Suffering from misted double glazed windows or doors is a common problem caused by the outside air temperature being different from that of the inside. In the first instance, it helps to keep the outside and inside temperatures reasonably balanced ie. open windows when using the shower, bath or when cooking.
Over time, the gasket (see picture below) looses it's effectiveness due to the sun drying out the oil which is within it when it is made. This causes condensation to run down the window or door into the window/door frame. Normally, the drainage holes take care of this problem, by letting the water run out and away from the glass.
On an old window/door frame (or a frame which receives a lot of either sunshine or heat changes), the drainage holes become blocked with dust and dirt. Thus the water has nowhere to escape to, which causes the glass to sit in water. This over time breaks down the chemical adhesive which bonds the two pieces of glass (double glazing) together.
In between the double glazing is a gas called Argon, which helps keep the heat in as it is an insulator. The amount of space (distance) between each pane of glass determines the sound barrier to the outside. For example, there are single glazed units, double glazed units varying is depth from 20mm, 24mm, 28mm and 28mm triple glazed.
As a property owner, it is really important to keep the area where the drainage holes are free from dust and dirt. A paint brush is good for cleaning this area.
People mistakenly think they need a new window or door every time they experience the problems spoken about above, and sometimes they are correct, as the frame has cracked and their only solution is a new window or door frame.
However, as we have had 30 years in the window glazing and manufacturing industry, we can remove the glass, without damaging the frame, clear and clean out the offending drainage holes or even add new drainage holes. Get the glazed unit remade (it does not matter whether the glass is leaded, patterned, double or triple glazed, tinted or clear). Replace the perished gaskets, refit back into the original frame, leaving it to function as good as new for a fraction of the price of a new window or door.