A whole life cost comparison was prepared by Heriot Watt University using data from the SLP model and a standard discounting technique. Using NPV (net present value) analysis, the whole life cost comparison for each option was also evaluated. With PVC-U windows indexed at 100, all timber based window options were indexed at less than 100: demonstrating that capital, installation, maintenance and replacement costs are lower for all building life options of 60, 80 and 100 years, and for all timber window alternatives. For mild exposures, timber windows offered the lowest lifetime cost option, while for moderate and severe exposures the more durable modified timber and aluminium-clad windows gave more favourable lifetime cost outcomes.
If lifetime cost is the primary concern, this analysis suggests timber offers the lowest cost option for properties in a typical urban/suburban setting, aluminium-clad timber options would be favoured on high-rise or multi-storey buildings, benefitting from their extended service life and low access requirement, while in coastal or moderately exposed locations modified timber or aluminium-clad timber windows may be optimal. If capital cost is the only criterion, PVC-U windows are the least expensive option. PVC-U windows are not suited to severe climate/construction scenarios and will significantly increase whole life costs due to a reduced service life.