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The insulation in my dormer roof is a bit iffy, however I only have 100mm joists where the flat roof is then above.
What would be the easiest way to insulate the roof? Take down ceeling and 50mm celotex between joists (leaving 50mm air gap) and some celotex then underneath? However I don't really want to loose 100mm + of headroom underneath my joists as the height to the curent ceeling is only 2.25m. So to loose 100mm plus plasterboard would bring the ceeling height down to 2.15m ish.
Not too fussed about current regs but any improvement would be nice?
I get the impression that the solution now is to add insulation directly on top of the existing roof as then the existing roof acts as the vapour barrier. However it does of course depend on what is around the edges of the existing roof e.g. drainpipes
Just had a loft conversion done - they did 100 Celotex between 150 timbers (50mm Air gap) and 30mm insulation backed plasterboard.
It's boiling up there so you'll probably be fine with 50 mm celotex*
* IANAB.
We had insulation on top =- made a no0ticable difference as out whole flat was under uninsulated roof space
I would fit 50mm celotex between the joists with battens to maintain the ventilation, then 25mm over the whole ceiling and add plasterboard.
This would take it to a 75mm fill with only a bit of height lost.
Doing the calcs the other day 75mm in celotex is about the same thermally as 150mm of loose fill (dependant on exact variants).
It will make a big difference.
Would I need ventilation underneath with the battens? Or just put 50mm celotex in the lower half if the roof joist, leaving 50mm gap in the top 50mm. Then screw 30mm celotex plasterboard underneath joists?
Is the existing roof watertight? If so. Overlay.50-100mm insulation bonded to the existing felt as vapour check, Timber around perimeter to act as a hard edge, one hard edge higher than the other to force rain to one side then two layers of felt over top, one as a base sheet, one as a cap sheet over the edges. New fascia fixed through timber with guttering. Job done.
If you do it internally use foilbacked plasterboard and/or vapour barrier to prevent interstitial condensation.
I've done 70+50mm rigid board insulation in my loft and it's grand. 50+50 would be acceptable I'm sure.
veedubba on here actually works for one of the insulation manufacturers so see if he pops up.
Indeed I do!
Not a roofer, but we do see a lot of projects where the boards are laid directly over the existing roof and a waterproofing system goes straight on top.
So Dabaldie has it as I would imagine that it'd be quicker to do it that way than faffing with removing the ceiling. You can get tapered boards to give the fall that he's talking about as well although not sure if you can as a punter, perhaps contact your builder's merchant.
Assume you'll not be walking on it other than to lay it? Also, what's the roof area?
The section to be insulated is the flat ceeling of a dormer rooms. Id prefer to do it from the inside otherwise I can see a scaffold being required.
There are two bedrooms and a landing to insulate, guessing about 30 to 35m2.
@veedubba, does the existing roofing felt or membrane needs to be removed first? I'm guessing so.
Airgap -> "celotex" -> insulated plasterboard would definitely help*. Agree that external + internal would give the best insulation**.
Just check that your rafters are strong enough to take the extra weight.
*Happening in my loft room as we speak.
**Happening to my new flat roof extension (as we speak).
Murray, no you only need to remove the old felt if the insulation underneath is wet as this will compromise the insulation properties and you'll get water trapped below the vapour check and inside the roof = mould/condensation.
FYI I'm a surveyor and also currently re-covering about 10000 square metres of flat roof projects doing exact this..
^^^what he said^^^
(IANAR, and IANAS)
What's all this obsession with Celotex anyway?!