18-28-308.6 Reduction table.

   The allowable clearance reduction shall be based on one of the methods specified in Table 18-28-308.6, Clearance Reduction Methods. If required clearances are not listed in Table 18-28-308.6, the reduced clearances shall be determined by linear interpolation between the distances listed in the table. Reduced clearances shall not be derived by extrapolation below the range of the table.

Table 18-28-308.6
Clearance Reduction Methods

Type of Protective Assembly a	Required Clearance to Combustibles with Protection (Inches) a
	Horizontal Combustible Assemblies Located Above the Heat Source				Horizontal Combustible Assemblies Located Beneath the Heat Source and All Vertical Combustible Assemblies
	Required clearance to combustibles without protection (inches) a				Required clearance to combustibles without protection (inches) a
	36	18	9	6	36	18	9	6
Galvanized sheet metal minimum nominal thickness of 0.024 inch (No. 24 Gauge), spaced 1 inch off the combustible assembly	18	9	5	3	12	6	3	3
Galvanized sheet metal minimum nominal thickness of 0.024 inch (No. 24 Gauge), spaced 1 inch off the combustible assembly	18	9	5	3	12	6	3	2
Two layers of galvanized sheet metal, minimum nominal thickness of 0.024 inch (No. 24 Gauge), having a 1-inch airspace between layers, spaced 1 inch off the combustible assembly	18	9	5	3	12	6	3	3
Two layer of galvanized sheet metal, minimum nominal thickness of 0.024 inch (No. 24 Gauge), having 1-inch of fiberglass insulation between layers, spaced 1 inch off the combustible assembly	18	9	5	3	12	6	3	3
0.5-inch inorganic insulating board, over 1 inch of fiberglass or mineral wool batt, against the combustible assembly	24	12	6	4	18	9	5	3
3.5-inch brick wall, spaced 1 inch off the combustible assembly	–	–	–	–	12	6	6	6
3.5-inch brick wall, against the combustible wall	–	–	–	–	24	12	6	5

 

For SI: 1 inch = 25.4 mm, °C = [(°F)-32]/1.8, 1 lb. per cubic foot = 16.02 kg/m 3 , 1.0 (Btu x in.)/(sq. ft. x hr x °F.) = 0.144 W/m 2 x K.

a Mineral wool batts (blanket or board) shall have a minimum density of 8 pounds per cubic foot and a minimum melting point of 1,500°F. Insulation material utilized as part of a clearance reduction system shall have a thermal conductivity of 1.0 (Btu x in.)/(sq. ft. x hr. x °F.) or less. Insulation board shall be of noncombustible material.