7 min read · Cost
Search for fiberglass windows and you'll find plenty of brands using the word — but only a handful of manufacturers actually pultrude true fiberglass window frames, and at least one famous 'fiberglass-adjacent' product isn't fiberglass at all. Knowing who really makes the material matters, because the manufacturer landscape is small enough that your quote will almost certainly involve one of four names. This guide maps who makes what, straight from the manufacturers' own materials pages, so you can tell true fiberglass from composite before you compare bids. For our take on which line to actually choose, see the companion best fiberglass windows guide — this page is about who builds them.
What makes a window 'true' fiberglass
True fiberglass frames are made by pultrusion: continuous glass-fiber strands are saturated with resin and pulled through a heated die, producing straight structural lineals that are then cut and joined into sashes and frames. The result is a material whose thermal expansion nearly matches the glass it holds — the frame and the glazing move together through temperature swings, which is why fiberglass units protect their seals so well in California heat. The Efficient Windows Collaborative covers the material class alongside the alternatives. The practical test when shopping: if the manufacturer describes pultruded glass fibers, it's fiberglass; if it describes a blended or extruded wood-polymer material, it's a composite — a different material with different behavior, covered in our Fibrex vs. fiberglass comparison.
Marvin — Ultrex fiberglass (Essential and Elevate)
Marvin is the manufacturer most identified with pultruded fiberglass, which it brands Ultrex. Per Marvin's materials page, Ultrex is made by pultrusion — long lineal glass fibers embedded and concentrated as the material is pulled through a die — and the company claims it is eight times stronger than vinyl, holds its shape at temperatures up to 285°F, and carries an acrylic finish three times thicker than competitive coatings (AAMA 624 verified). In the current lineup, the Essential collection is Ultrex fiberglass inside and out, while Elevate pairs an Ultrex exterior with a wood interior. For California heat and dark exterior colors, those are the claims that matter — verify any specific unit's numbers through its NFRC label rather than the brochure.
Milgard Ultra and Pella Impervia — the other true fiberglass lines
Two more national manufacturers pultrude true fiberglass. Milgard's C650 Ultra Series is its fiberglass line — Milgard is a longtime West Coast manufacturer with deep California dealer coverage, which in practice means competitive lead times and easy service here. Pella's Impervia line is built from what Pella brands Duracast, a five-layer pultruded fiberglass reinforced with a patented interlocking mat; Pella states the thermoset material won't expand, warp, or sag in summer heat, with a powder-coat finish meeting AAMA 624. Both sit in the accessible mid-tier of the fiberglass category — real pultruded frames without premium-custom pricing — and both are widely stocked through NorCal dealers and big-box special order.
Andersen — fiberglass-clad, but read the labels carefully
Andersen is where shoppers get tripped up. The A-Series, Andersen's top-performing line, uses a wood frame with a durable exterior of fiberglass and Fibrex cladding — so it's a fiberglass-clad wood window, not a full-fiberglass frame. The popular 100 Series, meanwhile, is not fiberglass at all: it's built from Fibrex, Andersen's proprietary composite of roughly 40% reclaimed wood fiber and 60% thermoplastic polymer. Fibrex is a legitimate material — Andersen states it is twice as strong as vinyl — but it is a wood-plastic composite, and anyone cross-shopping it against Marvin, Milgard, or Pella fiberglass should know they're comparing different material classes. Our Fibrex vs. fiberglass guide breaks down exactly how they differ.
Who doesn't make fiberglass — and why the list is short
Notably absent from the true-fiberglass list: most vinyl-focused brands, and regional wood-window makers like Sierra Pacific, whose lines are wood, aluminum-clad wood, and vinyl rather than pultruded fiberglass. Pultrusion requires dedicated tooling and dies, which is why the manufacturer set has stayed small — the material is harder to make than vinyl and harder to shape than composites, since pultruded lineals come out straight (curved and specialty shapes are where composites and clad wood have the edge). The short list is actually good news for shoppers: with essentially four manufacturers in the conversation, comparing fiberglass quotes is manageable. Weigh frame material against the full picture in our frame materials guide, and remember that on any line, the install detail decides whether the material's advantages survive — which is where a window replacement scope done right earns its keep.
Who makes true fiberglass windows — and what's actually composite
| Manufacturer / line | Frame material | As described by the manufacturer |
|---|---|---|
| Marvin Essential | Pultruded fiberglass (Ultrex) | Ultrex inside and out |
| Marvin Elevate | Ultrex exterior, wood interior | Fiberglass-exterior hybrid |
| Milgard Ultra (C650) | Pultruded fiberglass | Fiberglass line from a West Coast maker |
| Pella Impervia | Pultruded fiberglass (Duracast) | Five-layer pultruded, patented mat |
| Andersen A-Series | Wood, fiberglass/Fibrex-clad exterior | Fiberglass-clad wood — not full fiberglass |
| Andersen 100 Series | Fibrex composite | 40% reclaimed wood fiber / 60% polymer — not fiberglass |
Key takeaways
- Only a handful of manufacturers pultrude true fiberglass frames: Marvin (Ultrex), Milgard (Ultra C650), and Pella (Impervia/Duracast) lead the category
- Andersen's A-Series is fiberglass-clad wood; the Andersen 100 Series is Fibrex composite — not fiberglass
- Pultruded fiberglass expands at nearly the same rate as glass, which is why it protects seals through California heat cycles
- Pultruded lineals are straight — composites and clad wood serve curved and specialty shapes better
- Verify every unit's performance through its NFRC label, not the brochure, and weigh the installer as heavily as the manufacturer
FAQ
Quick Answers
Marvin (Ultrex fiberglass in the Essential and Elevate collections), Milgard (C650 Ultra Series), and Pella (Impervia, built from Duracast pultruded fiberglass) are the principal manufacturers of true pultruded fiberglass windows sold in California.
Mostly no. The Andersen 100 Series is Fibrex, a composite of about 40% reclaimed wood fiber and 60% thermoplastic polymer — not fiberglass. The A-Series uses fiberglass and Fibrex as exterior cladding over a wood frame, so it's fiberglass-clad wood rather than a full-fiberglass window.
A manufacturing process where continuous glass-fiber strands are saturated with resin and pulled through a heated die, producing straight structural lineals. It's what gives true fiberglass frames their strength and their near-zero thermal expansion relative to the glass they hold.
For long-tenure owners and sun-exposed elevations, usually yes — the dimensional stability protects seals through heat cycles that work vinyl loose. It costs more upfront, and our best-fiberglass-windows guide covers which line fits which project tier.
Sources
Authoritative references
- Marvin — Ultrex pultruded fiberglass (materials overview)
- Milgard — C650 Ultra Series fiberglass windows
- Pella — Impervia fiberglass windows (Duracast)
- Andersen — Fibrex composite material (40% reclaimed wood fiber / 60% thermoplastic polymer)
- Andersen — clad-wood windows & doors (A-Series fiberglass/Fibrex exterior)
- Efficient Windows Collaborative (NFRC) — window frame materials
- National Fenestration Rating Council (NFRC) — window performance ratings
External links to government, code, and manufacturer sources. Sierra Siding is not affiliated with these organizations; references are provided for verification.