[Update: A longer version of this post featuring some thoughts from our builder appears as a guest post here.]
As the landscape around our building site disappears under a blanket of snow, the sheathing on our houses has been disappearing under a thick layer of exterior insulation. Known as Comfortboard IS, this insulation is impressing 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 and water repellent. It also has excellent thermal properties and will add an R-value of 8.4 to our walls (at two inches thick – it also comes in thinner and thicker boards).
While our staggered stud wall has less thermal bridging than many standard walls, the top and bottom plates, rim joists, and plywood window boxes 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 18. This is better than the average for most 2×6 stud wall systems.
Mounted on the exterior like a rigid foam, Comfortboard has the added benefit of allowing walls to dry out to the exterior (research on this topic is available at buildingscience.com). 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 from greenbuildingadvisor.com to figure out what R-value you should have on the exterior of the sheathing (though the minimum thicknesses listed for foam are not as vital for the permeable Roxul).
Comfortboard is also made of natural, inorganic materials and has a high recycled content. In addition, the company has invested deeply in emission reductions and other green initiatives. (Mineral wool 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.)
Initially we were concerned that installation would be tricky due to the softer nature of the Comfortboard. (It’s less rigid than foam–about 5 PSI versus 25-30 PSI for some foams–and truly feels like a thick, wool blanket.) In the end the product is just rigid enough. Capped nails installed sixteen inches on center over the studs help prop out the 1×4 rain screen strapping and maintain compression consistency. The entire lower level is now covered and looking very warm indeed.
“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. It’s like a little piece if the Canadian Rockies (since it’s made of Canadian Basalt) covering your house, and nearly as durable,” said Albert Rooks in a GreenBuildingAdvisor.com article called “Choosing a Cost Effective Wall System” by Scott Gibson.
Not that we need convincing. With all this snow falling, we can see some of the benefits first hand. And we look forward to feeling the other benefits down the road.
For more information on mineral wool insulation, visit greenbuildingadvisor.com.