Hello, I have an old house (100 years). It is smaller size. I would like to build the Rigid foam channel for the attic insulation. I notice since I had a new roof with a ridge vent or in that the plywood under the new roof now may be getting moisture. 

Questions on doing the Rigid foam insulation on the old house. How thick does the Rigid foam have to be for the channels? Does it all have to be foil backed for the full channel or just the wind wash baffle part against the soffits? (I read the subscriber articles). Also the house is wood trim (lead paint) with vinyl trim over that. So how to handle not having the Rigid foam going up the wall to the channel in the rafter? Should I make a shorter rigid foam channel against the wood trim in the soffit or not bother?

I see on my old house someone covered all the rafter tails with plywood and just left soffit vents every 8 to 10 feet. Should every rafter bay have a soffit vent or is there such a thing as too many? (Make too much air moving through the attic?)

We do get snow and below 30 regularly through the winter here. The humidity is not too bad normally. I did notice ice dams on the new roof this winter.

Thanks for any help you can give someone having to do this retrofit. I would love to tear and redo the wall exterior but unfortunately I’m also looking at doing a foundation replacement. So trying to be effective with what funds I have. I appreciate all the resources here!