I've recently been involved in some interesting conversations about polyisocyanurate foam and its insulating performance:
Over the last several decades, reported R-values for polyisocyanurate foam insulation have been declining as the industry has adopted testing and reporting methods that more realistically reflect this material's tendency to decrease in insulating value over time. At one time, R-values as high as R-7.2 were reported for this material. At this time, the Polyisocynuarate Insulation Manufcturers Assocation (PIMA) recommends a Long-Term Thermal Resistance (LTTR) value of R-5.7 for one inch of foam, with the value per inch increasing slightly with greater thicknesses:
|Insulation Thickness||LTTR R-Value per Inch||Total R-Value|
LTTR values are intended to reflect the expected insulation value of the material at 5 years of age, and also its average performance over 15 years (higher in earlier years, lower in later years).
Polyiso insulation also loses insulation value as it gets colder, in some cases by as much as roughly 25 percent. Though it may seem counter-intuitive, most building insulation materials are tested at a mean temperature of 75 degrees F, that is, very close room temperature! (Code of Federal Regulations, Labeling and Advertising of Home Insulation, R-Value Tests.) In the case of polyiso foam insulation, this can lead to overstating its performance at colder temperatures. Tests of polyiso foam samples performed at a mean temperature of 25 degrees F have produced measured R-values ranging from R-3.9 to R5.6 per inch (see Figure 2 in the linked document).
The National Roofing Contractors Association (NRCA), has sponsered tests pf polyiso insulation samples extracted from roofs of various ages and adopted the following recommendations for this material: R-5.0 per inch for heating applications and R-5.6 per inch for cooling, regardless of thickness. These values are intended to account for both aging beyond 15 years and temperature dependent factors.
Polyiso insulation loses insulation value over time as the gasses, or blowing agents, used in its manufacturer gradually diffuse out of the material and are replaced by ordinary atmospheric gasses. The reason that thicker samples retain a higher value longer is because this exchange is slower when it must progress through more material. Impermeable facers, such as aluminum foil, applied to the foam may also slow the diffusion process and loss of r-value to some extent over time, though how much is uncertain.
Extruded polystyrene (XPS) insulation also experiences the same exchange of gasses and loss of insulating value over time, though to a lesser extent than polyiso. The standard value of R-5 per inch reported for this material reflects the results of aged testing. Oddly, a different stanard for determing aged values of XPS foam insulation used in Canada, reports what most agree are slightly inflated values for the performance of this material.
Expanded polystyrene (EPS) insulation is a more gas-permeable material that rapidly exchanges any blowing agents used in its manufacturer with ordinary atmospheric gasses. For this reason its insulation value quickly stabilizes and no aged testing is required. Standard insulation values for this material are in the range of R-3.5 to R-4 per inch.
And one more thing regarding temperature dependency: Fiberglass batts, XPS foam, and EPS foam--unlike polyiso foam insulation--all gain insulating value at lower temperatures.