Use And Care Manual

However, as the temperature drops below 59°F, its thermal conductivity increases substantially which results in a significant
decrease in R-value. In contrast, the thermal conductivity of XPS decreases as the temperature drops, resulting in an R-value
that is ultimately greater than R5.0 per inch.
Figure 1. Thermal conductivity test results from Building Science Corporation₄
Q: Why does most polyiso perform so poorly when the temperature declines?
A: The increased thermal conductivity and inconsistent performance of most polyiso is primarily attributed to the amount and
type of blowing agent used in the product. The blowing-agent (insulating) gases that are entrapped within the cells of most
polyiso begin to condense at colder temperatures and, as those gases condense, the thermal conductivity of the blowing agent
and in turn, the polyiso increases.
The blowing agent (insulating) gases that are entrapped within the cells of the XPS remain as a gas over the tested temperature
range. As the temperature gets colder the thermal conductivity of the gaseous blowing agent decreases and in turn, the
thermal conductivity of the XPS decreases. Figure 2 uses averaged data to illustrate the effect of blowing agent boiling point
on a foam’s insulation value. This data demonstrates one of the advantages of low-boiling blowing agents: improved insulation
performance over a broader operating temperature range. And, the better the insulation performance, the lower the energy
consumption of products that rely on these high-value foam insulations.₅
Due to this degradation of most polyiso’s thermal performance in colder temperatures, builders and specifiers are paying a
premium for a product that does not deliver on the intended thermal performance.
In contrast, XPS is manufactured in a continuous extrusion process that produces a homogeneous closed cell crossed section.
Even when exposed to freeze-thaw cycling conditions in colder temperatures, XPS maintains its closed cell structure and is
virtually impervious to moisture, preventing loss of R-value due to moisture penetration.
Figure 2. Foam Insulation Value Versus Temperature for Liquid and Gaseous Blowing Agents₅