Musty, earthy, damp. Wet soil, rotting leaves, old books in a wet cardboard box. It is caused by compounds mold releases as it grows, which means one important thing: if you can smell it, the colony is active and you are breathing what it puts out. The smell is often strongest when the furnace kicks on or when you come home after a few days away.
A musty smell with no visible mold is the classic sign of hidden growth, inside a wall, under flooring, or in a ceiling cavity from a leak that never fully dried. Do not paint over it, do not run air fresheners at it, and do not open the wall yourself. Nonstop Restoration traces hidden mold to its source with thermal imaging and moisture meters, then remediates it with proper containment so the spores stay put. The inspection is free: (403) 620-8489.