Insulating ceiling - flat roof

9 Jan 2007
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United Kingdom
Hi i was wondering could some one help me with my query?

I have a ground floor single story extension with a flat felt roof and want to insulate the inside ceiling.

I was thinking of installing insluated slabs and plastering , but friends have told me that it is easier to put rolls of insultion inbetween the joists and cover with standard plasterboard slab.

Is there must of a difference in insulation values in the 2 methods.

This is a diy job with a budget but would stretch budget if really need to

Also, if i was to use insulated slabs would it be recommended to remove the existing plastered ceiling slabs- or could i secure the insul slabs to existing plastered ceiling

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The foil-faced insulation boards are about twice as efficient as fibreglass, but much more expensive (though seconds are available - in uk at least).
You could fix the boards through the existing plastrrboard, and then re-plasterboard, but you would need longish fixings to go into the joists. A better job would be to remove the existing plasterboard and fix the new insulation plus plasterboard direct to the u/s of the joists.
If you decide to use fibreglass between the joists, don't fill the space up to the underside of the deck - leave a good air gap to reduce the possibility of condensation. With fibreglass, you must use foil-backed plasterboard to reduce vapour getting into the void.
If you can run to it, the best solution would be to remove the exiting plasterboard, fit 1" or 2" thick insulation board between the joists, and then 1" below, then fit plasterboard on to that.
PIR boards are 60% more efficient than fibre-wool and about 700% more expensive ( in UK ).

, you must use foil-backed plasterboard to reduce vapour getting into the void.

It is completely in order to use a poly sheet instead - cheaper but more work and you want to eliminate vapour getting in, not just reducing it.
Thanks for the replies

What about using rockwool (or sheets of foil backed kingspan foam) between joists instead of standard fiberglass rolls
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