by Patrick J Hislop RIBA, Senior consultant architect, TRADA Technology. Published in TRADA Timber Industry Yearbook 2001.
The thinking man’s choice
Over the last 30 years the expectations for the performance of all windows has changed considerably. Improved thermal and acoustic insulation, better draughtproofing and weather resistance, environmental credibility, greater security, a good choice of appearance, reduced maintenance and low cost are now all important features in the selection of any window design. Modern wood windows can satisfy these requirements better than any other material.
Wood windows get Greenpeace vote
Wood is a naturally renewable material and uses less energy in processing and manufacture than the alternatives. For instance a PVC-u window requires eight times more energy to manufacture than a hardwood window of the same size. Greenpeace, who have spent six years researching the environmental impact of manufacture and disposal of PVC-u and timber in windows are categorical in their argument:
“The production and disposal of PVC-u windows leads to the release of highly poisonous chemicals which threaten the environment and human health. PVC-u production involves no less than six of the fifteen most hazardous chemicals listed by European governments for priority elimination.
Timber is a sustainable resource. As long as the timber is sourced from properly managed forests and care is taken in the choice of preservatives, paints and stains, timber windows are by far the best environmental choice.
Developments in timber window design and finishing mean that modern, high performance timber windows need minimal maintenance and potentially have significantly longer life than PVC-u.
PVC-u windows do degrade, they are not maintenance free and worst of all they cannot be repaired when necessary.
High performance double-glazed timber windows need not cost more than PVC-u equivalents.”
From Greenpeace leaflet "Look out – Your choice of window frames could seriously affect the health of our planet".
In their June 1998 report, the National Housing Federation found that:
Low life cycle cost is at least partly attributable to the fact that wood windows benefit from a low thermal transmittance. Thermal transmittance through components is measured and compared by U values - the higher the U value the greater the loss of heat through the material, ie the lower the number, the better the insulation. Wood windows have lower U values than steel, PVC-u, or aluminium (even where thermal breaks are provided), although they are not differentiated in the building regulations. This reduces the risk of condensation on the frame, which is especially important when insulated double glazed units are specified.
Besides the good insulation properties inherent in wood windows, high performance weather resistance can be achieved by the use of efficient draught seals on all opening lights. These ensure that the window remains draught free even in the most severe exposure conditions. The greater stiffness of the timber frames means that these seals are equally compressed for the whole perimeter of the window, ensuring a tight fit at all points. With the correct choice of section, insulated double glazed units of any thickness can be accommodated - not always possible in metal or PVC-u window frames. The combination of insulated glass panels, efficient weatherseals, and wooden frames also ensures good acoustic performance.
Beaded wood windows with sections robust enough to take substantial locks, catches and bolts can ensure a high degree of security against break-in - and increasingly important aspect of modern window design. Wooden casements are inherently stiffer than PVC-u, making it far more difficult for intruders to ‘spring’ them out of the frame.
Wood windows are now available in a wide variety of styles and finishes to suit all types of building, both modern and traditional. However, it is also possible for special profiles to be designed in wood. This has the advantage that timber profiles can always be manufactured in the future to match existing windows or styles, either for replacement or extension of the original building. This is often not economically feasible with metal or plastic frames, if the basic sections or extrusions are no longer available.
Durability problems with wood windows are largely a legacy of the immediate post war years when shortages led to the use of unpreserved sapwood (the less durable part of a tree). The problem was compounded by the introduction of impermeable and often brittle gloss paints, which although they provided reasonable protection to the outside surface, also trapped in any moisture that entered the wood as a result of internal condensation on glass, moisture take-up through exposed end-grain, or breaks in the paint film. This trapped moisture made the windows susceptible to fungal attack.
Since the early seventies the National House-Building Council has required all softwood external joinery to be treated with preservative and this is now general practice. Preservative techniques have been developing rapidly and a life of 60 years can now be expected of softwood windows (British Standard BS 5589: 1989 ‘Code of Practice for preservation of timber’). Alternatively, durable or preservatively treated moderately durable hardwoods can be used. Thus wood windows now only require maintenance for reasons of appearance, rather than to prevent deterioration of the timber.
Maintenance? Not a problem
To maintain appearance the use of stains or paints will prevent weathering of the surfaces due to ultraviolet light. A major improvement in paints has been the introduction of ‘microporous’ types which, as well as being more flexible, are vapour permeable and thus prevent the build up of moisture within the timber sections.
Stains require more frequent maintenance than paints but the process is much simpler because with minimum preparation fresh coats can be applied over the existing finish, without the time consuming surface preparation necessary to ensure proper adhesion of paint. Stain finishes, because they are absorbed into the surface of the timber, show damage less than paint and the natural grain of the wood always remains visible, one of the visual attractions of wood windows. Factory finishing of windows, with either stain or paint, is now generally available and has the advantage that all surfaces are protected. Some manufacturers will guarantee 10 years maintenance free ‘life’ when the window is factory finished.
Wooden windows are reasonably resistant to impact or scoring in use and if they are damaged they can be more easily repaired and refurbished ‘insitu’. However, pre-finished metal and PVC-u windows cannot be easily repaired if damaged during construction.
The future for wood windows is a bright one. Timber is accepted as the only mainstream construction material that can be produced on a sustainable basis. It is tough, durable and with the increased use of improved factory-applied finishes, relatively maintenance free.
