Plasterboard and insulation for ceiling

4 Nov 2013
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United Kingdom
I have a section of ceiling in old airing cupboard to board. I have fitted 50mm celotex between the joists ( will also add 100mm roll on top when done)

I have my plasterboard ready ... But I was thinking... Should I be leaving a gap between the insulation and board? I also have small draughts coming from the gaps between the insulation and wall, I have tried to fill this with the roll insulation.

Should I also batten off the joists to make sure the new board sits below the tops of the walls? View media item 95439
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The foil facing on the board doesn't work to reflect radiant heat if there is no air gap nxt to it, but equally a draft running between your plasterboard and your foil face will do a great job of negating the effect of the insulation anyway

I'd seal the gaps wih expanding foam (dampen any surfaces it must stick to) to eliminate the drafts and put the PB in with a gap
It's not usual to leave a gap between plasterboard and insulation.

I assume you mean unusual Richard. Thanks for the reply. I will just make I stop any draughts and fill the gaps. I was just worried about saturated air condensing on the insulation foil if it were to get cold in the gap.
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I assume you mean unusual Richard.

No, I meant usual. It's not usual to leave a gap between plasterboard and insulation boards. It would make it more fiddly to install the plasterboard, and would serve no purpose. Bear in mind that you can buy insulated plasterboard, which has the two bonded together.

Ok sorry. I have already got my plasterboard so I won't get the insulated backed stuff now.

I don't understand how it will be more's just being screwed to the joists. I was thinking of battening of the joists by 2 inch though as some of the original ceiling is lower.

But I won't have a problem with what I've described? As long as I make sure it's all sealed. Foam in the gap at each side where the insulation meets the wall.
It shouldn't give you a problem if the insulation is doing its job. The plasterboard and the (unnecessary) air gap are both on the warm side of the insulation, and of the vapour barrier created by the foil backing. But make sure the air gap isn't ventilated from an uninsulated space, or you negate the point of the insulation. As has been suggested, you can fill any gaps with squirty foam.

Excellent - just as I thought. I noticed that cool air was getting in yesterday so I will fill with foam.. trim flat and then tape it all. Then plasterboard either off the joists or battens.

Any suggestions on the best expanding foam to use? I try stay away from the cheap stuff normally. I will have to be careful too as there are some cables feeding a light close to the gaps and it will cover my screw heads which joins the cut joists together.
Just thinking about it.

I could lift out the insulation and fit celotex into the gaps. Then drop the other insulation back down.. then join with tape...
Battened off the joists and actually started boarding today!


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