Fiberboard Rigid Insulation

Fiberboard rigid insulation is a product historically used in roof recover jobs, or jobs where a new roof is being installed over an existing roof. It has an R-value of 4.0 per inch, and is typically installed in thicknesses of ½” to provide a substrate between the old roof and the new roof. Fiberboard rigid insulation is known for being less expensive than other roofing insulation products.

However, Fiberboard has numerous flaws in flat roofing applications. The biggest flaw is its ability to soak up water like a sponge, which adds significant weight and moisture to the roof system. After being saturated for an extended period of time, it tends to break apart, shrink, or turn into dust, which can sometimes find its way inside the building. As fiberboard shrinks, the fasteners used to hold the insulation in place stay in the same location. However, the membrane still sits on the insulation, which causes the fasteners to poke holes through the membrane.

Cutting Fiberboard

Fastener Poking Through

Benefits of Fiberboard Insulation

  • Inexpensive relative to other rigid insulation products.

Drawbacks of Fiberboard Insulation

  • Soaks up water like a sponge.
  • Shrinks after being saturated with water for extended periods of time.
  • Can get into the building through openings in the structural deck.
  • Breaks apart over time.
  • Because of its dramatic flaws and known issues over time, Fiberboard Rigid Insulation is rarely used in roofing anymore. Especially since polyisocyanurate insulation is much better product, and has a more competitive price than in the past.