Wet-Spray Rock Wool and Slag Wool
Rock wool and slag wool fall within a group of materials historically referred to as man-made mineral fibers (MMMFs) or synthetic vitreous fibers (SVFs); however, a more appropriate name is manmade vitreous fibers (MMVFs). First patented as a commercial product in the United States in 1875, MMVFs were not used as a wet-spray insulation for residential open-wall cavities until approximately 10 years ago in Texas.
Rock wool and slag wool each use different raw materials in their manufacture. Rock wool is made primarily from natural rock such as basalt or diabase. Slag wool is made primarily from iron ore blast furnace slag. The base material of wet-spray rock wool is the same material used in rock wool loose-fill insulation (see Chap. 7), except that it is smaller in particle size and mixed with an adhesive.
Wet-spray rock wool insulation is applied in the same manner as wet-spray cellulose insulation. The mineral wool material, which contains a dry adhesive, is mixed with a small amount of water and blown into open wall cavities at a density of approximately 4 lb/ft3. The water activates the binder, which strengthens the bonding of the material to the sheathing and studs. After spraying onto the substrate, contractors screed the wall framing members using a motorized roller that runs down the face of the studs (Fig. 9.9). The mixture ratio is usually 1 gal of water per 55 lb of rock wool, but may vary among manufacturers. The vapor retarder, if applicable to the specific locality or building code, can be installed immediately after the wet-spray insulation cures.
Like other wet-spray insulation types, rock wool is very effective for insulating behind and around electrical boxes, wires, and pipes. It can fill the most difficult wall cavities, leaving virtually no voids. It will not support the growth of mildew, mold, or bacteria when tested in accordance with the specifications of the ASTM (C1338).
Although prices will vary depending on local supply and labor rates, wet-spray rock wool and slag wool are often less expensive than wet-spray fiberglass and wet-spray cellulose insulation.2
Figure 9.9 Rock wool wall scrubber. (.American Rockwool)