Sealing up a house can cut mold and still leave the air no better. In 944 adults living in English social housing, the most energy-efficient homes had far less mold, yet the people in them were more likely to have seen a doctor for asthma. Mold did not explain the gap.
If you have insulated, air-sealed or replaced windows, fewer mold spots tell you about the walls. To know about the air, check the ventilation and test the air.
The study
Richard Sharpe, Christopher Thornton, Vasilis Nikolaou and Nicholas Osborne, working from the University of Exeter, published the study in Environment International in 2015. The authors declare no conflicts of interest.
They mailed questionnaires to 3,867 social housing homes in Cornwall, in England’s damp, mild southwest, between August 2012 and January 2014. The answers were matched to the housing association’s own property records, including each home’s official energy efficiency rating. That rating, SAP, is a UK government score that runs from 0 to 120.
18% responded, which gave 944 adults with a mean age of 59. Ratings in the study ran from 24 to 88, with a mean of 65.7. 99% of the homes had double glazing, and 44% of participating homes had visible mold somewhere.
What they found
Mold went down as efficiency went up. In the least efficient homes (SAP 24 to 60), 60% of people reported visible mold. In the most efficient (SAP 71 and up), 35% did. A musty smell fell from 37% to 20%.
After adjustment, the odds of visible mold in the most efficient group were 0.4 (95% CI 0.2 to 0.7) and of a musty smell 0.3 (0.1 to 0.7). Each extra rating point went with 4% to 5% lower odds of both.
Asthma went the other way.
15% of adults in the least efficient homes had seen a doctor for asthma in the past year, against 28% in the most efficient.
Adjusted for age, sex, income, smoking, BMI, pets, length of tenancy and the dates of window, loft insulation and heating upgrades, the odds ratio for the most efficient group was 2.6 (1.3 to 5.3). Adding visible mold to that model moved it to 2.9 (1.4 to 6.0), so mold did not account for it.
In homes with no visible mold at all, each extra rating point still went with about 4% higher odds of asthma (1.04, 1.01 to 1.08).
Mold still mattered by itself. A musty smell went with about double the odds of asthma (OR 2.2, 1.3 to 3.8). Visible mold, once adjusted, fell short of significance (1.7, 0.9 to 2.9).
Why it might happen
The authors’ leading explanation is that a well-sealed home without enough heating and ventilation holds on to what a draftier house would dilute: moisture, combustion products, chemicals and allergens. That is their inference. The study measured no air, no ventilation rate and no pollutant.
They also point to gas, which adds moisture and combustion by-products indoors. In the paper’s comparison table, 85% of people in the most efficient homes used gas for heating or cooking, against 18% in the least efficient.
The limits, stated plainly
This is a snapshot survey, so it cannot show that efficiency caused asthma. Only 18% answered, and the authors note that people with breathing problems may have been likelier to reply, and to report mold. Mold and asthma were both self-reported.
The two ends of the rating scale held different people. The efficient homes had more unemployment, more deprivation and more gas heating. When the authors added those factors, the per-point link was 1.03 (0.98 to 1.08) and no longer significant. Comparing the top group with the bottom group, the odds ratio was 4.3 (1.3 to 14.2), an interval too wide to pin down.
Split by survey month, only January 2014 showed a significant link. The authors say their analyses lacked power and ask for a larger, more representative sample.
What survives: in these homes, less mold did not come with less asthma, and mold could not account for the difference.
The intervention evidence
This runs against a New Zealand randomized insulation trial we covered in August, where warmer, drier homes went with better health. The authors note that a review of housing interventions found health gains, and suggest their own result may reflect heating and ventilation that did not keep up with the sealing. That part is in a homeowner’s hands.
What gets lost in the energy-upgrade marketing
Insulation, air sealing and new windows are sold on comfort and lower bills, and sometimes as a healthier home.
The EPA’s page on energy, weatherization and indoor air quality, last updated June 23, 2026, says tightening a building “may also have the unintended consequence of allowing contaminants that would otherwise be diluted in leaky homes to build up to unhealthy levels.” It also says “Weatherizing without maintaining proper ventilation can negatively affect indoor air,” and that the minimum air protections in many retrofit programs “are generally not sufficient to improve IAQ.”
What the data supports
- Ask before you seal. Find out how the house will get fresh air once it is tight. The EPA’s recommended minimum for retrofit programs includes ventilation that meets ASHRAE Standard 62.2 and testing to confirm it works.
- Use your bath and kitchen fans. In a tight house, leaks no longer do that job.
- Treat a musty smell as a real warning. In this study it tracked asthma more closely than visible mold did.
- If you burn gas, a tighter house gives combustion gases fewer ways out. The EPA lists carbon monoxide among the contaminants that can build up.
Bottom line
An energy-efficient house can have less mold and still have air that is no better for the people in it. After sealing a home, check how it breathes.
First step: if you have had air sealing or new windows, find out this week whether the house has a mechanical fresh-air supply, and whether it runs.
illumenair does mold testing and indoor air quality testing in Portland and Vancouver homes. Call 971-363-5626, email info@illumenair.com or visit www.illumenair.com.
References: Sharpe RA, Thornton CR, Nikolaou V, Osborne NJ. “Higher energy efficient homes are associated with increased risk of doctor diagnosed asthma in a UK subpopulation.” Environment International, 2015;75:234-244. doi:10.1016/j.envint.2014.11.017. US EPA, “Energy, Weatherization and Indoor Air Quality,” last updated June 23, 2026.