What does polyiso board do?

It provides high-R rigid thermal insulation in a thin panel, reducing heat transfer through roofs and walls while the foil facers add a radiant and vapor-control layer.

Where is polyiso board usually found?

It is most common on flat or low-slope roofs directly above the roof deck, and as continuous exterior insulation on energy-code-compliant wall assemblies.

How does polyiso compare to XPS or EPS foam?

Polyiso typically delivers about R-6 per inch versus R-5 for XPS and R-3.8 for EPS, but its R-value drops at very cold temperatures, so designers sometimes use XPS in extreme cold climates.

Yes, and it loses R-value when saturated. Taped seams and proper slope or drainage are critical. Wet polyiso on a roof must be identified by a moisture scan and replaced before new roofing is installed.

Can I replace polyiso board myself?

Cutting and fitting polyiso boards is straightforward with a utility knife and straightedge. Roof applications usually require a licensed roofer for code compliance and manufacturer warranty purposes.

Polyiso Board — R-Value, Installation, and Replacement

A polyiso board is a rigid foam insulation panel made from polyisocyanurate foam laminated between two foil or glass-fiber facers, offering one of the highest R-values per inch of any board insulation.

Polyiso Board diagram — labeled parts and installation context

What It Is

Polyiso achieves roughly R-6 per inch at a mean temperature of 75 degrees Fahrenheit, making it the densest performer among common rigid foam types. Foil facers on both sides act as radiant barriers and vapor retarders and allow the boards to be taped at seams with foil tape for a continuous air barrier. The closed-cell foam structure traps a blowing agent that provides its high initial thermal resistance.

One important performance characteristic is that polyiso R-value decreases at cold temperatures. At a mean temperature of 25 degrees Fahrenheit, the effective R-value can drop to around R-4.5 to R-5 per inch. This temperature derating is why building scientists in cold climates sometimes specify XPS or EPS for below-grade applications and reserve polyiso for above-grade walls and roof assemblies where average service temperatures stay higher.

Types

Roofing polyiso is the most widely used form and is installed in tapered or flat layers above roof decks on commercial and residential buildings. Tapered systems create slope for drainage on flat roofs, eliminating the need for separate cricket framing. Wall-sheathing polyiso comes in thinner panels, typically 1 to 2 inches, for continuous exterior insulation over structural sheathing. Interior polyiso is used under roofing in cathedralized ceiling assemblies where it controls condensation on the underside of the roof deck.

Most panels are 4-by-8 feet and range from 1/2 inch to 4 inches thick. Glass-fiber-faced polyiso is used where a more porous facer is needed for adhesion to roofing membranes, while foil-faced versions are standard for wall sheathing and interior applications. Some manufacturers produce composite panels with polyiso bonded to OSB or gypsum for one-step sheathing and insulation.

Where It Is Used

Polyiso is used on low-slope commercial roofs, above roof decks on residential projects, as continuous exterior wall insulation over sheathing, and as insulation in structural insulated panel cores. It is also used as thermal breaks behind Z-furring on commercial metal stud walls and as insulation in walk-in cooler and freezer panel construction.

In energy code compliance, polyiso is frequently specified because its high R-value per inch allows thinner wall assemblies to meet IECC requirements. A single layer of 1-1/2-inch foil-faced polyiso provides approximately R-9 of continuous insulation, which satisfies many climate zone requirements for above-grade walls.

How to Identify One

Look for a bright silver or tan foil face on a pale-yellow to orange foam core. It is stiffer and heavier than EPS or XPS foam of the same thickness, weighing approximately 1.5 to 2 pounds per cubic foot. The foil face often carries a recycled-content or energy-code compliance label, and the board edges are typically square-cut or ship-lapped for overlapping joints.

On a roof, polyiso boards are visible during re-roofing as the layer between the roof deck and the membrane. In wall assemblies, they appear as the shiny layer outboard of the structural sheathing before siding is installed.

Replacement

Replace polyiso when it shows water absorption, delaminated facers, crushed corners from foot traffic, or thermal performance loss confirmed by a thermographic inspection. Wet polyiso loses R-value permanently because water displaces the gas trapped in the closed cells, and saturated panels must be removed and replaced before re-roofing.

During re-roofing projects, a moisture survey using infrared thermography or nuclear moisture meters identifies wet panels for targeted replacement. Salvageable dry panels can remain in place with new polyiso layered on top to meet current energy code requirements.