Will Welch has chosen to build his high-performance house in Nederland, Colorado. The site is at the border of Climate Zones 6 and 7, and it poses some challenges: it’s at an elevation of 8,600 feet; the area gets a generous amount of snow and wind; and the number of heating degree days tops 8,800 a year.
But Welch has one more concern: the threat of wildfires.
“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,” Welch writes in a post at the Q&A forum.
His plans currently call for a 2×6 framed wall rather than a double-stud wall, mainly as a way of preserving all the interior space he can. Wall cavities would be insulated with dense-packed cellulose or a mixture of closed-cell spray foam and cellulose, with 5/8-inch drywall on the inside and 5/8-inch plywood sheathing on the outside. On the exterior, Welch would use 4 inches of mineral wool insulation, covered with a mix of corrugated metal and fiber-cement siding.
“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,” Welch says. “That said, exterior mineral wool doesn’t look nearly as cost-effective as something like recycled rigid foam.”
Welch has three questions:
That’s the topic for this Q&A Spotlight.
The closed-cell foam option
GBA editor Martin Holladay thinks that selecting exterior mineral wool rather than rigid foam makes sense if fire safety is high on Welch’s list or priorities.
But the case for installing 2 inches of closed-cell spray polyurethane…