you will always get condensation on these windows
when you sleep at night warm air from you breathing will cause it
trickle vents will help but wont stop all of it.
the only way is to leave the top window slightly open.
Have you considered upvc inserts,you just remove the sliding sash window and are fixed against the outer bead,its much cheaper than full replacment windows and wil save money on your heating,plus no more condensation
OK, it seems like its 6 days out of 7, so we've managed to get some paint to dry on them.
Its not from moisture while we sleep as its permanently condensed, all day.
I'm going to fit a trickle vent to one of them, the worst, just to see what happens. I'll look into the UPVC insert for next spring - this is a replacement to the inner window rather than the whole unit? So glass & edging but not frame?
I can get double-glazed units made really cheaply so thats probably the same thing.
ventilation and air moisture are the issues here, as the windows are single glazed they will always be the coldest part of the room , assuming this temperature is below the dew point of the room you will have condensation