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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthorShevek
    • CommentTimeJan 9th 2022 edited
     
    Question:
    Is the best option in our case to use anti-condensation paint, with perhaps an anti-mould undercoat? Does anti-condensation paint actually work?

    I think if we were to use anti-damp paint on this surface it may make the problem worse, in that it would prevent the wall from drying out to the interior during summer.

    Situation:
    We're about to repaint a bedroom, which has one external wall facing west. It's a concrete frame block of flats and badly built, in that the floor slabs connect with the brick facing work.

    So we have this mildew problem in the winter, 200-400 mm down the wall and across the celling at the external wall (mostly across the ceiling rather than the wall). And there was some mould behind a piece of furniture up against the external wall.

    It doesn't look like moisture is making it's way to the interior from the exterior—at least not in any significant way—because there's no staining and the plaster is robust and feels dry, including at the skirting board. The mildew and mould simply wipes off. So, despite the slab linking with the brick, I don't think it's a damp problem. And, because of the slab linking with the brick, I think it's causing the interior face of wall to get cold, leading to condensation. We also have an intermittent extract fan in the adjacent bathroom which we're about to replace with a continuous fan.

    At the paint shop here in Portugal there are a few different applied treatments, but they all seem to fall into one or more of the following categories: anti-damp (anti-humidade) for blocking dampness making it to the surface, anti-condensation (anti-condensação) for insulating and raising temperature of wall face, and anti-mould (anti-fungos) undercoats and top coats with fungicides.
    • CommentAuthorJeff B
    • CommentTimeJan 9th 2022
     
    I have used two approaches in the past: thermal wallpaper (e.g. https://www.gowallpaper.co.uk/erfurt-mav-wallrock-thermal-liner.html) and Thermilate additive added to emulsion paint. Both were successful in eliminating condensation and black mould.
    •  
      CommentAuthorfostertom
    • CommentTimeJan 12th 2022
     
    Related, perhaps - whether in a similar condensation/mould risk situation, it's wise to apply any paint/finish at all. It's an old store room used for wholesale-quantity food dry-goods, half-brick construction and built into the bank, north side so brilliantly cool in summer, and in winter only the uninsulated lath and plaster ceiling is a (severe) mould problem - that's a separate issue. The lower walls, where below ground level, have been tanked internally with Vandex sand/cement render, which has nicely stopped penetrating water - and the raw s/cem finish doesn't seem to suffer at all from either condensation or mould, while the upper lime plaster does, a bit. There's domestic pressure to paint the whole interior, tho personally I like the workmanlike s/cem render. My guess is the s/cem is a bit vapour permable, tho resists liquid water. I fear that painting it may irreversibly disturb whatever mechanism is keeping it mould-free. What think?
    • CommentAuthorJeff B
    • CommentTimeJan 12th 2022
     
    As it is only a storeroom I would have thought that the finish provided by the tanking compound would be adequate, if not exactly aesthetically pleasing.

    If you are confident that there is no penetrating damp then presumably only condensation could cause the mould growth? I guess you are concerned that by using paint you might prevent the breathability of the walls and thus prevent “internal drying”?

    I presume you do not want to get into lining the wall with plasterboard/VCL/studwork/insulation etc as an alternative?
    •  
      CommentAuthorfostertom
    • CommentTime7 days ago
     
    All correct - but someone else in the house isn't content with the 'aesthetic' bit - and also I think hopes that a coat of paint will cure the mould generally.

    To me, not just that paint might affect breathability - I could make sure of breatheable paint - but I wonder whether the rough 'sandpaper' texture of the render may be helping, which would be a pity to clog up.
  1.  
    I've sometimes wished for a kind of universal invisible spray that would prevent condensation on anything.
    In an unheated room a glazed picture gets condensation on the glass. The room's freezing at the moment, but the wall and hence the picture are even colder, so the picture attracts all the condensation.
    If I warm the room slightly the air can hold more moisture, but the wall is just as cold so the condensation is even worse.
    If I leave the window wide open and let the arctic gale howl through there is no condensation at all. :)
    •  
      CommentAuthorfostertom
    • CommentTime6 days ago edited
     
    In spring when outsiude air gets warmer and holds more moisture (for same RH) but internal masonry etc hasn't warmed up yet, ventilation just imports even more moisture to condense! Can happen even on a warm day after a cold night with windows left open - the solution is then to prevent ventilation till temps equalise a bit.
    • CommentAuthorJeff B
    • CommentTime6 days ago edited
     
    Posted By: fostertomAll correct - but someone else in the house isn't content with the 'aesthetic' bit - and also I think hopes that a coat of paint will cure the mould generally.

    To me, not just that paint might affect breathability - I could make sure of breatheable paint - but I wonder whether the rough 'sandpaper' texture of the render may be helping, which would be a pity to clog up.


    I wouldn't think a couple of coats of a microporous breathable paint would alter the "roughness" of the render to any great extent?

    This looks like the kind of paint to do the job:
    https://earthbornpaints.co.uk/product/claypaint/
  2.  
    FT, might it be that the lower tanked area is kept warmer than the above ground and ceiling, due to the earth behind it? Being a little warmer, the moisture from outside air brought in, does not condense so readily? Assuming the mould is due to that process (cold wall, damp air), is there a method to very cheaply insulate those above ground parts? Ceilng - loft roll? Walls - might be more tricky?

    Outbuildings are always difficult with the day/night temp swings.
    •  
      CommentAuthorfostertom
    • CommentTime5 days ago
     
    Esp if they've got doors into the heated interior of the house but aren't themselve heated - then they become a condensing plate for the whole house - as moisture is condensed out in the outbuilding it creates a partial-vapour-pressure 'vacuum' which draws yet more water vapour in from the house (even if there's no bulk air movement).
    • CommentAuthorShevek
    • CommentTime4 days ago edited
     
    Posted By: ShevekQuestion:
    It doesn't look like moisture is making it's way to the interior from the exterior—at least not in any significant way

    I might have to take this back. We've sanded it back and cleaned (with bleach) and then, in the very top corners of each side of the external facing wall, efflorescence has formed. I'm not sure if it's because we cleaned and it's taking time to dry out or if we have do indeed have a damp problem.

    If we do have a damp problem in combination with a condensation problem in the bedroom, can you paint with both anti-damp paint + anti-condensation paint?

    We're going to to leave it for a week or two to properly dry out, to determine whether it's just drying out from having washed it or if damp is coming through the wall from the outside. My guess is the later, based on powdery mildew that was there before we started.

    (as an aside we've since installed continuous background ventilation to the bathroom, which already making a noticeable difference to the adjacent bathroom)
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