Insulating pitched roof between rafters rather than joists.

[nanuuk]

Can someone see anything wrong with this plan?
My drawing specifies insulation (100mm) between the joists in the loft (pitched roof). I want to move it all between the rafters (150mm) so I can have an insulated attic space. I know that in this case the roof needs to be ventilated so I will incorporate a ventilated air space (50mm) above the insulation and 25mm vent strip in the timber soffit and provide vapour control layer with aluminium tape on insulation boards joints.

I hope that is good enough and that I don't need tile vents inserted at top of pitch on both sides of the roof?
Did I miss anything?

Many thanks.

Details from the drawing:
100mm thk insulation (kingspan k7) between joists with
kooltherm k18 to the underside of the joists.
followed by 12mm thk foil backed plasterboard, skimmed.

 

[noseall]

You will still need an insulation layer across the underside of the rafters in addition to the 100mm betwixt.

 

[nanuuk]

Sorry for not being clear. I am going to use it all:
insulation boards between the rafters,
k18 and plasterboard on the underside of the rafters.

 

[freddiemercurystwin]

You still need vents at the ridge or tile vents on each side near the ridge.

 

[Trouble666]

You haven't specified what you're doing in the loftspace.
If nothing, it would be more beneficial to insulate between the joists as that creates a barrier between the habitable space (i.e house) and the loft.

This barrier precents heat rising into a loft where typically there is no use.

Insulating between the rafters would mean your heat would escape into a loftspace but the insulation in the rafters would keep the heat in the loft where in theory it doesn't help you heat the house and in turn affects the energy bill.