If your roof is on its last legs then the chances are that your neighbour's roof is in an equal condition. Have you thought about/talked to your neighbour about a joint solution? Then you could do the job properly without level differences and joining problems.

Posted By: tonyThis is only a small part of your insulation problems, likely the flat roof has no insulation and possibly none in the sloping parts of the roof either.

Yes insulate them, decide on a U-value then join up insulation in roofs walls and ceilings.

totally agree, no point in doing the flat roof alone, you will just push the problem to the next element. however it might be simpler to approach from the inside with a celotex and multi-foil combination?

The insulation barrier MUST always be all joined up, no gaps cracks or dislocations

Posted By: OnTheEdgeOfOk thanks for your reply.

Just to recap then if we went down the warm deck route we would need to block the access from the main roof to the flat dormer roofs??

Yep, and all the way around the perimeter of the warm deck preferably - so that no air can get in underneath the insulation - imagine having the window open in your bedroom and a duvet suspended 20 cm above your body - not much use unless you can stop the cold air from outside getting in underneath it. Sadly, loads of roofers seem to be too thick to realise this. Easiest method is to use PIR board (Celotex or similar), vertically between the joists and polyurethane expanding foam.

Preferably go for a low expansion foam (e.g. one which is used for window fixing or similar), since it insulates better, and lets less air through.

If we went down the cold deck route would we still be ok to insulate them even if they are unventilated?? My idea is to get the felt replaced & insulate between joist & below joists.

In most cases you shouldn't do this, because you'll risk condensation on the underneath of the deck, which will then spread downwards, and all the timbers will rot. OK if the roof is ventilated tho' - you don't need much ventilation below the deck to keep it dry enough so long as the ceiling is well sealed (and preferably vapour proof too).

The one exception to that is that in some circumstances an unventilated cold deck roof can shed enough water vapour around its edges to keep the central area sufficiently dry - (you shouldn't rely on that unless you've done an analysis and can prove that your roof will be OK tho' because it depends on quite a lot of different factors).

dream on!

Thanks for the advice & links.

I like the warm deck route as hopefully no come backs but will look odd unless the neighbours want there's done at the same time??

One thing I've found is when I've had trades people round for quotes none of them so far fill me with confidence that they know much about insulation. Had one roofer who said he would replace flat roof deck/felt & squash 200mm of Rockwool into the roof void another tried to flog me a warm deck but no mention of sealing the perimeter with PIR as was described above.