Hi, I recently acquired a 70's home with wood clapboard siding. I had planned on taking off the clapboard, adding tyvek and re-siding. To my surprise, thee clapboard is functioning as the sheathing. I know the best is likely to replace clapboard with sheathing and start over but would now like to come up with an option of leave the clapboard and go over top. It is in Ok condition I am thinking I have a couple of options for new vinyl siding 1) Strap the entire house and applying new siding to furring strips. Would tyvek be used in this situation? 2) Apply 1" rigid foam over the clapboard and vinyl applied directly on the foam. Any advice for this situation? P.S. window and door detailing is not a major concern as they will be replaced. Thanks!

Whatever you end up doing, make sure you have a water vapor drying path to either the inside or outside.

https://www.borstengineeringconstruction.com/Building_Assembly_Moisture_Analysis_Calculator.html

When you put that layer of foam towards the outside of the wall you create a place where moisture can condense inside the wall.  You don't want water sitting around inside the wall.