Roof Insulation Systems

August 21, 2015

Return to Blog Home


Roof insulation performs two basic functions:

  • Acts as a thermal barrier for the top of the building.
  • Serves as the base for the roof system.

Secondary functions include:

  • Enhancing drainage through the use of tapered insulation systems and stiffened roof decks.

Poly-Isocyanurate (ISO)

Far and away the most common roof insulation installed, ISO is available in flat or tapered panels and is the base for many roof systems with an average LTTR value of 5.56 per 1″ of product. Polyisocyanurate features a closed cell ISO core integrally laminated to fiber reinforced felt factors. ISO has a perforated facer on one side for use with hot asphalt applied systems while the non-perforated side is for use with single-ply systems.


  • High R value of 5.5 per inch and good compressive strength. Excellent dimensional stability and very lightweight.


  • Contains CFCs or HCFCs which may be released into the environment, causing ozone depletion. It is also expensive in comparison to other insulations. Contains a petroleum by-product increasingly in demand worldwide, resulting in availability problems and lengthy lead times for orders.


Perlite insulation is a rigid board insulation available in 2″ x 4″ or 4″ x 4″ panels. The panels are composed of expanded volcanic glass and wood fibers bonded with asphaltic binders. Until the late 1990’s, perlite was the most common insulation used in roofing. Perlite insulation is gradually being phased out of the commercial roofing industry due to its low R-value of 2.78 per inch and tendency to absorb moisture has diminished its frequency of use.


  • Perlite has an excellent fire protection resistance rating. It also has good compressive strength which allows normal roof foot traffic. It also has great dimensional stability and the ability to absorb outgassing common in foam and ISO insulations.


  • Very friable and relatively easy to break panels. Complete deterioration when in contact with moisture. Low R value and poor tensile strength.
  • Low wind uplift rating

Expanded/Extruded Polystyrene

As the lead times required for isocyanurate insulation increases, EPS has risen as a low-cost alternative for roofing insulation. Both types are manufactured from polystyrene, one as a fused board made of beads while the second is formed from a molten sheet pressed into shape.


  • Dimensionally stable with lower water absorption.
  • Extremely lightweight, minimizing roof dead loads.
  • Easy to handle, cut, and form.
  • The extruded form is used widely as a base for architectural shapes at wall parapets.
  • Decent R values, between 4-5 depending on type.
  • Relatively inexpensive


  • Poor fire resistance
  • Low compressive strength equates with poor wind load resistance
  • Releases potentially toxic combustion product if ignited


Dens-Deck is an ever increasingly popular roof insulating product in roof assemblies. It is the roofing version of an exterior board used most commonly as an exterior stucco backer board. This board is a mold resistant roof panel that consists of a moisture-resistant, non-combustible core of specially treated gypsum with glass mat facings.


  • Approved by all manufacturers.
  • Highest performance rating for fire, wind uplift, and moisture resistance.


  • Very heavy board increases handling costs and deck requirements.
  • Difficult to cut and form
  • Primed version must be used in hot applications to avoid blistering membrane.
  • Low R value

Wood Fiber

It is the oldest type designed for roofing. It is normally found in a “high density” form and is inexpensive and durable. It is also often used as a cover board over ISO in hot applications to reduce outgassing.


  • Durable, easy to handle, and relatively easy to cut and form.
  • Decent compressive strength provides reasonable wind uplift ratings.


  • Dimensionally unstable in contact with moisture.
  • Rapid deterioration when wet, also providing possible medium for mold growth.
  • Poor fire rating with low R value of 1.9 per inch.

Each project and roof assembly has its own subtle differences that may lend itself to a particular size, type, or thickness of insulation. Additionally, the expense at insulation should be cost-factored in comparison to your cooling bill over the expected life of the assembly to determine the most effective insulation for a particular project. Your contractor, consultant, or manufacturer can all play vital roles in helping with the final decision based on R value needs, anticipated length of ownership, and local codes.