Mold inspection and indoor air help in Portland, OR
We do not keep an office in Portland. Every job starts with a free call. For large and complex properties in the Portland area we travel to you; for smaller jobs we tell you exactly what to ask for and point you to a local inspector we are comfortable recommending.
What the Portland climate does to a house
Measured at NOAA's Portland weather station.
- At the NOAA Portland station, mornings average 82 percent relative humidity across the year, and afternoons 59 percent.
- The dampest mornings come in October, averaging 86 percent. Afternoons are driest in July, at 44 percent.
- A normal year brings 36.03 inches of precipitation over about 153 days, with the most in November (5.63 inches).
- In 9 of 12 months the average morning humidity is 80 percent or higher.
- Even in the afternoon, average humidity stays above 60 percent in 5 of 12 months. Indoors, 60 percent is the line where mold starts to find it easy.
| Month | Morning RH | Afternoon RH | Precip. (in) | Days with precip. | Mean temp (°F) |
|---|---|---|---|---|---|
| January | 84% | 76% | 4.88 | 18 | 41.4 |
| February | 84% | 66% | 3.66 | 16 | 43.8 |
| March | 83% | 60% | 3.68 | 17 | 48.2 |
| April | 82% | 55% | 2.73 | 15 | 52.3 |
| May | 80% | 53% | 2.47 | 12 | 58.3 |
| June | 79% | 50% | 1.70 | 9 | 63.6 |
| July | 77% | 44% | 0.65 | 4 | 69.2 |
| August | 78% | 44% | 0.67 | 4 | 69.5 |
| September | 82% | 48% | 1.47 | 7 | 64.5 |
| October | 86% | 62% | 3.00 | 13 | 54.9 |
| November | 86% | 74% | 5.63 | 18 | 46.6 |
| December | 85% | 78% | 5.49 | 19 | 40.4 |
Source: NOAA National Centers for Environmental Information, Comparative Climatic Data: average relative humidity (period of record through 2018), normal precipitation, mean temperature and cooling degree days (1981 to 2010 normals), and mean days with 0.01 inch or more of precipitation, 90 degrees or more, and 32 degrees or less. Last updated September 24, 2026.
Where moisture hides in Portland homes
Long, damp rainy seasons
Months of drizzle and mild temperatures mean building materials rarely get a chance to dry out fully. About 153 days a year bring measurable precipitation here.
Crawlspaces that never dry
Damp soil under the house feeds moisture into crawlspaces and, through the stack effect, into the rooms above. Mornings average 82 percent humidity across the year.
Bathrooms and bedrooms without enough ventilation
Mild weather means windows stay shut and heat stays low, so shower and breathing moisture condenses on windows and exterior walls. Winter afternoons here average 73 percent humidity outdoors.
Coastal fog and salt air
Near the water, fog wets exterior surfaces daily and salt air speeds up the failure of flashing and window seals. Summer mornings here average 78 percent humidity.
Common questions from Portland
- Crawlspace Smells Musty
- Mold in Crawlspace
- Bathroom Smells Musty After Shower
- Musty Smell in Bedroom at Night
Large, custom or multi-property situations: see Keystone.
Mold or air problem in Portland?
Start with a free 15 minute call. We will tell you what you need, whether that is a local inspector, a virtual consultation or an on-site investigation.