Insulating and boarding bathroom ceiling

[AureliaB]

Morning all,

Had my flat roof replaced, had a multitude of issues which I think are now solved. So now onto how to insulate and board this.

I was thinking of drilling holes through the wood length at the bottom, cross ways to create airflow then insulate below it with foiled backed celotex, tape all edges then water resistant plasterboard below?

Is that how people would do it? We are getting what I think is condensation on the chipboard guessing due to the temp drop.

 

[paulc]

That looks like a complete new roof rather than a basic refelt job. Building regs would require insulation to be incorporated (usually as a warm roof design), so you shouldn't need to be doing anything. But it looks like you have a cold roof without any insulation, so no surprise that you are getting condensation - It is also going to get unpleasantly cold in there when the big freeze starts.
Butting vents in the facia boards will help to provide some ventilation when you start to add insulation. Ideally, you also want some vents where the flat roof butts up to the main building to maximise air flow. But these additional vents need to be incorporated whilst the felt is being put down. Faced the same issue with the flat roof over my kitchen extension. I have some fibergalss insulation packed between the joists with an air gap to the boards. Then an additional layer of Celotex type material fixed to the underside of the joists. Joints are filled with expanding foam and then covered in aluminium foil tape. The 100mm of fibreglass and 50mm of Celotex doesn't get me anywhere near current u-values to meet building regs... Already limited on ceiling height, and going to ~150mm of Celotex would be too much (have plans to go to a warm roof next time the felt needs replacing).

 

[AureliaB]

Yes its bloody cold in the bathroom. Yeah I was thinking of drilling holes cross ways and like you suggested added vents. Luckily the flat roof overlaps the main loft space, so have a gap for air flow coming down from our very old roof. So hopefully that will sort that out. What did you use for the ceiling normal plasterboard?

 

[paulc]

What did you use for the ceiling normal plasterboard?

Regular plasterboard for the ceiling. Gave it a coat of Febond Blue Grit, and skimmed with a fine lime plaster. Finished it off with a (very) pale blue polished venetian plaster.

 

[Cubist]

Whatever you might finally decide upon regarding insulation and ceiling materials you might want to consider treating that OSB with something to kill off what appears to be black mould forming on some of its undersides and then sealing it to prevent future moisture penetration and mould growth. Much of the black staining I can see appears to be roughly aligned with the bath/shower tray where moisture laden air from the bath/shower would most likely condense out. Or were those stains present when the OSB was first installed?

Whatever you finally decide you should also consider installing a timed extractor to clear that bathroom of airborne moisture as rapidly as possible during and after its use.