[chris vickery]

For your consideration Phil might I offer this opinion...
I think it is more likely that your wood joists have shrunk over the years as the timber dries out and not the styrofoam itself. Food for thought.
BTW, your air exchanger invention is interesting. One thing I considered but did not do was to install an exhaust fan in a sidewall. I guess I am too picky so my shop does not see sanding dust or paint too much as I do most outdoors. If I were to build another shop or add an addition I think I would have a separate section walled off for “dirty work”.
Paul you alluded to perhaps sectioning off your shop, maybe you will take this as a consideration...