Clients are slowly coming to accept the importance of whole-life costing, particularly in Private Finance Initiative projects, but there is very little documentation to turn it into an exact science. One company that has tackled the issue is timber-protection specialist Sikkens. It has developed a computer model which, it claims, enables building designers to calculate precisely the whole-life costs for any given window project. This is particularly interesting because, in the past, manufacturers of plastic windows have used durability as one of their major selling points.
The Sikkens Life Cycle Inventory Model, or SLIM, is the result of extensive studies by Imperial College London, working in association with Sikkens, its customers and suppliers. The model helps specifiers arrive at the best economic and environmental solutions concerning the coating and maintenance of factory-finished timber windows, and allows SLIM calculates maintenance requirements them to make comparisons with PVCu. Not surprisingly, it shows up the advantages of specifying timber. For example, it underlines that end-of-life disposal of PVCu systems has far greater environmental impact than that of timber systems. In addition, PVCu window systems consume approximately twice as much fossil fuel over their whole life cycle.
So how does it work? Working in association with a Sikkens technical adviser, a building designer enters the following information on to the computer database:
ltotal service life required;
ltotal number of windows;
lpercentage of windows subject to a mild or severe climate;
lpercentage of windows located in an exposed or sheltered aspect; and lpopulation of the area serviced.
SLIM then uses these values to calculate specific real-life maintenance requirements for the duration of the project and the associated costs. It also analyses all environmental impacts, from global warming through fossil-fuel consumption to nutrient pollution.
Findings will no doubt be challenged by manufacturers of PVCu windows, but the introduction of a quantifiable method of assessment is a valuable tool.