Flat roof dormer condensation

[Bmitche92]

Have a flat roof dormer which is developing damp patches during this cold weather and cracking in the ceiling - assume this is where the PB joints are but some water is dropping down slowly. It’s foil backed PB and there’s an air gap between the OSB and Kingspan inside the rafters driven by vented soffit on the front . We have 4 downlights and the builders put rock wool there but when the lights were installed this has been pushed up and not put back.



I’ve pulled the lights down to have a feel and you can feel some air and also moisture / droplets are visible.

To try avoid taking some PB down to do a more intrusive fix, would a solution be:

1. remove rockwool completely so the airflow is restored but this allows warm air below up so will condense more (unless the increased airflow mitigates this)
2. Push rockwool down so lights are covered fully so most of the warm air is stopped from rising and condensing
3. Add fascia vents to increase airflow alongside existing vented soffit?

would 3 be worth doing even if 1 or 2 won’t help or is that overkill


Interestingly the en-suite ceiling isn’t leaking through but assume that’s down to the different paint used as you can feel damp when pull those downlights down and again see the droplets.

Builders coming back next week but any advise / suggestions would be most welcome!

 

[harrythefurrysquid]

My non-expert opinion, if I've visualized this correctly as a cold roof:

If the downlighters are penetrating the plasterboard, then it isn't going to be a useful vapour control layer.
I don't think the rockwool will be an effective VCL.

You'll have cold surfaces up there such as the rafters, so that moisture is going to condense on them.
To keep them dry, you need to have sufficient airflow through the space.

It sounds like the airflow through the space is obstructed by the rockwool, although I can't quite visualize where it is w.r.t the kingspan.
If it's restricting airflow, it needs to come out so the space can dry.

I can't really see how you fix this without taking the ceiling down.


You would at least want:
- Free-flow of air through the cavity
- A continuous vapour control layer

You said you have soffit vents. Can the air flow through the roof space and out the other side - e.g. into the loft space under a pitched roof?
If so, do you have vents and breathable membrane in the loft space?

From your description, I think you should have:
- plasterboard
- VCL without any penetrations (can you use surface-mount lights instead?)
- insulation across the joists (ideally, so you don't have cold-bridging)
- insulation between the joists
- 50mm airgap
- OSB
- waterproofing

So you could fix this by pulling down the ceiling, adding an additional layer of insulation and VCL, and then using surface mount lights so the VCL stays intact. Or keep the downlighters, but create fully-taped niches where you need to cut away the insulation above.