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Battling Moisture and Mold During Japan Winters

Mold is a serious health issue and is something you need to pay attention to.

By 3 min read 7

Here in the cold northernmost island of Japan, apartments and houses are very different when it comes to how their rooms react to the cold weather, and the issues of excessive moisture and mold tend to arise as an indirect result of this weather.

If there is one thing I’ve found is a fairly large problem in my apartment during the winter it is condensation, mainly on the windows and the entrance door which is made of steel. In houses with proper heating and ventilation which keep “room temperature air” circulating through all of its rooms (in addition to the use of dehumidifiers), this problem is almost non-existent.

There are a wide variety of reasons that condensation occurs, ranging from room temperatures that are too low (the cold air can’t hold the moisture so it deposits on the windows) and high temperature showers, boiling water, etc. that contributes to higher humidity levels indoors.

For people like who have a modest-at-best income and live in an apartment without central heating, having a heater every room is not a very efficient option money-wise. As expensive electricity bills make for an even more difficult winter, we have to be a bit more creative with our solutions to avoid this.


For starters, a 除湿器 (joshitsuki, otherwise known as a dehumidifier) is important for rooms that are cannot be heated most of the time. I personally reserve the joshitsuki for our bedroom so that water does not deposit heavily onto the windows, which then drip onto the walls and floor and lead to an even worse enemy than unwelcome excessive moisture: mold.

For other rooms that get neither the heater nor the joshitsuki, a bit more maintenance is required to prevent excessive moisture and mold. Every morning I wake up and use a 結露取ワイパー (ketsuro tori waipaa, or “condensation removing wiper”), an ingenious invention that lifts water from smooth surfaces like glass and stores it in the handle to be dumped later.

As the condensation also builds up around the window frame and the surrounding walls, a daily wipe with a rag is also necessary.

During the winter, windows are rarely opened because of the cold temperatures. This leads to stagnant, motionless air which is a large contributing factor to the development of mold. At the very least, you should open your windows at least once every couple days for a good 20-30 minutes to cycle in fresh new air and hopefully get a nice breeze through your rooms as well. Since this can be quite cold, I recommend closing off these rooms during the process.

For worst case scenarios where moisture and stagnant air have led to mold, you need to act quickly as the presence of mold in your home is detrimental to your health. My personal recommendation is to pick up a mold killing spray made for walls like カビキラー (kabi kiraa) or “mold killer”. It is recommended to spray on moldy areas until the spray gets all bubbly and leave it for 20-30 minutes before wiping it clean.

You additionally need to have a nearby window open to keep the area well ventilated as the chemicals in the spray include bleach, the fumes of which should be inhaled as little as possible.

You should always confirm that it is the correct kind of mold spray. Some types are only for bath areas and some are meant for the walls of your house, so walls could end up damaged if you choose the wrong kind. Asking a store employee これは壁の為ですか? (kore wa kabe no tame desuka) meaning “is this for walls?” while showing them the product should get you the confirmation you need.

No matter how you choose to deal with excessive moisture and mold, always keep these two basics in mind:

1. Temperature and humidity regulation is important for controlling moisture, so make sure your rooms either have a heater and/or dehumidifier. If this isn’t possible use the moisture removal methods listed above.

2. Mold likes moist areas with little to no air flow, so make sure you find these mold prone areas and keep them dry with moving air.

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  • Miya says:

    I rented an apartment on 2nd floor, and the flooring where I DIRECTLY lay my Futon/beddings are damp and moist as I woke up in the morning. I’m worried that my Futon/beddings will get mold and be damage sooner. Any recommendation on what should I do with it?

  • Reicheru Furiizaa says:

    Just wanted to ask with the dehumidifier do you run it all day or just when you’re in the bedroom? Wouldn’t it also rack up the electricity bill?

  • Globbi says:

    Hello from Poland where we’ve always been heating and insulating our homes .

    You should open your window way more often than once every couple days, even if it’s -20C outside. Open it at least once a day for no longer than 5 minutes, but preferably a few times a day like this. It will actually not make your bills higher, since heating water is harder than heating air. Lowering the humidity will make heating more efficient – that’s physics. If you’re heat-loving to the degree that you can’t stand window open for a couple minutes then do it when you go to pee or wrap yourself with blanket and get some hot tea.

    For chemicals, you don’t really have to use them usually. Wipe the wall well to make it dry and keep the room ventilated. There shouldn’t be much fungi in the air after you wipe it. Mold is not good to breath it in, chemicals killing it aren’t either. Eventually you will have to clean it more throughly when it’s warm and dry – change the wallpaper or scrub and repaint the wall. You’ll have to do it whether you use chemicals or not. Only reason to use chemical mold killer (though more generic toilet cleaners are good enough and often cheaper) is for bathrooms when you can’t get rid of something in the corner and it keeps staying wet.

    One thing to add: check for mold behind furniture. Sometimes you may not realize that some drawer in a corner has mold behind it. You make have to move it or at least make a small gap between it and the wall.

    • Kyle Von Lanken says:

      Thanks for the tips, Globbi. As a community I think we can all learn from each other.

  • Ashwin Campbell says:

    Most bleach cleaners like Kabi killer contain water so they kill the immediate mold on the surface but actually feed the mold deeper in the wall or flooring. UV light is good at killing mold on walls but that means having good window placement. For door seals, I spray hydrogen peroxide on the mold and let it sit until it dries. That usually solves the mold on doors and windows problem (don’t spray it on walls or wood though as it is mostly water once the hydrogen has interacted with the air and sunlight).

    • Kyle Von Lanken says:

      I may very well try the UV light thing if the mold comes back. Thanks for the tip, Ashwin!

  • Katherine Moser Kelly says:

    We use dehumidifiers and heaters but we only run our heaters when it is very cold in the house. Many here have told me to get a humidifier for skin and face but we are all okay in that regard AND once we lived in Colorado where it equally dry and we used a humidifier for our newborn and when it was time to move out we found a large patch of mold on the wall behind the clothes. NOT good!



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