Would it be cheaper to change the affected windows ?
Windows are less than 2 years old so no appetite to do that, hard to know exactly what the cause is but cold spots due to insufficient attic insulation is most likely bet as mentioned by Coles below. Or crappy gutters, or too much moisture generated by us, or a bit of all of the above.
Did you have this problem before you changed the windows? If not, it could be that drafty old windows were fulfilling the ventilation requirements but the better fitting new windows combined with the absence of 'normal vents' which you say you don't want in the bedrooms are leaving the stale humid air in the room to condense on cold surfaces.
If you didn't have this problem before changing the windows, the quickest and simplest thing you could do is to open bedroom windows every morning to allow a full air change in the room.
Think it was there before but has gotten slowly but steadily worse since draughty ones replaced.