The situation

Illustrative example. Not a real job or customer.

In this illustrative example, a family moves into a newly built two-story home on a lot where a mid-century ranch once stood. The primary bathroom has a large open shower, a soaking tub and a single exhaust fan near the toilet, on the same switch as the vanity light. By the second winter, fine gray spots appear on the ceiling above the shower, and the paint near the window begins to peel.

  1. The owners clean the spots with a mild detergent; they return within a few weeks.
  2. A hygrometer left in the bathroom shows humidity staying high for a long time after showers, and indoor humidity in the rest of the house is higher than expected for a Denver winter.
  3. A contractor checks the fan and finds it undersized for the room, with a long flexible duct run that sags in the attic and terminates through a roof cap partly blocked by insulation.
  4. No leak is found in the shower or roof, and the drywall is firm with growth limited to the paint surface.
  5. The owners have a properly sized fan installed near the shower, with smooth duct to a clear roof cap and a timer switch, and lower the setting on a whole-house humidifier.
  6. The ceiling is cleaned, allowed to dry, primed and repainted. The owners check the same spots the following winter.

With ventilation corrected and humidity lowered, the spotting did not return. Because the drywall was sound and the growth surface-level, no demolition was needed in this example.

Room fully sheeted in plastic containment with a dehumidifier in the corner

Room fully sheeted in plastic containment with a dehumidifier in the corner. One example of what this stage can look like; it is not a job connected to this site.

Clues that separated ventilation from a leak

Observation
Spots spread evenly over the shower area, not in one patch
Pointed toward
Condensation from humid air
Observation
Drywall firm, no stain rings
Pointed toward
No liquid water from above
Observation
High humidity long after showers
Pointed toward
Inadequate exhaust
Observation
Worse in winter
Pointed toward
Cold ceiling surfaces below the attic
Observation
Fan tied to the vanity light
Pointed toward
Fan rarely running long enough

Questions this case raises for new-home owners

  1. Is the bath fan sized for the room and shower, and does it actually vent outdoors?
  2. Is there a timer or humidity-sensing switch so the fan runs after showers?
  3. Does the house have a mechanical ventilation system, and is it set and maintained correctly?
  4. Is a humidifier on the furnace raising winter humidity more than the house can handle?
  5. Should the builder be told about it in writing, given how new the house is?

General information for Hilltop, not a quote or professional advice. In an emergency, call 911.