R-Value Metrics
R-Value Metrics

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, click here  to read about PIMA’s QualityMark Program.

The LTTR method provides designers, specifiers, and contractors several advantages:
  1. It provides a more representative measure of long-term thermal resistance for polyiso insulation and other foam insulation products.
  2. 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).
  3. It is an advanced test method, based on consensus standards in the United States and Canada.
  4. 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.

Thermal Resistance Following Conditioning

In the Unites States, polyiso manufacturers typically report R-values for wall insulation products measured on full thickness samples following a 180-day conditioning period at 73°F and 50% relative humidity (per ASTM C1289). This period can be reduced to 90 days by subjecting samples to a higher conditioning temperature (140°F and dry heat) prior to testing. In Canada, polyiso wall and roof products complying with the CAN/ULC-S704.1 product standard must test and report LTTR values.