AWF Blog

Blue Board vs Thermax. What you need to know.

Blue Board vs Thermax. What you need to know.

Posted by Martin Z on May 14th 2021

A common question we receive is what is the difference between Blue Board and Thermax™?

Blue Board

Officially Styrofoam™ Brand Extruded Polystyrene Foam Insulation, No wonder it’s commonly known as Blue Board

  1. Originally developed by Dow Chemical, Blue Board is an Extruded Polystyrene (XPS), its blue color is a trademark of DuPont (DuPont de Nemours, Inc.). Other manufacturers make similar products but in other colors. Officially this product is trademarked as Styrofoam™ however Styrofoam has come to be used with a much larger range of expanded foam and extruded foam products. Somehow getting your coffee in an “expanded polystyrene” cup seems a bit wordy.
  2. Blue Board is available in a wide range of board styles, and thicknesses which share common characteristics. Being closed cell foam it makes a good insulator (typically R5 per inch) that is resistant to water damage and mold growth.
  3. DuPont Styrofoam™ can be used below grade and DuPont has specific products to channel water away from foundation walls.

Are my eyes deceiving me?

No they aren't. Blue Board may no longer be blue. DuPont has been working on a more environmentally friendly version of Styrofoam™ that now qualifies as a Low Global Warming Potential product. Certain chemicals were removed from the product that make it compliant with environmental laws in all 50 states.

This new version of Styrofoam™ is now a stylish Gray color.


  1. Blue Board is combustible and should not be used in living spaces without a thermal barrier (typically gypsum board [wall board]). Maximum use temperature is 165° F. This may be a consideration when using in high temperature areas.
  2. Most XPS has been said to be unfriendly to the environment due to the chemicals used in manufacture. DuPont’s XPS products (Blue Board) are hydrochlorofluorcarbon free with zero ozone-depletion potential.


  1. Thermax™ Sheathing is a Polyisocyanurate (Polyiso) foam board infused with glass-fiber reinforcement. Polyiso boards are sandwiched between facer materials. In the case of Thermax™ these facers are 1.0 mil aluminum or a 1.0 mil white acrylic bonded to aluminum.
  2. Thermax™ is available in a range of board styles, and thicknesses. Polyisocyanurate (Polyiso) is also a closed cell foam which is more efficient per inch than blue board (R6.5 per inch). It is resistant to water damage and mold growth.
  3. The aluminum facers reflect back radiant heat in addition to foam insulation. This increases the heat savings over the rated R value.
  4. Thermax's™ fire rating allows it to be installed in interior spaces such as basements, attics and crawl spaces without an additional thermal barrier.
  5. White Finish Thermax™ offers a finished look without the need for additional covering or paint. White Finish Thermax™ has the exact same insulation properties as standard Thermax™ Sheathing.


  1. Polyiso insulation has a higher cost per square foot than XPS.
  2. A little more care in transportation and installation is needed than blue board and similar materials. Once installed the aluminum facers provide a durable easy to clean surface.
  3. Some sources claim Polyiso is not as efficient in frigid temperatures; exterior applications of Polyiso may not perform as rated in extreme cold. When installed in interior spaces this is not a problem and the full insulation value is not affected.


DuPont Styrofoam™ Blue Board is a cost effective insulation that can be used in a wide variety of applications, including foundation walls, under siding and exterior sheathing, and between wall studs. Blue board needs to be covered with a thermal barrier on the inside or with siding or other covering to the outside.

DuPont Thermax™ is a higher efficiency insulation that performs well in interior spaces. Since it doesn’t require a thermal barrier you can achieve a high R value without covering it and increasing wall thickness.

Update: It is important to note that the DuPont Styrofoam is no longer blue, it now has a gray finish, but although the color has changed you can expect the same consistent, trustworthy performance.