From the BBC website:

http://news.bbc.co.uk/1/hi/business/4035343.stm

How PC are your PVC windows?
Consequently, wooden window frames have given way to PVC framed double
glazing, which is cheaper to buy and install than double glazed timber
frames.

In fact, wood double glazing can cost up to three times as much as PVC
double glazing.

My wooden windows are the same the house was built with, they are starting
to rot, but are also nearly 70 years old, my neighbours have had three
different sets of double glazed windows in the last twenty years, how can
that be environmentally friendly?

I intend to replace my single glazed wooden frame windows with the same, I
can make them myself and even reuse the glass. Also, the odd draughts that
get around them keep the house ventilated, just as per original design, my
neighbours have had to add air bricks to stop black mould growth in their
house.

mrcheerful