Aluminum-Clad Wood Windows

An aluminum-clad wood window is a wood window with a factory-fitted aluminum skin on the exterior faces of the frame and sash. The wood supplies the structure and the interior appearance; the aluminum supplies a weather-facing surface that does not need on-site painting.

How is an aluminum-clad wood window built?

An aluminum-clad wood window is built from a wood frame and sash, an aluminum cladding profile fixed to the outside, and the glazing unit held between them. The Efficient Windows Collaborative describes wood cladding as a permanent weather-resistant surface, with vinyl or enameled metal cladding giving much longer protection to the wood. The interior stays wood and keeps its wood finish.

How the cladding is attached

Cladding is generally fixed to the wood mechanically rather than bonded. Exact methods vary by maker and are proprietary, so the useful questions for a specifier are:

• Ask whether the cladding is a formed or extruded section and how thick it is.
• Ask how the cladding is retained and whether an air space is kept behind it.
• Ask where water that gets behind the cladding or past the glazing seal is supposed to go, and request the section drawing that shows it.
• Ask how corners of frame and sash are sealed and whether the exposed end grain is protected.

These questions matter more than the word "clad." A good section drawing answers all four.

Where the thermal path runs

The thermal path runs through the wood core, not the aluminum. Wood frame U-factors fall in the range of 0.3 to 0.5 Btu/(hr-sq ft-F), according to the Efficient Windows Collaborative, far below bare aluminum at roughly 2.0. Because the aluminum is an outer skin rather than a structural member crossing from inside to outside, it does not need the thermal break that an all-aluminum frame requires. Whole-window U-factor still depends on glazing and spacers; U-factor for windows explains how the rating combines them. For the all-metal alternative see thermal break aluminum windows.

Which wood species are used?

Species selection varies by maker and market, and the useful distinction for a specifier is the function of the wood, not the name. Windows use softwoods and hardwoods chosen for dimensional stability, machining quality, availability and how well they take a finish. Decay resistance differs between species and with treatment, which matters most where the wood meets water, such as sills and bottom rails. A clad window's wood is hidden from the weather, so its decay resistance counts only if moisture gets in, but then it counts a lot.

On a specification, ask for the species of the core and of the interior face, whether the core is solid, finger-jointed or laminated, and what preservative treatment, if any, is applied. Interior face species is a visual decision; core construction is a performance decision.

What finishes do the aluminum and wood take?

The aluminum skin takes a factory-applied coating or anodized finish, and the wood interior takes a stain or paint chosen by the owner. The Efficient Windows Collaborative notes that metal and vinyl claddings come in a more limited range of colors than painted wood. Colors available are set by the maker's coating program.

For the exterior coating, the AAMA 2604 and 2605 specifications are the common high-performance tiers. AAMA 2605 is the stricter one, with 4,000 hours of salt-spray resistance against 3,000 for 2604 and a higher gloss-retention minimum (50 percent versus 30 percent). Typical use of 2605 is coastal, high-UV or high-rise work. Specify the tier and request the coating warranty.

How much maintenance do clad wood windows need?

Clad wood windows need less exterior maintenance than painted wood, and the work shifts to inspection. The U.S. Department of Energy notes that wood frames need regular maintenance and that aluminum or vinyl cladding reduces it. A reasonable routine:

1. Wash the aluminum exterior periodically with mild detergent and water.
2. Inspect exterior sealants, glazing seals and joints for cracks or gaps.
3. Clear any drainage openings in sills and frames of dirt and debris.
4. Refinish the interior wood when the coating wears.
5. Check the interior sills and lower rails for staining or soft spots, the earliest signs of moisture.

Follow the manufacturer's maintenance guide, since it governs the warranty.

Where does the moisture risk sit in a clad wood window?

The moisture risk sits wherever water can get behind the cladding and stay there. Aluminum protects the exterior face, but it also covers the wood, so a leak that would dry quickly on an open painted surface may remain hidden.

The conditions for rot are well defined. Canadian Building Digest 111 reports that wood-rotting fungi need about 35 to 50 percent moisture content to flourish, depending on species and wood, that fiber saturation is commonly reached around 25 to 30 percent, and that wood is not considered immune until its moisture content is below about 20 percent. Fungal growth is fastest at about 65 to 95 degrees F. Wood that is wetted occasionally and dries out stays safe; wood that stays wet does not.

For a specifier, the practical consequences are:

• Prioritize drainage: water that gets past a seal must have a path out.
• Keep the sill detail, flashing and sealant in good condition on site; the best product is wetted by a failed installation.
• Watch interior humidity and condensation, which can wet the lower interior sash and sill; see window condensation.
• Treat unfinished or damaged end grain and joinery as moisture entry points.

No cladding system makes the wood immune. It reduces exposure, and the section detail determines whether it also traps water.

What should a specification include?

A specification should name the cladding, the core and the verification the project needs. Include:

• The cladding coating standard (AAMA 2604 or 2605, or anodized), color and warranty.
• The core wood description, construction and treatment.
• A section drawing showing cladding attachment, air space and drainage path.
• The NFRC-rated U-factor and solar heat gain coefficient for the exact unit configuration.
• The structural performance class and design pressure at the unit sizes on the schedule.
• The maintenance and warranty terms in writing, including what voids them.

Aluminum-clad wood sits between all-aluminum and all-wood options. For the broader comparison see window frame materials compared, and for a frame without a wood core see aluminum vs fiberglass windows. Aluminum's own environmental profile is a point in the cladding's favor: FGIA reports that aluminum is 100 percent recyclable regardless of applied finish and that recycling uses roughly 5 percent of the energy of primary production.