Roxul Comfortboard IS: Exterior Insulation Wrap

Since I designed my tiny house to have stud framing instead of using SIP panels and wanted to have an exterior insulation wrap as a thermal break, I had assumed I would use rigid foam board to do so. Rigid foam is commonly used as exterior insulation in the U.S. and is easy to find in addition to being lightweight and having a high R-value per inch (polyiso has around R-6.5 per inch). However, after talking to an architect who I met via volunteering at a solar winery tour and who used Roxul for exterior insulation on his own home, I decided to research the product more to see if it might be a better option for my build.
I learned that Roxul is the leading company making mineral wool insulation. It accounts for around 75% of the insulation installed in Europe and around 50% of the insulation installed in Canada, but only around 5% of what is installed in the U.S. Part of this is because not many people know about it here and assume that it will be similar to fiberglass insulation, especially in batt form. However, I learned that mineral wool is much different than fiberglass. You can read a convincing argument about using mineral wool insulation here.
Mineral wool is made from spun molten rock and has many environmental benefits including the use of natural and recycled materials. It repels water, continues to insulate when wet (just like wool socks), is fire resistant, breathable and sound absorbent. It has an R-value of 4.6 per inch which is comparable to R-4 EPS and an R-5 XPS rigid insulation. The cost is similar to foam. I paid $23 for each 4'x8' 1 1/4" sheet. The tricky part was sourcing it since only a few places in town will have it in stock occasionally. Luckily my architect friend, Corey, connected me with Brett, a Roxul rep who was working on a project in Ashland, OR and needed to source a little more for his project. He connected me with a company in Tacoma, WA via Service Partners in Portland who had just enough material for my walls. He also had some 2" board available so I decided to use that for my roof. All together with the cost of material and shipping ($80), it was just over $600 for the exterior insulation wrap.
Installation was easy. We used a utility knife with a long blade with the guide of a T-square (sometimes) to slice through the material after marking our measurements. Then we pressed cap nails part way into the Roxul before picking in up and placing it on the wall so they were ready to be tacked in. Not many nails were necessary since the battens for the rainscreen are meant to hold the Roxul in place. We used full pieces whenever possible and took care not to damage the edges during install since the material is somewhat fragile. In retrospect, I probably should have had all of the windows trimmed out since the window face is now flush with the Roxul and will look a little odd with the layers of furring strips and then trim. I will probably end up cutting back the Roxul a little to make window frame boxes when I get the cedar this next week. Oh material sourcing!
*A note about fender flashing. I needed to make a decision about my fender flashing before installing the Roxul. This is something I had been putting off since I had not found a great solution. I decided to cut up some of the L flashing I had left over from the walls and piece it together around the fender starting with the lower pieces and lapping the upper ones so that water can freely flow down to the ground. I used the screws with the rubber gaskets left over from the undercarriage to attach the pieces to the sheathing and then used Fast Flash to cover the connection. I left a small gap between the flashing and the fender where I applied a bead of Dow 791 medium-modulus silicone caulk which creates a flexible building joint and should be a good choice in this application. (Ideally I would have applied this before the flashing went down but I had to wait to get it until after the weekend when Atlas Supply was open again.) Only time will tell.
For more photos, check out my flickr!