If those windows are in your upstairs (where you're saying it's 16.5º), it could also just be a temperature issue which will solve itself if you get the upstairs warmer. Basically, if the dew point of the air inside a room is above the surface temperature of the window, then you'll get condensation on the cold surface.
But if your room is warmer, that will warm the inside surface of the window slightly, which might be enough to raise it above the dew point. Just an example using a constant moisture level:
16º room temperature and 52% humidity = 6º dew point. So if the window surface temperature is 5º, you'll get condensation.
21º room temperature and 38% humidity = 6º dew point (same amount of moisture as the prior example). But because your room is 5º warmer, the window surface temperature is likely several degrees warmer, pushing it above the dew point and preventing condensation.
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