Extension bathroom mould/damp - how did you fix yours?

[IhateSPSS]

Our bathroom is so damp that I can scrape the paint off and the plaster is wet underneath.

Victorian terrace, Yorkshire stone, very wet area. Tiles floor. Bath with shower over. Roof has two wooden beams and sloped, vaulted ceiling. We put an extractor fan in last year, have a dehumidifier in there. Used mould remover spray and clean it a lot. The mould spores are back. I honestly could cry. It's so cold in there at night feels like going to the outside loo. Anyone cracked this eternal nightmare? Thanks!

Not really! Keeping it warm would help. We are in a 17c limestone cottage. The walls are lined. I run the extractor fan in the bathroom permanently (we installed a super quiet one) I have a moisture monitor which shows it goes up to 80% after someone has had a shower and takes at least an hour to go back down to 60%. It is painted in anti mould paint. We still get a small amount of mould settling round the edges of the ceiling which is low. This is treated with mould spray regularly. I open the window most days too.

When was it built?
What's the construction type?

It needs to be warmer and drier, obviously, but how you do that depends on why it is wet and cold.

I'd get a surveyor to assess and advise.
The surveyor can also check for leaks/water ingress.

Our bathroom is so damp that I can scrape the paint off and the plaster is wet underneath. This just sounds wrong - is this an exterior wall? Have you ever had it surveyed?

The house was built in 1894, the extension who knows. The owners who sold it to us had to get a building indemnity certificate as nobody has any idea when it was built. It's a typical Victorian terrace with stone and I'm guessing zero insulation.

Nope, never got a surveyor apart from. The basics one when you buy. Is there a specific survey I should be asking for?

Pics below of the paint coming off!

The pic with the mouldy wood is next to the shower/bath.

All the walls, not just external but also the internal one where the extension joins the original house are peeling.

Could there be water seeping in where the extension joins to the house? Have you had a look at the roof? You could get a builder out to have a look.

Your problem is the result of using modern materials in a traditional solid stone building. Damp walls are terrible insulators and the use of gypsum plaster and vinyl emulsion paint is stopping the walls from breathing as they should (and were designed to do). Anything you do, apart from stripping away the modern stuff and replacing with lime plaster and limewash or soft distemper, is unlikely to do anything but provided limited relief.

Is the bathroom in the extension or in the original house?

Bathroom is in the extension. As is the kitchen (underneath) which is absolutely fine.

Do you have any radiators in there? Our bathroom is also damp (1920s house) and (touch wood) we've managed to reduce the damp significantly with a large dehumidifier (it's a 10 litre one and I empty it every 12 hours).
But we did this in the early spring, so we'll have to see how the winter goes.

But like PPs said, you also need some heat in there too.

Our extension bathroom didn't have all the issues/design features yours has but we have done the following:-

Wiped down all the surfaces with white vinegar then wipe down with bleach scrubbing off mould spores;
Sand down all the surfaces where possible;
Paint with zissner mould paint (pic below);
Dehumidifier constantly and windows closed pic below;
Internal window dehumidifiers on all windowsills;
Open the windows first thing after showers;
Heat the bathroom overnight.

We are making the decision to stay or sell once my youngest has finished her GCSE's (June 2026) so we don't really want to start stripping everything back until we know what the long term plan is. I'm pretty certain that the extension went on quite a while after the original house was built but it's still stone walls.

I'll try the dehumidifier/heating/paint combination for the next two winters. And save up for the lime plaster and lime wash in the meantime. How depressing.