Condensation on the inside of a window

Condensation in its purest form is caused when moisture in the air
comes in contact with a cold surface and it condenses from water
vapour to a liquid again.
Condensation points to problems in humidity levels in the home
and a lack of adequate ventilation which in turn can lead to damage
to the property and mould growth as well.

The reason we notice condensation on our nice new thermally
efficient windows, particularly in the cooler months, is simple: they
are doing their job.
Older windows are usually less air tight, allowing moisture-laden
air to pass from inside the home to outside without causing any
condensation.
Modern windows will have been manufactured to minimise air
loss and sealed up nice and tight during their installation. Now the
moisture can’t escape and when it hits the glass, which is usually
the coldest part of the room, it forms condensation.
Condensation can also appear when there’s simply too much
humidity in a home, so it can manifest at any time of the year. Think
hot showers and steamed up mirrors. Likewise when boiling a kettle
or cooking.
So condensation is the result of relatively warmer and humid air
meeting a cold surface, in this case, glass.

All living organisms give off humidity simply by existing: humans,
animals, even plants.
Then there’s the humidity we create with our everyday activities.
Changing some habits or making minor lifestyle tweaks can make a
lot of difference and help reduce the amount of condensation that
occurs in a home.

The bathroom is the most humid place in the home. Taking a
shower releases huge amounts of humidity into the air – the hotter
it is and the longer it is, the more water vapour is released. So,
where possible, try to reduce this avoidable humidity source and
keep your shower at a reasonable temperature for as short a time as
possible. You’ll save on your bills as well!
If you have one, open a window. Ideally before or during the shower
but definitely after, and leave it open for 10-15 minutes at least to
let that humid air escape outside fast.
If you don’t have a window then you should have a mechanical fan.
Make sure this is running during the shower and for 15-20 minutes
after.
Make sure also to use a decently sized bathmat to avoid saturating
bathroom floors when getting out of a bath or shower. The bathmat
should help soak up some of the moisture, helping to reduce the
condensation in the room, especially if it is put in the dryer along
with the wettest towels.

Kitchens are the second biggest source of humidity in the home.
Cooking, kettles and other appliances all release steam, so be sure
to use pan lids when cooking to reduce moisture being created
from water boiling.
Make sure also to have a cooker hood that’s set to extract and use it
all the time while cooking and for a short time afterwards.

Bathrooms and kitchens are the worst culprits for condensation.
When cooking food, boiling the kettle or taking a shower, ensure
that the kitchen or bathroom door is kept closed to prevent the
moisture in the air from going into colder rooms which will cause
condensation to form if it touches a cold surface

It’s difficult in the winter, but try not to dry laundry inside on an
airer or radiator when it’s cold outside as every load releases pints of
water into the air.
If you can, dry clothes outside or if that isn’t a possibility dry them in
an exterior-vented or condensing dryer.

When natural gas or propane and butane burns it releases water
vapour, carbon dioxide and potentially carbon monoxide.
Make sure you have adequate ventilation to ensure it can escape
outside.

If you’re painting or plastering large walls in your house, these
surfaces will need to dry, and the moisture will go straight into the
interior air unless the space is well ventilated.
Keep your windows open where possible when decorating.