
• Coordinate field-paint compatibility if colors may change

Uses

Specific uses

• Cold and hot climates with large temperature swings
• Dark exterior colors where vinyl would warp
• High-performance and passive house housing
• Multifamily and light commercial needing slimmer frames than vinyl
• Coastal sites (no rot, rust, or corrosion)

Pros and cons

Pros

• Low thermal movement; stable seals and dark colors
• Strong, slim frames; larger sizes than vinyl
• Low maintenance; no rot or corrosion
• Good thermal performance

Cons

• Costs more than vinyl
• Fewer manufacturers and styles than vinyl or wood
• Field-cut ends and damaged finishes need repair
• Combustibility treatment varies by product; check for commercial use

Manufacturing and cost

Construction

• Profiles pultruded, cut, and joined with corner keys, adhesives, and screws
• Finished, glazed, and fitted with hardware at the factory

Main manufacturers and suppliers

• Marvin (Infinity and Elevate lines)
• Pella
• Milgard
• Andersen
• Alpen High Performance Products
• Cascadia Windows and Doors

Trade association: Fenestration and Glazing Industry Alliance.

Typical costs

• Fiberglass window, installed: $1,100 to $2,200 per window; budget lines from about $500
• Frame and sash only: about $250 to $600 per window

Sources

• Pella: replacement window cost by material, 2026, https://www.pella.com/ideas/windows/replacement-window-cost/
• Infinity from Marvin: vinyl vs. fiberglass windows, https://infinitywindows.com/blog/vinyl-vs-fiberglass-windows
• Inspect and Test: vinyl vs. fiberglass windows, https://inspectandtest.net/guides/vinyl-vs-fiberglass-windows/
• Oknoplast: window cost by material, https://oknoplast.us/?p=25119
• Sweets: 08 00 00 openings, https://sweets.construction.com/masterformat/companies/08-00-00-openings
• Wikimedia Commons: Detailed view of a cross-section of a modern window frame shows multiple materials and insulation layers.jpg, by Shixart1985 (CC BY 2.0), https://commons.wikimedia.org/wiki/File:Detailed_view_of_a_cross-section_of_a_modern_window_frame_shows_multiple_materials_and_insulation_layers.jpg