An insulation material’s resistance to heat flow is rated in terms of R-value – a higher R-value means greater insulating effectiveness (i.e., higher resistance to heat flow).

Note: Keep in mind that polyiso’s R-value increases with thickness. For example, a two-inch polyiso product has more than twice the insulating value than a one-inch product. The increase in thermal resistance for thicker products is due to the reduced contribution of more thermally conductive elements such as the facer and the denser cell structure in the foam near the surface.


The U.S. Federal Trade Commission’s R-value Rule (16 CFR Part 460: Trade Regulation Concerning the Labeling and Advertising of Home Insulation) requires manufacturers to test insulation products that contain a captive blowing agent on samples that fully reflect the effects of aging on the product’s R-value.

To comply with this requirement, the polyiso industry uses two primary metrics for determining thermal resistance:

  1. Long-term thermal resistance (or LTTR); and
  2. Thermal resistance following a conditioning period.

In both scenarios, the heat transfer through the insulation is determined at specific conditions. However, the specimen configuration, calculations, and the time at which the measurements are obtained differ.

For more detail on R-value metrics, click here.

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).