Fiberglass vs. Clad Wood Windows: The Premium Tier

Fiberglass and clad wood are the two premium choices, and the cost surveys put them close together. Modernize's calculator lists fiberglass at $900 to $1,400 and wood at $800 to $1,500 per standard double-hung window ²; This Old House's 2026 survey of 1,000 window customers found averages of $564 for fiberglass and $544 for wood ³. Price alone rarely decides between them.

What does decide it is the inside and the outside. Fiberglass gives a painted, low-upkeep frame that a manufacturer says moves with the glass; clad wood gives real wood indoors, in more species and stains, behind an aluminum or vinyl skin. The warranty terms differ too, and the manufacturers' documents show where. The sections below compare them point by point.

Cost: what the surveys report

None of these surveys separates clad wood from all-wood windows, so the wood column covers both. Fixr's ranges put wood higher at both ends ¹; Modernize's ranges overlap almost completely ²; and in This Old House's survey, fiberglass averaged $20 more than wood (our subtraction of its two averages) ³. The differences come from method: an average of what homeowners paid mixes every window size and grade ³, while the calculator is limited to one window type ². For fiberglass prices in more detail, see fiberglass window cost, and for wood, wood windows cost.

Expansion and stability

A frame that grows and shrinks at a different rate from the glass puts stress on the seals. Marvin says its Ultrex fiberglass expands at virtually the same rate as glass, which it says keeps windows tight and helps prevent leaks, seal failures and stress cracks ⁴. Marvin also says its Elevate fiberglass holds its shape up to 285°F ⁵. These are a manufacturer's claims about its own product, and the page does not describe the test.

Wood moves mainly with moisture. Modernize notes that wood expands and contracts with moisture and that painting the frame reduces the risk ⁸. Marvin, which makes both, says wood can resist warping under extreme temperature changes and that moisture is a window's worst enemy, wood windows included ⁹. Modernize adds that aluminum or fiberglass cladding can protect wood from rot and weathering ⁸.

Exterior finish and upkeep

Both are low-upkeep outside, but the finish is different.

• Fiberglass: Marvin says its Ultrex frames need no sanding, scraping or painting and that its acrylic finish holds dark colors without UV fading ⁴. Its Elevate casement comes in six exterior colors ⁵.
• Aluminum-clad wood: Marvin's Ultimate casement has an extruded aluminum exterior over a wood interior, in 19 exterior colors plus custom ⁶.

Neither is maintenance-free. Marvin says clad exteriors should be cleaned once or twice a year in dry climates and four or five times a year on the coast, and never with a pressure washer ¹⁰. Any wood left exposed on a clad window still needs sealing: Marvin's warranty excludes failures caused by not treating, sealing and maintaining exposed wood ⁷.

Interior look

This is the clearest difference, and often the deciding one.

A fiberglass window can still have a wood interior. Marvin's Elevate line pairs an Ultrex fiberglass exterior with a wood interior, offered as bare pine, with a clear coat or in factory-painted colors ⁵. Marvin's Essential line is fiberglass on both sides ⁴.

Clad wood offers more choice indoors. Marvin's aluminum-clad Ultimate casement lists six interior species (pine, two grades of Douglas fir, cherry, white oak and mahogany), two paint colors plus primer, and six stains plus a clear coat ⁶. If you want a stained hardwood interior to match floors or trim, clad wood is the material that offers it; if a painted or pine interior is enough, fiberglass with a wood interior may cover it.

Warranty terms, side by side

Marvin's limited warranty, effective January 12, 2026, covers both its aluminum-clad wood (Ultimate) and its fiberglass (Elevate and Essential) lines, which makes the terms easy to compare ⁷:

The aluminum figure is for chalk, fade and loss of adhesion, and the coastal figure applies within one mile of salt water ⁷. Removal, installation, finishing, refinishing and disposal costs are not included, and factory-applied wood primer must be painted within 90 days of installation ⁷. Other manufacturers' terms differ; window warranties explained sets six of them side by side, and a warranty term is not a lifespan.

Energy performance

The frame is a minority of the window. The Efficient Windows Collaborative, part of NFRC, says the sash and frame make up 10 to 30 percent of a window unit's area, enough to influence the whole window's performance ¹¹. The glass package usually matters as much as the frame. Compare the NFRC label on the exact products quoted: U-factor runs from 0.20 to 1.20 for windows, and lower means less heat lost ¹². The window U-factor explained article shows how to read it.

Which fits your house

Fiberglass may suit you if you want a painted or dark exterior color with little upkeep, a simple interior, and a frame that a manufacturer says moves with the glass. Clad wood may suit you if the interior wood matters (a stained hardwood, a historic room, matching trim) and you accept slightly more care around any exposed wood. If vinyl is also on your list, see vinyl vs. fiberglass windows; for a broader view, window frame materials compared and the types of windows guide.

The surest comparison is a quote on the same openings in both materials, with the product line, interior finish, glass package and warranty document named for each. Not every installer carries both, so ask early. When you are ready, you can compare quotes from window installers.

We are not affiliated with the manufacturers named.

About these figures

• ¹ Reported: Fixr per-window ranges for fiberglass ($400 to $1,500) and wood ($600 to $2,000), updated January 21, 2026, checked October 26, 2026.
• ² Reported: Modernize calculator ranges for fiberglass ($900 to $1,400) and wood ($800 to $1,500) for standard-size double-hung windows, updated September 22, 2026, checked October 26, 2026.
• ³ Reported: This Old House averages for fiberglass ($564) and wood ($544) from its 2026 survey of 1,000 window customers, updated August 11, 2026; the $20 difference is our own subtraction; checked October 26, 2026.
• ⁴ Reported: Marvin claims on Ultrex expansion, finish, color and its use in the Essential line, checked October 26, 2026.
• ⁵ Reported: Marvin Elevate casement exterior, 285°F shape statement, six exterior colors and interior finish options, checked October 26, 2026.
• ⁶ Reported: Marvin Ultimate casement aluminum exterior, 19 exterior colors, six interior species and finish options, checked October 26, 2026.
• ⁷ Reported: Marvin limited warranty terms for cladding finishes, interior wood finish, hardware, glass, primer and exclusions, effective January 12, 2026, checked October 26, 2026.
• ⁸ Reported: Modernize notes that wood expands and contracts with moisture and that cladding can protect it, updated August 12, 2026, checked October 26, 2026.
• ⁹ Reported: Marvin guide on wood, moisture and warping, checked October 26, 2026.
• ¹⁰ Reported: Marvin cleaning frequency for clad exteriors and pressure-washer advice, checked October 26, 2026.
• ¹¹ Official record: Efficient Windows Collaborative statement that sash and frame are 10 to 30 percent of a window unit's area, checked October 26, 2026.
• ¹² Official record: NFRC label definition of U-factor and its 0.20 to 1.20 range for windows, checked October 26, 2026.