What is XPS Foam Insulation?

by Erik North on May 6, 2011

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What is XPS foam board insulation? How is XPS foam board insulation used in my home? What is the R-value of XPS foam board?

Why is there a coffee cup here and did you really install 4″ of XPS board in your attic when it was 95 F outside (yes!, and probably 115 inside).


XPS stands for Extruded Polystyrene. XPS foam board is essentially liquified then molded and compressed styrofoam. Ever grab your Dunkin’ Donuts coffee and not get burned? Well the hot coffee is kept from scalding your mitts by 1/4” of polystyrene foam. Imagine how well 2 or 4 inches would insulate (Answer: much better).

XPS foam board is an insulation material similar to polyisocyanurate in its texture and rigidity. It is usually blue or pink in color, with a smooth plastic surface. Like fiberglass, the pink and blue colors are not natural but part of the manufacturing company’s marketing efforts.

Polystyrene is liquified to create XPS. This liquid is mixed into a foam then shaped in a flat mold. Once hardened, it is cut into the 4’x 8’ sheet boards that most customers have seen at Lowe’s as they walk past it to buy fiberglass.

XPS panels can come unfaced or faced typically with a foil air/vapor barrier. The R-value is about 5 per in. This type of rigid foam won’t absorb water and can act as an semi-impermeable vapor barrier. XPS is stronger and more durable than expanded polystyrene (aka Styrofoam) and it is comparably priced to polyiso.


The road to full Extruded Polystyrene started in the early 1800s. A German chemist, Eduard Simon, extracted an oily substance from sweetgum trees that he dubbed ‘styrol’.

Through several refinements of the process (and a tidy 80 years forward), the first production of long chain polymers were achieved by German chemist Hermann Staudinger based on the basic styrols. Shortly after German giant IG Farben began production of polystyrene for industrial and military use. This set the path for polystyrene in modern industry.

Current Uses

Polystyrene has almost infinite applications in modern businesses. More narrowly, XPS foam board has value in many building and insulating applications, especially in harsh environments (like Maine!). XPS is suitable for cold storage, insulating below slabs and slab edges, insulating foundation or cellar walls and exterior sheathing as part of the building shell. It provides water resistant layers and its durability ensures good long term payback.

Finally, XPS is widely used in weatherization work in conjunction with caulking and foam guns (think Great Stuff). Weatherization crew use XPS to help air seal soffits, insulate sill plates and seal leaky attic spaces.

Outside the insulation/auditing industry, XPS/Polystyrene is used for surf boards, manufacturing hobby models, creating architectural miniature models and of course Dunkin’ Donuts coffee cups.

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