A survey of 3,099 English homes logged living room and bedroom humidity and scored visible mold. Homes where bills were hard to pay, or the heating fell short, ran damper and had more significant mold, and bedrooms ran damper than living rooms.
If you have a musty smell this fall, or mold behind the bed, look at how warm each room is kept. In a study of low-income homes in England, households that found their bills hard to pay had significant mold about twice as often as those that did not, 14.4% vs 7.6%. Bedrooms ran colder and damper than living rooms, so measure humidity where you sleep, not only where the thermostat is.
14.4% vs 7.6%: share of homes with significant mold where paying bills was fairly or very difficult vs not (odds ratio 2.20, 95% CI 1.68 to 2.89).
The study
Tadj Oreszczyn, Ian Ridley and Sung H. Hong of University College London and Paul Wilkinson of the London School of Hygiene & Tropical Medicine, with the Warm Front Study Group, published the paper in Indoor and Built Environment in 2006. Warm Front was England’s grant scheme for heating and insulation in low-income homes, and the study was part of its national evaluation.
The team surveyed 3,099 homes in Birmingham, Liverpool, Manchester, Newcastle and Southampton over the winters of 2001-02 and 2002-03. Inspectors scored visible mold on windows, walls and ceilings with a standard mold severity index. In 1,604 homes, data loggers recorded temperature and relative humidity in the living room and the main bedroom every half hour for two to four weeks. The humidity analysis used the 1,095 homes recorded during cold spells, with each home’s readings converted to a “standardized” value for the same cold outdoor day so homes could be compared.
What they found
Significant mold, meaning a severity score above 1, which leaves out only the lightest “slight” cases, turned up in 10.1% of homes.
Heating that doesn’t do the job: 13.4% of homes where people were dissatisfied with their heating had significant mold, vs 7.8% of satisfied homes (odds ratio 2.05, 1.55 to 2.70).
The humidity readings moved the same way. Bedrooms in homes dissatisfied with their heating ran 5.27 points higher in standardized relative humidity (3.90 to 6.65), and bedrooms in homes struggling with bills ran 2.82 points higher (1.39 to 4.24).
The bedroom ran damper than the living room. Median standardized humidity was 49.2% in bedrooms vs 42.8% in living rooms. Median standardized temperature was 17.1°C (about 63°F) in bedrooms at night vs 19.1°C (about 66°F) in living rooms by day.
Humidity tracked mold. In the most humid quarter of homes, 16.1% had significant mold, vs 5.8% in the driest quarter (odds ratio 3.57, 1.84 to 6.94). More people, more mold: 16.3% of homes with four or more people had significant mold vs 6.3% of one-person homes.
Why it happens
Relative humidity depends on temperature. Warm air holds more moisture, so the same amount of water vapor reads as lower relative humidity in a warm room and higher in a cold one. The authors write that most of the drop in humidity after the Warm Front upgrades came “because of the increase in temperature,” not from changes in airtightness.
Walls make it worse. In the authors’ words, “the relative humidity at the surface of an outside wall will be higher than in the bulk of the room air.” Laboratory work they cite found mold grows when wall surface humidity stays above 80% for several weeks. They point to the places where the surface runs colder or wetter than the room: thermal bridges, behind furniture, and “around bed headboards where people exhale.”
They also give a seasonal example: on a damp autumn day at 12°C (about 54°F) with saturated outdoor air, a room held at 19°C (about 66°F) would sit near 72% humidity.
The limits, stated plainly
This is a snapshot survey, not a trial. Difficulty paying bills and satisfaction with heating were self-reported, and the mold comparisons for those questions were adjusted only for year and area, and the humidity comparisons only for year and region, not for each other or for the house itself. The bedroom figures cover night hours and the living room figures day hours, so the two rooms were not measured over the same hours. The homes were low-income English homes measured more than 20 years ago, not Portland or Vancouver houses. Mold was scored by eye, and the species was not identified. The “standardized” humidity is a calculated value set to a cold outdoor day; the authors note it reads lower than the humidity a home actually reaches in damper weather.
What survives: in these homes, a harder-to-heat house was a damper one and had more significant mold.
What the heating and insulation upgrades were associated with
The study compared homes measured after Warm Front upgrades with homes measured before, adjusted for area, year, deprivation and household size (the mold comparison used 2,155 homes with complete data, the bedroom humidity comparison 640). With a gas central heating system alone, the odds of significant mold were lower (odds ratio 0.47, 0.30 to 0.75). With heating plus insulation, 0.55 (0.38 to 0.81), and bedroom humidity was 7.62 points lower.
When the energy-efficiency rating of the house was added to the model, the bedroom humidity drop held (5.63 points lower with heating plus insulation), but the confidence intervals for the mold odds ratios now included 1 (0.81, 0.53 to 1.24; heating alone 0.66, 0.40 to 1.10). The authors put it more softly: adding that rating “weakened but did not abolish” the association. Read together, the upgrades were associated with clearly lower humidity; their link to less mold was weaker once efficiency was counted.
What gets lost in the mold-season marketing
Mold-killing sprays and cover-up paints do not fix the water problem. The US EPA’s mold guide (page updated February 18, 2026) says a biocide such as chlorine bleach “is not recommended as a routine practice during mold cleanup,” and it is blunt about paint: “Do not paint or caulk moldy surfaces.” It also warns: “If you clean up the mold, but don’t fix the water problem, then, most likely, the mold problem will come back.” Its first rule is the one this study supports: “The key to mold control is moisture control.”
What the data supports
- Keep the bedroom from running much colder than the rest of the house in the damp months. EPA lists “Increase air temperature” among the ways to prevent condensation.
- Measure humidity where you sleep, overnight, not only in the hallway. EPA: “keep indoor humidity below 60 percent (ideally between 30 and 50 percent).”
- Pull beds and dressers off cold outside walls, and look behind them.
- Run a fan when showering and cooking, and vent the dryer outside, as EPA advises.
- Treat condensation on windows as a warning. EPA: “Condensation can be a sign of high humidity.”
Bottom line
In these homes, a house that was hard to heat ran damper and had more significant mold, and bedrooms ran damper than living rooms. A single reading in the warmest room can miss it, which is why room-by-room measurement matters (see how our testing works).
First step: put a humidity reading next to the bed for a week.
Mold testing in Portland and Vancouver, room by room
illumenair does mold testing and indoor air quality testing in Portland and Vancouver. We test and report. We do not do remediation.
971-363-5626 · info@illumenair.com · www.illumenair.com
Sources: Oreszczyn T, Ridley I, Hong SH, Wilkinson P, Warm Front Study Group. Indoor and Built Environment 2006;15(2):125-135. doi:10.1177/1420326X06063051. Author manuscript, UCL Discovery, read October 25, 2026. US EPA, A Brief Guide to Mold, Moisture and Your Home, last updated February 18, 2026, read October 25, 2026.