| Kind
| Cost per sq.metre
| Light transmission
|
| K ( U ) =
| | Watts per sq. M. per degree difference
|
| Thickness
|
|
| Hence running cost
| | (assume £600 for glass + bubble plastic)
|
| Capital cost of glazing - for 24ft long, 17ft wide house, on a dwarf wall.
|
| Hort. Glass
| £4 per 2x2 piece.Assume 33% discount for quantity ,then = £7.25 p.sq.M.
| Assume 95%
|
| 5.5 for glass
| | maybe 4.2 for glass with bubble plastic lining
|
| 5mm ?
| 6.6kg
| £600
| £520
|
|
| £13
| 82%
| 3.2
| 10mm
| 1.7kg
| £450
|
|
|
| £19
| 78%
| 2.4
| 16mm
| 2.7kg
| £385
|
|
|
| £25
| 68%
| 1.6
| 25mm
| 3.4kg
| £228
|
|
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
