Window Frame Materials Compared: Aluminum, Wood, Clad, uPVC, Steel

Window frames are made from six main materials: aluminum, wood, aluminum-clad wood, uPVC or vinyl, fiberglass and real steel. Each trades off insulation, maintenance, sightline width, structural span, durability and finish differently, and the right choice depends on the glass size, climate and exposure of each opening.

Why does frame material matter?

Frame material matters because sash and frame make up 10 to 30 percent of a window unit's total area, according to the Efficient Windows Collaborative, and the frame's thickness, weight, durability and thermal behavior all follow from the material. The U.S. Department of Energy states that vinyl, wood, fiberglass and some composite frame materials provide greater thermal resistance than metal, and that there are advantages and disadvantages to all of them.

Frame choice is separate from the opening type and from the glass. The same tilt-and-turn or lift-and-slide hardware can be built in several materials; see European windows and doors in the USA for the full picture, and profile vs fabricator vs installer for who actually makes what.

How do the frame materials compare?

The table below rates each material on the criteria that usually decide a specification. "Frame insulation" describes the frame only, not the whole window.

These are generalizations drawn from public guidance. Individual products vary widely, so read the table as a starting filter and then check tested values.

How do conductivity numbers explain the thermal ranking?

Conductivity explains the thermal ranking directly. Under the ISO 10077-2 method, aluminum is modeled at 160 W/(m-K), steel at 50 W/(m-K), glass-fiber-reinforced polyamide 6/6 at 0.30 and PVC at 0.17. In US units, aluminum is about 92 Btu/(hr-ft-F), steel about 29 and PVC about 0.10. Aluminum conducts more than three times as much heat as steel, and steel conducts roughly 300 times as much as PVC. This is why both metals need a thermal barrier and why plastics and composites start ahead.

The Efficient Windows Collaborative gives category frame U-factors that line up with this: wood roughly 0.3 to 0.5 Btu/(hr-sq ft-F), aluminum with no break roughly 2.0 and aluminum with a standard break about 1.0. These are generic categories, not product ratings. Real ratings come from NFRC simulation; see U-factor for windows.

Aluminum windows

Aluminum frames are light, strong and durable, and they extrude into complex shapes, which gives the narrowest sightlines of the mass-market materials and the largest glass sizes. The weakness is conductivity: without a break, an aluminum frame can get cold enough to form condensation or frost on its interior surfaces in cold climates. A thermal break fixes most of it; how the barrier works, what it does to frame U-factor and what to put in a spec are covered in thermal break aluminum windows.

Aluminum is also the most recyclable frame material. FGIA reports that aluminum is 100 percent recyclable regardless of applied finish, and that recycling uses roughly 5 percent of the energy needed to make primary aluminum. For coatings, AAMA 2604 and 2605 are the common high-performance specifications; 2605 is the stricter, with 4,000 hours of salt spray versus 3,000 and a higher gloss-retention minimum.

Wood windows

Wood frames insulate relatively well and can be milled into complex profiles, but they expand and contract with weather and need regular maintenance. The Efficient Windows Collaborative notes that wood is susceptible to rot, while well-built and well-maintained wood windows can have a very long life. Decay needs sustained moisture: wood-rotting fungi need roughly 35 to 50 percent moisture content, and wood is not considered immune until it is below about 20 percent, according to Canadian Building Digest 111. Wood suits interiors where warmth and finish matter and where exposure and maintenance are managed.

Aluminum-clad wood windows

Aluminum-clad wood windows put a weather-facing aluminum skin on a wood core, which cuts exterior maintenance while keeping a wood interior. The cladding protects the outside, not the whole assembly, so drainage and sealant detailing decide long-term durability. Construction, finishes and the moisture questions are covered in aluminum-clad wood windows.

uPVC and vinyl windows

uPVC and vinyl frames are made of polyvinyl chloride with UV stabilizers; the color runs through the material, so no painting is needed, and moisture resistance is good. Multi-chamber and insulated profiles give thermal performance comparable to or better than wood. The trade-offs are a narrower color and finish range, wider sightlines at large sizes because of reinforcement, and a higher expansion coefficient than other materials, per the Efficient Windows Collaborative. "uPVC" is the European term for unplasticized PVC; in practice both words describe PVC frames, though product quality, profile design and reinforcement vary widely.

Fiberglass windows

Fiberglass frames are dimensionally stable and have air cavities that can be filled with insulation, giving them superior thermal performance compared with wood or uninsulated vinyl, according to the Department of Energy. They are a common alternative where aluminum's slimness is not needed. The detailed comparison is in aluminum vs fiberglass windows.

Real steel windows

Real steel windows are made from steel sections, and they are a distinct product from anything aluminum. Steel is stronger and stiffer than aluminum, which allows very slim sightlines and large grids of glass, and it conducts less heat than aluminum (about 50 versus 160 W/(m-K)) but still far more than polyamide or PVC. Steel frames therefore need a thermal break to meet modern energy goals, and they need a corrosion-protection coating that must be maintained. Real steel requires project-specific engineering and is usually specified from drawings rather than from a catalog.

Real steel versus steel-look aluminum

Steel-look aluminum is an aluminum window designed to resemble steel, with narrow profiles and often a dark finish. It is not steel: it has aluminum's weight, thermal behavior, corrosion behavior and recyclability, not steel's. Use "steel" only for products made from steel, and "steel-look" for aluminum products that imitate the appearance. Mixing the two on a specification leads to wrong structural, thermal and warranty assumptions. Dedicated articles for both are planned within this materials section.

How do you choose a frame material?

Choose by the criterion that cannot be compromised, then check the others:

1. Decide the largest glass size and the slimmest acceptable sightline. That narrows the field toward aluminum or steel.
2. Set the climate and condensation requirement. Cold or humid climates favor higher interior surface temperatures: wood, clad wood, fiberglass, uPVC or a well-broken metal frame.
3. Define the maintenance tolerance. Exposed painted wood needs the most; clad wood, aluminum, uPVC and fiberglass need the least.
4. Match the finish and color range to the design intent, particularly for dark exteriors and coastal exposure.
5. Confirm structural class and design pressure at the sizes on the schedule against site wind loads and local code.
6. Request tested whole-window ratings rather than generic material claims.

What should be checked on a specification?

On a specification, check what the product is tested to, not what material it is. Ask for the NFRC-rated U-factor and solar heat gain coefficient of the exact configuration, the condensation rating and its method, the North American Fenestration Standard performance class and design pressure (R at a 15 psf minimum, LC 25, CW 30, AW 40), the coating or finish standard, and the maintenance and warranty terms. Class R is typical for one- and two-family homes, and the higher classes add larger sizes, higher loads and deflection limits. Local codes vary, so confirm requirements with the building official.