Vapour barrier plasterboard ceiling?

[Roger Moss]

I have a bungalow with a very shallow (6 degree) felted roof and an old sloping ceiling attached to the underneath of the roof timbers. There is no loft access to most of it.

I am building a new, flat ceiling under the old one (gap of 2 to 20" between them). I don't think the old ceiling is insulated so I will put loft insulation over my new ceiling.

Do I need to use vapour-barrier plasterboard to stop condensation forming in the space above the insulation? There is no ventilation here. Or is it better to use ordinary board so any moisture can get out?

I am hoping to just paint the plasterboard. Do I need to tape the joins or just use filler?

I will be putting 9W CFL spotlights in the ceiling. Can one cover a low energy CFL spot with insulation or should this be avoided to stop it getting too hot?

Thanks

Roger

[Andrew Gabriel]

Normally vapour barrier board isn't necessary, but you've probably identified the one case where it really is -- a small poorly ventilated cold space. However, I would look to fix the poor ventilation whilst you have a chance for access there (but still use vapour barrier board). The old ceiling will almost certainly eventually get covered in condensation otherwise. You need external ventilation above the layer of insulation (both above and below the old ceiling, which you could punch lost of holes though).

I would guess filler on bevel-edged board is going to give the best result, if you aren't skimming the whole ceiling.

Generally not a good idea. Although they don't give off the same heat as filament lamps, they'll die before they get as hot, so they still need the same cooling.

However, first you're worrying about a vapour barrier, and now you're talking about punching holes right through it! Are the CFL spotlights sealed to airflow?

[Tim S]

Andrew Gabriel coughed up some electrons that declared:

The BCO recommended vapour barrier PB on my bay window ceilings. Those are also now insulated and ventilated (more or less - technically to half the building regs required area, but that's better than the previous zero).

Cheers

Tim

[NT]

Taping joins with paper tape is a quick and thus cheap method. However the tape is visible to a degree, filling is much better.

NT