I'm not trying to spend your money. This suggestion may seem extreme and it IS expensive, but hear me out. I would seriously consider removing the osb walls and having the existing foam board sprayed with 2 or 3" of closed cell foam. Not only will it increase your r-value, but it will also make the walls air tight - no more Michigan winter winds cutting through the cracks and sucking your heat away. And an additional benefit is how it gives the building a huge structural boost. I could not believe how much the walls tightened up on my pole barn when they sprayed it. It honestly made a serious difference.
Yes, I know it's a lot of money to have it sprayed. Trust me I know. I had to make some huge sacrifices to make it happen in my barn. But I can honestly say that I have no regrets. It cost me a ceiling for 2 years. Instead of the steel panels for my ceiling (plus a few other items), I put that money towards spray foam. So for the last 2 winters I have only had a sheet of thick mill plastic stapled to the trusses - far more effective than I expected but still, it was NO ceiling. I couldn't even blow in the insulation because the plastic certainly wouldn't have supported it. Even so, I had no problems keeping it warm enough to work in over the past 2 winters even with just a plastic ceiling. Along with being without ceiling panels in the shop, I was also without a garage (still am actually) because the front garage is still FULL to the rafters with bags of cellulose that I'd already bought before deciding on the spray foam and told the wife it would only be there for a week or so - oops.
All that to say that later today I should be driving the last of the ceiling panel screws and will then begin the work of finally blowing in the attic insulation. But already I have benefitted from postponing the ceiling in order to have it sprayed. I cannot recommend it enough. And in your situation, since you are covering up foam board, the spray foam wont be sticking to the back side of your siding - which is a PITA to deal with if you ever need to replace any siding later. So you'd be safe on that one. Also, as I said, the spray foam will make the walls basically air tight!
OK, enough. Sorry about the long post. I just can't say enough about the foam and it's advantages. I'm so so glad I went that route and like I said, I have no regrets about it. Even without any insulation in the attic yet, it's been so easy to heat it with a little propane heater while installing the ceiling panels so I'm really stoked to see what kind of difference the blown cellulose is going to make. While waiting 2 years on the ceiling, I was able to gradually add enough bags of cellulose to the pile to fill the attic to 20". My hope is that I'll be able to raise the room temperature one degree simply by breaking wind.
Best of luck on your barn and please be sure to post back here and let us know what you decide to do. We all enjoy reading about choices other guys go with to build their shops. Maybe you could also consider making a build thread and keep us all posted as you progress through the build. I love build threads. Again, at least consider the spray foam. I highly recommend it. As do many of the members here. In fact, it was here at GJ that convinced me to spray my own barn. Either way, let us know what you decide. I'll be watching this thread for sure. Us Michiganders gotta stick together ya know!
