Polyisocyanurate (polyiso) insulation has long been one of the most widely specified rigid foam insulation products in North American construction. For years, builders and designers relied on a published R-value of 6.0 per inch when designing roof and wall assemblies. Recent updates to testing standards and certification programs have revised that number, and understanding these changes is essential for anyone involved in specifying or installing polyiso insulation. When paired with other cost-effective strategies for energy-efficient homes, accurate R-value data helps builders deliver better performing buildings without unnecessary material expense.
The shift did not come from a change in the material itself. Advances in how long-term thermal resistance is measured revealed that the actual aged R-value of polyiso differs from previously accepted figures. The Polyisocyanurate Insulation Manufacturers Association (PIMA) responded by updating its QualityMark certified R-value program, and standards organizations adopted new test protocols. This article explains what changed, why it matters, and how builders can apply the updated values in their projects.
How Polyiso R-Values Are Tested and Why They Changed
The R-value of any insulation material represents its resistance to heat flow. Higher values indicate better insulating performance. For foam insulation products like polyiso, the R-value changes over time as the blowing agent used in manufacturing gradually escapes from the foam cells and air replaces it. This process, known as thermal drift, means the aged R-value is lower than the initial value measured immediately after production.
The Role of Long-Term Thermal Resistance Testing
Standard ASTM C1289-13, published in 2013, introduced an improved methodology for predicting long-term thermal resistance (LTTR) of polyiso insulation boards. Working in conjunction with Underwriters Laboratories of Canada standard CAN/ULC S770-09, the new testing protocol provides a more accurate picture of how polyiso performs over its service life. Under these standards, the LTTR of 25 mm (1 inch) of polyiso was determined to be 5.6, compared to the previously accepted value of 6.0.
This seven percent difference does not indicate any reduction in product quality. Rather, it reflects better science. Earlier test methods did not account for long-term aging effects as precisely as the LTTR approach does. Builders who have been satisfied with polyiso performance in the field can continue to specify it with confidence, using the updated numbers for code compliance and energy modeling.
PIMA QualityMark Program Updates
PIMA has operated its QualityMark certified R-value program for rigid polyiso roof insulation since 2004. The program provides third-party verification that polyiso products meet published thermal performance values. In June 2013, PIMA announced it would update the program to reflect the new LTTR data, with implementation scheduled for January 1, 2014. This transition ensured that all participating manufacturers report thermal values consistently under the updated standard.
The QualityMark program remains a valuable tool for builders. Products bearing the QualityMark label have been independently tested and certified to deliver the stated R-value. When specifying polyiso for any project, looking for this certification provides assurance that the product meets the standards it claims.
Comparing Polyiso to Other Rigid Insulation Materials
Polyiso competes with several other rigid foam insulation products in the building market. Each material has distinct thermal properties, installation characteristics, and cost profiles. Understanding where polyiso fits helps builders make informed specification decisions.
| Insulation Type | R-Value per Inch | Typical Applications | Moisture Resistance |
|---|---|---|---|
| Polyisocyanurate (Polyiso) | 5.6 (updated LTTR) | Roof assemblies, wall sheathing | Good with proper facer |
| Expanded Polystyrene (EPS) | 3.6 to 4.2 | Below-grade, wall systems | Excellent |
| Extruded Polystyrene (XPS) | 5.0 | Below-grade, foundation walls | Excellent |
| Stone Wool (Mineral Fiber) | 4.0 to 4.3 | Fire-rated assemblies, sound control | Very good (non-combustible) |
| Fiberglass Batt | 2.9 to 3.8 | Wall cavities, attic floors | Fair (absorbs moisture) |
Polyiso offers the highest R-value per inch among common rigid foam insulation products, even after the updated LTTR adjustment. This makes it a strong choice for applications where maximizing thermal performance within limited cavity depth is important, such as low-slope roof assemblies and continuous exterior wall insulation. For projects requiring a complete suite of high-performance insulation and housewrap products, polyiso combines well with other materials to achieve a continuous thermal barrier.
R-Value and Thickness: A Unique Relationship
One property that distinguishes polyiso from other foam insulations is that its R-value per inch increases with board thickness. A 76 mm (3 inch) panel delivers a higher R-value per inch than a 50 mm (2 inch) panel. This non-linear relationship occurs because the aging effect is proportionally less significant in thicker boards, where a smaller fraction of the total thickness is affected by thermal drift near the surfaces.
For builders, this means that thicker polyiso installations provide better value in terms of thermal performance per millimeter of material. When designing assemblies that require higher total R-values, specifying thicker polyiso boards rather than multiple layers of thinner material can improve both performance and installed cost.
Practical Implications for Building Code Compliance
The updated R-value for polyiso has implications for code compliance in both commercial and residential construction. Energy codes specify minimum insulation levels based on published R-values. When those values change, the total installed thickness needed to meet code requirements may also change.
Adjusting Assembly Designs
Consider a roof assembly designed to achieve R-30 using polyiso. Under the previous R-6.0 per inch assumption, five inches of material would meet the target. With the updated R-5.6 per inch value, reaching R-30 requires approximately 5.4 inches. Depending on available board thicknesses, this may mean specifying 6-inch boards or adding a supplementary insulation layer.
The practical impact varies by climate zone and the specific requirements of the applicable energy code. Builders should verify that their standard assemblies still comply with local code requirements when using updated polyiso R-values. In most cases, the adjustment is modest and does not require major redesign, but ignoring the change could result in underperforming assemblies.
Prescriptive vs. Performance Compliance Paths
The updated R-values affect both prescriptive and performance-based compliance approaches:
- Prescriptive path: Minimum insulation R-values specified by code tables may require additional thickness when using polyiso. Check local code amendments, as some jurisdictions may have already incorporated the updated LTTR values into their prescriptive tables.
- Performance path: Energy modeling software that accepts insulation R-values as inputs will produce different results when the polyiso value is adjusted from 6.0 to 5.6. Modeled annual energy consumption may be slightly higher, potentially affecting compliance margin.
- Trade-off compliance: Projects that offset lower insulation performance with higher-efficiency mechanical systems should be recalculated using the updated value to ensure the trade-off still satisfies code requirements.
For builders focused on comprehensive thermal comfort in residential construction, the accuracy of R-value inputs directly affects how well modeled performance matches real-world occupant satisfaction. Using the correct values from the start prevents field corrections later.
Specifying Polyiso Insulation With Confidence
The updated R-value for polyiso does not change the fact that it remains one of the most effective and widely used insulation products in North America. Builders can continue to specify it with confidence by following a few best practices.
Verifying Certified Products
When selecting polyiso insulation, check for the PIMA QualityMark certification label. This third-party verification program confirms that the product has been tested using the updated LTTR methodology and delivers the stated thermal performance. Products from all major polyiso manufacturers participate in the program, and the certification covers both roof and wall insulation products.
Reviewing Manufacturer Data Sheets
Manufacturers have updated their published literature to reflect the new R-values. The key figures to confirm on any data sheet include:
- The stated R-value per inch at standard conditions (75 degrees F mean temperature)
- The LTTR test method used (ASTM C1289-13 or equivalent)
- The QualityMark certification number for the product line
- The facer type and its impact on thermal performance
- The recommended design R-value for specific assembly types, such as Class 1 roof systems, which PIMA suggests at 5.7 per inch
Requesting updated data sheets from suppliers before specifying polyiso on a new project helps ensure that all parties are working from the same thermal performance assumptions. This is especially important when the project involves a high-performance building envelope design where every component contributes to the overall thermal barrier.
Coordinating With Code Officials
Building departments may not yet be fully aware of the polyiso R-value update, particularly in jurisdictions that adopt newer versions of energy codes slowly. When submitting plans, include manufacturer literature showing the tested R-value and the applicable ASTM standard. This proactive approach prevents plan review delays and demonstrates that the design uses current data.
Comparing Insulation Options for Each Application
Polyiso is not the right choice for every application. It performs well in above-grade assemblies but can absorb moisture if exposed to prolonged wetting. For below-grade applications or areas with high groundwater, EPS or XPS may be more appropriate. Evaluating the full range of available products, including stone wool insulation strategies for long-term performance, ensures that each assembly uses the material best suited to its conditions.
The Bottom Line on Polyiso R-Values
The revision of polyiso R-values from 6.0 to 5.6 per inch represents progress in measurement science, not a decline in product quality. Better testing methods produce more accurate data, and the industry has responded by updating its standards and certification programs in a coordinated manner. Builders who account for this change in their specifications, energy models, and code compliance documentation will avoid the surprises that come from relying on outdated numbers.
The key takeaways for builders are straightforward:
- Use the updated R-value of 5.6 per inch for polyiso insulation in all new designs and energy models
- Verify that products carry the PIMA QualityMark certification for tested performance
- Account for the non-linear relationship between polyiso thickness and R-value when designing thick assemblies
- Check local energy code requirements to ensure prescriptive assemblies still comply at the updated value
- Request current manufacturer data sheets that reference the LTTR test method
The polyiso insulation market has demonstrated its commitment to transparency and accuracy through these updates. By staying current with the new values and understanding how they affect design decisions, builders can continue to deliver the energy-efficient, code-compliant buildings their clients expect.