WINTERTIME HOUSEHOLD MOLD: Why It Happens & How to Stop It

Winter indoor mold

Cold winters present plenty of challenges, and one of the most widespread is indoor mold growth. Unlike warmer parts of the world where mold grows mostly during humid, summertime weather, winters are the primary household mold season for people like me who live in cold climates. And since windows are closed and we spend lots of time indoors, household mold can also bring significant indoor air quality issues, too.  Understanding the causes of winter mold growth and the solutions is something that can make a big difference to your health.

Temperature differences between indoor and outdoor spaces are why winter is a mold-prone time of year in cold climates. And the wider the temperature difference, the more mold pressure develops. The reason is because of a peculiar trait of air. The cooler the air is, the less moisture it can hold. Whenever warm, indoor air is allowed to make its way into cooler areas around windows, inside wall cavities and in attics, the ability of that air to hold moisture declines. Indoor air with a comfortable level of 50% relative humidity at 22ºC, for instance, will rise to 100% relative humidity when that same air cools to just 11ºC, all else remaining equal. Any further cooling will result in the formation of water droplets appearing out of nowhere on surfaces. Mold can only grow in the presence of sufficient moisture, but as soon as that moisture appears, mold flourishes. This dynamic of cooling and condensing is why your windows might get wet on the inside during cold weather, and why mold develops inside wall cavities that don’t have an effective vapour barrier. Even poorly insulated walls can develop visible mold on interior surfaces when the weather gets cold outside and furniture prevents the circulation of warm air in those areas. If mold ever grows on your walls in winter, it’s almost always behind a couch or dresser.

If your house grows mold in winter, the solution is two fold. First, you need to lower indoor humidity levels. This is something of a balancing act, because the level of humidity we want indoors for comfort is almost always higher than the level of indoor humidity that’s ideal for our home. A house that’s got an ideal humidity level for structural integrity during winter will usually feel somewhat too dry for the humans living there.

Winter Mold Attic Hatch

The ideal way to reduce indoor humidity levels in winter is with a heat recovery ventilator (HRV). This permanently installed ventilation device swaps stale indoor air for fresh outdoor air, all while retaining most of the heat invested in the indoor air before shooting it outside. Don’t bother trying to reduce indoor humidity levels in winter with a dehumidifier. They can’t reduce humidity levels enough to stop wintertime condensation, they use way more electricity than an HRV, and dehumidifiers make more noise. The only problem with an HRV is the cost. You’ll spend about $2000 to get one installed. If you don’t have that kind of dough handy, simply run your household exhaust fans more often. Bathroom fans and kitchen range hoods can do a lot to reduce indoor humidity levels. For every cubic foot of air they expel from the building, a cubic foot of fresh, cold outdoor air must come inside through gaps and cracks.  As this air warms up, its relative humidity plummets, making your home drier.

how to get rid of mold

The second part of the mold solution involves preventing warm indoor air from getting to places where it can cool and condense. Uninsulated attic hatches are a classic place for mold to grow in winter because they get so cold. I receive a constant stream of questions from homeowners about indoor mold growth, and that’s why I created a detailed course on how to get rid of household mold once and for all. Normally I sell this, course, but you can take it in for FREE right here. You’ll learn:

  • why mold grows in houses
  • how mold is an indication of underlying problems, not a problem on its own
  • the best way to boost household ventilation to solve mold problems
  • options for removing mold stains without bleach

Visit to take in this FREE course while it’s still free.