I have a pitched roof which forms part of the ceiling in the bedroom of a 1930's house. At the moment it consists of clay tiles on top, wooden rafters etc. and then plasterboard nailed to the underside of the rafters. No sarking board/felt or anything else.
I would like to add some insulation to this in above the part over the bedroom, but the rafters are only 100mm deep and I understand that a 50mm ventilation gap needs to be left between the insulation and the tiles. Therefore I can only add 50mm of insulation (kinspan/celotex/etc.) without losing headroom.
My plan is to add 50mm insulation board between the rafters and then replace the plasterboard underneath the rafters with the plasterboard with insulation laminated on the back that you can get in DIY stores (27mm together) to reduce thermal bridging (losing a few mm headroom).
My question is, this probably doesn't meet modern insultation standards required by building regs but, as it's an improvement over current state (no insulation at all) of an old house does this matter?
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