Spun from blast-furnace slag, this inorganic insulation does not burn or support the growth of mold or mildew. Highly sound-absorbent. Blown into place wet, it's trimmed flush with the studs after it dries; the trimmings are recycled.
Good for: New construction, attics
R-value per inch: 4
Vapor barrier needed: Yes
Cost: 19¢ per sq. ft.