Mold and Moisture: 4 Common Sources of Mold in a Home

We recently performed a visual fungal assessment and air sampling services at a home in Hillsboro, Oregon. Our investigation was requested in response to:

  • Observations of a musty odor
  • Concerns about hidden mold growth
  • Reports of allergy-like symptoms

During the assessment, our technician uncovered four of the most common sources of mold and moisture that can contribute to elevated levels of mold growth inside homes.


Mold and Moisture Issues Discovered

The residence was a two-level home built in 1930’s and last remodeled in the 1970’s. Results from our microbial air sampling indicated a slightly elevated presence of Cladosporium and Penicillium/Aspergillus fungal spore concentrations inside the home when compared to the concentrations measured from the control sample taken outside.

We also performed a visual assessment throughout readily accessible areas of the home’s:

  • interior,
  • crawlspace,
  • attic, and
  • the home’s exterior.

Our assessment included searching for visible mold growth, any evidence of water intrusion, and any mold and moisture-related issues.

A thorough inspection of the home revealed specific areas where mold growth was observed. Those areas are included in the list below, along with the most likely sources of mold and moisture that cause the mold growth, and what the homeowners can do to remedy the current situation and prevent excessive mold growth in the future.


4 Common Spots in a Home Where Mold and Moisture Occur

1) Inadequate Sealing of Bathroom Caulking

Our inspector observed a minor amount of mold growth along the caulking of the bath in the lower level bathroom. The most likely cause is inadequate sealant used when the caulking was installed, which allows water to penetrate the caulking during routine use of the bath.

Recommendation:  The affected portions of the caulking should be removed. New caulking should be applied by qualified industry professionals.

2)  Condensation Forming on Windows

Mold growth was also seen along the interiors of several window sills throughout the home. The most likely cause is from indoor conditioned air cooling on contact with cold windows, resulting in condensation. The homeowner reported that they regularly opened their windows to provide natural ventilation, and the window sills are routinely cleaned. The current windows are metal-framed single pane windows, which don’t provide sufficient insulation to prevent condensation.

Recommendation:  Replace metal-framed single pane windows with double pane vinyl windows that will provide sufficient insulation to prevent condensation.

3)  Leaky Gutters

Our technician observed that the outside gutter system was leaking in several areas along the north side.

Recommendation:  Have qualified industry professionals assess and repair the gutter system as needed to adequately divert stormwater drainage away from the house.

4)  Inadequate Exterior Paint

The exterior paint on the wood siding on all sides of the house was peeling and blistering. This was likely due to normal environmental degradation. Inadequate exterior paint can allow moisture to penetrate the home’s exterior and eventually lead to mold and moisture-related issues on the interior.

Recommendation:  Stabilize the current exterior paint. Confirm the siding is within acceptable moisture content levels to prevent enclosing wet building materials. Then, apply new exterior paint. Given the age of the residence, we recommend testing the exterior paint for lead content prior to performing any work.


The homeowners can follow our recommendations to rid their home of causes of excess water and mold and moisture issues inside their home. Remedying these situations will also help prevent mold from growing to elevated levels in the future.

Dan Rouse