Condensation and mould in 1930s semi

[AnotherEmma]

I have an annoying condensation and mould problem that's been going on for years and I am determined that this is year I will get it fixed.

DH and I have many talents but DIY is not one of them and I was hoping to call on mumsnet's collective wisdom for advice.

The house is a 1930s semi with external wall insulation that was installed about 5 years ago. The condensation and mould are in the two first floor rooms at the front of the house.

One room is a bathroom with 2 external walls, it gets cold. We installed an extractor fan with a humidity sensor a few years ago. DH scrubbed the ceiling with mould treatment and painted it with anti-mould paint. These two things seem to have helped a bit but the mould still grows. One problem is that we have to dry laundry in the bathroom in the winter (nowhere else to dry it) but we do run a dehumidifier in there pretty much constantly during the day (not at night because the noise disturbs me).

The other room is a bedroom with just 1 external wall with a bay window. (One wall is adjoining our neighbour's property). The bedroom had a fireplace which was blocked in by the previous owner with no ventilation. The worst mould is inside a built in wardrobe where there is obviously even less ventilation than the rest of the room.

It seems obvious that ventilation is the problem but we don't know how to fix it. DH spoke to a builder who suggested that the loft insulation might be a problem, if there isn't enough of a gap to allow air flow.

We have got a few quotes for damp surveys and we're willing to pay for one if necessary but I am wondering if there is anything obvious we are missing that we should try first.

@PigletJohn I hope you don't mind me tagging you but I have seen your posts on other threads and hoped you might be able to advise please?

The external wall insulation is polystyrene apparently.

DH has reminded me that one builder he spoke to recommended lining the roof tiles at the bottom of the roof. I've just dug out the survey we got when buying the house and it says this:
"The flat roof covering was not visible but no leaks were evident.
The main roof structure is a sound condition. The roof slopes are not lined and the underside of the tiles are generally in a fair condition and no leaks were evident. The roof slopes should be lined in order to prevent the possibility of rainwater penetration."
Probably should have done that, then

I'd cheerfully run it full-time for a week, and I'd expect all signs of damp or condensation (except existing stains) to be gone after that.

If you set it to lowest humidity, I think it will run constantly.

If that doesn't cure the damp, there must be some other problem, such as a roof or plumbing leak, or a blockage in the ventilation duct.

If it does cure the problem, you can ease off the settings gradually, as long as the damp and condensation doesn't come back. If it does, increase back to the level that keeps the room dry. You have a powerful fan, so as long as it is running enough, it should cope with any normal bathroom damp. Leave the door and window closed, it will suck air through the gap under the door.

You can use a pair of binoculars to inspect the roof and look for any slipped, cracked or missing tiles.

I've seen external wall insulation fitted in Switzerland and Italy, and it is very effective on concrete walls and on hollow-pot bricks (not used in UK). So far as I can tell it does not cause damp or condensation. Since the walls are now warmer inside, it would reduce it. The usual causes of condensation are excessive humidity from wet washing or ill-ventilated bathrooms. Sometimes a leak.

And breathing, of course.

I had another look at the bedroom pictures.

The ceiling looks like it might just be insufficient insulation in the bay and sloping areas (which are difficult or impossible to insulate without removing either the roof or the ceiling) but you mention condensation in a wardrobe. This is typical of excess humidity. Often it is enough to open the windows every morning, or the trickle vents permanently, but I'm wondering if water vapour is diffusing from the bathroom, or if there is some other source of moisture such as fishtank or leak, or steamy ironing.

A leak under the bath or shower tray, or in the wall behind a washbasin or radiator, can go unnoticed.

Water vapour is lighter than air so it will move into the upstairs rooms from downstairs.

Let's see what happens after blasting the bathroom extractor more. And check gutters and downpipes.

@PigletJohn
Thank for all your help and advice, much appreciated!
I will keep the bathroom extractor on, and check the main roof and roof above bay window, and report back.