I have searched the other topics and can't find an answer, so some help would be much appreciated.
I live in a victorian terrace and the loft has been converted into two bedrooms. This was done long before modern day building regulations as the sloping ceiling/roof is quite low and you have to duck a bit when you go up to the loft.
There is no insulation in the sloping ceilings, and is basically tile, felt, joist and lath and plaster, and therefore want to insulate them as heating bills are enormous!!! The roof joists are only 70mm deep and want to insulate as best I can and dont mind taking down the old plaster and replace with plasterboard.
Seeing as I only have a gap of 70mm, and cant lower the ceiling due to the restricted height already, can someone recommend the best way to insulate it please. I take it there would be condensation issue if the entire gap between the joists was insulated.
You are in for a really messy job removing the lathe and plaster. Once this is done fit urethene foam 50mm thick between the rafters leaving maximum air gap above the foam, then fit new plasterboard. If you get condensation problems above the foam you will have to cross ventilate the air space.
The removal of the existing lathe and plaster ceiling, and the provision of thermal insulation would require Building Regulation approval, as it effects Part L of the regulations (i.e. works to a thermal element). You will therefore need to submit a Building Regulations application and agree the method of insulation with the Building Control Department.
As you say you have a felt waterproofing membrane over the rafters you must maintain a 50mm ventilation void below the felt - therefore the maximum thickness of any insulation provided between the rafters (based upon the rafters being 70mm in depth) would be 20mm. As such one solution would be to introduce 20mm of Kingspan/Celotex insulation between the rafters, provide a vapour check membrane below the rafters, then plasterboard. YOu would also need to provide a reasonable degree of ventilation to the void between the insulation and the felt, which could be achieved by introducing high and low level roof vents.
Another method would be to remove the existing tiles, battens and felt, so that a breather roof membrane (such as Tyvek) could be provided over the rafters. This would then allow 60mm of Celotex/Kingpsan insulation to be insulation (with vapour check membrane) - maintaining a minimum 10mm ventilation gap. No supplementary roof vents would be required as the membrane allows the entire roof to breather.
Alternatively you could consider using a sprayed insulation, such as Renotherm, which can be sprayed between the rafters. Speak with Renotherm about this option, as it is my understanding that roof ventilation is not required.
Building Control would be keen to achieve the best possible insulation provision, but if you explain the issues relating to the headroom within the loft room, and the limited depth of the rafters they should adopt a reasonable approach.
Thanks for your comments guys. If we forget about building regs, if I was to have a seamless vapour barrier next to the felt, then insulate leaving no airspace, then vapour barrier then plaster board. Would this stop any condensation problems?
Building regs are there to protect your house. If your in stallation does not comply to them you can a) cause damage and b) make selling your property difficult. Air can enter through the tiles also and condense on the rafters in any airspaces available (Intersticial condensation) this can rot the rafetrs easily over time. Please check your plans with BUilding control as it is infinitely cheaper to do the job in the correct way the first time.