advice on partial attic conversion

mick121

I have 4.5×1.5 inch rafters and am looking for advice regarding insulation. I'm going to fix 2×2 battens to each rafter to increase the depth to near 6 inches.I can then fit 100mm solid insulation between them and still have airflow between insulation and felt.my question is what u value should I be looking for on the sheets.I dont want to get less than regs.cheers.Mick

dathi

mick121 wrote: »

question is what u value should I be looking for on the sheets.I dont want to get less than regs.cheers.Mick

this is a common mistake the figures quoted on kingspan/xtherm sheets or on fiber glass is not a u value figure it is the thermal resistance figure for the insulation. a u value is got by adding all the resistance figures for your roof structure together ie the resistance of the tiles, felt, timber rafters insulation, and plasterboard and then dividing 1 by the total of the resistances

building regs for existing buildings look for a u value of 0.16 on the flat section of your attic ie the area above flat ceilings which can be achieved with 300mm fiber and a u value of 0.25 for sloped ceilings which will require 100mm kingspan /xtherm between and 30mm below the rafters &plasterboard
to keep both sections the same and achieve 0.16 on the sloping ceiling you would need 100mm between and 60mm below the rafters
but insulation on its own is of little value unless you make the structure air tight at the same time google vario membrane for details of how to do this

mick121

cheers dathi.anymore insight would be appreciated.