Damp patch over blocked air brick

[bottle top]

Please look at the 2 pictures.

Hopefully you can tell that before I moved into this house an air brick was fitted but was left blocked internally. Maybe not obvious from the picture but I also have a damp.patch were the air brick is fitted.

The damp.patch has only started or become noticeable after a new bathroom was fitted. Part of the new bathroom fitting which maybe relevant was a towel radiator fitted below damp patch and wallpaper above picture rail removed.

I have checked guttering roofing and in attic and all seems very dry and sound.

My theory for the damp.patch is cold air coming in hitting the brick that blocks the air vent and hot air from the new towel radiator causing some mild condensation and spoiling decoration by salt problems.

What is the best way to resolve this. Maybe block vent externally. Or remove internal brick to open up air vent.

The house was built in 1890 and looking at brick work I don't believe it has a cavity.

Thanks for looking.

 

[John D v2.0]

In ours I just wedged a bit of celotex in then a few boys of brick with mortar then plastered over it.
If you think it's from cold you can get an ir thermometer cheaply online to confirm

 

[Doggit]

Your reasoning is spot on Bottle top. The vent has been covered inside, but the air vent is making the inner wall colder, so it's the excess moisture inside causing the issue. Do you have a fan in the bathroom; if not, then get one fitted. You could go out through where the old airbrick was fitted, then, and then fit a cover outside to fill any gaps.

Having said all that, I assume there are no issue from the pipe above the airbrick, as the disolouration seems to go a lot higher than the airbrick