Mold on attic sheathing — humidification meeting cold deck.
Black staining on the underside of attic sheathing, concentrated on the north slope and around penetrations, is one of Eden Prairie's most common winter findings. The cause is almost always interior humidification stacked on top of an under-ventilated soffit-to-ridge system. The fix is moisture-source first, ventilation second, surface treatment third.
Premium homes humidify hard. Cold roofs do not forgive.
Eden Prairie's premium tier runs whole-house humidifiers aggressively through Minnesota winter — frequently at 40 to 45 percent indoor relative humidity. That moisture has to go somewhere. When the ceiling air-seal is imperfect and the soffit-to-ridge convection is starved, it migrates upward and condenses on the cold underside of the deck. We see this pattern on roughly one in four homes inspected from December through March.
Six contributing patterns.
Attic-sheathing mold is a symptom. The diagnostic value of an inspection is identifying the specific contributing patterns so the remediation actually holds. These are the six we document most frequently in Eden Prairie attics.
Black staining colonies
Visible growth on the cold side of the deck — north slope, around plumbing stacks, around skylight curbs. Confirms moisture has been condensing repeatedly.
Bath fan terminating in attic
Common premium-home shortcut. Insulated flex routed to nowhere. Each shower dumps a pint of vapor onto the deck. Reroute to roof or eave.
Buried soffit baffles
Blown-in insulation packs the eave. The 1:300 ratio works on the plan; in the attic it is closer to 1:600. No intake, no convection, no drying.
Decorative ridge vent
Cap shingles installed over the slot, or no slot cut at all. We have pulled cap shingles off premium homes to find a continuous deck behind a vent that never breathed.
Whole-house humidifier overrun
Bypass humidifier dialed to 45 percent in February. Every percent of indoor RH adds load to the ceiling plane. Outdoor-temperature reset is the right answer.
Recessed-can ceiling leakage
Non-IC-rated cans punching the ceiling plane. Each one is a stack-effect chimney pulling humid air directly to the deck. Air-seal or replace.
How we tag the finding.
Not every staining pattern is the same finding. Severity drives the negotiation conversation; classification drives severity.
| Pattern | Substrate MC | Extent | Severity |
|---|---|---|---|
| Surface staining, dry sheathing | <14% | Localized, north slope only | Monitor |
| Active growth, elevated MC | 16–22% | Multi-bay, north slope | Major |
| Active growth, both slopes | 18%+ | Whole attic | Critical |
| Delaminated OSB or rot | 25%+ | Structural compromise | Critical |
| Asphalt resin (not mold) | Normal | Pattern around fasteners | Cosmetic |
Three steps. Cause-first, not stain-first.
Attic survey with lighting and substrate readings
Direct visual inspection of decking, rafters, soffit intake, ridge slot, and bath/dryer terminations. Pin-type and pinless moisture meter readings on suspect bays. Photographic log of every staining pattern.
Thermal scan of the ceiling plane from below
Infrared survey of the conditioned ceiling identifies air-leak signatures — recessed cans, attic-access hatches, plumbing chases, party walls — that explain where the moisture is migrating up.
Severity tag and cause-ranked recommendation
Same-evening report includes severity tag per the table above, photo log, and remediation recommendation in cause-first order: source correction, ventilation correction, surface treatment.
Concerned about staining you have already seen?
Two-minute quote. Same-evening report. We tell you the cause, not just the symptom.