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.

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?

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.

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.

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.

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)

We had condensation and mould issues above a shower because ceiling was frequently dripping wet. Paint was peeling etc

I strongly recommend Zinsser paints. Worked brilliantly for us. Not had a problem since despite ceiling still getting wet.

Ideally fix the cause of the cause of the condensation first

https://www.zinsseruk.com/

Posted By: fostertomThat's interesting. Do you know (not on the website) if they make any claims to environmental/health-virtue etc (apart from "low VOC primer-sealers", mentioned)? Sometimes high performance means exotic/harmful chemicals.

What is on their website are all the SDS so you can answer your question yourself

<blockquote><cite>Posted By: fostertom</cite>That's interesting. Do you know (not on the website) if they make any claims to environmental/health-virtue etc (apart from "low VOC primer-sealers", mentioned)? Sometimes high performance means exotic/harmful chemicals.</blockquote>

This is what we used..

https://www.zinsseruk.com/product/bulls-eye-1-2-3/

Safety data.

https://www.zinsseruk.com/core/wp-content/uploads/2016/12/ZN7040001-0002-Zinsser-Bulls-Eye%C2%AE-1-2-3-UK-gb-2.pdf

Says its water based with a biocide.

I haven't got one for the mould :-)

Actually the more I think about it the more I think the plaster has actually become saturated over time and that's why my plaster filling is flaking off. Which suggests I need to bathe the room in heat to try and dry it out first.

Well I've had a bit of an unmitigated disaster. The undercoat I used is this "universal" undercoat for all sorts of surfaces, wood, plastic, metal, plaster, etc. It's unusually thick. It says to dilute with 10% water but this is not nearly enough. It's extremely difficult to apply and has made a mess of my beautifully prepared smooth walls. It dries after a few rolls of the roller and it goes on thick. I've got dimples and hardened drips. I've got spots where the old paint has peeled off as I rolled on the undercoat. I've got spots where the new undercoat is peeling off and hasn't adhered well to the surface.

I don't know how much of this is to do with bad preparation or bad application or bad undercoat product or bad/damaged old paint/plaster. Most of my problems are in the area where there's been a problem with condensation or damp, but I've also experienced problems on the internal walls.

So it looked a lot worse than it seemed. I kept scraping any loose stuff off and then replastered those areas. The areas where I removed a lot of plaster are looking a lot more robust now too. No more powdery mildew.

I've pre-coated these areas with a anti-fungal coating, then an undercoat layer and then I'm going to use an anti-condensation top coat.

I also watered down the undercoat little more and it was fine. I went over with another coat in the areas that weren't coated properly and they look fine now. I think I was just applying it too thick and at the upper end of temperature range, so I should have just watered it down more.