Heatilator BIR42 Owner's Manual page 45

Combustible Wall
2 x 4 stud wall
Standoffs
Non-combustible
Decorative Facing
such as:
Steel, iron, brick,
tile, concrete, slate,
glass, plasters.
Seal joint with
non-combustible
sealant
of ANSI/NFPA211.
Combustible
Decorative Facing
Grid represents
1 in. squares
12 in./305 mm
Mantel
22 in./559 mm
minimum
12 in./305 mm
minimum
6 in./152 mm
minimum
1 1/2 in./
38 mm
maximum
Measured from top of fireplace opening
7 ft (2134 mm)
minimum
base of fireplace
to ceiling