Insulation and plasterboard... how bigger gap?

[Kargx]

hi,

I've been abandoned by my builder and I'm now completing the project on my own.
I have a carport with a bedroom and bathroom above it.
In the ceiling of the carport I have pipes for the bathroom.
Yesterday I installed my celotex fire rated insulation in between the joists of the carport ceiling.
Today I want to put up the plasterboard.

Ideally I'd place the plasterboard under the pipes so the new carport ceiling is completely flat, but this would mean I'd have a fairly large air gap between the insulation and the plasterboard, is this ok? Or do I need to put the plasterboard directly onto the insulation and then board in the pipes separately?

Thank you so much for your help.

 

[rssteve]

Does building control not want to see the insulation before you board? Not sure if you need a vapour barrier in there too.

 

[rssteve]

The pipe might need insulating too. What are they serving?

 

[noseall]

Most scenarios (similar to yours) require additional insulation across the underside of the joists to provide a continuous layer of insulation.

 

[cjard]

+1 for noseall's comment, and also note that you must make really good efforts to fill any gaps between your celotex board and the joist edges. If cold air can get round to the "warm" side of the insulation, the celotex might as well not be there. As the joists are much better at conducting heat than the celotex, you should look to insulate them as per noseall but if there is then a gap between the top of the board that runs over the joists and the bottom of the celotex between the joists, that too should be filled with insulation to improve effectiveness of the underboarding.

Mineral wool held in place with cuts of roofing felt stapled to the joist undersides would be ideal, as it will deform to fill the gap well