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Apologies, could not see where I could amend initial post


We have a new build extension and now ready to insulate between the rafters and get the electrics sorted. The “drawing” states

“Slates over 50x25 battens and 38x38 counter battens, breather membrane over 150x47 rafters …150mm insulation between rafters with gap over 25mm insulation to u/s of rafters with taped joints 12.5mm plasterboard and scim ceiling”

If someone would be kind enough to clarify

Do we install both the 100mm and the 50mm board between the rafters then perhaps need to install a batten strip to create the 25mm void or the “gap” as mentioned on the drawing notes (above) prior to the 12.5mm plasterboarding ?

Where would the best place for the 1st fix electric cabling to run

Appreciate any assistance comments with thanks

Hi bokvis,
Suggest this should be slates on 25x50mm battens on breathable membrane on 150x50 rafters with 125mm insulation between rafters - leaving a 25mm gap under the breathable membrane. 50mm insulation under rafters. 25x50mm counter battens with 12.5mm plasterboard.
Your electrics will be run in the space between the counter battens and the extra depth provided by the counter battens will mean that if you fit downlighters they will not be protruding above the insulation between the rafters.
Regards S

Thank you very much for the response. I received the following from the architect (finally)

The max allowed U value for your roof is 0.18.

Please see EcoTherm calculation to provide this value with the least
amount of insulation.
(Any good insulation brand of rigid PIR insulation can be used
instead-this may depend on what your local merchants have in stock)

The 125mm insulation to be fixed between the rafters
and under your membrane and perhaps held in place with
sections of small battens. This would leave a gap of 25mm under
the insulation for electrical cables prior to fixing the 25mm thick
insulation continuously over the ceiling to be finished with the
12.5mm plasterboard.

The 125mm could consist of 75 + 50 if available cheaper.

Trust this makes sense.

Hi bokvis,
Ok so the original 150mm insulation fitted between the rafters is now 125mm, this makes more sense because provided the insulation is a close fit between the rafters, it can be flush with the bottom leaving a 25mm gap under the breathable membrane. I would still recommend fitting insulation directly onto the bottom of the rafters and to match the original spec of 175mm (150+25mm) I suggest you use 50mm. Then counter batten and fix plasterboard to this, the space between the counter battens can carry your cables.
If you go with the cables being located between two layers of insulation you will have to downrate their current carrying capacity to comply with electrical regs.
Regards S