wrote in message
oups.com...
I had some double glazing fitted last year, it was fitted by a
FENSA registered company, but was the cheapest quote
we found - approx £4000 for 6 leaded windows and a
patio door.

As others have said..it's due to a lack of ventilation. Replacing old
windows with double glazing can sometimes make condensation worse because it
eliminates ventilation through the badly fitting old windows.

When people breath they put water vapour into the air. The amount of water
vapour that the air can hold depends on the temperature. If the temperature
drops - for example when the damp air comes into contact with cold glass the
excess can condense out (=rain).

There are two main solutions....

1) Add ventilation (open windows or trickle vents) to let the damp air out
and dry air in (outside air is normally dryer even in winter).

2) Use a dehumidifier to remove the water vapour from the air.

Personally I feel the only "perfect" solution is to install whole house
ventilation system with heat recovery. Think of it like opening a windows
without loosing the heat.

http://www.toolbase.org/techinv/tech...chnologyID=202

http://www.villavent.co.uk/domestic-...on-systems.htm

Note that UK Building Regulations require replacement windows to meet the
Building Regulations. This can include the need for them to have built in
trickle vents (if the house doesn't have a ventilation system). If your new
windows have trickle vents in the frames make sure they are open - that
might be sufficient but I suspect not as they are frequently a bit on the
small side.