Polyiso Insulation – Rigid Boards

Polyiso insulation is short for polyisocyanurate and is also known as PIR and ISO. It’s a closed-cell foam rigid insulation. The closed cells eliminate moisture absorption, making this insulation a top choice for exterior and interior applications.

Polyiso(PIR) Insulation

How Polyiso is Made

Polyiso manufacturing begins with a polymer, which is turned into a liquid. Manufacturers inject blowing agents such as pentane into the mix. A chemical reaction between the polymer and pentane causes the pentane to expand–forming large gaseous cells.

Manufacturers run the mixture through a laminator, where the chemical reaction hardens the foam. Then they apply facing to both sides. The most popular facings are kraft paper and aluminum foil – kraft paper for roofing applications and aluminum foil for most other applications.

95% of the resulting product is air. Air–which is a gas–is one of the better insulators. Most insulation achieves its R-value from trapped air, not from the product itself. This is true of all rigid foam board insulation and batt insulation like fiberglass.

Polyiso Insulation R-Values

Polyisocyanurate has an R-value of R-6.2 per inch, which remains consistent regardless of thickness. The advantage of thicker insulation is that it takes longer for heat to penetrate more material.

Rigid foam insulation boards also eliminate thermal bridging at all stud locations when on wall exteriors. For example, a fiberglass insulated wall may be rated at R-20, but each 2 x 6 stud is only R-6.88. A blanket of one-inch polyiso will almost double each stud’s insulation value.

PIR also covers gaps, cracks, and holes in house sheathing. If you tape all the joints, polyiso rigid boards will also eliminate convective heat loss.

Aluminum foil-wrapped polyiso provides reflective insulation in addition to the R-value. Reflective insulation is beneficial in hot climates where radiant heat from the sun adds heat to the house. Reflective insulation is not as effective in colder climates because it can prevent heat gain.

Thickness (Inches)R-value (in I-P/Imperial)RSI (R-value in Metric)

Where to Use Polyiso Insulation

Contractors use Polyiso on roofs, walls, inside and outside. It provides quality insulation and R-value without using a lot of space.

Polyiso Use On Roofs

Poorly insulated roofs can allow up to 50% of a building’s heat to escape. PIR panels keep more heat inside the building, with up to 75% of commercial flat roofs using polyiso panels.

  • Compatible with most roofing systems.
  • Excellent fire performance. Approved for direct-to-steel attachment.
  • Low global warming blowing agent (pentane)
  • Long-term thermal performance certified by a third party. Polyiso panels can shrink, leaving gaps between them. The certification restricts shrinkage to less than 1%. Multiple-layer installation with non-aligned joints reduces the problem.
  • Meets energy code requirements without excess thickness and weight.

Polyiso Use On Walls

Contractors often use Polyiso to complete the energy blanket on new houses. Some building codes require higher R-values than a 2 x 6 fiberglass insulated wall provides. Adding PIR panels to the exterior increases the overall R-value of the entire wall by adding insulation and eliminating thermal bridging.

Polyiso Below Grade

Contractors can glue PIR rigid panels to the exterior of basement walls to add insulation value and reduce moisture accumulation in the basement. Adding a waterproof membrane or waterproof spray keeps the basement even dryer. 

Polyiso rigid panels provide excellent insulation for the interior of basement walls. Applying 2” foam to the wall will save space, add R-12.4, and act as a vapor barrier. You must fill all joints, gaps, and holes with spray foam insulation or caulking. Acoustic caulking works best, but it never dries, so you must cover it.

Most adhesives will work well on foil or kraft paper-backed ISO panels. Avoid adhesives with solvent additives – the solvent will melt any foam it comes in contact with.

Polyiso is also very effective under concrete slabs–either basement floors or slabs on grade. A 6” concrete floor has an R-value of R-0.48. Four inches of polyiso provides over R-24 insulation, keeping the heat in.

Insulating under slabs is even more important for heated concrete since heat can pass into the earth below an uninsulated concrete floor.

Polyiso Above Grade

Existing 2 x 4 walls are only R-12, and very old walls may have no insulation. Adding polyiso to older houses during exterior renovations is a popular way of increasing the R-value. Even adding only one inch brings an R-12 wall to over R-18, which is close to a 2 x 6 wall.

Adding 2” of foam beneath vinyl siding does not require strapping. Anything heavier–such as cement board siding–needs strapping to prevent sagging or worse. You should attach strapping directly to existing studs. 

Polyiso HVAC Ducting

Manufacturers use polyiso panels covered with pure embossed aluminum for pre-insulated ducting. They are a one-step installation eliminating the need to add insulation to metal ducts.