Polyisocyanurate (polyiso) exterior wall insulation is a high-performing closed-cell, rigid foam continuous insulation that is bonded to various organic and inorganic facers on both sides. Polyiso accounts for over 75 percent of the insulation used in new and retrofit residential and commercial construction in the U.S. 

What is Polyiso Exterior Wall Insulation?

Manufacturers mix methylene diphenyl diisocyanate (MDI), polyol, blowing agent, and flame retardant with small amounts of catalysts and surfactants to produce polyiso insulation. The mixture creates a heat-generating chemical reaction that causes the liquid-blowing agent to boil, which expands the foam and creates gas-filled cells with a high thermal-resistance value.

Depending on the application and thermal resistance (R-value) requirements, you can buy polyiso insulation in various board sizes and thicknesses.

Builders generally use 4’ by 4’ boards to attach or serve as a substrate for adhered membrane applications. 4’ by 8’ boards are used for loosely laid and mechanically attached membrane systems.


The R-value of polyiso insulation correlates to its thickness. Therefore, manufacturers offer tapered polyisocyanurate and polyiso thermal insulation in thicknesses ranging from 1” to about 4” in 0.1-inch increments to accommodate a project's specific thermal resistant needs. In addition, they provide high-density polyisocyanurate cover boards in ¼”, ⅜” and ½” thicknesses.

Why Buildings Need Exterior Wall Insulation

Polyiso exterior wall insulation enhances a building’s performance and contributes towards code compliance.  

Polyiso exterior insulation provides thermal benefits that significantly increase a structure's energy efficiency. 

In addition, in cooler conditions, building science tells us that wall systems with sufficient cavity insulation of R-13 (but no exterior insulation) will experience condensation as warm/moist air from the interior moves toward the cold sheathing, which can lead to structurally damaging and unhealthy mold. Adding exterior insulation will warm up the sheathing and minimize the potential for condensation to protect the wall system's durability. 

Building Code Exterior Wall Insulation Requirements

Building professionals should refer to their local and state jurisdictions for their region's energy building codes; however, most jurisdictions use the codes set by the residential and commercial International Energy Conservation Codes (IECC). 

The IECC establishes codes and standards (I-Codes) for the minimum design and construction requirements for energy efficiency (e.g., insulation requirements) for new and renovated buildings, utilizing the ASHRAE 90.1 as their reference standard. 

The ASHRAE 90.1 standard furnishes the minimum requirements for energy-efficient structures, except for low-rise residential buildings, and specifies the amount of insulation required to prevent thermal bridging and establish an effective R-value in a wall system. The amount of cavity and exterior insulation depends on the climate zone, the wall assembly type, and the structure type (non-residential or residential).

  • Framed above-grade exterior walls typically require R-values ranging from 13 to 20 cavity insulation. In addition, those in climate zones 4 and above also require exterior insulation. 

  • Below-grade walls in warm and humid climate zones 0, 1, and 2 do not require basement insulation. Climate zone 3 requires either R-5ci on the wall's exterior or interior or R-13ci on the interior side.

2021 IECC Climate Zones

The climate zone map divides the U.S. into eight temperature zones. Further divisions in the map separate the regions by moisture levels labeled A, B, and C, to create 24 climate designations within the IECC map. In addition, the IECC created five climate categories: Subarctic, Cold/Very Cold, Hot-Humid, Hot-Dry/Mixed Dry, Mixed-Humid, and Marine 


The parameters of climate regions include precipitation, heating degree days, and average temperatures.