[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).
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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.
can this be laid on the upper side of the ceiling in a house as insulation instead of using real sheeps wool as some of us do here in Australia.
Yes. But the batt kind of roxul would be cheaper for that kind of application. The rigid roxul boards can be used over the roof decking. If you have a vented attic that is vermin proof and has a flat floor (above ceiling), blown in cellulose insulation is one the best performing and greenest insulations available for that application (that is what we will be using). It has borax added to resist insects and fire and mold. The website greenbuildingadvisor.com has a Q&A section and experts will respond to questions there.
Pictures, please! We wanted to use this (or Dow’s structural insulated sheathing – SIS) but at the time (2.5 years ago) they were not yet available through building supply stores. Also, the building department would not approve the use of the SIS instead of sheathing instead of plywood.
Great progress! And Happy New Year.
Gail
Thanks for your interest, Gail. We’ve added some more pictures above. We ordered this product through Pacific Homes, but our local building supplier said they could also order it. It is very new.
Hi, I am in the process of building a home. I am considering using the comfortboard product. If you don’t mind me asking. What is your total cost for the comfort board? Also did you order your windows and doors with extra wide jamb extensions?
Hi Joe. It cost $4,600 for enough 2″ Roxul Comfortboard IS to cover the entire 2900 sqare foot project (main house is two storeys and 950 sqft suite is one storey). That was part of the house kit, so I think our supplier (Pacific Homes) got a better price than we could get at the building supply store. Also, allow a few weeks for ordering. We also got a quote for $8.70 for each 2′ x4′ piece of 2″ Roxul Comfortboard IS from the local builder supply contractor’s desk where we have an account (who offers some discount due to bulk buying).
Yes we had plywood window boxes made that extend out the 2″ beyond the sheathing for accomodating the Roxul exterior boards. Our windows are outies (sit towards the outside of the wall. )We will do another post on window sealing, flashing and trim details as that required some careful attention (we installed a frame of a few inches of xps foam around each window to nail the window flanges into and tape the peel and stick membrane to).
[...] Weeks before our windows arrived, we had discussions with our lead contractor about how to flash the windows on this air tight house and in-law suite. We were already committed to using two inches of exterior insulation, along with plywood boxes extending all window and door frames by a corresponding 2″ on the outside. However, rather than the XPS foam suggested by our house plans, we opted to use a new product produced in Canada known as Roxul Comfortboard IS. (More on this insulation choice here.) [...]
Very nice application, the pictures look great. Cutting the Comfortboard is not too difficult, did you use a bread knife or something similar? It looks great the way you cut it to shape to fit around the window frames in the last photo.
Hi Jeff,
The builders used a long utility knife to cut the Comfortboard. They said it was easier than cutting styrofoam.
Can you show how your builder did the step flashing details on the roof on wall segments? On our home the builder forgot this detail and the rain water is directed directly behind the insulation where typer was also forgotten!
Thank you,
Simon
Hi Simon. I can check if I have any photos of those details. I recall that the typar house wrap had a slit cut in it, a bead of caulk was place along the bottom of the slit and along the back of the flashing, the flashing inserted into the slit and taped or peel and stuck to plywood (so water would run down the typar and over flashing or if needed, down the plywood and over the flashing). The typar was then taped to the flashing along the top of the slit to complete the air seal / water seal. I know with the windows, the top of the flashing had peel and stick right to the plywood (see our window flashing post), so I imagine that the roof flashing was similar where it meets a wall.