Researchers have known for years that most types of insulation — including fiberglass batts, extruded polystyrene (XPS), and expanded polystyrene (EPS) — perform better at low temperatures than high temperatures. The phenomenon was described by Chris Schumacher, an engineer and researcher at Building Science Corporation, at a conference in 2011: “If you measure the R-value of an R-13 fiberglass batt, you’ll get different results at different outdoor temperatures. If the outdoor temperature rises, the R-value goes down. If the outdoor temperature drops, the R-value rises. Why? Because as you move to a higher temperature, you get more radiation happening, and therefore a lower R-value. But at lower temperatures, there is less conduction, less convection, and less radiation — and therefore a higher R-value.”
Polyisocyanurate does not follow the usual pattern for other types of insulation. When tested at mean temperatures below 50°F, polyiso performs worse than it does at a mean temperature of 75°F. The reason for this declining performance, according to Schumacher, is that “the trapped blowing-agent gases start to condense at cold temperatures.”
The standard ASTM test methods for determining a material’s R-value are performed at a mean temperature of 75°F. According to the Federal R-value Rule, the U.S. law that regulates how insulation products are labeled and marketed, R-value claims for insulation must be based on these ASTM tests. It could be argued that these test procedures tend to favor polyisocyanurate (which ends up with a labeled R-value of about R-6 per inch) over XPS (which ends up with a labeled R-value of R-5 per inch). Many builders probably specify polyiso because of its high R-value per inch, without considering the fact the the performance of polyiso suffers at low outdoor temperatures.
Achilles Karagiozis, the director of building science at Owens Corning, decided to use WUFI, a hygrothermal…