We are having a flat roof replaced on an old extension of our house. The flat roof construction is unknwon until we remove the fibreglass. The flat roof of the garage (built I suspect at the same time) was solid concrete, we expect that the utility room is built the same, but if we're lucky its an insulated timber construction.
If the utility room is solid concrete, we can either remove the concrete (approx. 150mm thick) and replace the ceiling with a timber ceiling insulated with rockwool. A new tiled, hipped roof will be installed. The utility room will be made a right mess and there's a risk to damaging a brand new boiler and 8 year old units.
Alternatively we can insulate internally under the concrete roof (we have limited space available for this) or we could insulate on top of the concrete slab (or even both).
Is insulating on top of the concrete slab, or sandwhiching it in insulation a bodge, or is it fairly straightforward to do?
I feel that removing the concrete is the 'proper' way to do it but it will increase cost and make a real mess.
I'm aware that without pictures, site insepctions etc then this is a bit of a crystal ball question, but any thoughts / advice much appreciated. We're meeting out builder to discuss tomorrow.
You normally insulate above a concrete deck.
Torched-on layers were used on our school 10 years ago, dunno what current thinking is
Should the vapour barrier be placed in front of the concrete slab, or between the slab and the insulation? so =
1) Plasterboard, vapour barrier, concrete slab, insulation
OR
2) Plasterboard, concrete slab, vapour barrier, insulation
Warm roof is the modern way. Vapour barrier is usually immediately above the concrete.
Waterproof layer usually above the insulation.
Cold roof is frowned upon and I think illegal in Scotland
http://mannokbuild.com/insulation-boards/flat-roof/insulating-flat-roof-with-concrete-deck/
So I'll end up with a warm roof as it will be (existing structure) plasterboard, concrete roof, then vapour barrier, insulation, new tiled roof?
Is there any necessity insisting on PIR insulation rather than rockwool if it meets the necessary U values and enough space?
Apologies. I missed the one-line specifying the tiled roof! DOH. 🤡
In that case I would ring the helpline at rock wool and check with them,
Email: technical.solutions@rockwool.com
Telephone: 01656 868 490
You need to push the cold above the concrete deck or you'll get interstitial condensation above the plasterboard, which isn't great.
I'm thinking don't touch inside. Concrete deck, vapour barrier, any sort of insulation of suitable thickness, then ventilated roof space, but IANAE and this needs to be right
