Japanese summers are famous for their heat. It’s not just daytime temperatures that makes it unpleasant, but also the high humidity and nighttime temperatures that don’t cool down.
Further, the same features of Japanese houses that make them uncomfortably cold in winter—single-paned windows with aluminum frames and minimal attic and wall insulation—also make it challenging to keep them cool in summer.
It isn’t the heat that will get you, it’s the combination of heat and humidity.
Warm air has the capability to hold a lot of moisture, and combined with Japan’s rainy season and generally humid climate, it fulfills that potential.
What can you do about it?
The first is to keep humid air out of your apartment. The same cracks and gaps that let cold air in during winter also let humid air in during summer. If you have particularly leaky windows you can cover them with plastic sheeting to block all air movement.
Gaps in walls or around door or window frames can be sealed permanently with caulk.
The second is to avoid generating any additional moisture. Run exhaust fans by the stove while cooking (especially boiling water) and run fans in the bathroom during and after showers to remove excess moisture. Keep in mind that for air that is exhausted, replacement air needs to come in from outside, so if you have blocked off windows with plastic, you need to crack a window or door to let the exhaust fans function.
The third is to use a portable dehumidifier. These units are widely available between (10,000 and 30,000 yen) depending on the amount of air you need to dehumidify. Dehumidifying is expensive and consumes large amounts of electricity, as the units function similarly to an air conditioner. Collected water needs to be regularly dumped out from an internal storage tank. As with winter heating, you can reduce the amount of money you spend by only dehumidifying living or sleeping areas and closing doors to the rest of your apartment. Opening windows or balcony doors to air out a room will very quickly return a room to outdoor levels of humidity and negate any benefit from the dehumidifier. It’s possible to get by with just a dehumidifier and fans, but it will be quite warm inside.
The last method is to use an “aircon,” mini-split heat pump. These are true air conditioners, as unlike a dehumidifier they are connected to the outdoors and expel heat through the outdoor unit. Aircons also dehumidify as they cool, and direct water vapor outside through a condensate line, so there is no need to empty them manually.
Aircon are sized based on the amount of area they are used to cool and heat. If you are buying a new unit, look for a unit that can also work in low winter temperatures, as aircon are the most affordable and healthy method of heating throughout most of Japan. Note that sales start with the end of the heating season in September and also when new models are introduced around the new year. Their cooling capacity is greater than their heating capacity, so if in doubt, get a unit able to heat and cool a larger area. The efficiency of aircon is measured by APF (higher is better) and all units have a printed estimate for the amount they will cost to operate over the course of a year (assuming it is run year-round for heating and cooling in Tokyo’s climate for 18 hours a day.)
Mold thrives in moist, dark spaces. Expose its hiding places by keeping closet doors open, and regularly opening the drapes to let in sunlight.
Eliminate excess moisture. Keep your apartment under 55% relative humidity in the summer by always running an exhaust fan when cooking or showering, avoiding the use of humidifiers, and running a dehumidifier or aircon until humidity levels drop below 55%. Dry laundry outside instead of inside.
Tatami rooms are susceptible to mold growth, and beds or futons should not be left in contact with the mats. In addition, tatami mats should be regularly vacuumed.
Mold on windows, walls or elsewhere can be killed and regrowth can be reduced by cleaning with a mixture of water and baking soda or a diluted white vinegar solution. Bleach can discolor surfaces, is harmful to breathe and only kills mold on the surface, but can be useful for stubborn patches of mold.
After showering, wipe up water with a bathroom sponge, wring it into the drain and run an exhaust fan until moisture is no longer visible. Regularly clean the walls, floor and ceiling of the shower area with baking soda.
When purchasing a dehumidifier, make sure the model has a large enough capacity for the space you want to heat. There are some more efficient units that are inverter-driven, meaning that instead of oscillating between full power and off, they can adjust themselves gradually to meet your dehumidifying needs. The main expense with dehumidifiers is the amount of electricity needed to run them, which can easily be 10,000 or more per month. You can track your spending with a cheap electricity monitor.
The lowly fan is actually quite effective at keeping people cool by accelerating evaporation from the skin and by convection so long as the room temperature is lower than your body temperature. Note that fans do not cool rooms, so there is no need to keep them running when you are not there.
Fans use negligible amounts of electricity (a normal floor fan is 30 watts or less), so run them as long as you like, including all night without worrying about the charges. If they keep you comfortable enough not to use an aircon during times of modest outside temperatures, it will save money. Disable ion “purifying” features on fans (and aircon) as there is little evidence they improve air quality and may increase it by reacting with pollutants in the air.
Aircon are able to dehumidify (and cool) larger areas than dehumidifiers, so if you have one you can likely stop using the dehumidifier. Modern aircon also have high quality particle filters which can reduce dust, pollen and pollution from the outside air, making them a better ventilation option for asthma and allergy sufferers than opening windows.
The key with aircon is to set them properly. If you are not familiar with the aircon control, please see this guide. http://blog.gaijinpot.com/using-japanese-air-conditioner/
The best way to operate them is to set the remote to the desired temperature, set the fan speed to auto and let the unit run. Aircon are run most efficiently at less than their full power thanks to advanced electronics and inverter-driven variable-speed compressors. Instead of forcing the unit to put out large amounts of cool air at once, aircon work best by running for hours at partial capacity and continually adding cool air to the room. Running for long amounts of time also maximizes their ability to reduce moisture in the air, improves indoor air quality, and increases comfort.
Note that if the aircon measures room temperature at the unit, you may need to set it several degrees lower to achieve that temperature in the farthest reaches of the room. Advanced aircon also have a temperature sensor in the remote control, which eliminates this problem if the remote is kept separate from the aircon.
Turn off the aircon if residents will be gone for many hours or longer. Frequently changing the temperature on the aircon or turning it on and off will significantly increase energy usage. Disable “eco” temperature setback or auto-off occupancy sensing modes on the aircon.
Advanced aircon report the amount of electricity used per day so you can experiment with different settings and find the best balance of comfort and cost. Alternatively, for 100V units you can connect it to a cheap electricity monitor to see how much electricity it uses and experiment by running it in different ways.
Periodically clean the aircon filter. For aircon with auto-cleaning systems, periodically empty the dustbin.
Do not pile items on or around the aircon. If you hang laundry nearby, make sure it will not get sucked against the aircon’s outdoor unit.
Contact the author of this guide, Roger Smith, at