Bathroom Ceiling PB Questions

23 May 2017
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United Kingdom
Excuse my ignorance...

When we moved into our current house I noted that I could see the joists in the bathroom ceiling, the ceiling was very mouldy, no bathroom fan. I assume it's been like this for a very long time.

Have since killed the mould, fitted a strong fan, painted the ceiling with Zinsser (which worked well for a while), but the paint is starting to crack, electrician told me the plasterboard was shot.

Due to ongoing issues with condensation (which I have been attempting to fix with vents etc.) want to stop moisture from entering the loft from the bathroom area.

Can I safely overboard the bathroom ceiling using drylining with a Duplex foil back board or Knauf moisture panel or similar (which would you suggest)?

Am assuming I would also have to tape the edges of the room as well as the joints if I want to stop moisture?

What type of tape should you use with this plasterboard I'm assuming paper, to prevent moisture from going through?
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A photo would help.

It may simply be a case of getting the board skimmed.

But yes you can put moisture resistant plasterboard over the existing and tape and skim the joins.

Loft insulation on top will stop any moisture going in to the loft.
You could also use a polythene sheet and then normal plasterboard, overboarded, to stop moisture.

If you have downlighters in the ceiling, this can also be source of allowing moisture into the roof space.
Loft insulation on top will stop any moisture going in to the loft.

I wish it did, have redone the insulation and added roof vents. Still see condensation in the roof space above the bathroom during that -6 degree period at Christmas. It appears only low down on the plywood of the new valleys in this area.

Trying to troubleshoot and eliminate all issues to try to resolve this, bathroom ceiling seems like my next/last step.
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If you've put in a fantastic fan and vents and things, are you sure it's definitely condensation and not something else? Or could the water vapour be coming from elsewhere in your home and settling above your bathroom? Ie if that spot is the coldest bit of your loft?
If you have the headroom, consider a false ceiling (say 200mm below existing). Stuff the void with rockwool, then vcl (polythene sheet sealed to the walls as best you can) then pb. Or if headroom is more limited, fix 50mm of Celotex to the existing ceiling, foil tape the joins and board over (yes you can get 100mm plasterboard screws).

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