We live in an 80 year old horse barn converted to a beautiful home.
The upper half has a gambrel roof with exposed redwood reverse trusses under exposed ruff 1X12  sheathing .  We need to re roof. currently there is no insulation. House is in SF bay area hills climate zone 3.
We heat with a pellet stove and no AC.

I was thinking of adding 2″ of rigid foam on top of redwood  sheathing then 1/2″plywood  under composition roofing. 
I know this is far from ideal but it should be an improvement.
My questions are ;should I use any moisture barrier or backing on the foam or plywood? Will the shingles get too hot on top of invented foam?
Will rot from condensation be a huge problem? We keep the Windows open most of the year and every night.
Thanks for your input.
Danny