I think that by "screen room," an enclosed, screened-in porch (or rear deck) is what is meant. Such a place would have the outer siding of the house as one of the walls; the others would probably be of minimal construction.
I also recently installed a heater on an open terrace. Somehow the whole idea of 'standing outside' while seeking shelter from the elements fails to make any sense to me!
In the case of the heater I installed, the instructions were quite explicit as to clearances expected in different mounting positions.
