Roof insulation for a log cabin

[Fastelectrix]

I would like to build a log cabin in my garden to use as a home office/workshop. To keep the cabin warm during the winter months I would like to add some additional insulation to the roof. The attractive tongue and groove wood used for the roof would be hidden by any insulation fixed to the inside and so it would be my preference to add a layer of insulating material to the outside of the cabin under the roofing felt. Can anyone suggest a suitable material that could be used for this purpose?

 

[foxhole]

How is the roof constructed, what’s the heat source.

 

[Fastelectrix]

The roof is 19mm tongue and groove covered with standard roofing felt. The heat source will be an electric radiator.