A common use for our log cabins are to be used as garden offices. Normally, the log thickness for this use would be 45mm or above. However, we do also have some cabins in a 40mm thickness. The thickness of the log will help with the thermal capacity of the cabin, particularly if the cabins will be used as an office for all months.
To help to retain the heat within your cabin, it is also possible to insulate both the roof and floor. With our flooring either a 40-50mm insulation would work well.