How to prevent moisture and mold in your apartment during Japan’s rainy season

During Japan’s rainy season, moisture build up in your home can make life a bit uncomfortable and if left unchecked could lead to a much more serious problem, in the form of mold.

Mold growth in your home is, of course, a health hazard. It can also cause permanent stains on walls and ceilings. If you’re not able to remove these stains yourself, you may be charged a cleaning fee when you move out.

Many rental apartments in Japan also don’t come equipped with a washer and dryer, so it’s typical to air dry laundry on the balcony. However, during the rainy season, which usually lasts from the beginning of June to the end of July in Tokyo, most people hang up their laundry indoors to dry. This can lead to even more moisture in the air.

Dreaded black mold! Image: iStock 1129349088

Since moisture build up in homes is a ubiquitous problem during the rainy reason, there are many products sold at this time of year to help dehumidify living spaces.

Below we go over five things you can do to help prevent the build-up of humidity in your apartment or house. Many of these don’t cost any money or only a nominal amount.

Let’s take a look at what to do in the rainy season and let’s stay as dry as possible out there, folks.

Ventilate your home

The simplest way to ventilate a room is to open the windows, but of course, on a rainy day, an open window would only increase the humidity in your home.

On days (or the few hours) when it’s sunny, try opening at least two windows in order to increase air circulation. You can also improve ventilation by running a floor fan, if you have one, with the windows open.

Another common method is to run your bathroom ventilation fan with the bathroom door open, even if you haven’t just showered or bathed.

If you have a wall-mounted air conditioner, you can also keep the air moving around in your room by using the ventilation function. This function uses less electricity than using the cooling function, while still circulating the air.

Use the button labeled 送風 (soufuu) or 換気 (kanki) on your A/C remote.

The 送風 (soufuu) or literally “blower” function button is circled in red. Use this function to circulate air through your room. The dehumidify (除湿, joshitsu) function is circled in blue. Most A/C units come with a dehumidifier function. Image: Real Estate Japan

Click here for a guide to using a Japanese air conditioner with English translation.

Use a dehumidifier

Dehumidifiers, joshitsu-ki (除湿機) in Japanese, are one of the most popular summertime purchases for the home.

They can be quite expensive if you spring for a high-capacity model, but you can also find compact units for about ¥6,000 to ¥7,000 ($55 to $64 USD) on or one of the big home appliance retailers like BIC Camera or Yamada Denki.

As shown in the diagram of the A/C remote, most wall-mounted A/C units come with a dehumidifier function, but they may not be as energy efficient as standalone dehumidifier units.

Dehumidifier. Image: iStock 1151153967

Another consideration is the running cost of a dehumidifier, which works using a compressor, desiccant, or hybrid of the two. When shopping for one, you should consider not just the upfront cost of the unit but also the impact on your electricity bill. Compressor models are less costly in terms of the impact on your electricity bill, while hybrid models are more pricey.

However, it may well be worth the investment in terms of upping your comfort and keeping your home, clothing and other personal possessions from picking up too much moisture and creating ideal conditions for mold growth.

Wipe down and prevent condensation on windows

Along with wet shoes and ¥100 vinyl umbrellas, condensation on windows is a fact of life during the rainy season.

Make it a habit to wipe down your windows and sliding glass doors in order to prevent mold from growing in the corners of the window frames and to keep the moisture level down in your home, in general.

To prevent condensation, wipe down windows with a solution of dishwashing liquid and water (using a ratio of 1:10 to 1:20). Apparently, this will minimize condensation but you do have to periodically re-apply the solution. The added benefit being that you will have extra clean windows!

Baking soda and/or Mr. Elephant

Baking soda, called juso (重曹) in Japanese, can be used to absorb moisture around the home, not just in your fridge.

Fill a small bowl or container with some baking soda and cover it with lightweight gauze to make your own demoisterizer packet. You can place these in your closets, drawers, under the sink and the corners of rooms.

There are also many demoisterizer products you can buy that quite effectively soak up moisture.

Probably the most widely used and most effective is called Mizu-tori Zou-san  (水とりぞうさん). These small plastic tubs filled with a chemical desiccant can be found at drugstores and supermarkets. “Zou-san” means elephant. Look for pink packaging with a cartoon elephant. Place the plastic containers in your closet and the corners of rooms; and replace them as the container fills up with water.


Charcoal, called sumi (炭) in Japanese, can be used as both a deodorizer and to soak up moisture.

Charcoal and charcoal powder.  Image: iStock 672325124

Place a small bowl of charcoal briquettes on a shelf or in your entryway (genkan), for example, to soak up moisture and bad odors in a confined space.

Also very good to know and do

Here are other articles related to dealing with the rainy season and keeping your apartment squeaky clean!

Dealing with laundry and mold during the rainy season in Japan

Japan apartment maintenance: Mold prevention in the bathroom

Japan apartment maintenance: Cleaning your air conditioning unit

How to clean tatami mats in a Japanese apartment

Guide to essential home cleaning products in Japan

Lead photo: Drying laundry indoors, iStock 1237067754