Clips and fixings
Polyamide or stainless clips are screwed to the timber at regular centres and the cladding snaps over them. Plastic clips also reduce thermal bridging between metal and wood.
Timber-aluminium window systems
Aluminium-Clad Wood Windows
An aluminium-clad wood window has a solid or engineered timber frame on the inside and a powder-coated or anodised aluminium shell on the outside, so the room sees warm wood while the weather only ever meets metal. Done properly, the cladding is clipped on with a ventilated gap that lets the timber breathe and move. Metline supplies extruded cladding profiles, clips, sheet and accessories to timber-window makers and project buyers worldwide.
- Interior
- Pine, oak, larch, fir, meranti
- Cladding
- Extruded 6063 / 6060
- Typical Uw
- about 0.8 – 1.4 W/m²K
- Standards
- EN 14351-1 · NAFS
What an aluminium-clad wood window is
Timber is a good insulator, pleasant to look at and easy to repair, but on the outside of a building it needs repainting every few years and rots if water is trapped against it. Aluminium is durable and nearly maintenance-free, but it conducts heat. A clad window puts each material where it performs best: wood carries the structure and the glass inside, aluminium takes the sun, rain and pollution outside.
In a typical unit the load-bearing frame and sash are machined from laminated timber, usually three-layer finger-jointed scantlings that are far more stable than solid boards. The glass unit is glazed into the timber sash. On the exterior face, extruded aluminium sections are fixed to the timber with plastic or metal clips that hold them a few millimetres away from the wood. That small gap is the most important detail in the whole window: it lets water drain away and air circulate, and it allows the aluminium (which expands about twice as much as timber across the grain and many times more along it) to move without cracking paint or straining fixings.
The system is known by several names: aluminium-clad timber, timber-aluminium, wood-aluminium composite, or simply alu-clad. In German-speaking markets it is "Holz-Aluminium-Fenster"; in North America it is "clad wood" and is offered by most large window brands. Whatever the name, the same questions decide quality: what kind of cladding is used, how it is attached and ventilated, how the timber is protected, and how the window performs in tests.
How the cladding is attached, and why it matters
Two very different products are sold as "aluminium-clad". The difference is the cladding itself and how it meets the wood.
| Feature | Extruded cladding | Roll-formed cladding |
|---|---|---|
| Material form | Profiles pushed through a die, typically 6063 or 6060 alloy | Thin coil bent through rollers into a shape |
| Typical thickness | About 1.2 – 2.0 mm | About 0.4 – 0.6 mm |
| Fixing | Clipped to the timber with a ventilated gap; corners mitred and joined with cleats or welded | Often wrapped tight or nailed to the timber; corners folded |
| Dent resistance | High; holds sharp, crisp lines | Lower; dents more easily during handling and in service |
| Moisture behaviour | Gap drains and ventilates, protecting the timber | Tight contact can trap water against the wood if seals fail |
| Finish | Powder coating, PVDF or anodising applied to the profile | Usually pre-painted coil |
| Typical market | Premium and project windows in Europe and North America | Mid-price residential windows |
For project work, extruded cladding is the safer specification. It is stiffer, keeps its shape at mitres, and its clip fixing creates a drained, ventilated cavity. Good systems also leave the cladding free to slide along its length on at least one fixing so it can expand in hot sun without bowing. Dark colours can reach 70 – 80 °C on a south-facing facade, and a 2 m length of aluminium grows by roughly 2 – 3 mm over that temperature swing.
Ventilation gap
A gap of a few millimetres, open at top and bottom, lets condensation and any leakage evaporate or drain. Blocking it with sealant defeats its purpose.
Bottom drainage
The sill cladding carries water away from the timber end grain, the most rot-prone part of any window. Concealed drainage keeps outlets out of sight.
Glazing interface
The cladding often forms the outer glazing bead or covers the edge of the glass seal, so the timber never meets rainwater at the glass line.
Corner joints
Mitred cladding corners are cleated, crimped or welded; sealed corners stop water entering behind the shell.
Replaceability
Because it clips on, the shell can be removed for recoating or replaced after damage without dismantling the window.
Clad wood compared with aluminium, timber, uPVC and fibreglass
| Property | Aluminium-clad wood | Thermally broken aluminium | Painted timber | uPVC | Fibreglass |
|---|---|---|---|---|---|
| Interior look | Natural wood | Metal, any colour | Natural or painted wood | Plastic, foils available | Painted or wood-look |
| Exterior maintenance | Very low | Very low | Repaint every few years | Low | Low |
| Typical frame U-value | About 1.0 – 1.4 W/m²K | About 1.4 – 3.0 W/m²K | About 1.2 – 1.6 W/m²K | About 1.0 – 1.4 W/m²K | About 1.2 – 1.6 W/m²K |
| Slimness of sightline | Medium | Slimmest | Medium to wide | Wide | Medium |
| Large sizes | Good | Excellent | Good | Limited without steel | Good |
| Exterior colour stability | Excellent with PVDF or anodising | Excellent | Fades, needs repainting | Light colours best | Good |
| Relative cost | High | Medium to high | Medium | Low to medium | Medium to high |
Frame U-values are indicative ranges for current products and vary widely with design. Compare whole-window Uw values calculated to EN ISO 10077 or NFRC 100 at the same reference size.
The honest summary: clad wood costs more than aluminium or uPVC, and the timber must be protected inside like any fine joinery. In return it gives the interior quality of timber, the exterior durability of aluminium, and thermal performance that comfortably meets most energy codes. Where the interior look does not matter, a thermal break aluminium window gives slimmer frames and lower cost.
Types of aluminium-clad wood windows
Casement (inward or outward)
Side-hung sash closing onto compression seals; the most common clad format for bedrooms, kitchens and villa facades. Outward casements are usual in the UK and North America, inward in continental Europe.
Tilt-turn
Inward-opening sash that tilts for secure ventilation and turns for cleaning; standard in German and Scandinavian timber-aluminium systems.
Push-out casement
Opened by a lever handle rather than a crank, giving a cleaner traditional look on period and luxury homes.
Awning and hopper
Top-hung sash opening outwards at the bottom, ventilating even in light rain; good above fixed lights, in bathrooms and basements.
Double-hung
Two vertically sliding sashes on balances, the classic North American window; tilt-in sashes allow cleaning from inside.
Sash (box sash style)
British-style vertical sliders that keep heritage proportions while adding insulated glass and modern seals.
Fixed / direct-set
Glass glazed directly into the frame for maximum daylight; available in rectangles, arches, trapezoids and custom shapes.
Sliding
Horizontal sliders for openings where a sash must not swing; more modest air tightness than hinged types.
Corner and pass-through
Mitred glass corners for panoramic views, and service windows for kitchens, bars and terraces.
Choosing the timber species and interior finish
Inside a clad window the timber is protected from weather, so durability against rot matters less than stability, appearance and how well it takes a finish. Engineered (laminated) scantlings are the norm: three or more layers of kiln-dried timber, often finger-jointed in the hidden core, glued with alternating grain so the section stays straight. Timber for window frames is normally dried to about 10 – 14 % moisture content before machining.
| Species | Character | Typical use |
|---|---|---|
| Pine / spruce (laminated) | Light, even grain; economical and takes paint and stain well | Standard choice for painted and light-stained interiors |
| Larch | Denser softwood with warm colour and strong figure | Natural-finish interiors in Alpine and Nordic styles |
| Douglas fir, vertical grain | Straight, tight grain and a reddish tone | Premium North American interiors with clear finishes |
| Oak (red or white) | Hard, heavy, prominent grain; very durable surface | High-end residential and heritage projects |
| Meranti | Mid-density tropical hardwood, uniform reddish colour | Painted or stained windows where a hardwood is preferred |
| Maple, cherry, mahogany, alder | Fine-grained hardwoods with distinctive colour | Bespoke interiors matched to furniture and joinery |
Interior finishes range from clear satin lacquer that shows the natural grain, through tinted stains such as golden oak or chestnut, to fully opaque white or black paint. Factory finishing with water-based coatings, applied in a controlled booth and cured before glazing, gives a more consistent and durable result than painting on site. If interiors will be painted on site, order the windows primed and protect the timber from building moisture until it is sealed. Real timber varies in colour and grain from batch to batch, so approve physical samples rather than printed colour cards.
Where certified timber is required, specify FSC or PEFC chain-of-custody material at enquiry stage, because it affects sourcing for the whole order.
Cladding alloys, profiles and exterior finishes
Exterior cladding is almost always extruded from 6063 or 6060 alloy in T5 or T6 temper. These alloys extrude into thin, crisp sections, have a smooth surface for coating, and resist atmospheric corrosion well. Because the timber carries the structural load, cladding walls can be thinner than in an all-aluminium window, typically 1.2 – 2.0 mm. Sheet in 3003, 5005 or 5052 is used for flashings, sill covers and panel infills, and 5005 is the usual choice where the sheet will be anodised to match extrusions.
| Finish | Description | Where to use it |
|---|---|---|
| Polyester powder coating | Standard architectural powder to Qualicoat class 1 or AAMA 2603/2604 | Inland and suburban buildings in moderate climates |
| Super-durable powder | Qualicoat class 2 (and higher) with better gloss and colour retention | Strong sun, dark colours, demanding facades |
| PVDF (fluoropolymer) coating | Multi-coat liquid finish meeting AAMA 2605, with long resistance to chalking and fade | Coastal, high-UV and prestige projects |
| Anodising | Integral oxide layer, natural or bronze to black; class 20 (20 µm) for exterior per Qualanod | Metallic appearance and high abrasion resistance |
| Wood-grain or metallic effect | Sublimation transfer or special powders | Matching heritage or decorative schemes |
The cladding profiles themselves need the same care as any architectural extrusion. Ask for precision tolerances (EN 12020-2 for 6060 and 6063 sections) so clip grooves engage consistently along the full length, and agree a visible-surface definition so that die lines and handling marks are judged on the faces that will actually be seen. Long sill and head profiles should be packed with interleaving and edge protection, because a dent in a 1.5 mm cladding wall cannot be dressed out once it is coated. Where the shell is fixed with polyamide clips, check that the clip material is UV-stabilised and rated for the surface temperatures a dark colour will reach.
Exterior and interior colours are fully independent, so the facade can be dark grey while the room shows natural oak. Order a coated and an anodised sample for approval, and on coastal sites ask for pretreatment that meets the marine (seaside) class of the coating specification. More background is in our pages on 6063 extrusions, 5005 sheet and anodised extrusions.
Thermal, weather and acoustic performance
Timber frames insulate well without a separate thermal break, so the glass unit and the frame depth dominate the result. Typical European timber-aluminium systems use timber depths of about 68, 78, 88 or 92 mm. The deeper the timber, the wider the glass unit it can hold and the lower the frame U-value.
| Timber depth | Typical glazing | Indicative Uw (1.23 × 1.48 m) | Typical application |
|---|---|---|---|
| 68 mm | Double Low-E, argon, 24 – 28 mm | About 1.2 – 1.4 W/m²K | Mild climates, refurbishment |
| 78 mm | Double or slim triple | About 1.0 – 1.2 W/m²K | Mainstream new-build housing |
| 88 – 92 mm | Triple Low-E, warm-edge spacer, 40 – 52 mm | About 0.7 – 1.0 W/m²K | Low-energy and passive-house projects |
Indicative values only; certified results depend on glass, spacer, frame geometry and size and must come from the system's calculation or test report.
| Market | Standard | What it covers |
|---|---|---|
| Europe / UK | EN 14351-1 (CE / UKCA marking) | Air permeability (EN 12207), watertightness (EN 12208), wind resistance (EN 12210), U-value (EN ISO 10077), acoustics (EN ISO 717-1) |
| Europe | EN 942, EN 14220, EN 13307-1 | Quality of timber in joinery, timber for windows, laminated and finger-jointed scantlings |
| North America | AAMA/WDMA/CSA 101/I.S.2/A440, NFRC 100 / 200 | Performance class and grade; certified U-factor and SHGC |
| Coatings | Qualicoat, Qualanod, AAMA 2603 / 2604 / 2605 | Durability of the aluminium cladding finish |
Acoustic performance depends mainly on glass: an asymmetric triple unit or acoustic laminated glass can lift a window from about 32 dB to around 40 dB Rw or more. Four-seal systems (glass seals plus two or three sash seals) help both air tightness and sound reduction.
Glass, grilles, screens and hardware
| Option | Choices | Points to confirm |
|---|---|---|
| Glass units | Double or triple insulated glass, Low-E, argon or krypton, solar control, laminated, toughened, acoustic, vacuum insulated glass | Climate zone, target Uw or U-factor, SHGC, safety glazing zones and acoustic targets |
| Grilles and bars | Grilles between the glass, simulated divided lites with bonded bars, true divided lites | Bar width, pattern and colour on both faces; decide at drawing stage |
| Insect screens | Removable, retractable or fixed screens with standard or high-visibility mesh in aluminium frames | Inward or outward opening determines screen side |
| Hardware | Lever handles, folding cranks, multi-point locks, concealed hinges, tilt-turn gear, restrictors | Sash weight limits, security rating, finish (black, white, bronze, satin chrome) |
A clad window is a system: the cladding, glass, seals and hardware must be from tested combinations. Changing to heavier glass, for example, may need stronger hinges and a deeper sash. Ask for confirmation that your chosen combination falls within the tested range.
Where clad wood fits, and how to specify it
- Villas and custom homes: large casements, corner glazing and fixed shapes with oak or fir interiors and dark exterior colours.
- Residential replacement: upgrading old timber, vinyl or single-glazed windows while keeping a wood interior and matching sightlines.
- Hotels and resorts: consistent exterior colour across many rooms, acoustic glazing and warm, natural interiors for guests.
- Heritage and conservation areas: sash and push-out casement styles with slim glazing bars that satisfy planning requirements.
- Low-energy buildings: 88 – 92 mm timber with triple glazing reaching passive-house-level window values.
- Offices and clubs: premium interiors where timber joinery is part of the design language.
- Define performanceLocation, exposure, target Uw or U-factor, wind and water classes, acoustic and security requirements.
- Choose types and sizesOpening type per window, sizes, combinations and any special shapes on the elevations.
- Select materialsTimber species and interior finish, cladding colour and finish, glass build-up, grilles, screens and hardware finish.
- Approve samplesA corner sample with real timber and coated cladding, plus colour and finish panels.
- Confirm logisticsDelivery sequence, labelling by schedule reference and export packing that keeps the timber dry.
Installation. Clad windows are fixed through the timber frame into the structure with frame anchors or screws at the centres given by the system supplier, packed under the jamb and sill so the frame is not distorted. The gap between frame and wall is sealed in three layers: a vapour-tight membrane or tape on the warm side, insulating foam or mineral wool in the middle, and a breathable, weather-tight membrane or sealant on the outside. This "tight inside, open outside" principle stops warm indoor air carrying moisture into the joint, where it would condense against the cold reveal. The aluminium sill flashing must lap over the wall below and slope outwards, and its end caps must be sealed to the reveals; most water problems in clad windows start at the sill ends rather than in the window itself.
Site protection. Timber that has been dried to window moisture content will pick up water quickly on a wet building site. Store units upright, under cover and off the ground, and keep factory protective film on the cladding until scaffolding and wet trades have finished, but not so long that sunlight bakes the adhesive onto the coating. Plaster, mortar and cement splashes are alkaline and will etch both anodised and coated aluminium, so clean them off with water straight away rather than waiting for the final clean.
Life cycle. A well-made clad window can serve for several decades. The interior timber finish is the part that ages, and it can be refreshed in place with a light sand and recoat. The exterior shell rarely needs attention beyond washing; if the building is repainted in a new colour, the clip-on cladding can be removed, recoated and refitted. At the end of life, the aluminium cladding is easily separated and has high scrap value for recycling, and the timber can be recovered for energy or reprocessing. This separability is one reason clad wood scores well in environmental product declarations compared with frames that bond dissimilar materials together.
Typical maintenance schedule
- Exterior cladding
- Wash with mild detergent and soft cloth once or twice a year; every three months within a few kilometres of the sea
- Drainage slots
- Check and clear each spring and autumn
- Hardware
- Lubricate moving parts and check locking adjustment once a year
- Seals
- Inspect for flattening or splits; replace gaskets when they lose elasticity
- Interior timber
- Clean with a damp cloth; refresh clear or stained finishes when the surface becomes dull or worn
References and standards
- EN 14351-1 – Windows and doors: product standard, performance characteristics
- EN 942 – Timber in joinery: general requirements
- EN 13307-1 – Timber blanks and semi-finished profiles for non-structural uses
- AAMA/WDMA/CSA 101/I.S.2/A440 – North American Fenestration Standard
- AAMA 2605 – High-performance organic coatings on aluminium extrusions and panels (FGIA)
- Qualicoat – Specifications for a quality label for coatings on aluminium for architectural applications
Aluminium-clad (aluminum clad) wood windows: frequently asked questions
Are aluminium-clad wood windows worth the extra cost?
They are when the interior appearance of timber matters and the exterior must be low-maintenance and colour-stable for decades. If interiors are painted or the look of wood is not important, a thermally broken aluminium window usually costs less and has slimmer frames.
Will the timber rot behind the cladding?
Not if the cladding is designed and fitted correctly. Extruded cladding clipped on with a ventilated, drained gap keeps rainwater off the timber and lets any moisture evaporate. Rot problems are typically linked to tight-fitting cladding that traps water, or to the gap being sealed during installation.
What is the difference between extruded and roll-formed aluminum cladding?
Extruded cladding is a thicker profile (about 1.2 – 2.0 mm) pushed through a die, stiff and dent-resistant, normally clipped on with an air gap. Roll-formed cladding is thin coil (about 0.4 – 0.6 mm) bent to shape and often fixed tight to the wood. Extruded cladding is the better choice for exposed and project work.
What U-value can an aluminium-clad wood window reach?
With 68 mm timber and double Low-E glazing, typically about 1.2 – 1.4 W/m²K. With 88 – 92 mm timber and triple glazing, values of about 0.7 – 1.0 W/m²K are achievable. Always check the calculated value for the actual product and size.
Can the interior wood and exterior aluminium be different colours?
Yes. The two are finished separately, so you can pair, for example, a natural oak or white-painted interior with a black, grey or bronze exterior.
How do I maintain aluminium-clad windows?
Wash the exterior cladding with mild detergent and water once or twice a year (more often near the sea), keep drainage slots clear, lubricate hardware annually and refresh the interior timber finish when it shows wear, usually after many years.
Do you supply cladding profiles to timber-window manufacturers?
Yes. Metline supplies extruded cladding profiles, clips, sheet and accessories in mill, anodised or coated finishes to window makers; the window maker builds and glazes the clad windows. Export packing and mill certificates are included.
Specifying aluminium-clad wood windows?
Send your window schedule, timber and colour choices, glazing targets and destination. We will propose cladding sections and quote profiles, clips and accessories.
