Fiberboard insulation is only available in flat boards in 24" x 48", 48" x 48", and 48" x 96" sizes. The common thicknesses of fiberboard insulation are 1/2" 1", 1 1/2" and 2", although not all thicknesses are available from all manufacturers. Fiberboard insulation is also available in two densities, normal (Grade 1) and high (Grade 2).

The good compressive strength of fiberboard insulation, listed at 80 pounds per square inch, makes it sufficient to resist deformation from traffic loads, hailstone impact and dropped tools. Its good compressive strength is one reason fiberboard insulation is commonly used to overlay foam plastic insulations under built-up and modified bitumen roof membranes.

The thermal resistance of fiberboard insulation (R-value = 2.78 per inch thickness) is relatively low when compared to other insulation types. However, its thermal resistance retention remains constant unless exposed to moisture. At 100% moisture content, fiberboard insulation will lose 40-60% of its thermal insulating value.

Fibrous, organic insulations are especially vulnerable to moisture damage. The equilibrium moisture content (% by weight @ 75 degrees Fahrenheit 90% RH) of fiberboard insulation is 15% compared to 5% for perlite, 2.9% for polyisocyanurate, or 0.8% for extruded polystyrene. When exposed to moisture, fiberboard becomes dimensionally unstable. Long-term exposure to moisture will cause this insulation to warp, buckle and decay. As the insulation deteriorates from moisture absorption, it loses its shear strength and peel resistance. A reduction in insulation shear strength and peel resistance leaves the roof membrane highly susceptible to splitting from stress concentrations and displacement or blow-off from wind uplift forces.

As previously stated, fiberboard insulation is commonly used to overlay foam plastic insulations. Fiberboard is also commonly used in recover roof applications. However, caution is required when specifying its use as any moisture entrapped within the existing roof, or moisture released from the roof surface will adversely affect the insulation's performance.

Although fiberboard insulation has desirable properties, the design criteria for a roof will determine if fiberboard is appropriate for use. If fiberboard is appropriate, it is imperative that it be adequately protected against moisture damage prior to installation. Once installed, it must be covered with the roofing membrane the same day.