Extruded Polystyrene vs. Expanded Polystyrene vs. Polyisocyanurate

Rigid foam boards provide some of the best insulation R-values per inch but aren’t suitable in all situations. For instance, rigid foam is not an efficient replacement for fiberglass batts or mineral wool batts in stud wall cavities.

The three most popular rigid foam insulations–EPS, XPS, and ISO–have different R-values, strengths, and weaknesses.

Most Popular Rigid Foam Insulation Boards

Choosing the correct product for your application involves educating yourself about the properties of each type of rigid board insulation. The following lists provide a detailed comparison of these popular and effective products.

Expanded Polystyrene Rigid Insulation (EPS)

Manufacturers create expanded polystyrene by blowing pentane gas into polystyrene cells to make them larger closed cells. Almost all polystyrene insulation is 95% – 98% air. Dead air provides the insulation value of most products–including rigid foam and fiberglass batts.

Expanded Polystyrene Rigid Insulation (EPS)

Expanded polystyrene insulation is often called bead board or Styrofoam, but Styrofoam is the trademarked name of Dow Chemical’s products. Most EPS rigid insulation is white, but sometimes color is added.

Several million tons of polystyrene are produced every year–not only for insulation. A few uses of polystyrene include CD cases, disposable cutlery and take-out food containers, packing peanuts, molded packaging, bottles, trays, lids, etc.  

Here are some of the uses and benefits of expanded polystyrene:

  • Most versatile of the three rigid foam insulation products.
  • Suitable on walls, roofs, floors, and below grade.
  • Insulation of choice for construction of Insulated Concrete Forms (ICF) and Structural Insulated Panels (SIP)
  • The least expensive product BUT provides the highest average R-value per dollar spent.
  • Approximately R-3.6 to R-4.2 per inch depending on product density.
  • Meets or exceeds most building codes and energy loss requirements.
  • Approved for ground contact and below-grade applications.
  • Comes in insect-resistant panels. (Not a food source but can become a nesting site for termites and other pests.)
  • Absorbs very little moisture and does not retain it over the long term.
  • Will not act as a house wrap – install on the exterior of walls over house wrap.
  • Available faced or unfaced. Faced products act as a vapor retardant, and some specialty facings will act as vapor barriers.
  • Among the most eco-friendly insulation products.
  • The warranty covers 100% of R-value retention, and the product does not degrade over time.

For all of its advantages, EPS is the least used insulation board. It has a lower R-value per inch and is perceived as cheap. It also requires more space because of the extra thickness needed to achieve higher R-values.

Building codes are becoming more stringent, and to reach an R-40 roof insulation value, EPS needs to be approximately 12” – 15” thick.

Extruded Polystyrene Rigid Insulation (XPS)

Most people are familiar with blue expanded polystyrene rigid insulation.  Dow Chemicals manufactures it and stamps it with the Styrofoam trade name. Other colors–yellow, pink, or green–are appearing as more manufacturers make XPS foam boards. 

Extruded Polystyrene Rigid Insulation (XPS)

  • Both cost and R-value of XPS are about in the middle range of rigid foam insulations.
  • R-5.0 per inch
  • Costs approximately $0.42 per square foot for a 4 x 8 sheet of 1” thick material.
  • Suitable on exterior walls under siding renovations and for below-grade insulation applications.
  • Available unfaced or with a number of plastic facings.
  • Semi-permeable – one-inch-thick unfaced product has a perm rating of about one.
  • Thicker-faced XPS can have lower perm ratings.
  • Extruded polystyrene is stronger and more rigid than expanded polystyrene boards.
  • XPS is recyclable.
  • Acts as a vapor retarder, not a vapor barrier.
  • Will absorb more moisture than other rigid insulation boards over time.
  • The warranty does not cover long-term R-value retention because of moisture absorption.

Historically, manufacturers used hydrofluorocarbons to create XPS–a very environmentally unfriendly process. Some manufacturers, like Dow Chemicals-Blue Styrofoam, now use hydrochlorofluorocarbon (HCFC) as a blowing agent. Not perfect, but a significant improvement–with 94% less ozone depletion.

Polyisocyanurate Rigid Insulation (ISO, PIR, Polyiso)

Polyisocyanurate rigid insulation starts out as a liquid, and manufacturers spray it onto the backing material, meaning that all ISO boards are faced–typically on both sides. Some of the uses and benefits of polyisocyanurate rigid insulation include:

Polyisocyanurate Rigid Insulation (ISO, PIR, Polyiso)

  • Costs approximately $0.70 per square foot for 1” thick boards.
  • R-5.8 per inch. Some products like Thermax claim R-values to R-6.5.
  • Most popular as roofing insulation.
  • Allowed as exterior sheathing in some jurisdictions.
  • Suitable on exterior walls under siding retrofits. (Not as effective in cold climates. May lose up to 30% more heat in cold climates.)
  • Different facings affect R-value, durability, and perm ratings.
  • Foil-facing is by far the most common and is impermeable sheathing–especially with taped joints (Do not create a double vapor barrier–one inside and one outside.)
  • Fiberglass-faced ISO is more permeable and won’t trap moisture in the walls.
  • Foil facing becomes especially valuable in hot climates, adding reflective insulation value to the product.
  • ISO is not recyclable.
  • R-value degrades over time. May be noticeable in 10 – 15 years.