The ability of insulation products to resist the flow of heat changes with temperature. Most insulation products — including fiberglass batts, extruded polystyrene (XPS), and expanded polystyrene (EPS) — perform better at low temperatures than high temperatures. At lower temperatures, there is less conduction, less convection, and less radiation — so insulation materials usually work better than they do at warmer temperatures.
However, one type of rigid foam, polyisocyanurate, doesn’t follow this pattern. At temperatures below 50°F, polyiso performs worse than it does at a mean temperature of 75°F.
Although many building scientists have known about this phenomenon for years, published data from recent testing by Chris Schumacher and John Straube — two researchers at Building Science Laboratories in Waterloo, Ontario — have sparked new questions about the performance of polyiso at cold temperatures. (See links to previous GBA reports on the topic in the “Related Articles” sidebar, below.)
There are two ways that polyiso can disappoint
Some people confuse the problem of polyiso’s poor cold-weather performance with a different problem — that of “thermal drift.” The phrase “thermal drift” refers to the gradual dissipation of gaseous blowing agents which are replaced by air as they exit the foam. This process takes several years.
Gaseous blowing agents are chosen for their thermal properties, so the escape of these gases causes the R-value of polyiso to decline. Polyiso manufacturers have faced decades of criticism from those who assert that thermal drift makes the R-value labels on polyiso unrealistic. Responding to these critics, polyiso manufacturers agreed in 2002 to adopt a new method of R-value testing, the “long-term thermal resistance” (LTTR) method. This testing method strives to come up with a more realistic R-value for polyiso — one that takes thermal drift into account. (For more on this issue, see Thermal Drift of Polyiso and XPS.)