Q:

High humidity in bedroom and condensation

I have a Bungalow build 1950s cavity walls not sure if they have insulation in the cavity.
Suffering from really high humidity in main bedroom 74% overnight dropping to 59/60 in day. Window and external wall is dripping in the morning with moisture. Also for the first time in 5 years had mold in the 2 bottom corners. Not too sure how to combat this. 
Tried sleeping with window open even though its freezing in UK at this time but still have the issue.
There is an old fire place that has been boarded over thinking of adding a vent into that to help with airflow?
Also maybe adding thermal wallpaper wallrock to that wall to keep it warm?
Dont suffer with this anywhere else in the house? Any ideas appreciated!

A:

In general, high moisture levels can be reduced by exchanging the room's air only during night, best time in the morning before sun rise.
Best method would be to open 2 windows at 2 different walls of the apartment/building, west and east sides would be the best option. 
If there is a noticeable air flow, just 1-3 minutes are enough and should not be exceeded. 
That way statistically colder and dryer air is replacing the warmer air without cooling down walls, furnitures etc. 
This method is extremely counter-intuitiv since most people do not want to get colder air into their home. 
But warming up dry air needs only a fraction of the energy needed to heat up moist air.
Without this air exchange the building's material, furnitures and interior have to be heated up above the dew point what needs much more energy.   
Since the humidity level in the mentioned bedroom is reduced during daytime, it could be due to the regime "airing during daytime, reducing temperature during nighttime", which is likely causing high humidity levels in the nighttime.
If the hull is not tight enough, statistically moist air comes into the building, since normally the wind blows stronger during daytime. Could be checked via candles, smoke, thermography or 30cm long hanging strings scotched to door and window frames.

