1 size designed for 24" and 16" rafter spacing. Prevents ice buildup on roofs and keeps houses warmer in the winter and cooler in the summer. Made of extruded polystyrene foam. Economical option for new construction. Wide flanges for easy stapling. Perforated for easy separation. Installing attic ventilation channels between the rafters or trusses provides an unobstructed air channel through the insulation. Without attic ventilation channels air cannot flow freely from the soffit to the exhaust vents. Poor air flow reduces insulation efficiencies and accelerates problems due to moisture. 1 ventilation channel per rafter or truss cavity is recommended. Dimensions: 22" x 48". Air channel depth, 1.4". Net free vent, 18.7 sq. in.