As the landscape around our building site disappears under a rare blanket of snow, the sheathing on our houses has been disappearing under a thick layer of exterior mineral-wool insulation. Known as Comfortboard IS, this insulation has impressed us with its green virtues, versatility, and price.Made in Canada by Roxul, Comfortboard is one product in a line of “stone wool” products that combine the power of rock with the characteristics of insulation wool. Originally inspired by the way wind spins molten lava into fibrous material during a volcanic eruption, stone wool is fire-, mold- and insect-resistant. It’s also water-repellent. This latter feature is attractive in the Pacific Northwest, where building tends to happen year ’round despite the weather.Comfortboard also has excellent thermal properties. The 2 inches of Roxul we installed will add R-8.4 to our walls. (Comfortboard is also available in thinner and thicker boards).Exterior insulation keeps the sheathing warmWhile our staggered stud wall has less thermal bridging than many standard walls, the 2×10 top and bottom plates and rim joists do have some. This exterior insulation helps reduce heat loss in those places and brings the weakest parts of the wall (aside from windows and doors) up to R-20. (The 2×10 wall plates have an R-value of about R-11.6, and 2 inches of Roxul has an R-value of R-8.4.) This level of insulation is better than the average for most 2×6 stud wall systems. Shannon Cowan and Patrick Walshe are building an energy efficient, multi-family house in Qualicum Beach, British Columbia. They chronicle this and other adventures in sustainable family living at their blog, A Green Hearth. RELATED ARTICLES Installing Mineral Wool Insulation Over Exterior Wall SheathingMineral Wool Boardstock Insulation Gains GroundWrapping an Older House with Rock Wool InsulationWindwashing in Exterior Mineral WoolGBA Product Guide: Roxul Mineral Wool Insulation Batts Residential Insulated Sheathing – Design Guide Roxul Report: Fastener Guidelines Roxul: Cladding Attachment and Support Structural Testing of Screws Through Thick Exterior InsulationQ&A: Fastening methods for Roxul ComfortBoard IS Q&A: Installing outie windows with 2-inch exterior Roxul board Q&A: Mineral wool exterior insulation, humidity, and windwashing Most of our wall assembly has a higher R-value, of course. The walls will have 9 1/4 inches of cellulose insulation (R-32) plus the exterior mineral wool insulation, for a total of about R-40.Mounted on the exterior like rigid foam, Comfortboard mineral wool has the added benefit of allowing walls to dry out to the exterior (research on this topic is available at the Building Science Corporation website). It helps prevent condensation on the inside of the sheathing by keeping it warm most of the time. (You can use some rules of thumb to figure out what R-value you should have on the exterior of the sheathing — the information is available in an article titled Calculating the Minimum Thickness of Rigid Foam Sheathing — though the minimum thicknesses listed for foam are not as vital for mineral wool, which is more vapor-permeable than rigid foam.)Comfortboard is made of natural, inorganic materials and has a high recycled content. The manufacturer has invested deeply in emission reductions and other green initiatives that help offset the energy used during production. Mineral wool generally has a minimum recycled content of 75%, making products in this category a nice alternative to petroleum-based foams and their greenhouse-warming blowing agents and flame retardants.Recently, some green builders have expressed concerns over reports that Roxul mineral wool products may contain formaldehyde. When we sent an inquiry to Roxul about the formaldehyde question, a company spokesperson responded, “Although a formaldehyde-based organic binder is used during manufacturing, a high-temperature curing phase virtually eliminates volatile compounds. The result is no measurable free-form formaldehyde in the final product and no volatile organic compounds that can off-gas.”Initial installation concernsInitially we were concerned that installation would be tricky due to the softer nature of the Comfortboard. It’s less rigid than foam; its compressive strength is about 5 psi, compared to 25 to 30 psi for common types of rigid foam. It truly feels like a thick wool blanket.Our builders shared some of the same concerns. Gerard Dubenski of Big Coast Construction, working with Penbay Construction in Nanaimo, British Columbia, led the crew who installed the product. None of them had ever used Roxul Comfortboard before. “Comfortboard is softer than foam, but we found that cap nails helped to consistently prop out the strapping at the required distance from the sheathing,” he said when interviewed at our building site.Dubenski and his crew attached the insulation with 2 1/2 inch cap nails tapped in by hand until they were flush with the surface of the Comfortboard. They installed these nails in vertical rows over the studs, which are 16 inches on center. The vertical spacing between nails was 24 inches, so that each 2’ x 4’ piece of Comfortboard (installed with the 4-foot dimension running horizontally) had one nail per stud.In addition to securing the Comfortboard, the nails helped to mark the studs for later installation of the rainscreen strapping (which was anchored to the studs).Adjusting the furring strips to be sure they’re plumb and co-planarWhen adding the 1×4 strapping over the Comfortboard, Dubenski and crew placed one screw at the top and and once screw at the bottom of each furring strip, and then checked it with a long level to be sure it was plumb before adding the remaining screws. “Once we got the hang of it, we didn’t need to fiddle around much backing screws out,” Dubenski said. “We also took care when installing the fiber-cement siding to make a few adjustments.”Now that the installation of the Comfortboard is almost complete (and with more than half of the siding on), Dubenski has a better understanding of the product. He cites a few challenges that arose when mounting boxes for penetrations through the wall. “We installed solid wood backing in places as needed, such as around doors where we wanted to install security screws,” he says. “Now that we’ve worked with it, I can only see it as being easier and easier because we have worked out methods for using it.”Dubenski sees many benefits to mineral wool. “We liked it more than foam. It was easier to cut with a large utility knife or bread knife. We also took pieces [of mineral wool] and submerged them in the pond as a test, because we were so worried about the wall being soggy. The water beaded off. As it turns out, the Roxul will actually help keep the wall dry.”Dubenski recommends the product for a variety of applications, including renovation jobs where the siding is stripped and the owner wants more insulation. “You could do it from the outside without any inside mess. I was definitely skeptical at first, but I am sold on it now. I will definitely use it again.”What about window flashing?Because Comfortboard is made from fuzzy mineral wool, it’s difficult to attach tape or peel-and-stick to its surface. For this reason, our builder and architect advised us to install “picture-frame” strips of 2-inch-thick XPS foam around each window — just enough to nail the window flanges to, and to provide a surface for sealing the rough openings with peel and stick membrane.We’ll provide further details on our approach to window flashing in an upcoming blog.It won’t rot and it won’t support moldAs owners who have watched the construction progress, we have been impressed by the versatility of Comfortboard IS. Its green points are important to us. The product’s affordability adds to its appeal, since it costs about the same as XPS foam.“I’m a fan of mineral wool because it doesn’t settle, doesn’t rot even if continually wetted, is fireproof, and won’t support mold or bugs,” summed up Albert Rooks. (Rooks was quoted in Scott Gibson’s article, “Choosing a Cost-Effective Wall System.”) “It’s like a little piece if the Canadian Rockies (since it’s made of Canadian basalt) covering your house, and nearly as durable.”Not that we need convincing: with this rare snowfall, we can see some of the benefits first hand. And we look forward to enjoying the other benefits down the road.