After using a host of other insulations on the exterior of homes, the forward-thinking design/build firm, Hammer & Hand, has settled on Roxul mineral wool. In this video, one of the company owners, Sam Hagerman, discusses what it is and why they're using it in the wall system for their Madrona Passive House.
Roxul is essentially stone - basalt - and is fairly non-toxic, easy to work with, extremely fire-resistant (it won't burn, but will finally melt above 2000°F), provides great sound resistance and is completely impervious to water. Watch the video.
For a detailed look into the use of Roxul on a super-insulated house in a much colder climate, see the JLC article by Lee McGinley ("On the Job: Working With Roxul Insulation," 3/14).