Readers of this column may be aware that I’ve been critical of some of our foam-plastic insulation materials. I’ve come down hardest on extruded polystyrene (XPS), which is made with both a blowing agent that contributes significantly to global warming and a brominated flame retardant, HBCD, that’s slated for international phaseout as a persistent organic pollutant.
So I’m always keeping an eye out for alternatives. I’ve written here about two of those alternatives that I’ve used in our own home: a cellular glass material, Foamglas, that works very well below-grade; and a boutique, all-natural rigid insulation material made from expanded cork.
I like both of those materials a lot, but they have two big problems: high cost and limited availability. They just won’t be able to enter the mainstream home building industry since they cost more than twice as much as XPS and polyisocyanurate and are very hard to get hold of.
mineral wool boardstock
With this context, I was thrilled to learn last month that Roxul, a Canadian manufacturer of mineral wool insulation and part of the global, Denmark-based Rockwool International, has been gaining traction with their residential ComfortBoard IS in the U.S. Rigid boardstock mineral wool has been available in the U.S. for decades from at least four manufacturers and it is widely used in commercial construction. But it’s never been widely available for home building.
That is changing as Roxul ramps up national distribution of ComfortBoard IS, which was first introduced about a year ago.
ComfortBoard IS has a density of 8 pounds per cubic foot (pcf) and is available in four thicknesses: 1-1/4", 1-1/2", 2", and 3", though the company has the capability to produce it up to 6" thick -- which could offer an attractive option for Passive House builders and those interested in deep-energy retrofits. Because thicker panels requires a special production run, that is available only in truckload quantities.
The insulating value is a very respectable R-4.0 per inch. That’s lower than XPS (R-5 per inch) and polyiso (about R-6.0/inch), but there will be no "R-value drift" (reduction in R-value over time), which occurs with foam insulation materials that rely on lower-conductivity blowing agents that slowly leak out.
A very attractive property of ComfortBoard IS is the high permeability to water vapor. A two inch layer of the insulation has a permeability of about 30 perms, which means it’s highly breathable. If the ComfortBoard is installed on the outside of the wall (with the sheathing layer providing the continuous air barrier -- an approach that more and more building science experts are recommending), the high permeability will allow excellent drying potential to the exterior. ComfortBoard IS has a textured outer surface, which may even aid moderately in that drying potential -- acting like a rainscreen.
Another feature of mineral wool that I hadn’t appreciated before is the very low coefficient of thermal expansion with temperature. According to Roxul, the coefficient of thermal expansion of ComfortBoard is just 5.5 (10-6 m/m°C), compared with 80 for XPS and 120 for polyiso. In applications where temperatures fluctuate significantly (like on the outside of a wall in a cold climate) this could make a big difference.
Availability and price
I was pleasantly surprised recently when I asked Leader Home Center in Brattleboro to price a number of insulation materials for a BuildingGreen report we’re revising. The contractor pricing for ComfortBoard IS came to $0.64 per board-foot, compared to $0.48/bd-ft for standard polyiso, $0.75 for fire-rated polyiso (Thermax), and $1.07 for XPS.
While pricing will doubtless differ in other regions and for different quantities, the fact that ComfortBoard is in the same ballpark as these other materials is great. Even after correcting for the lower insulating value (you need more thickness of ComfortBoard to achieve R-10 than with the foam plastics), Comfortboard IS is a better deal than XPS: roughly $1.59 per square foot at R-10 for ComfortBoard vs. $2.14/sf @ R-10 for XPS.
on the way
Just as exciting as the increased availability of ComfortBoard IS is a commercial version that’s about to be introduced: ComfortBoard CIS. It is similar to the residential product, but produced at a higher density of 11 pcf. It can be ordered up to 6" thick, but standard thicknesses will be up to three inches.
Alex Wilson is the founder of BuildingGreen, Inc. and the Resilient Design Institute (www.resilientdesign.org), both based in Brattleboro. Send comments or suggestions for future columns to email@example.com.