I have a recently completed pole barn with steel roof & siding. I ended up about $8k over-budget, so I had to pass on having it sheeted with OSB before the siding was installed and as a result I've got plenty of areas where I can see daylight and cold air can freely creep in. I know that it's not a good idea to use spray foam (Great Stuff, et al) since it traps moisture and could lead to premature rusting of the siding. I was thinking about spraying in some rubberized undercoating first to protect the siding and then following that up with spray foam to seal up the 'leaky' areas. Sound like a good idea? Building is not insulated at the moment, but I plan to add fiberglass batts as I can afford to until I've got the whole thing done.