Updated for 2026 • Based on manufacturer specifications, third-party performance data, and St. Louis dealer experience
If you’ve been researching replacement windows and run into the term “Fibrex,” you’ve almost certainly been looking at Andersen Corporation products — either the Andersen 100 Series (sold through retail dealers) or Renewal by Andersen (sold through in-home consultations). Fibrex is Andersen’s proprietary composite window material, and it’s the single feature their marketing emphasizes most. Whether it’s the right material for your project, and whether it justifies the prices Andersen and Renewal charge, is a different question.
This guide explains what Fibrex actually is, where it’s used, what it does well, what its marketing overstates, and how it compares to the fiberglass and vinyl alternatives you’ll likely cross-shop against. We’re an exclusive Marvin dealer in St. Louis, so we don’t sell Andersen or Renewal products. That’s worth saying upfront because it shapes both our recommendations and our limits.
| Worth saying up frontForshaw doesn’t sell Andersen or Renewal by Andersen products. We’re an exclusive Marvin dealer. If Fibrex turns out to be genuinely the right material for your project after reading this, we’ll point you to an authorized Andersen retail dealer in the St. Louis market for the 100 Series, or to your local Renewal by Andersen affiliate. For most projects, though, Marvin Ultrex (an alternative fiberglass material used in Marvin Essential, Elevate, and Infinity) is a better fit at typically lower cost. This guide explains how to tell the difference. |
| The short answerFibrex is Andersen Corporation’s proprietary composite window material, patented in 1992. It’s made of 40% reclaimed wood fiber bonded with 60% thermoplastic polymer. Andersen markets it as 2x stronger than vinyl, low-maintenance, and dimensionally stable. The material is genuinely well-engineered and durable. Where the marketing overstates the case: Fibrex isn’t structurally equivalent to fiberglass (Marvin Ultrex is 8x stronger than vinyl per published specs versus Fibrex’s 2x), and the energy performance ratings on Renewal’s Acclaim series are competitive but not category-leading. Fibrex is used in two product lines: the Andersen 100 Series (sold through retail dealers, $400–$1,500/window installed) and Renewal by Andersen (sold through in-home consultations, $1,500–$5,000/window installed). Same material; very different sales channels and pricing. |
| Considering a Marvin Ultrex alternative?Forshaw’s St. Louis showroom features operable Marvin Essential, Elevate, and Modern windows — all using Marvin’s Ultrex pultruded fiberglass. Free consultations, transparent quotes, no in-home sales pressure.Call (314) 993-5570 |
What Fibrex actually is
The composition
Per Andersen Corporation’s own documentation, Fibrex is a composite material made of:
- 40% reclaimed wood fiber. Andersen sources this wood fiber from offcuts and byproducts of their own wood-window manufacturing operations — a real sustainability angle that Andersen markets heavily.
- 60% thermoplastic polymer. The binding agent that holds the wood fibers together and provides the moisture resistance and dimensional stability vinyl alone wouldn’t deliver.
The two components are combined under heat and pressure to form a rigid, extrudable composite that can be formed into window frame profiles. It’s structurally different from both vinyl (pure thermoplastic) and fiberglass (glass fibers bonded with resin) — it’s a hybrid that splits the difference.
The history
Andersen Corporation patented Fibrex in 1992. The material was developed specifically to address a gap in Andersen’s product lineup: a low-maintenance alternative to wood that performed better than vinyl, which they didn’t want to put their brand on. Fibrex first appeared in Andersen’s Renewal by Andersen division (launched 1995) as a way to offer composite windows through the in-home installation channel. Andersen later extended Fibrex to the 100 Series retail line.
The material has been in continuous production for over 30 years at this point, with real-world performance data across millions of installations. It’s a mature engineered material, not a marketing fiction.
Where Fibrex is used
- Andersen 100 Series — the retail Andersen line that uses Fibrex frames inside and out. Sold through authorized Andersen dealers. Typical installed range $400–$1,500 per window.
- Renewal by Andersen — the in-home replacement window division of Andersen Corporation. The Acclaim series (Renewal’s only window product) uses Fibrex frames. Sold exclusively through Renewal’s in-home consultation channel. Typical installed range $1,500–$5,000+ per window.
- Andersen A-Series — Andersen’s premium architectural line uses Fibrex exterior cladding over a wood frame (similar in concept to how Marvin Ultimate uses aluminum cladding over wood).
This means same material, very different products and price points. A Renewal Acclaim window and an Andersen 100 Series window use the same Fibrex — the price difference reflects the sales channel and bundled service, not the raw material.
What Fibrex does well
Fibrex is a legitimately engineered composite with real performance advantages. The marketing isn’t inventing strengths — most of the claims hold up. Specifically:
- Twice as strong as vinyl per Andersen’s published specs. Fibrex resists the warping and dimensional movement that affects budget-tier vinyl, particularly in extreme-climate conditions like St. Louis summers.
- Lower thermal expansion than vinyl. Less frame movement across temperature cycles means less stress on the glass seal over decades. Better long-term energy efficiency retention than vinyl.
- Won’t rot, corrode, or attract pests. The wood fiber is fully encapsulated in the polymer matrix — it can’t absorb moisture the way solid wood can. Unlike aluminum, it won’t corrode.
- Supports darker exterior colors than vinyl. Higher heat tolerance allows Andersen and Renewal to offer black, bronze, and dark gray exterior options that would warp budget vinyl.
- Strong transferable warranty terms. Both the Andersen 100 Series and Renewal Acclaim warranties cover the Fibrex frame for 20 years with transferable coverage to subsequent homeowners — a real advantage over Pella’s non-transferable warranty.
- Genuine sustainability angle. The 40% reclaimed wood fiber from Andersen’s own wood manufacturing represents a credible recycling story — not greenwashing. Andersen Corporation has earned ENERGY STAR Partner of the Year recognition consecutively for over a decade.
- 30+ years of in-market track record. Fibrex has been in production since 1992. The product has had time to prove itself, and the long-term performance data is real.
Where Fibrex marketing overstates the case
A balanced read also notes where the claims diverge from the data. These aren’t hidden problems — they’re things the marketing emphasizes less.
Fibrex isn’t structurally equivalent to fiberglass
Andersen markets Fibrex as 2x stronger than vinyl. Marvin’s Ultrex pultruded fiberglass is published as 8x stronger than vinyl. The 4x difference between the two composite materials matters — it’s the reason fiberglass typically achieves longer lifespans (30–50+ years for Ultrex vs Andersen’s warranted 20-year Fibrex coverage) and supports larger window units without frame reinforcement.
This isn’t a knock on Fibrex; it’s a structural reality about composite materials. Pultruded fiberglass (glass fibers bonded with resin) is inherently stronger than wood-fiber-and-polymer composites. The trade-off Andersen made was to use reclaimed wood fiber for the sustainability story — a defensible choice with real environmental value, but one that produces a material that’s structurally less robust than pure fiberglass.
Energy performance is competitive, not category-leading
Per Replacement Windows Reviews’ March 2026 testing data, the Renewal Acclaim double-hung window’s performance ratings are: U-factor 0.33, SHGC 0.28, air infiltration 0.17, visible transmittance 0.39. Industry editor commentary describes these ratings as “mediocre for a premium-priced product” — they’re competitive but not best-in-class.
For context: high-performance vinyl from quality brands (Okna, Simonton, Soft-Lite) can match or exceed those numbers at substantially lower per-window cost. Marvin’s fiberglass-based lines with high-performance glass packages also match or exceed those ratings. The Fibrex frame doesn’t magically produce better energy numbers than the alternatives — the energy performance comes mostly from the glass package, which is similar across brands.
The pricing premium isn’t about the material
A homeowner researching Fibrex naturally assumes the higher prices for Renewal by Andersen reflect a premium material. In reality, the material cost between Andersen 100 Series ($400–$1,500/window installed) and Renewal Acclaim ($1,500–$5,000/window installed) is similar — both use Fibrex. The price difference reflects the in-home sales model, single-source installation, and bundled service that Renewal provides, not a different material quality.
If you specifically want Fibrex for your project and want to pay less for it, the Andersen 100 Series through an authorized retail dealer offers the same material at a fraction of the Renewal price. The trade-off is that you arrange installation separately rather than through a bundled service. For full detail on the Renewal pricing dynamic, see our analysis of why Renewal by Andersen windows are so expensive.
Fibrex vs. fiberglass: the most important comparison
Most shoppers researching Fibrex are comparing it to fiberglass alternatives — either Marvin Ultrex (Essential, Elevate, Infinity, Modern) or Pella Impervia. Here’s how they actually compare.
| Fibrex (Andersen) | Ultrex Fiberglass (Marvin) | |
|---|---|---|
| Composition | 40% reclaimed wood fiber + 60% thermoplastic polymer | Continuous glass fibers + resin (pultruded) |
| Strength (vs vinyl) | 2x stronger per Andersen specs | 8x stronger per Marvin specs |
| Thermal expansion | Lower than vinyl; higher than fiberglass | Near-identical to glass (lowest in category) |
| Frame warranty | 20 years, transferable | 20 years standard; lifetime on Marvin Infinity for original owner; transferable |
| Finish standard | Proprietary; integral to the material | AAMA 624 (fiberglass-specific finish standard) |
| Typical lifespan | 20–30 years (industry composite category) | 30–50+ years (fiberglass category) |
| Sales channel | Andersen authorized dealers (retail) and Renewal by Andersen (in-home) | Marvin authorized dealers (retail showroom); Infinity through certified dealers (Lakeside in St. Louis) |
| Typical installed cost | $400–$1,500 (100 Series); $1,500–$5,000+ (Renewal Acclaim) | $500–$1,200 (Essential); $650–$1,600 (Elevate); $700–$1,800 (Infinity) |
The pragmatic read: at equivalent price points, Marvin Ultrex fiberglass is structurally stronger (4x the vinyl-strength benchmark), more dimensionally stable (lower thermal expansion), and has longer typical lifespan than Fibrex. Where Fibrex wins: the sustainability story (genuine reclaimed-wood content), Andersen’s brand recognition, and the Renewal single-source installation model for shoppers who value that experience.
Fibrex vs. vinyl
The most common comparison Andersen and Renewal marketing makes is Fibrex vs. vinyl — because vinyl is the budget alternative they want shoppers to upgrade from. The comparison is generally accurate but worth unpacking.
Where Fibrex beats vinyl
- Structural strength: 2x stronger than vinyl per Andersen’s published specs. Real advantage on large window units and in extreme climates.
- Dimensional stability: Lower thermal expansion. Less frame movement across temperature cycles, which means less stress on the glass seal over decades.
- Color options: Fibrex tolerates darker exterior colors that would warp budget vinyl. Black, bronze, and dark gray options are widely available.
- Lifespan: Typical 20–30 years for Fibrex vs 8–15 for cheap vinyl, 20–40 for premium vinyl. Comparable to premium vinyl, longer than budget vinyl.
Where vinyl can still make sense
- Cost: Premium vinyl from brands like Pella 250 Series, Andersen 200 Series, Simonton, or Okna typically costs 30–50% less per window than Fibrex equivalents. For budget-conscious projects, that gap matters.
- Energy ratings: High-performance vinyl can match or exceed Fibrex on U-factor and SHGC. The glass package matters more than the frame material for energy performance.
- Wider availability: Many more vinyl product lines and brands. Easier to find the right product/price match.
For a deeper look at the vinyl-vs-premium decision, see our fiberglass vs. vinyl windows guide.
Same Fibrex, different products: Renewal vs. Andersen 100 Series
This is the part most shoppers don’t realize: the same Fibrex material is sold through two completely different Andersen Corporation channels with very different prices and customer experiences.
| Andersen 100 Series | Renewal by Andersen (Acclaim) | |
|---|---|---|
| Frame material | Fibrex composite | Fibrex composite |
| Sales model | Retail through authorized Andersen dealers | In-home consultation through Renewal affiliates |
| Customization | Standard sizes from inventory, limited custom options | Fully custom-manufactured to opening measurements |
| Installation | Arranged separately (dealer installer or independent contractor) | Bundled with the window in single Renewal contract |
| Typical installed cost | $400–$1,500/window | $1,500–$5,000+/window |
| Pricing transparency | Published retail pricing, itemized quotes | In-home consultation pricing with same-day discount model |
If you’ve been quoted Renewal by Andersen at $30,000+ for a project and you specifically want Fibrex windows, the Andersen 100 Series through an authorized retail dealer typically delivers the same Fibrex material for substantially less — often 50–70% less on equivalent-scope projects. The trade-off is that you arrange installation separately rather than getting Renewal’s bundled service. For many homeowners, that’s a worthwhile trade.
Decision framework: is Fibrex right for your project?
Choose Fibrex (Andersen 100 Series or Renewal) if…
- You specifically want a composite material with reclaimed wood content for the sustainability angle
- You want Andersen’s brand specifically (110+ year company, strong recognition)
- You want a 20-year transferable warranty on the frame
- For Renewal specifically: you value the single-source bundled installation experience and your budget supports the in-home-sales premium
- For Andersen 100 Series specifically: you want Fibrex at a more competitive price point through a retail dealer
Consider Marvin Ultrex fiberglass alternatives if…
- You want a stronger, more dimensionally stable frame material (8x vs 2x vinyl-strength benchmark)
- You want the longest typical lifespan in the category (30–50+ years for Ultrex vs 20–30 for Fibrex)
- You’re comparing prices and Marvin Essential or Elevate quotes are coming in lower than Renewal on equivalent projects (common in our St. Louis market)
- You want real wood interior options (Marvin Elevate pairs Ultrex exterior with Pine wood interior — Fibrex doesn’t offer this)
- You want the retail showroom experience with transparent itemized quotes rather than in-home consultation
Consider premium vinyl if…
- Pure lowest cost is the deciding factor
- You’re selling within 5–10 years
- Quality vinyl brands (Pella 250 Series, Andersen 200 Series, Simonton, Okna, Soft-Lite) at 30–50% less per window than Fibrex equivalents meet your performance and aesthetic requirements
Why Forshaw if you’re shopping alternatives to Fibrex
Forshaw is a family-owned St. Louis business since 1871. We’re an exclusive Marvin dealer for our window and door division — which means we work with the full Marvin Ultrex lineup (Essential, Elevate, Infinity, Modern) every day.
- Transparent, itemized quotes. Line-by-line pricing showing specific product, glass package, installation type, and warranty terms. Take the quote home, compare it to Renewal’s or Andersen 100 Series quotes, decide on your timeline.
- No in-home sales pressure. We don’t run the 2–3 hour in-home consultation model with same-day discount tactics. Quotes are what they are.
- Bring us your Renewal quote. We’ll walk through it line by line and show you what comparable Marvin Ultrex work would cost. Many Renewal-quote-holders save substantial amounts on equivalent-scope projects.
- Right-fit product matching. If Fibrex is genuinely the right material for your project after our consultation, we’ll tell you and point you to an authorized Andersen retail dealer. We’d rather give you accurate guidance than push the wrong product.
- Inserts and full-frame, both done well. We sell roughly even numbers of insert and full-frame replacements. Fibrex through Renewal is typically insert-only — we can do insert work and full-frame work depending on what your project actually needs.
- Showroom with operable Marvin Ultrex windows. See Essential, Elevate, and Modern lines side by side. Compare colors, hardware, and operating mechanisms in person before committing.
| Compare Marvin Ultrex to Fibrex in personVisit our St. Louis showroom to see operable Marvin Essential, Elevate, and Modern windows. Bring your Renewal or Andersen 100 Series quote and we’ll walk through a side-by-side comparison.Call (314) 993-5570or schedule a free consultation online |
Frequently asked questions
| What is Fibrex made of?Per Andersen Corporation’s own documentation, Fibrex is composed of 40% reclaimed wood fiber bonded with 60% thermoplastic polymer. The wood fiber comes from offcuts and byproducts of Andersen’s own wood-window manufacturing operations — a genuine recycling story. The two components are combined under heat and pressure to form a rigid composite that can be extruded into window frame profiles. Andersen patented the material in 1992. |
| Is Fibrex the same as fiberglass?No. Fibrex is a composite of wood fiber and thermoplastic polymer. Fiberglass is glass fibers bonded with resin. The two materials behave differently: fiberglass (especially pultruded fiberglass like Marvin Ultrex) is structurally stronger (8x vinyl per Marvin specs vs 2x for Fibrex), has lower thermal expansion (near-identical to glass), and typically achieves longer lifespans (30–50+ years vs 20–30 for Fibrex). Both materials are durable and resist moisture damage, but they’re engineered differently for different performance trade-offs. |
| Is Fibrex better than vinyl?Yes, by Andersen’s own published specs. Fibrex is twice as strong as vinyl, has lower thermal expansion, supports darker exterior colors that would warp budget vinyl, and lasts longer (20–30 years for Fibrex vs 8–15 for cheap vinyl, 20–40 for premium vinyl). The trade-off is cost: Fibrex windows typically cost 30–50% more than premium vinyl. Whether the upgrade is worth it depends on your timeline, climate, and design priorities. |
| Why is Renewal by Andersen so expensive if it uses the same Fibrex as the Andersen 100 Series?Same material, different sales model. The Andersen 100 Series is sold through retail dealers with itemized pricing and separate installation arrangements; typical installed cost is $400–$1,500 per window. Renewal by Andersen is sold through in-home consultations with bundled installation and a same-day-discount sales structure; typical installed cost is $1,500–$5,000+ per window. The price difference reflects the in-home sales model, bundled installation labor, custom manufacturing, and brand marketing investment — not the raw material. If you want Fibrex windows for less, the Andersen 100 Series through a retail dealer is the same material at a fraction of the price. For full detail, see our analysis of why Renewal by Andersen is so expensive. |
| How long do Fibrex windows last?Andersen warrants Fibrex frames for 20 years (transferable to subsequent owners), which is the manufacturer’s minimum commitment. Real-world lifespan is typically 20–30 years — longer than cheap vinyl (8–15 years), comparable to premium vinyl (20–40 years), and shorter than premium fiberglass (30–50+ years for Marvin Ultrex). Installation quality affects real-world outcomes substantially. |
| Is Fibrex really sustainable?The sustainability claim has real substance. The 40% reclaimed wood fiber comes from Andersen’s own wood-window manufacturing waste stream — a legitimate recycling loop rather than virgin material. Andersen Corporation has earned ENERGY STAR Partner of the Year recognition for over 10 consecutive years (including “Sustained Excellence” awards), which is a meaningful third-party validation of their broader sustainability practices. This isn’t marketing greenwashing — it’s a documented practice. That said, the energy performance of the resulting windows is competitive but not category-leading; the sustainability story is about manufacturing inputs, not necessarily operating energy efficiency. |
| Which is better: Fibrex or Ultrex?Both are engineered composite/fiberglass materials with real performance advantages. Ultrex (Marvin) is structurally stronger (8x vinyl vs 2x for Fibrex per published specs), has lower thermal expansion (near-identical to glass), achieves longer typical lifespans (30–50+ years vs 20–30 for Fibrex), and meets the AAMA 624 finish standard. Fibrex has the sustainability story (40% reclaimed wood fiber), Andersen’s brand recognition, and the Renewal single-source installation model some homeowners prefer. At equivalent price points, Ultrex typically delivers more material per dollar; at the Renewal pricing tier, you’re paying for the sales/installation experience as much as the material. |
| Does Forshaw sell Fibrex windows?No. Forshaw is an exclusive Marvin dealer in St. Louis, so we don’t sell Andersen or Renewal by Andersen products. If Fibrex is genuinely the right material for your project after a consultation, we’ll point you to an authorized Andersen retail dealer for the 100 Series, or to the local Renewal by Andersen affiliate for the Renewal Acclaim line. For most projects, we recommend Marvin Ultrex alternatives (Essential, Elevate, Infinity) for structural and pricing reasons explained in this guide. |
| Is Fibrex used in all Andersen windows?No. Andersen Corporation makes multiple window lines using different materials. Fibrex is the primary material in the Andersen 100 Series and Renewal by Andersen Acclaim products. The Andersen A-Series uses Fibrex as exterior cladding over a wood frame. The Andersen 200 Series and 400 Series use vinyl-clad wood construction. Andersen E-Series uses aluminum-clad wood. If you’re cross-shopping Andersen products, ask which series and which material — the differences matter. |
| Can Fibrex be painted?Per Andersen’s documentation, Fibrex accepts paint reasonably well, similar to fiberglass. However, Andersen offers a wide range of factory-applied colors that are bonded to the material during manufacturing — these factory finishes are more durable than field-applied paint. For homeowners who want to change colors years after installation, Fibrex is paintable; for the longest-lasting finish, choosing the right factory color upfront is the better strategy. |
