I do measure mine, before I added air over fire (AOF) it hit temps up to 1500F, with AOF it on ocassions gets to 1050F. Any temperatures at 300F can in time make the wood ignite at 300F, once the heat turns it into charcoal. To protect it any of those methods will do the job. 1 layer of tin, spaced on non combustible spacers cuts the needed distance in half. With none you need 36", so 1 makes it 18", 2, each spaced an inch makes it 9" and 3 cuts it to 4.5". An inch of ceramic blanket will do the same. Remember, this not only means the trusses or rafters but also roof boards.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.