Guide · 6 min read
Why Portland-Area Homes Get Mold: A Season-by-Season Guide
Mold is a moisture problem, and Portland supplies moisture on a schedule. NOAA's 1991 to 2020 normals for Portland International Airport put the year's precipitation at 36.9 inches, and 74 percent of it, 27.3 inches, falls from October through March. July and August together average about one inch. A house here spends half the year wet from the outside and the heating season then adds moisture from the inside. This guide shows that pattern month by month and what to check in each season.
The Portland moisture calendar
These are NOAA's 30-year monthly normals for Portland International Airport. Neighborhoods away from the airport differ, and the city pages for each community give local rainfall figures, but the shape of the year is the same across the metro area.
| Month | Average temperature | Precipitation |
|---|---|---|
| January | 41.9°F | 5.03 in |
| February | 44.1°F | 3.68 in |
| March | 48.3°F | 3.97 in |
| April | 52.8°F | 2.89 in |
| May | 59.4°F | 2.51 in |
| June | 64.2°F | 1.63 in |
| July | 70.2°F | 0.51 in |
| August | 70.6°F | 0.54 in |
| September | 65.4°F | 1.52 in |
| October | 55.6°F | 3.42 in |
| November | 47.1°F | 5.45 in |
| December | 41.6°F | 5.77 in |
Three things stand out. November through January alone bring 16.25 inches, 44 percent of the year. The wet months are also cool, so wet materials dry slowly. And the dry months, July and August, give the structure a long stretch to dry out.
Why wet, cool months cause trouble inside
Oregon OSHA's mold fact sheet says mold needs an organic substrate, moisture, and oxygen to grow, and a house is full of wood, paper, and fabric. Moisture is the one thing you can control. The EPA says it is important to dry water-damaged areas and items within 24 to 48 hours to prevent mold growth.
In the wet season, moisture arrives from the roof, the walls, and the ground. In the heating season, it also builds up inside, because showers, cooking, laundry, and breathing add water vapor to the air and the closed-up house keeps it there. When that warm, humid air reaches a cold surface, it condenses. PNNL's guide explains the mechanism for attics: condensation forms when surface temperatures drop below the dewpoint of the air they are exposed to, and it recommends indoor relative humidity between 30 and 50 percent during the heating season. The EPA says below 60 percent, ideally 30 to 50, and the CDC says no higher than 50 percent all day long.
That is why many Portland-area mold problems show up between November and March and are first noticed as a musty smell, window condensation, or a stain on a ceiling. The fixes differ by where the moisture is, and each has its own guide.
Where the moisture comes from and where to read more
Five paths account for most of what builds up in a house here.
Roof water. A square foot of roof collects about 0.62 gallons for each inch of rain, so a 1,000-square-foot roof footprint sends roughly 23,000 gallons a year toward the house. Gutters, downspouts, and the slope of the ground decide where it goes. See gutters and drainage.Ground moisture. Wet soil releases vapor and, after storms, water. Under a house with a crawl space, that vapor reaches the framing. See crawl space and subfloor.Indoor humidity. Showers, cooking, laundry, and breathing, held in by a closed house. See bathrooms and walls for fans and cold-wall condensation.Cold surfaces in the attic. Moist air escaping from the rooms below meets cold roof sheathing. See attic mold.Leaks. Plumbing, roofs, and appliances. A leak stays hidden longer in a wet season, because everything is damp anyway. See water damage and moisture control.What to check each season
None of this needs a professional. It is a routine you can do yourself, and the point is to find a problem when it is small.
- 1
September to early October: before the rain
Clean the gutters and check that each downspout carries water away from the house. Look at the ground next to the foundation for low spots. Check the crawl space hatch and vents, and look for wet insulation. Test the bath and kitchen fans, and look at where the ducts end. The September average is 1.52 inches, and October jumps to 3.42.
- 2
November to January: the peak
These are the three wettest months, and December and January are the coldest, so the heating season and the rain overlap. Watch indoor humidity with a hygrometer, run bath fans for an hour after a shower as PNNL recommends, and look for window condensation, damp closets on outside walls, and ceiling stains after storms. Dry anything that gets wet within 24 to 48 hours.
- 3
February to April: the tail of the wet season
Look in the attic if you can do so safely, and under the house, for stains, frost residue, or fresh growth. Check the roof and flashings after the winter. Spring is a good time to book inspections or repairs.
- 4
May to June: drying out
Rainfall falls from 2.51 inches in May to 1.63 in June. Open up the house, repair what the winter showed, and fix gutters or grading.
- 5
July and August: the dry months
At about half an inch a month, this is the driest stretch. It is the best time to do exterior work, to dry out a damp space, and to do repairs that need dry weather.
How you know it is time to call someone
A hygrometer reading that stays above 50 percent after you have run the fans and fixed the obvious sources, a musty smell that returns, staining that spreads, or any visible growth on more than a small area are all reasons to ask for an inspection. The inspection page explains what that involves. If you are not sure whether what you see is mold, see Is it mold, mildew, or dirt?.
Frequently asked questions
Is mold more common in Portland than elsewhere?
This guide does not make that comparison. The point is that Portland's rain is concentrated in the cool months, which is when houses are closed up and heated, so moisture problems tend to show up between November and March.
Why does my house smell musty in winter?
A musty smell means there is moisture somewhere. The usual places are a damp crawl space or basement, a closet on an outside wall, the attic, or a slow leak. A hygrometer and a look at the sources on this page will narrow it down.
What should indoor humidity be?
The EPA says below 60 percent and ideally between 30 and 50 percent. The CDC says no higher than 50 percent all day long.
Does a dehumidifier solve it?
It lowers humidity in the air. If water is entering through the roof, the ground, or a leak, the water needs to be stopped too. See the water damage page.
When is the best time to fix drainage or the crawl space?
July and August are the driest months, with about half an inch of rain each, so exterior work and drying-out are easiest then.
Sources
- NOAA NCEI, U.S. Climate Normals 1991 to 2020, Portland International Airport (monthly temperature and precipitation)
- U.S. EPA, A Brief Guide to Mold, Moisture and Your Home
- Oregon OSHA, Mold fact sheet FS-10
- CDC, About Mold and Your Health
- PNNL Building America Solution Center, Condensation Control in Attics and Roofs in Cold Weather
- PNNL Building America Solution Center, Bathroom Exhaust Fans
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