When I am looking a radiant (tube) heaters there is always a large clearance to combustables above. Does a metal lined (barn tin) reduce this requirement?
Typically a surface is considered combustible unless the substrate is non combustible. So metal over wood is combustible and clearances wood apply.
If this were a stove there would be a formula in the manual itself for reducing clearances... Might start there. Its usually a combination of non combustible material and airspace
I had one of those heaters in a 26x26. It was mounted on an angle in the front of the garage. For some reason(this was many years ago) I had to go in the attic in the winter. The trusses above the heater were so hot I had a hard time keeping my bare hands on them.
If I were to have another I would put an extra layer of heat shield similiar to a wood stove or something between.
