Chimney in our bedroom upstairs has had a some damp patches appear over the last few months – nothing drastic but its something i really want to fix! Patches are around higher part of the wall around where the old fireplace has been blocked off.

I have had a new cover put on the top of the chimney stack, which allows air so it shouldnt be leaking there. Also the flashing looks good and i can’t see any sign of damp in the loft where the stack passes through the tiles. So i concluded it must be condensation.

As a result i cut a vent hole in the plasterboard which was covering the old fireplace, but it looks like whoever covered over the fireplace also loosely bricked up inside the flue. There is some airflow, but i am tempted to open the airway up a bit more. Does this seem the most sensible approach? Or shall i just see how it goes with the addition of the air vent over the next few weeks.

Any bright ideas / advice out there?

Any airflow is better than none. Blocking a disused chimney is up there with blocking underfloor vents as a no-no.

if there’s a bit of ‘draw’ through the vent I’d wait and see what happens before taking any further steps.

If you have access to a dehumidifier it’ll speed things up a bit.

Cheers guys. There’s defo some flow thru it so I will see. As to the actual damp bits, I have taken back the wallpaper a few weeks ago and the residual glue is slightly has remained sticky in these areas. I did run a dehumid for a few weeks a couple of hrs a day, room seems to have RH approx 50 now. Didn’t actually remove the damp completely though.

Re the vent, will a single brick size be sufficient, or worth going up to a double?

Forgot to say it’s an old house so no cavity walls.

Soot and such from a coal fire is hygroscopic, ie it absorbs water from the air. So maybe it’s damp through the thickness of the brickwork and will take time to dry out.