by pmgltd »
Sun Jan 25, 2009 9:53 am
Hi,
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.
Hope this helps