Mysterious Water Damage

Water damage on the basement ceiling

Have you ever seen water damage in an un-expected location? There was no plumbing above the mold spot that was located in a basement ceiling. The spot was about the center of the house and there was no evidence of a roof leak.

No plumbing or visible moisture source above.

The spot was below a dining room floor with no visible damage. In, addition, the area was quite dry according to my moisture meter.

Less than 10% moisture content.

If you noticed the chase in the ceiling you are on the right track to the source.

This angle shows that there is an HVAC duct in the chase enclosed with gypsum (drywall)

Note, the HVAC register at the top of this photo. The supply branch duct runs below the floor and above the damaged ceiling.

When you realize that there is a supply duct above the water damage, the cause becomes more apparent. The water that damaged the ceiling must have been associated with the HVAC ducts. How can this occur?

This can happen when the ducts run through and un-conditioned space and are not insulated. In summer, when the air is humid in the basement and the ducts are cool the ducts can "draw" moisture out of the humid air. Water condenses and drips on whatever surface is below. In this case, the moisture was hidden until it worked its way through the gypsum ceiling.

Since the gypsum was dry at the time of these photos, the conditions have been removed that encouraged this condensation. Perhaps, the basement was closed up for a long time without ventilation or a basement dehumidifier prevented more condensation after the event.

Solutions:

1. Allow the basement air to circulate by opening interior doors and/or HVAC registers.

2. Insulate ducts to prevent the humid air from contacting the cold ducts.

3. Dry the air in the humid space using a dehumidifier.