I might look into that shield. Better than the Osburn option.. Single digits is cold! Only had teens so far here so the most I could test. Actually in the 50's today and sayin might get to 70 tomorrow-crazy!i just went with the manufacturer specs from the home saver heat shield , no real scientific method. It comes packaged much larger you just cut to size , I wish I would have made it alittle bigger and for roughly $70 I can order a new one and do it again. I did do the air all the way closed and had a better coal bed to start with in the am but useful heat was gone in the eight hours it was running because the furnace was on, it was in the single digits at this time .