Long-Term Thermal Resistance (LTTR)

Thermal Resistance of Insulation.

Insulation is of paramount importance to reduce the energy consumption of buildings. In the summer, insulation reduces the heat flow from the hot exterior to the cool interior environment. In winter, insulation reduces the heat lost from the warm interior to the cold exterior. The resistance to heat flow through an insulation is called “thermal resistance”. Thermal resistance is commonly referred to as R-value. A complete understanding of the R-value of insulation over its lifetime is critical to designing buildings that achieve reduced energy consumption.

Most people understand that the higher the R-value, the greater the insulating power of insulation. However, many people do not understand that the R-value of Polyiso and XPS is lost over the lifetime of the product

Polyiso and XPS Lose R-value Over Their Lifetime.

It is well understood that Polyiso and XPS insulations trap gas in their cells other than air and Polyiso and XPS will lose the gas over time. This is a natural process of materials coming to equilibrium with the environment. If this encapsulated gas assists with providing R-value then the R-value of the insulation will drop over time.

The trapped gases in the cells of Polyiso and XPS foam assist to provide an initial high R-value. During the life of these foams, air from the atmosphere diffuses in and the trapped gases diffuse out. The result is Polyiso and XPS lose R-value over their lifetime.


The LTTR value commonly published from testing to ASTM C1303 or CAN/ULC-S770 is an estimate for the R-value of the insulation after 5 years. Many insulation manufacturers are promoting LTTR without providing a clear understanding that LTTR is an estimate for the R-value of the material after only 5 years.

The concept of a 5 year R-value being equal to the “time-weighted 15 year average” is also often used by Polyiso and XPS manufacturers. This approach assumes that the higher R-value established in years 1-4 is weighted by the inevitably lower R-value of the insulation in years 6-15. Neither the 5 year R-value, nor the time-weighted 15 year average approach is appropriate for use in building design. This is due to the fact that the R-values of Polyiso and XPS continue to decline below the LTTR published 5 year numbers. Starting in year 5 and for the remaining life of the insulation, the R-values of Polyiso and XPS are below LTTR published R-values.


Most insulation users are interested in a true long-term thermal R-value for their insulations. A time period appropriate for building application is 50 years. In order to avoid confusion with the existing LTTR numbers commonly published, we recommend the use of the term “LTTR50” be used for insulation specifications. LTTR50 designates the actual R-value after 50 years.

LTTR50 is a more suitable long-term R-value for use in building design. The LTTR50 value can easily be determined using the existing protocol described in ASTM C1303 or CAN/ULC-S770.