In 64 homes logged all winter, people who said they always ventilated during and after showers did not have measurably drier bathrooms than everyone else. Median bathroom humidity was 67%, above the EPA’s 60% ceiling.
If you run the fan or crack the window every time you shower, you probably assume the bathroom dries out afterwards. A New Zealand study that logged bathroom humidity every 15 minutes through a whole winter found that what people said they did had no significant link to how damp their bathrooms stayed. The only way to know about yours is to measure it.
67%:
the median bathroom humidity across 64 homes over the 2016 winter. The EPA says indoor humidity should be kept below 60 percent.
The source
BRANZ, New Zealand’s building research organization, ran the Occupant Ventilation Behaviour study over the 2016 winter, funded by the Building Research Levy. Data analyst Vicki White and senior physicist Manfred Plagmann reported the bathroom results in “Bathroom habits falling short,” Build magazine issue 159 (April/May 2017).
Sensors logged temperature and relative humidity in the bedrooms and bathrooms of 64 houses across New Zealand every 15 minutes, about 20 million readings in all. Sensors on internal doors and windows recorded how often and by how much they were opened. When the sensors came back, each household answered a questionnaire about its ventilation habits and any damp, mold or comfort problems. All but five of the households had also been in the BRANZ 2015 House Condition Survey.
What they found
Bathrooms stayed humid. The median bathroom humidity across all homes was 67%, and the per-house medians ranged from 43% to 88%. One-third of the houses had a median of 70% or higher, and six houses (10%) were at 75% or higher. In over half the homes, bathroom humidity sat above 65% at least half the time. The authors expected peaks while the bathroom was in use, and said those peaks “should be brief.”
Bathrooms were cold. The median bathroom temperature was 16 C (about 61 F), ranging from 8 to 20 C (about 46 to 68 F). Nearly half of households (48%) had a median under 16 C, and 19% were under 14 C (about 57 F).
Ten houses were both. Ten houses (16%) had median bathroom temperatures in the coldest quarter (8 to 14 C) and median humidity in the dampest quarter (72% to 80%). Those are the bathrooms the authors flag as most prone to condensation and mold.
What people said did not match what was measured. 73% said they always ventilated the bathroom while showering or bathing, and 70% always did afterwards, by fan or window. 27% had no extractor fan at all, and 2% had no window that opened. There was no significant difference in median bathroom humidity based on the ventilation habits people reported. Bathrooms with heating available were not significantly warmer than those without it.
The authors conclude that where fans, windows and heaters are available, they “are not being used sufficiently or effectively and/or are not functioning adequately,” and that householders may “think they are doing the right thing.”
Why it would work this way
Relative humidity depends on temperature. The same shower puts the same water into the air, and in a cold bathroom it shows up as a higher relative humidity. Cold ceilings, outside walls and window frames are where that water condenses, and condensation is where mold starts. A fan that is switched on may still move too little air, run for too short a time, or lose to a door left open while the steam moves into the rest of the house.
The limits, stated plainly
This is a trade magazine report of a preliminary analysis, written by the researchers, not a peer-reviewed paper. It does not give a p-value or the size of each habit group for the comparison that matters most here.
It covers 64 New Zealand homes in one winter. The bathrooms in this study were often cold, so results may differ in warmer Portland or Vancouver bathrooms. Habits were self-reported. No mold was sampled and no health outcome was studied.
What survives: in these homes, what people believed they did in the bathroom did not predict how damp it stayed.
What happens when someone tries to fix it
There is no trial in this report. The closest local example is small. In a paper by four authors from the University of Oregon Department of Architecture, the team logged a single two-person apartment in Eugene in February and March 2010. Its 110 cfm bathroom fan ran on a timer about four hours a day. They swapped the timer for a humidity-sensing control. The fan ran more, 30.2 hours over five days against 22 hours on the timer, and humidity still spent more time above 70%: 34.2 hours against nearly 25. The authors conclude that the bathroom ceiling stayed above 70% “even longer” than with the timer. They also saw humidity rise in the kitchen and living room after showers. It is one apartment and not a published journal study, so read it as an illustration.
What gets lost in the bathroom fan marketing
Bathroom fans are sold by rated airflow, and humidity-sensing switches are sold as the fix for a steamy bathroom. Neither number on the box tells you whether your bathroom actually dries out. The EPA’s Mold Course (page updated December 1, 2025) says indoor relative humidity should be kept “below 60 percent” and “ideally between 30 percent and 50 percent, if possible,” and notes that a gauge that reads temperature and humidity is “generally available for less than $50.”
What the data supports
Measure first. Put a humidity gauge in the bathroom and read it an hour after a shower, and again the next morning.
Run the fan during the shower and keep it running afterwards until the moisture has cleared, with the bathroom door shut so the steam does not move into the rest of the house. That is BRANZ’s own advice.
Warm a cold bathroom before showering, and wipe condensation off windows and walls.
If it still reads high, have the fan’s real airflow checked, and look closely at ceiling corners and any cold outside wall, where condensation collects.
Bottom line
Running the fan is not proof the bathroom dries out. In 64 homes, saying “we always ventilate” did not mean a measurably drier bathroom. If your bathroom smells musty or the ceiling keeps spotting, see how our room-by-room testing works, or book mold testing and indoor air quality testing in Portland or Vancouver.
First step: a humidity gauge on the bathroom shelf tonight.
Mold testing and indoor air quality testing in Portland and Vancouver, room by room.
illumenair does indoor air quality testing and mold testing in Portland and Vancouver. We test and report. We do not do remediation.
971-363-5626 · info@illumenair.com · www.illumenair.com
Sources: White V, Plagmann M. Bathroom habits falling short. Build 159, April/May 2017, pages 68 to 70, BRANZ. Read November 1, 2026. Clark M, Craig-Bennett L, Deel C, Skoog C. Help, it’s running and I can’t turn it off! An investigation of humidity control and timer operated fans. Department of Architecture, University of Oregon, 2010. Read November 1, 2026. U.S. EPA. Mold Course Chapter 2: Why and Where Mold Grows. Last updated December 1, 2025; read November 1, 2026.