Mineral Wool Boardstock Insulation Gaining Ground in the Homebuilding World

Roxul ComfortBoard IS has some important environmental and performance advantages over XPS and polyisocyanurate insulation

ComfortBoard IS, Roxul's exterior insulation board, is being distributed nationwide in the U.S. at thickensses up to 3".Photo Credit: Roxul

Readers of this Energy Solutions blog 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 both with a blowing agent that contributes significantly to global warming and with 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 called Foamglas with high compressive strength that works very well below-grade; and Thermacork, an 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—not yet, anyway—since they cost more than twice as much as XPS and polyisocyanurate and are hard to get hold of.

Enter ComfortBoard mineral wool boardstock

With this context, I was thrilled to learn recently that Roxul, a Canadian manufacturer of mineral wool (or rock wool) insulation and part of the global, Denmark-based Rockwool International, has been gaining traction with its residential ComfortBoard IS in the U.S. Plus, the company has a new, even higher-density boardstock product coming out this month for commercial applications.

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. (A few years earlier the company began national distribution of its ComfortBatt product for cavity-fill applications.)

ComfortBoard IS from Roxul’s Milton, Ontario factory is third-party-certified to have a minimum recycled content of 75%, and product can be specified with recycled content up to 93%.

Residential installation of ComfortBoard IS.Photo Credit: Mark Gorgolewski

ComfortBoard IS, the residential product, 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". The company has the capability to produce the product up to 6" thick—which could offer an attractive option for Passive House builders and those interested in deep-energy retrofits—but because thicker panels requires a special production run, those options are only available in truckload quantities.

The insulating value of ComfortBoard IS 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 or allow air to leak in.

Permeability

A very attractive property of ComfortBoard IS is the high vapor permeability. A two-inch layer of the insulation has is about 30 perms, which means it’s highly "breathable." If the ComfortBoard is installed on the outside of the wall the high permeability will allow excellent drying potential to the exterior. This approach, in which the sheathing layer provides the continuous air barrier, is gaining many fans in the building science community.

ComfortBoard IS has a textured outer surface (see photo), which may even aid moderately in that drying potential (acting like a rainscreen). When asked about this, Paraic Lally, the North American Manager for Specifications at Roxul, told me that the texturing is a function of the manufacturing process and not designed to provide a rainscreen; thus, the orientation of installation is not important..

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 can be a real problem.

As Martin Holladay has reported on GreenBuildingAdvisor.com, shrinkage of XPS insulation used as an outer sheathing layer can be significant enough to totally separate the tongue from groove at XPS joints, thus eliminating that thermal break role of the exterior insulation.

Formaldehyde binder

All North American mineral wool today is produced with urea-extended phenol formaldehyde binder. This raises the prospect that the material could release formaldehyde, and it means that the insulation cannot be used in Living Building Challenge projects, because formaldehyde is a "red-list" chemical in that rating system.

From what I understand, however, the high-temperature processing of the mineral wool during manufacture drives off any free formaldehyde, and test data I've reviewed (for the ComfortBatt product, not ComfortBoard) showed formaldehyde levels to be at or below background levels. So, other than the concern that mineral wool can't be used in Living Building Challenge projects, I don't consider the formaldehyde binder a big deal. But I'd love to hear other thoughts about that.

Typically installed over sheathing, if structural bracing is provided, ComfortBoard can be installed without exterior sheathing.Photo Credit: Roxul

Availability and price

I was pleasantly surprised recently when I asked Leader Home Center in Brattleboro, Vermont to price a number of insulation materials for an update to BuildingGreen's encyclopedic report on insulation. 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 locally was more affordable than XPS: roughly $1.59 per square foot at R-10 for ComfortBoard vs. $2.14/sf @ R-10 for XPS.

Dimensions and installation

Although Roxul literature shows ComfortBoard IS being available in three sizes—24" x 48", 36" x 48", and 48" x 96"—it is most commonly stocked in the smaller sizes. This may be because the larger panels will be fairly heavy. At 8 pcf, a three-inch-thick, 4' by 8' panel weighs 64 pounds—not an insignificant weight to wrestle into place.

To achieve a reasonably thick, four- to six-inch layer of exterior insulation for a deep-energy retrofit of Passive House wall system will require a double layer (unless you have the ability to order by the truckload). This can be an advantage because is allows overlapping the panel joints (only square-edge product is produced), but it will likely increase labor costs.

Rigid mineral wool may also take some getting used to from an installation standpoint. It can be cut with a hand saw, though I can’t (yet) report on cutting the product from personal experience. Minimum one-inch-diameter washers or nail/screw heads are recommended for attachment, and when strapping is installed on the outside to produce a rainscreen, that strapping has to be screwed into wood studs through the insulation. Because it is mineral-fiber product, a dust mask and gloves should be used when working with it.

Commercial ComfortBoard 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. Like Comfortboard IS, it can be ordered up to 6" thick, but standard thicknesses will be only up to three inches.

While I am pleased to have used Foamglas and cork insulation on my home, I suspect that Roxul’s ComfortBoard will find its way into my next project.

Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. In 2012 he founded the Resilient Design Institute. To keep up with Alex’s latest articles and musings, you can sign up for his Twitter feed.