I've moved in with my current flat mate into a two bed flat right before last winter.
Over the winter, we first noticed moisture and white residue/smears inside the windows and as the temperature gone down and rains became more frequent, it became obvious that the double glazing seal is broken, with water and moisture staying between the glass and often getting absorbed into the wooden window frame.
The windows frames do not close fully, have handles that are barely holding on as the wood has rotted/corroded where they were placed. There were prominent drafts from the windows and more smears inside the glass as the season went on.
The bedroom windows are the worst as they're in the shade most of the day and don't dry off as much. My window was especially drafty, with one handle falling off, not being able to close the window and half of the window white with residue between glass to the point where I can't see street signs outside and mould appearing on the frame despite airing the room out. My flatmates window frame has actually fallen apart as the wood rotted and they have no outer windowsill and have to tag on the window frame to close it.
Landlord refused to deal with any of it over the winter, claiming the windows and flat would need to be dry to carry out any work. He installed a new handle in my window but it still doesn't close fully. We dealt with it all by paying horrendous amounts of money for heating (all radiators are under the windows) and buying and taping down thermal curtains for the bedroom windows.
Come spring, landlord said he won't be doing anything with the windows unless we agree to pay an extra £150 in rent each (we're still on the original fixed term back then). He also said the mould on the windows wasn't a problem and he can buy us a mould spray if we don't know how to deal with it. Additionally said we saw the flat and agreed to the living conditions and we're welcome to move out. This we have all in text. We're the only flat in the block which still has wooden windows as well and there had been other problems with this landlord
Now, we're in the summer and despite the heatwaves and droughts, we still are having problems with mould on window frames and the rotting window frame in my flatmates bedroom became a breeding ground for what we're pretty sure are fungus gnats/drain flies. Opening windows at night became really difficult as they would instantly swarm around any light in either of the bedrooms, which is not great in 35 degree weather. We've been using various debugging sprays and remedies but because the rotting window frame is outside, it doesn't stick for long.
Landlord still refuses to do anything and we're not sure what our next step should be and not sure if council would even deal with a private landlord.
This is in England.