What bears on project planning
Michigan's climate is what actually drives this, and the heating season is the bigger half. Across a long winter, warm moist indoor air reaches cold attic sheathing and condenses there — so a bath fan discharging into the attic rather than through the roof produces staining, then growth, then delaminated sheathing, repeating every year until the duct is corrected. Humid summers reverse the flow onto cool basement surfaces. The flood overlay matters as history: a property inside it warrants checking wall cavities for contamination from earlier events, not only for what is visible now.
Official sources
Separate past intrusion from present condensation
Citywide records cannot tell you whether your basement has backed up, what is inside a finished wall, or where your bath fan actually terminates. Establish the history and check the fan before assuming the source.
Last reviewed 2026-07-19. External requirements and maps are subject to change over time.