wrote:

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.

We are happy with the windows etc, but in winter sometimes get
condensation on the windows. This only happens on mornings where it is
particually cold and wet out, it burns off by midday, and is worse in
rooms where people have been sleeping.

The water is not inbetween the pannels (as I can wipe it off) and the
panels dont feel particually cold.

The links below are some pictures of the condensation, I just wanted to
know if it is a result of buying cheap double glazing (in which case
fair enough) or if it means they are faulty. Is there some kind of
measureable leagal minimum that the windows need to comply with?

http://www.websphereusergroup.org.uk/temp/IMGP1572.JPG
http://www.websphereusergroup.org.uk/temp/IMGP1573.JPG
http://www.websphereusergroup.org.uk/temp/IMGP1574.JPG


I had the same thing with my patio door, a metal-framed one installed
circa 30 years ago. Moisture would condense on the frame, and on the
lower part of the glass, eventually leaving small patches of mould on
some of the hardwood frame. It took me a little while to work out what
was happening. The pictures you took show exactly the problem here.

I traced the problem to a 'reverse chimney' effect due to the curtains
being closed. There was circa a 1" gap between the top of the curtains
and the ceiling, and similar one at the bottom.

As air trapped by the curtains cooled, it sank and emerged into the
dining area, to be replaced by warm moist air at the top. There comes
a point somewhere near but not at the bottom of the glass where the
temperature falls below the dew-point, and moisture condenses out. The
pattern of condensation is exactly as depicted in your photos. This
continues ad infinitum, drawing moisture from the room and depositing
it on the glass and frame. The colder the night, the farther up the
glass the condensation started.

Putting 'sammy snakes' (draught excluders) along the bottom of the
curtains stopped the problem completely. Although cutting off the flow
of air made the glass colder, the very limited airflow meant that
condensed vapour could not be replaced, and the problem has gone away.
The dining area is also a lot warmer!

I put a wireless thermometer between the curtains and the glass. With
no draught excluder in place, the temperature drop from ambient was
about 5 degC or so. With the sammy snakes in place, the thermometer
read just above the outside temperature, a drop on the night in
question of over 20 degC.

My suggestions: make sure that obvious sources of dampness are removed
from the rooms concerned (towels, clothing, large plants, etc) and cut
the air-flow off with draught-excluders. Your window-cills look ideal
for this. It may take a day or two for your rooms to reach any new
humidity equilibrium. HTH