Insulating the roof makes perfect sense - this is the first place warm air will try and escape, warm air rises after all ! So insulate the roof to reduce losing heat into the outside air.
The simple way of insulating the roof can be seen here in our article "Easy Roof Insulation" If, however, you have a cabin with a full cavity such as the Valmorel DS Log Cabin with double wall construction then it is likely the roof void will house some form of insulation material such as Rock Wool.
In either case it is good practice to provide ventilation across the top of the insulation material to avoid the build up of moisture and risk this getting to such a state that this moisture then falls back into your cabin as water.
Depending on how the roof cavity has been constructed make sure each area, if segregated by roof joists, have their own ability to ventilate moisture out of the building.
By cutting holes in an appropriate place adjacent to the void above the insulation layer, glue a plastic soffit vent cover to keep the job looking tidy and to prevent vermin from accessing the roof void.