Ask a tradesman

Restoration & Refurbishment

Blocked air vent - damp in cavity wall?

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)? Another options would be to fit an acoustic vent but I don't know how easy that is (as there is no access from the outside) and if it would sort out the issue (holes in the outside walls are on a larger area than the acoustic vent). How do I identify where the smell comes from? Does anybody have any suggestions as the smell is driving me crazy!

Thank you!

8 Answers from MyBuilder Restoration & Refurb Specialists

Best Answer

in all honesty you should never blocks vents they are there for a reason, try unblocking or go for a passive fresh vent there are ones that block sound.good luck


Answered 23rd Mar 2015

I would unblock the vent straight away. If it's a flat you live in ask your neighbours above, below and either side of you if they have the same problem. If they do then there is definitely damp in the cavity wall somewhere. Your tenant drying her clothes in an unventilated room won't have helped matters and this could contribute to damp in the room. If there is wall paper I would completely strip it all off and check underneath laminate flooring. U need to get this problem solved asap before it spreads. Damp can be caused by many different things so I would recommend having a qualified reliable tradesman with experience to come and fix the problem it maybe something simple like a leaky roof or cracked brickwork but it's important to get it nipped in the bud before it's too hard to get rid of. I hope this helps and good luck!!


Answered 7th May 2018

Check curtains. As these would hold smells as cold then warm.
Did lodger leave or vacated as him might have left nice surprise behind radiater.
Other vent at chimney brest. Need to unblock that as your stoping that breathing.


Answered 14th Mar 2018

Could it be the laminate flooring damp on the edges . Worth checking . .how many layers of walpaper on walls ?


Answered 26th Feb 2015

Get some dehumidifiers, change your windows for trickle vented windows, make sure your bathroom and kitchen extractor is working. You need to have good air circulation


Answered 28th May 2018

If the windows are old upvc with know vents you can put trickle vets in the sashes. Remove wooden floor and keep the heating on as you would the rest of the flat. if not the room will act as a cold room to all the heat will head to that room and cause condensation


Answered 12th Jan 2019

Vents shouldn’t be blocked they are there to allow walls property to breath stop situations like what accured to prevent condensation damp unpleasant smells answer is poor ventilation no ventilation property gets humid sweats causes condensation then causes mould etc has no we’re to go because ie ( air vent is blocked )
I suggest fit new air vent inside & external side hit & miss so external vent has flaps just flap away freely allow air to flow that way you won’t be seeing right threw to outside external vent open closes as it wishes allowing air flow into the property

Never block vents unless going replace them else we’re in same room that there not covered either with wardrobes must be able have clear way it to be used fall Potiential I suggest use dehumidifier take away any other condensation may be accumulating open windows when you can always good gives property fresh cycle of air

Hope this has helped regards

HB5 group


Answered 4th Jul 2018

Your walls are suffering from hot air meeting cold air, open the vent, this should have a sleave in it, place thermal. Boards over these walls, but do wash them first, this will keep heat in and a barrier between the two tempitures, a skimmed finish. Is always best, and put a vent cover on that you can adjust depending on time of year,


Answered 13th Apr 2021

Post your job to find high quality tradesmen and get free quotes

Can’t find an answer? Ask a new question

Question Categories