The clearance reduction methods specified in Table 18-28-308.6 shall not be used to reduce the clearance required for solid fuel-burning appliances that are labeled for installation with clearances of 12 inches (305 mm) or less. If appliances are labeled for installation with clearances greater than 12 inches (305 mm), the clearance reduction methods of Table 18-28-308.6 shall not reduce the clearance to less than 12 inches (305 mm).