On Thursday, 17 December 2020 at 22:12:50 UTC, Monique Castellani-Kraan wrote:
Hi all, could use some input on this because I'm a bit of a novice and need to make sure I'm using the right tools for the job!

Our house was built in the 1940s and we've lived there for almost 30 years, and little did we know there was a badly filled-in air brick inside my build-in clothes wardrobe. In recent years I wondered why I had such a bad clothes moth problem and why my clothes smelled fusty.

The air brick is 6 ft above the floorboards, directly inside my enclosed wardrobe. It is in a contained space consisting of only my clothes wardrobe and bookcase inside my bedroom. Photos here. https://imgur.com/a/KCKaehO

It seems the previous people did a bodge job and filled it in from the inside using only plaster filler, so of course over the years the plaster has deteriorated and crumbled - and has now had an explosion of mould growth that spread all over the walls and ceiling in the upper storage section of my wardrobe. Had to absolutely blitz it with bleach and throw a load of stuff away. (and for anyone wondering how I didn't notice for so long - I use the upper level for storage so I couldn't see what was happening until it was too late)

So needless to say this air brick needs filling in because of its ridiculous location. Now the question is what do I use to seal this flipping thing up for good so that this doesn't happen again?

It would have to be sealed from the inside as this is an upstairs room. Would it be feasible to scape off all the old stuff, seal it up with repair cement, then re-plaster and paint? Any help and tips would be greatly appreciated. Brands and recommended products especially. Thanks!

Some years ago, I lived in a house which was massively over-supplied with similarly position air vents. The excess air flow, especially cold, wet air in winter, made what was otherwise a very dry and reasonably warm house feel cold and unpleasant.

One room was particularly bad and, as were switching things round, using it as a home office meant it was occupied much of the day. So the cold air was far more important.

As a strictly temporary measure, I bashed a bit of Celotex into the hole on the inside leaf. Covered with a bit of plasterboard - just pushed in place.. Then slapped on some sort of plaster-based filler (I really cannot remember what it was). Made an acceptably smooth and flat finish - sufficiently good that our eyes were not drawn to it.

Some time later, as that had worked well, I did the second air vent in that room. Followed by three others over the next year or two. All using much the same technique.

Given the relatively poor fit of the double-glazed windows, there was still plenty of airflow but the improvement was huge.

Around ten years after doing the first one, we moved on. All air vents still filled adequately, untouched other than redecorating.

But you have got a ceiling problem. And, I suspect, condensation. Is the attic insulated properly? Is there rain ingress?