UK article talking about PVC and timber windows.
http://www.woodforgood.com/lwwpdf/window_of_opportunity.pdf
Very detailed report on Timber versus PVC windows in the UK - based on facts.
Lifecycle, (20 years for PVC) environmental impact, durability, cost, maintenance, cleaning etc.
And as for Aluminium
These figures are very interesting re the base material impact on the environment.
Material Amount of CO2 released during the manufacture per kg of material
Aluminium 27.50 kg
Plastic 3.40-11.00 kg
Copper 8.5 kg
Glass 1.30 kg
Iron 1.75 kg
Lead 2.50 kg
Rubber 4.80 kg
Steel 3.20 kg
Timber N/a
And the following table compares the atmospheric carbon released per cubic metre in the production of timber and some other common building materials.
Material Carbon Released (kg/m3)
Rough Sawn Timber 15
Steel 5320
Concrete 120
Aluminium 22000
Source data came from from here
http://www.ab.deakin.edu.au/online/vgallery/2004/srt251/team28/timber.htm
Steve