One of the most frequently stated objections to the use of wood windows instead of aluminium or PVCu is that they will eventually need to be repainted.

This premise goes back to the time when the majority of wood windows were site-painted, which, at the worst, could mean repainting at about three-year intervals. However, since the introduction of microporous flexible penetrating paints and stains, the period between repainting has been considerably extended. An even more significant step forward, though, was the introduction of factory painting. Here, painting is carried out on freshly cut, clean, dry wood in controlled conditions, which has extended the maintenance intervals to eight years or more. Repainting may only be required for visual reasons rather than because of any breakdown in the protection to the wood. Many window manufacturers, therefore, will now offer maintenance-free guarantees of up to 10 years on opaque finishes, which provide the best protection against UV light.

This reduced level of maintenance obviously adds to the appeal of wood windows, particularly for residential developments. However, particularly on larger multi-storey projects, even this less frequent maintenance may still inhibit the choice of painted wood windows, not only because of the cost of labour and materials required, but also because of the cost and disruption of scaffolding to reach upper storey windows.

Scandinavian window manufacturers were quick to bite this bullet many years ago and, as a result, aluminium-faced wood windows were introduced. At a stroke they eliminated the need to repaint during the life of the windows. Powder-coated aluminium sections on the outside of wood windows offer an extensive range of colours, yet still provide the visual and thermal insulation advantages of wood internally. The inner wood framing for composite windows can be coloured to choice, but more usually specifiers opt for a clear lacquer which shows the natural colour and grain of the wood. Because the wood is protected from direct UV light by the aluminium and glass, the natural colour of the wood will be maintained.

Although some of these composite Scandinavian wood windows have been available in the UK for many years, they have not been used extensively. The situation, however, is changing. There is now a growing interest from architects in specifying aluminium-faced wood windows for relatively small projects, including some one-off homes, even if there has not as yet been much interest from the large-scale housebuilders. If these builders consider the advantages of low long-term maintenance on their homes at all, they generally only offer PVCu or possibly aluminium windows, which are claimed to be maintenance free.

This type of composite window is also often used in the form of a fully glazed wall rather than as individual windows set in openings in walls

However, for authorities responsible for the long-term upkeep of their properties, such as housing associations, local authorities or health trusts, aluminium-faced wood windows offer an attractive alternative to PVCu in terms of low maintenance and a much wider range of colours. The wood frames also offer better insulation than either PVCu or all-aluminium frames, even when these incorporate thermal breaks.

The evidence of this increased interest is in the number of UK window manufacturers now either offering Scandinavian designs, or developing their own composite window systems. Early UK designs had some initial problems of condensation behind the metal faces before it was appreciated how important it was to provide sufficient ventilation between the aluminium and the wood. As the aluminium represents a total barrier to vapour, it is very important to ensure that any vapour diffusing through the wood frames can be dissipated by adequate ventilation between the metal cladding and the wood.

Another important aspect of these composite windows is that aluminium will move considerably due to temperature changes, whereas wood is more or less unaffected by thermal variations. For this reason, the aluminium sections tend to be ‘clipped’ to the wood in such a way that they can expand or contract, rather than being mechanically fixed with screws. Where screws are necessary, they need to be fitted into slots in the metal to allow for any differential thermal movement between the two materials.

There are now broadly two types of composite wood windows on the market. The first type consists of a wood sub-frame similar to a conventional wood window onto which aluminium cover pieces and aluminium glazing beads are fitted to cover the wood entirely externally. These windows are generally almost identical in design to the wood windows offered by the same manufacturer, other than that aluminium glazing beads are substituted for wood. Perhaps the most prestigious examples of this are the one-off stainless-steel faced laminated oak window frames designed for the new Scottish Parliament building.

The second type is more radical in design, in that it comprises a simple wood frame internally to which an aluminium glazing system is fixed on the outside, again concealing all the timber sections externally. This is essentially an aluminium window with timber sections on the inside. One particular Scandinavian design of this type has become increasingly popular with architects in the UK market and this particular window has been frequently specified on large projects for several years.

There is growing interest in the UK in this type of window

This type of composite window is also often used in the form of a fully-glazed wall rather than as individual windows set in openings in walls. Even as a full curtain wall a structural or laminated timber mullion may now take the place of tubular aluminium sections, although the glazing is entirely within the aluminium part of the window. These glazed walls often include insulated opaque panels.

All types of composite window offer drained, ventilated glazing systems, now recommended for insulating glass units, and can accommodate the increased thickness of glass necessary to meet current insulation standards. Glazing is generally with rubber gaskets rather than butyl tapes or mastics, and these are particularly suited to glazed curtain walls in allowing for the possible thermal movement of large insulated glass panels.

Like all prefinished components, these composite windows are best installed into buildings at a relatively late stage in the contract, when all the wet trades have been completed and the risk of consequent damage is decreased.