Years ago I had a similar setup. I bit the bullet and carefully remove the wall sheathing, filled between the studs with batt insulation and replaced the sheathing. I wasn't nearly as careful removing it as I should have been and it showed - lol. But WOW! the difference was AMAZING! And this was with no ceiling and zero insulation in the roof. Absolutely worth the cost and the effort to do it! After seeing the benefits of the wall insulation, I eventually decided to do something with the open ceiling too.
Since I kept things stored on top of the roof joists, I didn't want to seal the attic off so I took the minimal approach and stapled some heavy mil plastic to the bottoms of the roof rafters. No insulation - just heavy plastic. And DOUBLE WOW! I couldn't believe the difference AGAIN. Even without adding insulation (hey - I was broke. Don't judge me
lol) it was a huge improvement. I went from spending most of my shop time stuffing the wood stove, to barely ever adding wood after the first stoveful. And then rigging up a fan to circulate the brutally hot attic air back down to ground level made it a t-shirt only zone!
Now years later, while building my pole barn, I am concentrating on insulation. It's worth every penny and every bit of work! If I was you I would either insulate and/or slap some heavy plastic on your rafters to keep as much of that heat in as possible. Plus, I like your idea about the cement board above the heater. It could work as both a protective barrier as well as a heat mass. I'd worry about it drying up or becoming brittle and then raining down on you though - I'm not sure about how that stuff works with heat. Maybe a sheet of corrugated roofing steel? Pictures would certainly allow for more and better advice when it comes to a unique setup like you've described - just sayin.
Glad to hear that you got it up and running.
Best of luck on the shop. Keep us posted as to how it works out in the future.
