PVCu
Third-party assessed window ranges, PVCu hollow profiles, multichamber frames and sashes, internally reinforced as necessary with close-fitting stainless steel, aluminium or pegalvanised (min G275) steel sections. Plastic to BS 7413 (impact modified plastics) and construction to BS 7412. British Board of Agrément-approved window systems supplied by BBA-approved manufacturers. Property Services Agency-approved window ranges/suppliers (MOB 08 specification). Windows supplied under local authority buying group LHC's (formerly London Housing Consortium) bulk quotation arrangement U4 window hardware to be tested/approved only by the BBA, PSA or LHC, or kitemarked to BS 7412.
Capital cost £133/m2
Net present value for 60-year life £174/m2
Service life 40 years
White PVCu hollow profile, multichamber frames and sashes internally reinforced as necessary with close-fitting stainless steel, aluminium or pre-galvanised (G275) steel sections. Plastic to BS 7413 (impact modified plastics) and construction to BS 7412. Also, windows manufactured to the British Plastics Federation/Glass and Glazing Federation Trade Standard.
Capital cost £150/m2
Net present value for 60-year life £198/m2
Service life 35 years
Film coloured PVCu hollow profile, multichamber frames and sashes, internally reinforced as necessary with close-fitting stainless steel, aluminium or pregalvanised (G275) steel sections. Plastic to BS 7413 (impact modified plastics) and construction to BS 7412. Colour film to comply with BS 7722.
Capital cost £129/m2
Net present value for 60-year life £196/m2
Service life 25 years
Steel
Windows to BS 6510. Hot-dip galvanised to BS 729 including all fixings to a minimum of 460g/m2 zinc coating weight. Polyester powder-coated to BS 6497 after fabrication, minimum 60 micron coating.
Capital cost £230/m2
Net present value for 60-year life £285/m2
Service life 40 years
Windows to BS 6510. Hot-dip galvanised to BS 729 including all fixings to a minimum 460g/m2 zinc coating weight. Painted on site.
Capital Cost £188/m2
Net present value for 60-year life £292/m2
Service life 30 years
Windows, non-specific steel sections and/or strength of less than 355 N/mm2, and/or hinges, pivots, hardware, sills and so on not specified to requirements of BS 6510. Steel sheet pregalvanised and site painted cut ends painted with zinc-rich paint.
Capital cost £140/m2
Net present value for 60-year life £254/m2
Service life 20 years
Aluminium
Windows manufactured to BS 4873. Polyester powder-coated to BS 6496, minimum 40 micron coating. Compatible hardware and fittings.
Capital cost £244/m2
Net present value for 60-year life £295/m2
Service life 40 years
Windows to BS 4873. Anodised to BS 3987, minimum 25 micron coating.
Capital cost £216/m2
Net present value for 60-year life £216/m2
Service life 40 years
Windows manufactured to BS 4873. Mill finished.
Capital cost £192/m2
Net present value for 60-year life £274/m2
Service life 25 years
Factors included in whole-life cost over 60 years
Capital cost; installation cost; replacement of weather stripping every 10 years; replacement of ironmongery every 20 years; redecoration of powder-coated and anodised finishes at 20 years and every five years thereafter; redecoration of PVCu at 30 years and every five years thereafter; replacement of window frame at the end of its service life; costs are discounted at a rate of 6% a year.
Factors that affect durability
PVCu compound Almost all white PVCu compound now used in windows complies with BS 7413: 1997. This standard covers Type A material (impact modified). BS 7414: 1991, which covered non-impact modified material (Type B), has been withdrawn. The standard allows for up to 10% recycled material but as the recycled material also has to comply with the standard, the manufacturers' own reworked material must be used rather than general PVCu scrap. Early PVCu windows may have been made from unmodified material and may fail under impact in low temperatures.
Colour PVCu has a relatively high coefficient of thermal expansion, so the use of darker colour PVCu windows should be carefully considered. Note that only white PVCu can comply with BS 7413: 1991. Foil-coloured PVCu sections should be manufactured in accordance with BS 7722. To avoid problems of sashes jamming and thermal stressing of corner joints, strict attention should be paid to fixing positions and the maintenance of a 6-10 mm perimeter clearance.
Exposure Wind-blown grit and grime and ultraviolet light can degrade the PVCu surface finish to produce a matt surface that retains dirt. Windows should be washed every six months with a non-alkaline detergent. Polluted or marine environments may accelerate deterioration of metal finishes.
Fixing PVCu windows should be mechanically fixed and not fixed solely with polyurethane foam.
Protection For enhanced durability, aluminium windows should be anodised or polyester powder-coated. Either coating can be expected to last about 20 years, after which site redecoration will be required every five years. Steel windows should be hot-dip galvanised after fabrication and polyester powder-coated or site painted.
Maintenance Anodising and powder coatings may need painting after 20 years, PVCu after 30 years. The use of aluminium frames that rely on timber sub-frames will introduce a maintenance liability and limit durability.
Durability tip
Washing PVCu window frames every time the glass is cleaned or at least every six months will preserve the surface finish and prevent the surface being prematurely stained with atmospheric dirt. Early neoprene glazing gaskets and weather stripping may become brittle and crack even after very few years of exposure. Replace with new EPDM gaskets.
Types of failure
Joint failure Joint failure in PVCu is now 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.
Joint failure in aluminium windows is equally rare. Joints are usually machine-made, with the aluminium extrusion being crimped around a flat stainless steel angle. The mating surfaces should be sealed during manufacture to avoid the possibility of water ingress and deterioration of the surface coating.
Ultraviolet degradation Prolonged exposure to ultraviolet light can lead to yellowing, embrittlement and loss of surface finish in PVCu frames. Self-coloured PVCu will fade when exposed to sunlight.
Hardware failure Although not strictly a failure of the window itself, hardware failure is a common limit on durability. Until recently, hardware was the "weak link" in window specification, but corrosion resistance is now controlled by BSEN 1670: 1998. In PVCu windows, hardware should be fixed through at least two walls of the extrusion and preferably to the metal reinforcement.
Further information
The Housing Association Property Mutual's Component Life Manual, written by Construction Audit, the technical audit arm of Building Performance Group, provides lifespan assessments of more than 500 building components. It is updated twice a year to reflect industry feedback and changes to standards and codes of practice. Published by E&FN Spon, it is available in loose-leaf format, price £175, or on CD-ROM from June. The BPG Building Fabric Component Life Manual and BPG Building Service Component Life Manual will be available from E&FN Spon, in loose-leaf and in
CD-ROM formats, from summer 1999. BS Handbook HB 10141: 1997: Buildings – Service Life Planning: Part 1: General Principles is available from the British Standards Institution on 0181-996 9000. For further information, contact Gary Moss at BPG on 0171-240 8070.
