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The choice of window frame materials on the market is enormous, yet for most specifiers or users the choice comes down to softwood- or PVCu-framed windows.
Broadly speaking, the criteria that need to be considered when specifying a window system with durability and whole-life costs in mind include:

• The durability of: frame materials; window components such as ironmongery; and finishes.
• Compatibility of the components and finishes.
• The location of the window.
• The maintenance, repair and cleaning strategy.

<B><font size="+1">Softwood window frames</b></font>
<B>Factors affecting durability</b>

• Adhesives
• The most durable adhesives designated weather- and boil-proof to BS 1204:1993 or D4 to BS EN 204:2001 are not widely used in timber window manufacture, yet adhesive failure is a common limit on durability. Insist on the use of an adhesive formulated for external conditions.
• Exposure
• Exposure to sunlight and driving rain will result in movement of the frame and coatings. These thermal and moisture movements will lead to eventual adhesive failure and breakdown of protective paint coatings.
• Timber species
• Softwoods such as western red cedar or Douglas fir are naturally durable without preservation. Ensure that sapwood is excluded if these timbers are used. Refer to table NA1 of BS EN 942:1996 for a list of options.
• Preservation of timber and end grain protection
• Most softwoods used for frames do not have natural durability, so a preservative treatment to Hazard Class 3 to BS EN 599-1 should be specified. To increase durability specify a higher desired life; 60 years rather than 30 years.
• As water is most likely to enter at joints it is essential to preserve and seal the joint surfaces. All cut ends should be factory treated. Use of water-repellent grades of preservative and sealing end grain before assembling joints will reduce the risk of moisture ingress.
• Timber quality
• Timber quality is defined in terms of knot size and location and width of growth rings. Five timber qualities are listed in BS EN 942-1.
• Protection
• A good quality primer should be applied at the factory. The full protective system – that is one specifically suited for external timbers – should be applied as soon as the window is installed. Paints offer better protection than stains; this is especially important where less durable D3 adhesives have been used.
• Specify paint systems to BS EN 927, which classifies finishes by "appearance" and bases selection of coatings on "end use" and "exposure conditions". There is a trend towards factory finishes, which provide enhanced initial reliability of coating systems. This typically means eight years before first repainting cycle.
• Maintenance
• Planned preventative maintenance regimes typically require windows to be repainted at a maximum of five-year intervals or restained at three-year intervals. Economies and longer service in use may be obtained by employing an annual condition-based maintenance and repair strategy. Glazing compounds, insulating glass units, weather stripping and ironmongery may need replacing before the window frame.
• Joinery design
• All horizontal surfaces should be adequately weathered to shed water efficiently. BS 644-1:1989 suggests a minimum slope of 7º. Incorporate steps and drip grooves, appropriately located, in the detailed design, to reduce the risk of water being driven into joints.
• <B>Durability tips</b>
• Setting windows back away from the face of the brickwork as far as possible will reduce exposure and improve durability. To reduce stress on sash joints, specify "run-up" blocks or wedges to take the weight of the sash while in the closed position. Consider using aluminum bottom glazing beads and fix wooded beads with non-ferrous nails or screws.
• A BS kitemarked window offers assurance of quality. The British Woodworking Federation has instigated a "timber window accreditation scheme", which specifies minimum standards with a third-party accreditation. A softwood window manufactured to the BWF timber window accreditation scheme would have a service life of 25 years or longer depending on the timber treatment and adhesives used.
• <B>Modes of failure</b>
• Joint failure
• Adhesives not primarily for external conditions are frequently used for reasons of cost and convenience. This leads to joints failing under normal conditions of weather exposure. The problem is exacerbated by the widespread use of timber stain.
• Glazing not tightly wedged
• Joints in opening casements can be stressed by the omission of setting and location blocks when fitting heavy insulating glass units. Where the insulating glass unit is tightly wedged in the casement with setting and location blocks, a diaphragm action is achieved, which adds to the strength of the window. The single glazing in traditional putty glazing would contribute significantly to the rigidity of casements in this way.
• <B><font size="+1">PVCu window frames</font></b>
• <B>Factors affecting durability</b>
• Colour
• PVCu has a relatively high co-efficient of thermal expansion therefore the use of darker coloured PVCu windows should be carefully considered. Only white PVCu can comply with BS 7413:1991. Foil-coloured PVCu sections should be manufactured to BS 7722. To avoid problems of casements jamming and thermal stressing of corner joints, strict attention should be paid to frame fixing positions and the maintenance of a 6-10 mm perimeter clearance between the frame and the wall opening.
• Exposure
• The combined effect of wind-blown grit, grime, pollutants and ultraviolet light can degrade the PVCu finish to produce a matt surface that retains dirt. Windows should be washed every six months with a non–alkaline detergent.
• Fixing
• PVCu window frames should be mechanically fixed in openings. Polyurethane foam should not be solely relied on to provide fixing.
• Maintenance
• PVCu may require decoration after 30 years. Use of a darker colour than the original frame is not recommended as this may cause thermal stresses the frame was not designed to cope with. Inappropriate paint systems may reduce the impact resistance of the PVCu frame. Use a specifically formulated paint system; typically these are elastomeric with a repainting cycle of four to five years.
• <B>Durability tips</b>
• Washing PVCu window frames every time the window glass is cleaned or at least every six months will preserve the surface finish and prevent premature stains.
• Early neoprene glazing gaskets and weather stripping may become brittle and crack. Replace with new propylene diene rubber gaskets.
• <B>Modes of failure</b>
• Joint failure
• Joint failure in PVCu is now very rare because of general compliance with BS 7412. However, remember to fix PVCu windows 150-250 mm from the frame corners to avoid thermal movement stress to the joints.
• Ultraviolet degradation
• Prolonged exposure to ultraviolet light can cause the frame to yellow, become brittle and lose its gloss finish.
• Self-coloured PVCu will fade when exposed to sunlight.
• Ironmongery failure
• The materials and coating quality of window ironmongery should be expressly specified. BS EN 1670:1998 specifies the requirements of coated and uncoated hardware surfaces applicable to service in a range of environments.