7 min read · Cost
A lot of California homeowners cross-shop the Andersen 100 Series against fiberglass windows believing they're comparing two fiberglass products. They aren't. Fibrex is Andersen's proprietary wood-polymer composite — a genuinely good material, but a different one — and the confusion is worth clearing up before you sign a quote, because the two materials behave differently in heat, take different shapes, and carry different price postures. This is the honest side-by-side: what each material actually is, where each wins, and what to know so you buy what you think you're buying.
What Fibrex actually is
Fibrex is Andersen's proprietary composite: per Andersen, a blend of roughly 40% wood fiber by weight — mostly reclaimed from its own manufacturing — and 60% thermoplastic polymer, with each wood-fiber cell encased in polymer. Andersen states the material is twice as strong as vinyl, resists fading and peeling, and performs well in temperature extremes. Because it's extruded rather than pultruded, Fibrex can be formed into curved and specialty shapes that straight fiberglass lineals can't match. It anchors the Andersen 100 Series, one of the most widely installed replacement windows in the country. None of that makes it fiberglass — it's a wood-plastic composite, closer in family to composite decking than to pultruded glass strands.
What true fiberglass actually is
True fiberglass frames are pultruded: continuous glass-fiber strands saturated with resin and pulled through a heated die into straight structural lineals. The material class — Marvin's Ultrex, Milgard's Ultra, Pella's Duracast — is stiffer and stronger than composites, and its defining trait for California is thermal behavior: fiberglass expands and contracts at nearly the same rate as the glass it holds, so the frame and glazing move together through valley heat cycles instead of stressing the seals. Marvin, for its part, claims Ultrex is eight times stronger than vinyl and shape-stable to 285°F. The trade-off of pultrusion is geometry — lineals come out straight, so curved-top and specialty-shape windows are harder to get in full fiberglass. Our manufacturer landscape guide covers who actually makes it.
Strength, heat, and California sun
Both materials are meaningfully stronger and more stable than vinyl, which is the right baseline for this comparison — neither is a budget compromise. The distinction shows at the margins. Fiberglass is the stiffer material, carries thinner frame profiles with more glass area, and its expansion match with glass is the gold standard for seal longevity in hot-summer climates; independent comparisons like Brennan's call out that expansion match as fiberglass's key advantage. Fibrex's wood-fiber content gives it solid rigidity and good temperature performance per Andersen, and in normal California residential service it holds up well. For dark frames on full-sun elevations — the demanding case — fiberglass's thermal headroom is the more conservative spec. For typical exposures, both materials are durable choices, and the install detail matters more than the material gap.
Shapes, finishes, and where each wins
Fibrex's extrudability is a real advantage: curved tops, circles, and specialty shapes come easier and cheaper than in fiberglass, where those shapes often force a different product line entirely. Andersen's factory finishes on Fibrex are durable and warranted against flaking and peeling per the company's literature. Fiberglass counters with paintability — most fiberglass lines accept field repainting down the road, and factory finishes like Marvin's AAMA 624-verified acrylic are engineered for UV. On sightlines, fiberglass's strength allows slimmer frames and more glass. Which Andersen is which matters here too: the 100 Series is Fibrex, while the A-Series is a wood window clad in fiberglass and Fibrex — so even within one brand you can land on either side of this comparison. Check any unit's actual numbers against your climate zone with our U-factor and SHGC explainer.
Price posture and the honest bottom line
Qualitatively: Fibrex windows — effectively the Andersen 100 Series — sit in the strong mid-tier, typically above quality vinyl and usually a step below true fiberglass lines, though dealer pricing and glass packages move individual quotes around. Fiberglass carries the premium posture of its category. Both are legitimate mid-to-premium choices, and we'd install either with confidence in the right application. The honest bottom line is not that one material is a trap — it's that you should know what you're buying. If a bid says 'fiberglass' and the product is a 100 Series, that's a mislabel worth correcting before you compare numbers. Line up the bids by material class, verify NFRC ratings and ENERGY STAR eligibility for your zone, and weigh the installer's flashing detail as heavily as the frame material — the full decision framework is in our window replacement guide and window replacement scope.
On install day: how the two materials handle
Installers experience these materials differently, and the differences explain some quotes. Pultruded fiberglass arrives stiff — long lineals that hold square, resist racking in the opening, and don't bow when a big unit is walked across a yard. That stiffness is mostly virtue, but it comes with a demand: the rough opening has to be right, because a fiberglass frame will not flex to disguise an out-of-square wall the way a softer frame might. Shimming is where a fiberglass install is won or lost, and careless fastening near a lineal's edge can crack material that would merely dimple on a composite. Fibrex handles closer to dense wood-plastic: it takes fasteners forgivingly, trims predictably, and tolerates the small persuasions real-world openings demand. Movement allowances differ too. Fiberglass barely moves against the glass, so its perimeter sealant joint leads a quiet life; Fibrex sits between fiberglass and vinyl in thermal movement, so on large, sun-struck units the joint design — gap width, backer rod, sealant choice — earns more attention. None of this makes either material hard to install; it makes them different to install, and a crew fluent in one can be clumsy with the other. When interviewing bidders, ask what share of their installs use the frame they're quoting you — familiarity with the specific material is worth more than any brochure claim.
A decade on: repainting, repairs, and parts
Fast-forward ten or fifteen years and the ownership stories diverge in specific ways. A fiberglass frame's color can be renewed: with proper surface prep and a bonding primer, a field repaint is a normal maintenance event, which means the frames can follow a future exterior palette change — a real option if a re-side sits anywhere in your long-range plans. Fibrex takes the opposite posture: Andersen engineers and warrants the factory finish, and field repainting is not part of the material's design intent — before assuming a Fibrex frame can change color someday, get Andersen's current finish guidance in writing, because the manufacturer's position is what the warranty rides on, not a painter's confidence. On repairs, both materials age around their frames rather than through them: the parts that actually wear across a decade are glass seals, balances, locks, and weatherstrip. Andersen's scale is an asset here — its parts operation and dealer network can match hardware to units decades old. The fiberglass makers service through their dealer channels, and a fogged unit is typically cured by replacing the sash or the insulated glass without touching the frame. The question worth asking either way is not whether the frame will last — both will — but who supplies the twentieth-year sash and how you identify the unit to them, which is an argument for keeping the order paperwork with the house documents.
Fibrex vs. true fiberglass at a glance (qualitative)
| Attribute | Fibrex (Andersen composite) | True fiberglass (pultruded) |
|---|---|---|
| What it is | ~40% reclaimed wood fiber / 60% polymer | Pultruded glass-fiber lineals in resin |
| Strength vs. vinyl | 2x (per Andersen) | Stiffer still — thinner frames, more glass |
| Heat behavior | Good temperature performance (per Andersen) | Expansion nearly matches glass — best seal protection |
| Shapes | Extrudable — curves and specialty shapes | Straight lineals — specialty shapes limited |
| Price posture | Strong mid-tier | Premium tier |
| Where you find it | Andersen 100 Series | Marvin Ultrex, Milgard Ultra, Pella Impervia |
Key takeaways
- Fibrex is Andersen's proprietary composite — about 40% reclaimed wood fiber, 60% thermoplastic polymer — not fiberglass
- True fiberglass is pultruded glass strands; it expands at nearly the same rate as glass, protecting seals in California heat
- Fibrex extrudes into curved and specialty shapes more easily; fiberglass lineals are straight and stiffer
- Andersen 100 Series = Fibrex; Andersen A-Series = fiberglass-clad wood — know which side of the comparison you're on
- Both are solid mid-to-premium materials; the real risk is paying for one while believing it's the other
FAQ
Quick Answers
No. Fibrex is Andersen's proprietary composite of roughly 40% reclaimed wood fiber and 60% thermoplastic polymer. Fiberglass is pultruded glass-fiber strands in resin. They're different material classes with different strength, heat behavior, and shape capabilities.
Both handle California service well, but fiberglass has more thermal headroom — it expands at nearly the same rate as glass, which protects the glazing seals through hot-summer cycles. For dark frames on full-sun elevations, fiberglass is the more conservative spec; for typical exposures, either performs.
Yes — it's a well-regarded, widely installed mid-tier replacement window, and Andersen states Fibrex is twice as strong as vinyl. Our only caution is labeling: it's a composite window, not a fiberglass one, so compare it against other composites and quality vinyl as well as fiberglass when you line up bids.
Usually the reverse — Fibrex (the 100 Series) generally sits a step below true fiberglass lines in price posture, though glass packages, sizes, and dealer pricing move individual quotes. Get both bid on the same opening spec and compare directly rather than relying on category generalizations.
Treat the factory finish as the finish. Andersen warrants its applied color and does not position Fibrex for field repainting the way fiberglass makers do — so if changing frame color down the road matters to you, that alone points toward fiberglass. Confirm any exception against Andersen's current written guidance, not installer assurances.
Different rather than harder. Fiberglass is stiffer, so it demands a square, well-shimmed opening and careful fastening; Fibrex is more forgiving of real-world walls. The installer's fluency with the specific material matters more than the material itself — ask bidders how often they install the frame they're quoting.
Usually neither frame fails at all — what wears out on both is glass seals, balances, and weatherstrip. Fiberglass holds the edge in long-term heat stability and seal protection, but install quality and glass package decide most real-world lifespans, which is why the crew deserves as much scrutiny as the material.
Sources
Authoritative references
- Andersen — Fibrex composite material (40% reclaimed wood fiber / 60% thermoplastic polymer)
- Andersen — 100 Series windows & doors (Fibrex composite)
- Andersen — clad-wood windows & doors (A-Series fiberglass/Fibrex exterior)
- Marvin — Ultrex pultruded fiberglass (materials overview)
- Brennan Enterprises — Fibrex vs. fiberglass windows (independent comparison)
- 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.