Mould on ceiling edges

[womble12]

Good evening all, hopefully someone can give me a bit of advice on the following.
I'm a Mechanical Engineer by trade but pretty novice when it comes to household stuff, so apologies in advance!

We have lived in a house built in approx 1960 for a few years now and have noticed that we keep getting reoccuring damp/mould issues on the curved edges of our first floor ceilings (shown between the red arrows on the attachment).

Presumably to increase the ceiling height without increasing the roof height, the outside-wall edges of all of our upstairs rooms have 'chamfered' edges (the attachment might make this clearer!). The 'chamfer' is just done with plasterboard.

Especially noticable during the winter months, the bathroom and our master bedroom start going black on those 'chamfered' sections. A scrub with mould remover gets it off, but it'll be back within a few weeks/months. It makes sense that these are the two rooms with the most moisture. I have recently added an extractor fan to the bathroom which I think has helped but not solved.

Recently I have also added a layer of additional insulation to our loft which can be seen in the attachment. I have recently just been thinking though - has this made the problem worse? Now the outside of the plasterboard 'chamfer' section is presumably even colder now as there is no escaping warmth from the main loft area. Should I have filled the green area on the attachment with normal loft insulation first, and in which case, do I need to be careful of blocking anything in the orange arrowed area? Or should I look to add some ventilation in the eaves perhaps?

Any ideas would be welcomed. Happy to accept criticism of what I've already done too!!
Thanks in advance

 

[RandomGrinch]

Hi,

I'm only a DIY'er, so may be wrong!
I have had similar issues with cold bridging, after I had my house rewired.
The electrician's had displaced some loft insulation; the first we knew was when we found a perfectly square shaped patch of mould forming on the ceiling!

Personally, I would insulate the inside face of the chamfer with a decent amount of PIR insulation. You don't want to block any ventilation to the eaves.

Wait around for some better ideas!
Good luck

 

[JohnD]

this will be condensation on the cold surface. It is sometimes possible to poke insulation down the gap between the rafters, between the ceiling and the tiles, but unless the roof has felt or membrane, it is very likely to catch and ruck up

edit: yes, like the green in your pic. Is your house timber-framed?

You might have more luck insulating the room side of the ceiling.

It's all very difficult unless you are renewing either the roof or the ceiling.

To reduce condensation, improve ventilation, especially in the bathroom. You need a really powerful and effective fan.

 

[womble12]

Thanks for the replies so far. It is certainly condensation causing the problem, yes - you can see the droplets beading on the angled surface in the bathroom and just sitting there for days.
In the bedroom I imagine it is the same thing but just at a much finer scale, so much that you can't obviously see the droplets. We do get a lot of moisture on our bedroom windows though (3yr old PVC double glazing).

The house is not timber-framed, but the roof does appear to have a black membrane between the loft space and the underneath of the tiles - might be felt, not quite sure.
It's just quite a thin gap to squeeze some insulation into, and I was worried about removing this problem but creating a bigger one elsewhere.
I guess at the bottom of the 'chamfer' (where I drew the orange arrow on the original attachment) is the top of the wall cavity?
If I ended up covering that opening with insulation I'm guessing that's not ideal either?

The extractor in the bathroom was one of the Manrose spotlight/vent types which I exhausted through a vent tile in the roof.
Seems to clear the room after a steamy shower pretty quickly, but not sure if it's as powerful as is ideal.