Blocked air vent - mould inside cavity wall?

Gabriela67

Hi there,

I hope you can help! I live on the top floor of an ex-council flat built in the 50s. The building has cavity walls and the windows have upvc double glazing (of poor quality). There is gas heating in the flat with the boiler in the kitchen. The living room has a vent in the old fire chimney but there is high humidity in the room because mould is forming on the plants soil. There is no visible mould on the walls and no unpleasant smell in the room. The main bedroom had an air vent in the old fire chimney but I blocked this up (for soundproofing reasons). Again, I have no issue with damp or unpleasant smells in this room.
The problem I have is the spare bedroom!! When I moved in my flat there was an air/brick vent in the external wall of the bedroom, aprox 30 cm from the ceiling. This was made of a few holes in both cavity walls so if I looked through I could see outside. I had to block the vent for soundproofing reasons. I simply stuffed some paper inside the holes and then I filled the holes on the inside wall with filler. I couldn't reach to fill the holes in the external wall as the filler would just fall in the cavity. I then let the room out and did not notice until months later that my lodger was drying her clothes in her bedroom without ever opening the windows. As a result there was a bit of mould growing on the walls and windows. My lodger eventually moved out and I cleaned the walls and left the window open all summer. I did not have any mould since then. However, every time I go in the bedroom I notice of strong smell... I can't really identify it, it doesn't smell clearly of mould or damp, it smells of dirty wet socks, as if I never ventilate that room. The smell is much worse when the heating is on (the heater is on the external wall). I have laminate flooring and that the curtains have been cleaned and aired. I also noticed that the wall surrounding the blocked up air vent is cracked, but I am not sure if the crack was there when I moved in.

I wonder if it is possible that the smell comes from the blocked out vent? If I only filled in the internal wall and the holes remained in the external wall is it possible that rain water got in and mould formed in the cavity?? But could this be possible if the air in the cavity is as cold as outside, plus there would be too much surrounding ventilation to allow mould to form? Or perhaps the smell lingers since the time when the room had mould (unlikely as this was a year ago). Is there a way to block the holes in both walls given that I can't get access to the external wall from the outside (flat is on 4th floor)? How do I identify where the smell comes from? Does anybody have any suggestions as the smell is driving me crazy!

Thank you!