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. The rest of the wall will be over R40 with 9.25″  of dense packed cellulose insulation and the R 8.4 Roxul board.

 

Mounted on the exterior like a rigid foam, Roxul Comfortboard IS 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 the greenhouse-warming, blowing agents and flame retardants associated with XPS and EPS.)  Melting rock certainly takes energy so the insulation is not as green as straw or cellulose but those products can’t be used in this way. The boards are also softer than the foams so care must be taken to maintain an even plane when installing the strapping (we put cap nails under the strapping to help prop it out.) The fuzzy boards can’t be taped and air sealed like foam, so we used housewrap for that (sheathing is even better for that purpose.)

For more information and photos and to follow the project on our blog, please visit  http://agreenhearth.com/?p=553

Patrick Walshe, R.P. Bio.