||Cost per sq.metre
|K ( U ) =
|Watts per sq. M. per degree difference
|Hence running cost
|(assume £600 for glass + bubble plastic)
||Capital cost of glazing - for 24ft long, 17ft wide house, on a dwarf wall.
||£4 per 2x2 piece.Assume 33% discount for quantity ,then = £7.25 p.sq.M.
|5.5 for glass
|maybe 4.2 for glass with bubble plastic lining
Note: polycarb is guaranteed for 10 years, but will at some time after that need replacing due to crazing and yellowing from UV.
The S factor, is the time taken to recover the extra cost .(i.e. extra cost in glazing divided by annual savings).
If it is less than 10 years - ( it always is) then it is economically efficient.
Also note that the extra cost of the 25mm over the 16mm is recovered faster - the sharply improved K factor is responsible as compared to the smaller increase in cost.
Geoff Hands, 2001