If you have shopped for rigid foam board and noticed one type with foil facing that costs more and claims the highest R-value of the bunch, you were looking at polyisocyanurate insulation. Usually shortened to polyiso, it is the closed-cell foam board that dominates commercial flat roofs and increasingly shows up on the exterior of homes as continuous insulation. It offers the best R-value per inch of the common rigid foams and excellent fire performance, but it has a well-documented quirk in cold weather that you need to understand before you specify it.
What Polyiso Is and Where It Is Used
Polyiso is a rigid, closed-cell foam insulation board, typically manufactured with a foil or fiberglass facer on both sides. The foil facing serves as a radiant barrier and, when the seams are taped, as an air and vapor control layer. You will find it in several roles: as the standard insulation layer on commercial low-slope and flat roofs, as continuous exterior insulation applied over wall sheathing, on the interior of basement and foundation walls, and as sheathing on some residential wall assemblies.
Boards commonly come in 4-by-8-foot sheets in thicknesses from 1/2 inch up to 4 inches or more, so you can dial in the R-value you need for a given assembly and climate.
The R-Value Advantage
The headline number is R-value per inch, and polyiso leads the rigid-foam field. At standard testing conditions it delivers roughly R-6 to R-6.5 per inch, compared with about R-5 per inch for extruded polystyrene (XPS) and R-3.6 to R-4.2 per inch for expanded polystyrene (EPS). That higher density of insulation means you can hit a target R-value in a thinner board, which matters when wall or roof depth is limited.
For a practical sense of scale: 2 inches of polyiso yields about R-13, and 3 inches gets you to roughly R-19 to R-20, comparable to a filled 2×6 wall cavity but as continuous insulation with no thermal bridging through studs. That continuous coverage is a big part of why builders wrap exteriors in polyiso.
The Cold-Weather Catch You Must Know
Here is the crucial caveat that separates informed buyers from disappointed ones. Polyiso’s R-value is not constant across temperatures. Unlike XPS and EPS, which actually perform slightly better as it gets colder, polyiso loses R-value at low temperatures, a phenomenon called thermal drift or cold-weather derating. As the mean temperature of the board drops toward and below freezing, its effective R-value can fall from the rated R-6 per inch down toward R-5 or even lower in very cold conditions.
This matters most in cold climates and in roof applications, where the outer layers of insulation sit at outdoor temperatures in winter, exactly when you need the insulation most. Some cold-climate designers account for this by using a “hybrid” approach, pairing polyiso with a layer of XPS or EPS on the cold side, or by simply derating polyiso to around R-5 per inch in their calculations for cold-climate roofs. If you live where winters are severe, do not assume the labeled R-value at your coldest design temperature.
Pros of Polyiso
- Highest R-value per inch among common rigid foams, saving thickness.
- Strong fire performance: polyiso chars rather than melting and generally meets fire codes with less concern than polystyrene foams, often without a thermal barrier in some assemblies (always verify local code).
- Built-in radiant barrier and air barrier from the foil facing when seams are taped.
- Made with a blowing agent that has low global warming potential compared with older XPS formulations, giving it a favorable environmental profile.
- Rigid and stable, providing a solid continuous-insulation layer over sheathing.
Cons of Polyiso
- Cold-weather R-value loss, the thermal drift described above, is its biggest drawback.
- Moisture sensitivity: the foam core and facers can absorb water and degrade if they get and stay wet, so polyiso is a poor choice for below-grade exterior use or anywhere in direct ground contact, where XPS is preferred.
- Higher cost than EPS, typically running about $0.70 to $1.20 per square foot for a 1-inch board, more than EPS and comparable to or above XPS depending on market.
- Not ideal for ground contact or persistently damp locations.
How Polyiso Compares to XPS and EPS
Choosing among the three rigid foams comes down to the application. Polyiso wins on R-value per inch and fire performance and is the default for above-grade roofs and exterior walls in moderate climates. XPS, with its consistent R-5 per inch and superior moisture resistance, is the go-to for below-grade foundation walls, under-slab insulation, and other damp locations. EPS is the budget option with the lowest R-value per inch but good moisture drying ability and the lowest cost, often used under slabs and in exterior insulation and finish systems.
A simple way to remember it: use polyiso up high and dry where you want maximum R in minimum thickness, use XPS down low and wet where moisture is the enemy, and use EPS where cost is the priority and the assembly can dry.
Installation Notes
Polyiso installs much like other rigid foam. Cut it with a utility knife and a straightedge, fasten it to sheathing with cap nails or screws with plastic washers, and tape all seams with a compatible foil or acrylic tape to create the air barrier. When using it as exterior continuous insulation, detail flashing and water management carefully, since the board sits outboard of your sheathing and any bulk water must be directed away. Keep it out of standing water during storage and installation. Used in the right assembly, and with its cold-weather behavior accounted for, polyisocyanurate insulation delivers more thermal performance per inch than any other common foam board on the market.
Continuous Insulation and Thermal Bridging
One of the strongest arguments for polyiso is its role as continuous exterior insulation, which addresses a hidden weakness in conventional wall construction. In a standard wood-framed wall, the studs themselves conduct heat around the cavity insulation, a phenomenon called thermal bridging. Wood framing has an R-value of only about R-1.2 per inch, so every stud, plate, and header is a thermal shortcut that can cut the real-world performance of a well-insulated wall by 15 to 25 percent. Wrapping the exterior in a continuous layer of polyiso covers those framing members with unbroken insulation, breaking the thermal bridge and raising the effective R-value of the whole assembly. Even a modest 1-inch layer of polyiso over the sheathing, adding roughly R-6, makes a meaningful difference and is increasingly required by energy codes in colder climates. This continuous-insulation strategy is where polyiso’s high R-per-inch really pays off, since you get substantial added performance without adding much wall thickness.
Moisture, Dew Point, and Getting the Details Right
Adding exterior foam does more than insulate; it changes where moisture condenses inside the wall, and getting that wrong causes rot. When you install polyiso on the outside of the sheathing, you keep the sheathing warmer, which moves the dew point outward and reduces the risk of condensation forming on the back of the sheathing in winter. For this to work safely, the foam layer must be thick enough for your climate zone so the sheathing stays above the dew point, and building codes and manufacturers publish minimum thicknesses for exactly this reason. Tape the polyiso seams to control air movement, integrate the foam layer with your water-resistive barrier and flashing so bulk water drains to the exterior, and avoid trapping moisture with a second vapor barrier on the interior in cold climates. Because the foil facers are vapor-impermeable, polyiso should generally not be sandwiched between two vapor barriers where the wall cannot dry in either direction. When these details are handled correctly, polyiso gives you high R-value, an air barrier, and improved moisture performance all in one product, which is why it has become a staple of high-performance wall and roof assemblies.
