Ever since the 1990s, R-values required by U.S. energy codes have been creeping upward. Exterior insulation—applied over the studs and sheathing—offers a cost-effective way to comply with these tougher rules. And these days, rigid plastic foam isn’t the only product available for that purpose. Some builders are experimenting with rock wool, a mineral fiber made from blast furnace slag or naturally occurring basalt rock. The material comes in standard batts, but you also can get it in semi-rigid sheets rated at about R-4 per inch.
Rock wool is virtually fireproof, with a high 2100 F melting point, so it can protect walls against combustion in case a nearby structure burns. It’s also vapor-open, which means that exterior walls covered with it have the ability to dry toward the outdoors, reducing the risk of rot.
Builders apply rock wool sheets like rigid foam insulation, attaching the insulation to the sheathing first, then applying strapping over the insulation as nailing for siding. It’s a good idea to apply housewrap to the sheathing before attaching the insulation. The key concern is to protect the wood sheathing, because the water-repellent, drainable rock wool won’t be harmed by incidental moisture.