High-R Wall Design in a Wildfire Zone?
I am in the process of designing a high-R wall for my future home build in Nederland, Colorado. The property is located at 8600 ft. in climate zone 6/7 and in a high wildfire danger zone. My priorities are a tight building envelope with high-R walls and more environmentally friendly materials that also resist fire, but I’m also trying to get the most bang for the buck on those materials. I’ve read various articles on this site and others, and they’ve been very helpful, but I’d love some input on my design. Here’s what I’ve come up with:
I’m leaning toward a 2×6 frame, rather than a double-stud wall, to preserve square footage. The frame would be filled with 3.5 pcf dense-packed cellulose, sandwiched between 5/8” gypsum and 5/8” plywood. Studs would be 24” oc with California corners to maximize the cellulose fill. The exterior insulation would be 4” of ~6 pcf density mineral wool (Rockboard, CavityRock, VersaBoard, CurtainWall, etc.). I’d go with corrugated metal and fiber cement siding, attached to the exterior with plywood-ripped furring strips. Note: I’m leaning away from rigid foams for the exterior insulation because a number of them either don’t perform well in fires or in the cold, have thermal drift over 5-10 years, or are rough on the environment. That said, exterior mineral wool doesn’t look nearly as cost effective as something like recycled rigid foam.
A few questions that I’m hoping folks can help me with:
1) Exterior insulation: Does my thinking on exterior mineral wool here make sense compared to other options? (I just saw Peter Yost’s post on Kooltherm, a potentially promising product …)
2) House wrap/barriers: I’ve read a number of GBA articles on house wrap and air/vapor barriers, but I’m not clear on exactly how all of the principles translate into materials and placement. Nederland has low humidity, but it snows a decent amount, is fairly windy, and there are a large number of heating days. For the wall design described above, and in this climate, what kind of wrap/barrier should I use and where would it go?
3) Closed-cell spray foam: Given that the mineral wool exterior insulation would allow drying to the outside, could I add 2” of closed-cell spray foam to the inside of my plywood sheathing (and then fill with cellulose) to increase the R-value of the wall? If so, where would I then put my house wrap/barrier?
4) Flaws/tips: Any other flaws in the design or tips for improvement?