Well I have a radiant tube heater in my recently built pole barn, at the present time it's also non-insulated too. Will be doing the insulation soon, I hope. Anyways, it's a two stage furnace 40' long and 150,000 / 100,000 BTUs, The barn size is 40 x 60, with 5 garage doors and +10 windows, along with a 12 foot leanto. I've only been running it on the weekends and at daytime. I'm in SE Michigan, it was 14 degrees earlier last week and it took about 4-5 hours to get to 40 degrees. So, it needs to be insulated for sure. Hope this helps. Oh, on warmer days like 30 degrees it just takes just couple of hours to get it up to 45-50 degrees. For what I'm doing out there I really don't need it any hotter.