I have made only moderate progress since my last post. I'm about 90% done with all the framing, and I am waiting for only one more window. Today I visited Lowes again and as usual I investigated the plywood bunks- probably the 10th time in a month. I hit the mother load of perfect plywood. 3/8" CDX that was either knot free on one side or had only one small knot- less than a 1/2", and tight. I bought six sheets, one after the other was like that as I pulled them off the pile. That will be enough for the exterior and I already had one sheet at home. They told me it will be a long time before foam board insulation (made from oil) comes down in price, even though oil has dropped by over 40% in price, the price of foam insulation is still the same. I paid $14.90/sheet for the plywood after my 5% Lowes card discount. My 4 windows came to about $120.
I will need to decide how to finish the walls= paint or maybe stain & spar varnish, since the plywood is so nice. I have no experience in the use of spar varnish.