Blistering paint & mould on external wall

[mrrusty]

Unfortunately bathroom doesn’t have extractor, but window is left open most of the day

I think @23vc had it right at the beginning of the thread. Any wall surface that gets below about 12-13 degrees at any time (like the middle of the night when the heating is off and it's freezing outside) will attract condensation from room air, and condensation causes mould. If you have a bathroom without an extract, you'll have loads of water vapour in the house air and an open window is as likely to blow in and push that water vapour around the house, as take it outside.

Fit an extractor. If you still have a problem, run a dehumidifier when it's cold outside and I reckon the problem will go away.

Nice warm draught free houses where the air can carry a lot of moisture, and cold solid walls is not a good combination. I expect there is nothing wrong with the fabric of the wall, flashing, render at all, but a coat of paint wouldn't go amiss

 

[Ivor Windybottom]

Cooker hob too.

I'd add that some people think they have a hob extractor but it's actually sucking in from below and blowing out back into the room directly above, from the outlet that should have had ducting fitted to it. Utterly pointless, but many thousands are fitted like this. I suppose it prevents the adjacent wall cabinets from rotting, otherwise you might as well not bother. It's a job that kitchen fitters would rather not do, and many customers don't even realise it should have been done. There are usually filter cartridges inside, which most people never change. Get it ducted out, remove the cartridge if one is fitted and you'll remove litres per week of water from your home.

Get both main sources of home moisture extracted then look elsewhere, probably at laundry. Drying clothes on racks indoors shouldn't really be a thing, especially not if you have solid walls. We use a heat pump tumble dryer for almost everything - they cost almost nothing to run, and don't cook your clothes to death like the olden days tumble dryers did. They dry with only warm dehumidified air, unlike traditional dryers which essentially boil the water out of the fabric. Many have had experience of traditional dryers, shrunken clothes and huge bills so sadly overlook the heat pump versions which, despite looking identical, are totally different machines.

 

[ree]

OP,
You still definitely have penetrating damp - silly wishful thinking that there's nothing wrong & it will go away is still silly wishful thinking.

 

[b21playa]

I understand no bathroom extractor can cause issues, but it’s been like this for ages and the issue has only started. I have a dehumidifier that runs in the hallway to remove all moisture.

I will check the wall on the outside over weekend and fix any holes and look to paint it