Did you have building regulations in place, as this would have been stipulated, and all of the works including insulation would have been checked by bco.
I think you will have to use Celotex or Kingspan, but make sure that no air flows are restricted.
Best to check with bco as these works will need to be signed of, without certificate of compliance it could affect the sale of the house, should you sell it, as the rooms couldnt be classed as habitable.

There is nothing wrong with that method in princaple is just very tricky, hard work and itchy !!!!! best material to use is 100mm " kingspan" or " celotex" foil backed insulation board, easily cut to size to fit between rafters. 100mm based on 150mm rafters leaving 50mm air flow.

Regards

Gary

You need 50mm airspace between felt and insulation. We use 100mm kingspan inbetween and 50mm fixed underneath

Ok what ever you do , do not insulate directly under the tiles . No matter how well you try and do it you will cause a conflict of areas ( the area that should be warm and the area that should be cold ) your insulation should go above your loft ceiling down the walls and along the lower ceiling . the area you are looking into ( although drafty ) is needed to keep your roof ventilated in the eves and up hopefully to the ridge . It keeps it free from condensation and most types of rot ! :)
Now you asked what to use . Retro fitting to the walls you should use a 100 mm 4 " "cavity battern " like fiberglass but its cheap enough and slightly ridged .
The floor level / ceiling to get it up to regs you now need as much as 300 mm 12" thick of fibreglass " earth wool " and B and Q had it on offer for a unbelievable 3 £ per roll . good luck hope this made sense .
thanks woody apex lofts barnsley

I would personally use celotex cut tightly in-between the rafters. You need to leave a gap to the rear of the insulation between the membrane and the insulation to allow air to circulate around the roof space. You could then simply clad the area with plasterboard etc.

Make sure there are no huge gaps where the breeze is felt as there could be other problems that need to be addressed first.

Hope this helps

Christiaan

well its not covered by building regs now ,but by the sounds of it niether is your loft or you wouldnt be having this problem ,so yes insulate with that ,it will stop the cold to a point ,but not as good as celotex or kingspan 120mm thermo board ,and of course rockwool is a lot cheaper

I presume that as you are concerned about legal requirements and ventilation regs etc,that your loft conversion has been done officially and been signed off by building control.If so,the living space should be as insulated as well as it needs to be already.If it has not been done officially,i would get someone to come take a look at the structural side of things before you insulate anything as your conversion may not be safe or legal.