Small damp patch in re-plastered room, what to do?

[MisterBoy]

This is a small pantry about 2x2m, it is old solid brick prone to condensation and damp and mould which was not good for food storage.
We had the walls stripped back, treated with damp-proof render inside and then an insulated stud wall built and skimmed. All great and condensation issues resolved.

BUT then it turned out the roof was leaky, in a major storm we got water actually running down the inside of the brickwork in this corner. Had the whole roof replaced but this one corner remains wet 3 months on (everything else is fine).

The photo makes it look worse, from using a moisture meter its the circled bit which is actually damp the rest is just not properly painted yet. It's not much bigger than the palm of your hand but constantly damp and mouldy - this corner is where the water was running down.

I wanted to use stain-block and then paint on top but I'm pretty sure the area needs to be dry first and wet plaster likes to stay wet. Is there anything I can do or am I going to have to cut this section out and redo it? It's a hidden corner in a pantry so it doesn't have to be perfect it'll just bother me having black mould constantly.

Thanks.

 

[Tigercubrider]

I'd take advantage of the predicted heat wave and ensure that the damp issue is sorted first.
I assume that the grey bit is the floor.
From what I understand as a DIY'er damp can take ages to dry after the cause has stopped.

I don't think that the damage is excessive and would be easy to resolve once dried out.

 

[MisterBoy]

For a small area is actively drying out out feasible e.g a small heater/hot air?

 

[JohnD]

You don't need hot air

An ordinary fan blowing on the damp patch will speed evaporation. Like a washing line on a windy day.

Just ventilate the room to blow away the damp air

I can't see your photo. Is it brick-shaped? Or round? Or mountain-shaped?

Show us the outside of this area please

 

[MisterBoy]

Any better @JohnD , honestly I tested in a private browsing session the image worked too!

It is a low down corner so it doesn't get much ventilation. We already have constant trickle fan and dehumidifier installed so if I can GET it dry the room is now pretty fresh.

I might make a small hole to double check there's nothing horrible behind it that fell down when the roof was being fixed, that I can fill easily.

 

[JohnD]

OK, I can now see a mountain-shaped wet patch in the corner.

you need to pull off the skirtings to expose the wet wall and let it dry. They are liable to rot.

draw round the wet patch with a pencil, and date the line, then photograph it. In a week or a month, draw round it again and see if it is getting smaller.

Why do you need to drill the wall to look behind it? Why can't you just walk round to the other side?

Or do you mean it is a cavity wall that might be full of wet sand and rubble?

The floor appears to be concrete, so may also be damp. You may see better after removing the skirtings.

Blow the corner with a fan

 

[JohnD]

oh, I see it's an "an insulated stud wall"

so it is probably damp behind.

I do think you need to expose the cavity and ventilate it.

Do you remember the constructon?

Fibreglass holds water. Foam doesn't. Foil-backed plasterboard holds damp behind it, but looks dry on the front, except at joints.

You might be able to open a gap when you take off the skirting. If you have a hole saw, you can cut a decent disk out to have a look, and stick it back afterwards.

 

[MisterBoy]

Yea I wondered if the cavity might be wet inside in which case better to find out and sort it. There IS some damp in the floor but not much, and the damp is only in that corner which is where the rain was literally running down the inside of the bricks and making a puddle.

It was fun to find this out AFTER doing the internal work!

It's a 3" stud with rigid insulation -Kingspan or similar boards.

 

[opps]

An air brick might help. I appreciate that it sounds counter intuitive but you have a problem that will not resolve itself. You need air flow. That
said, a fan will help. Corners don't tend to get proper air flow as the circulating air bounces around the corner on a radius.

BTW, I do use moisture meters but they are terribly unreliable when working with plaster. They often give false positives.