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Long-Term Thermal Resistance (LTTR)



LTTR is a scientifically supported, consensus-based method used to estimate the thermal resistance for insulation products with captive blowing agents at an age of 5 years (which corresponds closely to the average thermal resistance over a 15-year service life). Polyiso manufacturers have adopted the LTTR method as the means to measure the thermal resistance of polyiso roof insulation with permeable facers. For additional detail on LTTR values, read about PIMA’s QualityMark Program below.

The LTTR method provides designers, specifiers, and contractors several advantages:

  • It provides a more representative measure of long-term thermal resistance for polyiso insulation and other foam insulation products.
  • It applies to all foam insulation with captive blowing agents other than air, such as polyiso, extruded polystyrene (XPS) and closed-cell spray polyurethane foam (SPF).
  • It is an advanced test method, based on consensus standards in the United States and Canada.
  • It has been the subject of a rigorous study by Oak Ridge National Laboratory

For the United States market, the ASTM C1289 product standard recognizes two LTTR test methodologies – ASTM C1303 (Standard Test Method for Predicting Long-Term Thermal Resistance of Closed-Cell Foam Insulation) and CAN/ULC-S770 (Standard Test Method for Determination of Long-Term Thermal Resistance of Closed-Cell Thermal Insulating Foams). Both test methods employ a technique called "slicing and scaling” to accelerate the aging process and provide a relevant and consistent prediction of a product’s thermal resistance after 5 years. This 5-year value corresponds closely to the average thermal resistance over a 15-year service life. For the Canadian market, the CAN/ULC-S704.1 product standard requires CAN/ULC-S770 exclusively.

QualityMarkcm Certification Program

The QualityMark certification program is a voluntary program that allows participating manufacturers to certify LTTR values for roof insulation products (ASTM C1289, Type II, Class I, Grade 2). The program is administered by FM Approvals and requires all products to be sampled and tested by independent and accredited third-party entities and laboratories. Polyiso is the only insulation product to publish certified LTTR values.

Polyiso manufacturers that participate in the QualityMark program include:

PIMA recommends that you ask the polyiso manufacturer for information on published QualityMark LTTR values.

Accredited laboratories that have been approved for QualityMark LTTR testing include:

Product Standards

The requirements for measuring and reporting thermal resistance as well as minimum R-values for polyiso products are described in two relevant product standards. For the United States market, the applicable standard is ASTM C1289 (Standard Specification for Faced Rigid Cellular Polyisocyanurate Thermal Insulation Board). For the Canadian market, the applicable standard is CAN/ULC-S704.1 (Standard for Thermal Insulation, Polyurethane and Polyisocyanurate, Boards, Faced).

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