Polyisocyanurate (Polyiso) Insulation
Polyisocyanurate (polyiso) rigid foam insulation offers the highest R-value per inch among rigid board insulations at approximately R-6.0 per inch under standard test conditions. It is typically sold with aluminum foil facers on both sides, which provide a radiant barrier and serve as a vapor retarder. Polyiso is widely used for continuous exterior wall insulation and commercial roofing applications. However, its R-value decreases in cold temperatures, dropping to approximately R-5.0 per inch at 25 degrees Fahrenheit. This thermal drift makes polyiso less ideal for the outermost layer in very cold climates (Zones 6-8) unless derated appropriately. For walls in moderate climates, polyiso provides excellent thermal performance at a competitive cost per R-value.
Polyisocyanurate (Polyiso) Specifications
Best Applications for Polyisocyanurate (Polyiso)
Polyisocyanurate (Polyiso) is recommended for the following building components. Each application link shows the specific R-value requirements by climate zone and how Polyisocyanurate (Polyiso) compares against other insulation options for that component.
Thickness Required by Climate Zone
The table below shows the thickness of Polyisocyanurate (Polyiso) needed to achieve the DOE recommended R-value for each building component in every climate zone. Cells showing "N/R" indicate that insulation is not required for that component in that zone. Use this table to quickly determine how much Polyisocyanurate (Polyiso) you need for your specific zone and application.
| Zone | Attic | Wall Cavity | Wall Cont. | Floor | Basement | Crawlspace | Cathedral | Garage |
|---|---|---|---|---|---|---|---|---|
| Zone 1 | 5" | 2.2" | N/R | 2.2" | N/R | N/R | 3.7" | 3.2" |
| Zone 2 | 6.3" | 2.2" | N/R | 2.2" | N/R | N/R | 3.7" | 3.2" |
| Zone 3 | 6.3" | 2.2" | N/R | 3.2" | 0.8" | 0.8" | 5" | 3.2" |
| Zone 4 | 8.2" | 2.2" | 0.8" | 4.2" | 1.7" | 1.7" | 6.3" | 5" |
| Zone 5 | 8.2" | 2.2" | 0.8" | 4.2" | 1.7" | 1.7" | 6.3" | 5" |
| Zone 6 | 10" | 2.2" | 1.7" | 4.2" | 2.5" | 1.7" | 8.2" | 6.3" |
| Zone 7 | 10" | 2.2" | 1.7" | 4.2" | 2.5" | 1.7" | 8.2" | 6.3" |
| Zone 8 | 10" | 2.2" | 2.5" | 4.2" | 3.2" | 1.7" | 8.2" | 6.3" |
Cost Estimates by Climate Zone (per 1,000 sq ft)
Material cost estimates for Polyisocyanurate (Polyiso) at the DOE recommended R-value for each climate zone and component. These are material costs only and do not include labor. Professional installation typically adds 30-50% to the material cost shown below.
| Zone | Attic | Wall Cavity | Wall Cont. | Floor | Basement |
|---|---|---|---|---|---|
| Zone 1 | $6,600 | $2,860 | N/R | $2,860 | N/R |
| Zone 2 | $8,360 | $2,860 | N/R | $2,860 | N/R |
| Zone 3 | $8,360 | $2,860 | N/R | $4,180 | $1,100 |
| Zone 4 | $10,780 | $2,860 | $1,100 | $5,500 | $2,200 |
| Zone 5 | $10,780 | $2,860 | $1,100 | $5,500 | $2,200 |
| Zone 6 | $13,200 | $2,860 | $2,200 | $5,500 | $3,300 |
| Zone 7 | $13,200 | $2,860 | $2,200 | $5,500 | $3,300 |
| Zone 8 | $13,200 | $2,860 | $3,300 | $5,500 | $4,180 |
Advantages and Limitations
Every insulation type has trade-offs that affect its suitability for different applications. Understanding these trade-offs helps you select the right insulation for your specific project requirements, budget, and installation constraints.
Advantages of Polyisocyanurate (Polyiso)
- High R-value per inch reduces required thickness
- Suitable for DIY installation, reducing labor costs
- Well-suited for: Wall Continuous (Exterior), Cathedral Ceiling, Attic
Limitations of Polyisocyanurate (Polyiso)
- Combustible material requires thermal barrier coverage in occupied spaces
- R-value decreases in cold temperatures — derate to R-5/inch below 25 degrees F
Compare Other Insulation Types
Browse specifications and cost analysis for other insulation materials to find the best option for your project.