Fire Resistance of Wood-Framed Exterior Walls: The Effect of An External Air Cavity and External Insulation

The paper describes a heat transfer model for wood-framed exterior walls with the exterior siding exposed to fire. These exterior walls have nominal 2x4 wood-studs protected by gypsum board on the ambient side (inside the building), and OSB (Oriented Strand Board) sheathing and exterior siding on the fire-side (outside the building). Between the OSB sheathing and exterior siding there is an external air cavity for air circulation and a 50-mm thick layer of external insulation. This type of exterior wall is becoming popular, because it prevents moisture condensation in the walls. The present paper investigates the fire resistance of this type of exterior wall, when the exterior siding is exposed to fire. The model suggests that the air cavity behind the siding and the external insulation layer play important roles in improving the fire resistance of the walls