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Exterior Cladding Layers on Wood Windows

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The skin that shields the wood

Bare wood left outdoors needs regular painting or sealing, because sun, rain and temperature swings slowly break down its surface. Cladding solves that by wrapping the exterior of the wood frame and sash in a weatherproof shell. It is what turns a wood window into a low-upkeep wood clad window. Not all cladding is the same, however, and the way it is made, attached and finished affects how well the wood underneath is protected. This guide looks at the two main cladding materials, how the layers are assembled and where problems can start.

Extruded aluminum cladding

Aluminum is the most common cladding. It is formed by extrusion into shaped sections that cover the face of the frame, the sash and sometimes the sill. The profiles can be crisp and thin, with the sharp lines that many modern and traditional designs call for. They are coated with a baked-on finish, either a liquid paint cured at high temperature or a powder coating, so the color is durable and resists fading when the finish is good. Aluminum does not rot, swell or absorb water, and it handles wide temperature swings without trouble.

Aluminum can dent if struck, and a scratch that reaches bare metal should be touched up to prevent corrosion. Corners are either mitered and sealed or joined with hidden clips, and the quality of those corner details is something to examine closely.

Vinyl cladding

Some lines use a vinyl cover instead of aluminum. Vinyl cladding is formed in a similar way to vinyl windows and has color mixed through it, so scratches are less visible. It does not corrode and it tends to cost less than aluminum. On the other hand, vinyl expands and contracts more than aluminum and is sensitive to heat at dark colors, so the color range is more limited and the profile may be a bit thicker. The look is less crisp than metal on a close inspection.

• Aluminum: crisp lines, broad color choices, can dent.
• Vinyl: color through the material, more limited palette, moves more with heat.

How the layers are assembled

From the outside in, a typical clad assembly consists of the finished skin, a small gap or a spacer, a moisture barrier or sealant, and the wood. The gap matters. If cladding sits tight against the wood with no space, water that gets behind the skin has nowhere to go and may stay against the wood. Better designs provide a ventilated space or drainage path so moisture can escape and the wood can dry. Spacers or standoffs hold the skin away from the wood at calculated points.

Fasteners are another detail. The skin may be held on by hidden clips that grip a groove in the wood, by staples or by screws. Clips allow the cladding to move slightly with temperature changes without cracking the finish or loosening, and they avoid punctures through the skin that could let water in.

Sealing the joints

Where two pieces of cladding meet, or where cladding meets glass or a sill, a sealant fills the gap. High-quality sealants stay flexible for years and bond to both the metal or plastic and the wood. Poor sealants harden and crack, allowing water to find a way in. Corners are particularly vulnerable because they combine joints from two directions. Look at the corner of a sample and see how neat the seam is, whether there is a visible bead of sealant and whether it is smooth and continuous.

Protecting the wood underneath

Even when the cladding is excellent, the wood behind it is typically treated at the factory. A water-repellent preservative is applied to the wood to resist moisture and decay, and edges that will be hidden are often sealed or primed. Treatment is especially important along the bottom of the sash and at the corner joints, where water is likely to linger. Ask whether the wood is treated and what the treatment guards against.

How moisture can get in

1. A failed sealant line at a corner or around the glass.
2. A puncture in the cladding from a fastener or impact.
3. Poor installation that leaves the window without proper flashing or drainage.
4. Condensation forming on the inside of a poorly sealed unit and running into the sill.

None of these happens often with a well-made, well-installed window, but each one is a reminder that cladding is a system rather than a single material. Inspecting sealant lines every year or two and repairing any cracks promptly is cheap insurance.

Finish durability

The coating on cladding is its first defense. Baked-on finishes rated for exterior use typically resist chalking and fading for many years, and the best have warranties that specify how much color change counts as a failure. Anodized aluminum, an alternative, forms an integral oxide layer with a metallic look and long life, though the color choices are narrower. Whichever is used, ask what the finish warranty covers and for how long, and keep the documents handy.

Cladding and appearance

From the street, clad windows look like crisp painted windows. Up close, the profile of the cladding differs from traditional wood moldings, so some owners with historic houses prefer specially shaped cladding profiles that imitate the look of wood. Many manufacturers offer such options, including putty-glaze profiles, simulated divided lights and decorative sill shapes. Compare these at close range, because small differences in shadow lines and edge details change the impression.

Care of cladding

Routine care is minimal. Rinse dust and salt-laden film from the surface, wash with mild soap and a soft cloth and avoid abrasive products. Keep weep holes clear. Touch up chips with a matching paint from the manufacturer. Check the sealant once in a while, especially at the lower corners and where the window meets the wall. If you see cracking, a bead of compatible sealant can restore the barrier before water has a chance to enter.

Weighing the choices

For long-term durability and a refined look, aluminum cladding is usually the preferred option, while vinyl cladding suits those who prefer a more modest approach with color throughout. In both cases, the quality of the assembly, the ventilation behind the skin and the sealing of the joints do more to protect the wood than the material alone. Ask to see a cutaway if the maker can provide one, and examine how the layers are built before you decide.

Reading a cutaway

If a cutaway sample is available, trace the path water would take from the outside surface to the drain slot. A well-considered design shows a clear route, a gap behind the skin and sealed wood faces. Notice whether the cladding wraps around the edge of the wood or simply sits on its face, because wrapped edges leave fewer places for moisture to enter. A few minutes with a cutaway tells you more about durability than any brochure statement about the quality of the exterior shell.