Should I leave open top of cavity when laying insulation?

22 May 2005
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United Kingdom
We have a single story "protrusion" from our 1976 house...(can't really call it an extension) for the front door lobby and downstairs toilet/cloakroom. I've pulled down the plasterboard ceiling to do some wiring and anyway it had nasty stippled artex so the replacement platerboard ceiling will be skimmed.

I found there's no insulation at all in between the ceiling plasterbaord and the T&G boards that support the flat roof felt. No wonder the room is so cold!

I've fitted 50mm foil backed polystyrene (cellotex?) taken (with permission!) from a skip between the joists down the middle of the cloakroom but was planning to squeeze some loft insulation into the gap between the outside wall and the joist that is about 10mm in from the room wall so its difficult to fit celotex. The wall is a cavity wall.

Couple of questions....
1. Is it ok to extend the loft insulation over the cavity, or should I leave the cavity free to ventilate?
2. Should I leave a small gap between the celotex and T&G or press it up hard to the T&G.?

As always, would be most grateful for any advice.
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The tops of the cavity should be stuffed with glass fibre, to allow a little ventilation but not a free flowing gale.
With a wooden deck , it should be ventilated, so there should be some vents built in. So the foam insulation should be down as low as possible (against the plaster board) with free air flow above. Yes I know its not possible!! You MUST use foil backed plasterboard or put in a plastic membrane to stop the warm MOIST air in the house percolating through the plasterboard into the roof space.
Thanks just saved me from a big mistake...that'll teach me to skip raid :oops:

I'd got as far as one panel pushed right up to the T&G....and clearly thats a NoNo.

Actually it will be a bit of a pain dropping it down to ceiling plasterboard level and it will be real messy with the downlighters and cabling so i'll dispense with the celotex all together and just use foil backed plasterboard.

I'll be pushing some fibre loft insulation into the gap on the left of the above photo (outside cavity wall as described above) to cut down on some of the heat loss.

Do you think it would be worth putting some fibre loft insulation loosely in the gap between the T&G and the foil backed ceiling plasterboard (If I can defeat gravity for a few minutes to do it!!!)
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If I can defeat gravity for a few minutes to do it!!!
When I redid the boarding under the recess in front of the garage door, I used string and a staple gun to hold the glass fibre up while doing the boarding. Staple end of string to end joist and cut so as to reach the other end joist with a bit extra - and repeat along the length of the joists. Lift fibre into place in end gap, pull string across under it and staple to next joist - repeat for each string. Repeat with fibre in next gap, and so on.

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