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Based on consensus standards in both the United States and Canada, Long Term Thermal Resistance (LTTR) is a scientifically supported way to calculate the 15-year, time-weighted average R-value of roof insulation. Most Polyiso manufacturers have adopted the LTTR method as the exclusive means to measure thermal performance of permeable-faced Polyiso roof insulation.

The ASTM C1289 standard is used to predict a foam’s R-value equivalent to the average performance of a permeably faced foam insulation product over 15 years. The LTTR method provides users, specifiers and designers several advantages including:

  • A technically supported, more descriptive measure of the long-term thermal resistance of polyiso insulation -a 15-year time - weighted average;
  • An advanced test method, based on consensus standards in the United States and Canada;
  • A method that applies to all foam insulation with blowing agents other than air, such as Polyiso, extruded polystyrene and polyurethane; and
  • A better understanding of thermal performance of foam. 

QualityMarkcm Certification Program

The PIMA QualityMarkcm certification program is now reporting LTTR values in accordance with ASTM C1289-11 2014. To participate in the certification program, a Class 1 roof is suggested to have a design R-value of 5.7 per inch. PIMA recommends that you check with the manufacturer for their current published LTTR values for polyiso roofing insulation.

The PIMA QualityMarkcm certification program is a voluntary program that allows participating Polyiso manufacturers to certify the Long Term Thermal Resistance (LTTR) values of their Polyiso roof insulation products through an independent, third-party. This program was designed to be a benchmark for all roof insulations and gives consumers an easy way to identify quality products that meet or exceed listed LTTR values.

Polyiso is the only insulation to be certified by this unique program for its LTTR value.

For over ten years, LTTR values for roof insulation under the QualityMarkcm certification program have been reported and certified in accordance with the testing requirements of ASTM C1289 "Standard Specification for Faced Rigid Cellular Polyisocyanurate Thermal Insulation Board.” A recent update to the standard, ASTM C1289-11, featured improvements to the testing method for the prediction of LTTR values for a variety of polyiso roofing boards that are included in all subsequent versions of ASTM C1289, including the most current.

Polyiso is unique in that the R-value increases with the thickness of the foam, so three inches of polyiso has a higher R-value per inch than two inches.


PIMA QualityMark LTTR Value Requirements


LTTR Value/Inch

LTTR Value












The PIMA QualityMarkcm certification program was developed by PIMA and is administered by FM Global.

The following is a list of polyiso manufacturers participating in the PIMA QualityMarkcm certification program:

Atlas Roofing Corporation

Firestone Building Products


Carlisle Construction Materials

Johns Manville


ASTM C1289

A key product standard for polyisocyanurate ("polyiso") board insulation, ASTM C1289, has been updated by ASTM International – the recognized leader in the development and delivery of international voluntary consensus standards. This recent update of ASTM C1289 Standard Specification for Faced Rigid Cellular Polyisocyanurate Thermal Insulation Board (ASTM C1289-11) features important improvements regarding the prediction of long-term thermal resistance ("R”) value for a variety of polyiso insulation boards primarily used in roofing applications.

In order to provide a comprehensive approach to predicting long-term R-value throughout North America, the updated ASTM C1289 standard now incorporates two test methods, ASTM C1303 and CAN/ULC-S770-09, which offer a similar approach to predicting the long-term thermal performance for foam insulation materials that exhibit air and blowing agent diffusion or aging over time. Both test methods employ a technique called "slicing and scaling” to accelerate this aging process and provide an accurate and consistent prediction of product R-value after 5 years, which is equivalent to a time-weighted thermal design R-value for 15 years. The update to ASTM C1289 in no way impacts polyiso’s physical properties.

Accredited Testing Labs

The following accredited testing labs have been approved for use by participating polyiso insulation manufacturers in the QualityMarkcm program

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