Andy Chappell-Dick is at work on a house in Climate Zone 5 where the task at hand is to upgrade a crawl space by adding insulation as well as a membrane to block the infiltration of moisture. The catch? The owners want to avoid the use of rigid foam insulation if at all possible.
The floor of the crawl space is about a foot below grade, Chapell-Dick writes in a Q&A post at GreenBuildingAdvisor, and the area seems to be well drained. Foundation walls are made from concrete block (CMUs).
He plans to foam in pieces of rigid extruded polystyrene in the rim joist area. To insulate the crawl space walls, Chappell-Dick wonders whether Rockwool Comfortboard 80, a rigid mineral wool insulation, would make a good substitute for rigid foam. Rockwool Comfortboard 80, according to the manufacturer, is non-combustible and chemically inert, and it’s made from natural and recycled materials, including rock. Rigid foam is a petrochemical.
A second issue is how the waterproof membrane should be installed: should it be run up most of the crawl space wall, or can it be terminated at the base of the wall? And, Chappell-Dick wonders, does this detail have any bearing on the performance of insulation?
That’s the backdrop for this Q&A Spotlight.
This is not the place for Rockwool
The inherent air-permeance of mineral wool insulation makes it inappropriate for this application, writes GBA senior editor Martin Holladay.
“The mineral wool can’t prevent humid interior air from contacting the cold crawl space walls,” he says. “The likely result will be moisture accumulation and mold.”
The best route, Holladay says, is to use an air-impermeable insulation — either rigid foam with seams that have been carefully sealed or closed-cell spray foam.
In theory, mineral wool and a clean cement…