The high density rigid rock wool IS quite heavy, (at 8 lbs/cubic foot it's 5x the density of Type-II EPS) and while air-permeable from a moisture-transport point of view it is QUITE air-retardent, of extremely low consequence from thermal loss point of view
It isn't quite 2x the cost of EPS in all markets, but it's not super cheap. According to Alex Wilson's article, in contractor quantities in VT it's about 16 cents/R-foot, compared to a typical ~10 cents R-foot for EPS, about the same $/R-foot as spray polyurethane. There's definitely a cost premium, but in some apps it's a premium worth paying. Unlike EPS it is highly dimensionally stable over time & temperature, extremely fireproof, and insect-proof, and extremely vapor-permeable- it won't inhibit drying. In some stackups the high permeance is an asset relative to any foam product, in others it's a liability, so like foam, it has to be used judiciously.
Rock wool batts cost about the same as high-density "cathedral ceiling" fiberglass, but are far more fire resistant, with none of the friable fiber contaminant issues. They still have the same installation-fault issues inherent with any batt insulation though. They're not nearly as dense or air-retardent as the rigid goods, about the same air retardency of 2.5lb cellulose. |