Ceiling insulation advice needed

12 Dec 2018
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United Kingdom
I am in the middle of a refurb and I have taken down the plasterboard ceilings in the rooms on the top floor of my house in order to access cabling routes and get rid of an ugly artex problem. I would like to take the opportunity to put some Rockwool in the ceiling before I put up new plasterboard. I have used it in my stud walls and am very impressed with the difference it has made to sound and heat.

I have a flat roof which has a gentle pitch, so whilst the roof joists are deep at the front of the house (24cm), they are only 19cm at the back of the house.

There is no insulation between the joists at the moment. The current insulation sits on top of the joists, but is a 1960’s material made of wood strips and isn’t very effective.

I have read that there should be a ventilated void in the roof, and although there is no sign of any ventilation gaps, I am assuming it is a good idea to leave an air gap. There is no sign of any condensation or damp in there at the moment.

So my question is: If I use 100mm Rockwool leaving a 90-140mm air gap, would that work?
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If you have no eaves ventilation, then why would you need to leave a "ventilation" gap?

Fill it with insulation, tight to the underside of the deck, no gaps, and ensure to fit a vapour check membrane at ceiling level, or foil backed plasterboard.

Btw celotex or similar will be much more effective as a heat insulator. Or if sound is an issue, use top half celotex and then rockwool.
With the depth you have available, Celotex etc is the only way to achieve the current standard for roof insulation- by thickness it is about twice as efficient as rockwool.
But by price per cubic metre, Rockwool is much cheaper- if you could afford to lose 100mm ceiling height (deepen the joists with ripped ply or rough boards) you could stuff the joist spaces with 300mm rockwool
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Thank you both for your advice. I haven’t deepened the void as the ceiling is not that high as it is, but I am going to use celotex instead of Rockwool. Thank you especially for the point re the membrane. I am going to use the foil backed plaster board as that sounds nice and easy.

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