Most conversations about a damp house start at the building: insulation, windows, air sealing, a crawl space. Those matter. But the water in the air of a Pacific Northwest home in January is mostly water the household put there, and one activity puts in far more than the rest.
What they did
Younhee Choi, Younghoon Lim, Joowook Kim and Doosam Song, at Sungkyunkwan University in Korea, published this work in Sustainability in September 2020. It is open access.
Two measurements ran side by side. The first was long-term: 17 apartments in a single public rental complex in Gyeonggi-do, with indoor and outdoor temperature and humidity logged every 15 minutes through January and February 2019. Seven of the units were built in 2013; the other ten were 27 years old. None of them had a mechanical ventilation system. Each unit is 21.78 m², laid out as a 10.53 m² combined bedroom and living room plus a 10.73 m² kitchen and utility space.
The second was controlled. Doors shut, bathroom fan off, one activity at a time, water either weighed or calculated from the humidity change. Showering and bathing five times each, about 20 minutes each. Cooking twice: one liter of water boiled for 30 minutes as a stew, pot weighed before and after. Laundry twice: about 3 kg dried on a rack in the living room, weighed wet, then weighed again once dry.
What they found
Drying one load of laundry indoors released 1,643.6 grams of water, at an average of 173.6 grams per hour.
The other activities, measured the same way: cooking released 290.5 grams per event, which the authors convert to 871.5 grams a day across three meals. One shower released 125.3 grams, at 358.1 grams per hour, the highest hourly rate they reported. One bath released 51.1 grams, at 153.2 grams per hour.
Add the shower, the bath and a full day of cooking together and the total is 1,047.9 grams. The single load of laundry released 1,643.6 grams on its own, about 1.6 times all three combined.
The newer apartments were the damper ones
Air leakage, measured in a representative unit of each construction age, was 0.1 air changes per hour for the 2013 build and 0.42 for the 27-year-old one, so the newer stock was roughly four times tighter. Their internal moisture excess ran about 2.4 g/m³ higher.
Internal moisture excess is the amount of extra water vapor the indoor air carries compared with the outdoor air. Across the study it averaged 4.69 g/m³, which the authors describe as about twice typical European values. For scale, the wettest room in a comparable Norwegian dataset, a bathroom, sat around 3.6 g/m³. The top decile of these Korean homes reached 7.42 g/m³; the equivalent figure for Finnish dwellings is close to 4.0 g/m³. Outdoor daily means stayed below 8 °C for the whole period.
Why a low-moisture household can still be a wet one
Moisture excess is production divided by air exchange. These households produced less water than design assumptions predict: one or two occupants, small kitchens, lunch often delivered rather than cooked. They were damp anyway, because the water had nowhere to go. No mechanical ventilation, an open studio plan that carries kitchen and laundry moisture straight into the sleeping area, and heat that costs money, so windows stay shut.
The authors’ own conclusion is blunt: improving the building envelope further is not the fix, and controlling indoor humidity is.
The limits, stated plainly
Seventeen apartments, one complex, one winter, and unusually small units. The controlled runs were repeated twice for cooking and twice for laundry, five times each for shower and bath, which is enough to be indicative rather than precise. Load size, spin speed and fabric all move the laundry figure, and the authors say so. And nothing in this study measured anyone’s health: no asthma, no allergy, no mold count. The paper’s health language cites other people’s work, not its own findings, and we are not going to present it otherwise.
What survives the caveats is narrow and useful: drying laundry indoors is a large, repeated, entirely voluntary moisture load, and tightening a building does not shrink it. It makes it stay.
What gets lost in the damp-house marketing
Heated drying racks, anti-condensation paint, moisture-absorbing tubs and dehumidifiers all get sold against this problem. Only the dehumidifier takes water out of the house, and only if it is emptied. Anti-condensation paint nudges a surface temperature slightly; it does not change how much water is in the air. Absorber tubs pull grams, where a single load of laundry here was 1.6 kilograms. A heated airer dries clothes faster and puts every gram of that water into the same room, sooner.
What the data supports
Dry laundry where the water can leave: outdoors, in a vented dryer, or in one small room with the door closed and the extract fan running.
Run the bath fan during and after a shower. The shower had the highest hourly rate the authors reported, at 358.1 grams per hour.
Treat air sealing as half a job. If a house is tightened, it needs a deliberate way to move air out, or the moisture that used to leak away simply stays.
Watch cold surfaces as well as the humidity reading. Condensation shows up first on the coldest ones, which in a Portland or Vancouver home usually means window frames, north-facing closet walls and the corner behind furniture.
The bottom line
Dampness is arithmetic: make less water indoors, or move more air out. In a Pacific Northwest winter, the laundry rack is often the biggest term in that equation. The one small first step: dry one load this week somewhere the water can actually leave, and see whether the windows still stream in the morning.
Before you blame the building, find out where the water actually is
illumenair measures moisture, humidity and airborne mold room by room in Portland and Vancouver homes. We measure air. We do not diagnose or remediate.
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Sources: Choi Y, Lim Y, Kim J, Song D. “Why Does a High Humidity Level Form in Low-Income Households Despite Low Water Vapor Generation?” Sustainability, 2020;12(18):7563.