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Brown stains on external white plaster wall

  • 10-02-2023 6:15pm
    Registered Users Posts: 6,829 ✭✭✭

    Hi folks

    As title suggests i am currently dealing with 1 small patch of a light brown stain on a white sand / cement plastered wall. The stain is located around a vent for a bathroom extractor fan and directly under it. Im putting it down to the moist warm air hitting the surface of the cold external wall. I cant find a lot online to confirm this though. Any ideas? Will get photos up. The rest of the wall is fine.

    Extension was built about 18 months ago.


  • Registered Users Posts: 33,601 ✭✭✭✭listermint

    Any air flow can carry with it growth. You will have some moisture gathering on the surface of the grill and the tube and it will drain down the wall. This carries bacteria which causes the discolouration.

    You could try gluing a small plastic piece or metal to the very bottom so the water trickles out beyond the wall down to that point and drips off the end.

    Otherwise it's just regular painting.

  • Registered Users Posts: 6,829 ✭✭✭Gusser09

    Thanks. Yeh i was going to extend the vent about 300mm away from the wall so the warm moist air isnt hitting the wall directly. I see other houses with the vent extending well beyond the wall.

    Ive tried bleaching the stains to remove but doesnt really help.


  • Registered Users Posts: 33,601 ✭✭✭✭listermint

    Try the stick trick first. Extending vent is alot of messing around.

    Ya bleaching won't take that out tbh. That's only for green growth. Once the brown staining is in its generally in. Could prob sand blast it possibly. Maybe steam

  • Registered Users Posts: 6,829 ✭✭✭Gusser09


  • Registered Users Posts: 6,829 ✭✭✭Gusser09

    Hi Folks,

    Just resurrecting this. Any ideas what this patch might be? I notice it has got darker and grown over the last couple of months. Definitely not leaks so I am at loss as to what is going on. Cheers.

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  • Registered Users Posts: 6,630 ✭✭✭10-10-20

    Mould by the look of how it's following airflow lines. Scrub an area in a line and see is it sitting on the external surface of the render? Is it coming out of a kitchen or what room is behind it?

  • Registered Users Posts: 6,829 ✭✭✭Gusser09

    It's a bathroom behind it so that would make sense. I've noticed it started around the vent initially. I've tried cleaning it previously but it doesn't come off at all. The bathroom is fully tiled. Wall insulted with 100mm insulated plasterboard. Attic space 300mm.

    I've always suspected the vent is the issue when it is sucking out the warm moist air. I tried venting it away from the wall a little but I don't think it has helped much. In my opinion it definitely gets worse when the extractor fan is being used.

    I've noticed 2 settling cracks in the render. Is it possible the warm moist air is entering through them? I'll upload a photo of them later.


  • Registered Users Posts: 6,630 ✭✭✭10-10-20

    I'd visually inspect the vent piping - it needs to be sealed to the render outside and plaster/tiling all round and the pipe needs to be complete from interior to exterior. Also are the vents opening and is there an extractor fan on the opposite side?

  • Registered Users Posts: 6,829 ✭✭✭Gusser09

    yea it's an extractor fed vent if you know what I mean. The exterior vent cover opens when the vent is on for sure and I can feel moisture hitting my hand when I examine it. I do think there could be a seal issue for sure and I will check this out. I think there is warm moist air getting in somewhere. Would the hair line cracks be causing an issue? They traverse the vents in a diagonal manner. about 1.5mm wide.