Thanks for the suggestion...just sent an inquiry off to them. So far here is my train of thought: If I'm taking away 2.25 of air space then that is removing the equivalent of R=3.2175. With that mentality (lots of people think I'm a mental case, anyhoo...) then I will need the prescribed R=1.1 value PLUS the R=3.2175 value that was taken away by removing the 2.25" of air space. This is kind of like the Chinese do...reverse engineer it. The original configuration: F3CB Requirements Tall Legs Inches R-Value Required ? 1.10 Air Space (ventilated) 1.00 1.43 Air Space (ventilated) 1.00 1.43 Air Space (ventilated) 0.25 0.36 Totals 2.25 4.32 These are the clearances that I came up with between the top of the rear exit flue and the lintel: Height Height F3CB Tall Legs Short Legs Lintel 27.00 27.00 Top of Flue -25.56 -23.31 Flue Clearance/ Hearth Thickness 1.44" 3.69" And this is what I think it would take to get to the required R=4.32 which would leave roughly .6" clearance between lintel and pipe...might be enough for double-wall pipe?: Inches R-Value Ceramic Tile 0.25 0.01 Cement/Thinset 0.25 0.05 Durock Cement Board 0.50 0.26 Ceramic Board (Micore) 0.50 1.10 Ceramic Board (Micore) 0.50 1.10 Ceramic Board (Micore) 0.50 1.10 Ceramic Board (Micore) 0.50 1.10 3.00" R= 4.72 Maybe that wasn't too confusing, but I think that would work. Is having 2 inches of micor too much in regards to structural integrity (will it mash/sag)? Ah, well I see the formating didn't work out very well... Again...clear as mud, Ed ....one cell at a time.