What is Polyisocyanurate Insulation?

by Erik North on May 16, 2011

What is Polyisocyanurate Insulation?


What is polyisocyanurate insulation? How is polyiso used as an insulation? And why don’t we say polyisocyanurate (actually, ok, that one is pretty obvious)?


Polyisocyanurate or polyiso if we’re getting familiar is a closed-cell foam with an injected gas trapped within the (wait for it) closed cells. Polyisocyanurate insulation is formed as a rigid foam, most often found as 4′ x 8′ boards. Polyiso is used is almost all phases of construction, often providing insulation and vapor resistance (depending on the thickness) to wall assemblies. Polyisocyanurate insulation can act as a vapor barrier when an applied foil face is present.

Polyiso foam is created in a very similar manner to extruded polystyrene (XPS or blue/pink board). The polyisocyanurate is melted down into a liquid, foamed with a gas (usually the highly safe sounding hydrochlorofluorocarbons) and poured into a mold.

Once it has hardened, polyiso can be cut and add the foil facings that most customers have seen at Lowe’s as they shop for fiberglass (I know, I’ve used that line before but it’s true). Because polyisocyanurate insulation is a thermoset polymer (I looked it up), it has a much higher ignition point and melting point than other styrene based foams.

The gasses (again, the not at all scary sounding HCFCs and CFCs) trapped within the polyiso foam are poor conductors, slowing heat transfer. The resultant R-value typically varies from R-6 to R-8 per inch, the highest of any regularly available insulating material. Over time some of the gas escapes, lowering the final R-value by approximately 1 (a final aged value of around 5-7 per inch).


Normally I’d give an in-depth (or as in-depth as one can pack into two paragraphs) review of the insulation’s history. But polyisocyanurate insulation’s history so closely tracks that of polystyrene’s that you can click this link, and read up on it. Just cross out each instance of polystyrene and add polyisocyanurate.

Current Uses

Polyisocyanurate insulation is used through out modern industrial and residential construction. When foil faced, polyiso is air and vapor impermeable and has a much higher melting and ignition point than either extruded or expanded polystyrene.

Foil faced polyiso is used for insulating the duct work on heating, ventilation and air conditioning systems, for the construction of insulated panel roof and wall assemblies and pipe insulation.

For energy audits and residential work, foil faced polyisocyanurate insulation is often recommended where an effective air and vapor barrier is needed (in New England, the interior but this varies by climate and differing heat and moisture conditions).

The polyisocyanurate panels do shrink over time. This can cause gaps that compromise the insulation value and function as a air/vapor barrier. This requires particular attention during installation to securely tape the paneling seams. Polyisocyanurate, like other styrene insulation, will not stand up over time to UV rays. If used on exteriors, polyiso panels should have a cover shielding it from

Outside the insulation/auditing industry, polyisocyanurate is used as a material for machined cuts and architectural models.

Previous post:

Next post: