Most insulation materials have an R-value lower than R-5.6 per inch. As David Yarbrough, a nationally known insulation expert, explains, “At 75°F, the theoretical maximum R-value of a product is 5.6 per inch. That represents the maximum R-value if there is no convection and no radiation — it represents the pure conductivity of air. That’s as high as you can go unless you are talking about a product that incorporates encapsulated gas, or a vacuum, or nano-scale powders.”
Since this is a theoretical maximum, it isn’t surprising to learn that cellulose, mineral wool, fiberglass, and EPS generally have somewhat lower R-values (in the range of R-3.5 to R-4.2 per inch).
Why have many manufacturers of polyisocyanurate, at least until recently, been touting R-values that are higher than this theoretical maximum? Because polyiso (like extruded polystyrene) includes encapsulated gas; as Yarbrough noted, insulation products with encapsulated gas can exceed the theoretical maximum R-value of R-5.6 per inch.
If the bubbles of air found in foam insulation are replaced with bubbles of a different gas — a gas with lower vapor conductivity than air — it’s possible to achieve a higher R-value. The blowing agents used to produce polyiso and extruded polystyrene (XPS) fall into this category.
Although the blowing agents used to produce polyiso and XPS successfully increase the R-value of newly manufactured rigid foam, they have an Achilles’ heel: over time, these blowing agents dissipate. The gases gradually diffuse through the rigid foam, and as they dissipate, they are replaced by air. (Covering the rigid foam with a vapor-impermeable facing like aluminum foil slows, but does not prevent, this process.) Because this process results in a gradual reduction in the foam’s R-value, it’s often referred to as “thermal drift.”
The blowing agents used to make expanded polystyrene…