Exhaust ducts shall have a clearance from unprotected combustible construction or other combustible material of not less than 18 inches. This distance may be reduced if protection is provided for all combustible surfaces within 18 inches according to the following schedule:
1. 28 gauge sheet metal on 1/4-inch insulating millboard 12 inches.
2. 28 gauge sheet metal on 1/8-inch insulating millboard spaced out 1 inch on noncombustible spacers 9 inches.
3. 22 gauge sheet metal on 1-inch rockwool bats reinforced with wire mesh or the equivalent 3 inches.
4. Any other approved material of equal of greater fire-resistance.