Insulation when installing a Velux

yllw.ldbttr

Around 3 or 4 years ago I converted the garage attached to our house, I now use this as an office.
The room does not get sun until late in the evening, so even when the rear of the house is warm, that garage can still be quite cold (one window at front of garage only). The wallls were slab insulated, and the floor of the attic is also insulated.
I am getting a builder to put a Velux in the rear facing part of the roof, with a tunnel through the attic into the garage.
This will allow some light in and hopefully some heat from the sun also.
My question is - the builder said it's better to insulate the roof between the rafters, with kingspan.
I had thought best to just insulate the back of the tunnel that he creates with kingspan.
By insulating the inside of the roof, will this not just lead to heat being trapped in the attic, rather than the room? Also, if I don't insulate the back of the tunnel, is it not likely to end up having issues with damp etc?
Would apprecaite any advice you can offer.

homer911

Sounds odd - unless you want to keep the attic warmer, why insulate between the rafters? I would ensure the attic floor is well insulated, then insultate the tunnel, as you have indicated