This article is written by a Japanese local.
For foreign employees relocating to Japan, the most serious living environment trouble they face a few months into their new apartment life is a “massive outbreak of mold on walls and window frames.” Japan’s climate—with subtropical-like high humidity in the summer and severe dryness causing condensation in the winter—is an unknown territory for many foreigners.
The biggest cause of this mold damage is foreign employees turning off the indoor “24-hour ventilation system” because they feel “it’s too cold” or “the noise is annoying.” To prevent situations where employees are billed hundreds of thousands of yen for restoration costs (deposit disputes) upon moving out, we explain the objective defense procedures that HR managers should announce before departure.
1. Why Do Japanese Apartments Have “24-Hour Ventilation”?
[Summary] It is a legally required facility installed in buildings since 2003 to prevent Sick Building Syndrome and mold in highly airtight homes.
Modern Japanese apartments are built with “extremely high airtightness (sealed state)” to improve heating and cooling efficiency. However, when humans live, cook, and shower in a highly airtight space, the moisture with nowhere to go fills the room, turning it into a breeding ground for mold in the blink of an eye.
To prevent this, the Japanese Building Standards Act mandates the installation of a “24-hour ventilation system” in all homes built since 2003. This system is physically designed to completely replace the indoor air every two hours, acting as a “lifeline” infrastructure in the Japanese living environment.
2. The Risk of “Expensive Move-out Fees” Caused by Turning Off the Switch
[Summary] Mold generated by stopping ventilation is considered “tenant negligence,” and the cost of replacing wallpaper and flooring becomes entirely out-of-pocket.
Many foreign employees mistakenly believe that the “24-hour ventilation” switch on the wall is simply an extension of the toilet or bathroom exhaust fan. As a result, immediately after moving in, they turn off the switch and completely close the air supply vents on the walls, complaining that “cold wind comes in during winter” or “the slight operating noise is bothersome.”
Consequently, massive “condensation (water droplets)” forms on windows in the winter, and within a few weeks, pitch-black mold multiplies on window frames and behind wallpapers. Under Japanese real estate rental rules, mold damage caused by turning off the 24-hour ventilation is ruled as a “breach of the tenant’s duty of care (negligence).” Upon moving out, they will be billed a massive penalty of hundreds of thousands of yen to replace all the wallpaper and for specialized cleaning.
3. The “3 Ironclad Rules” to Prevent Mold and Condensation
[Summary] The ironclad rule is to keep the ventilation switch “Always ON.” Open the air supply vents to physically expel moisture generated by daily habits.
To protect the employees’ living environment and company funds (housing allowances and security deposits), strictly enforce the following three rules as a guideline upon relocation.
- Rule 1: “Never” turn off the 24-hour ventilation switch.
The button labeled “24-hour ventilation (24時間換気)” on the control panel in the bathroom or toilet must be kept ON 24/7, 365 days a year. *The electricity cost is only a few hundred yen a month.* - Rule 2: Keep the “Air Supply Vents” on the walls of each room open.
To exhaust air, you must bring in outside air. Push or slide the round or square plastic “air supply vents (registers)” installed on the walls of the living room and bedrooms to keep the air flowing at all times. - Rule 3: Run additional exhaust fans when performing tasks that release lots of moisture.
After taking a shower, boiling water, or cooking, you must run the main “exhaust fan (High)” in addition to the 24-hour ventilation to physically expel moisture outdoors.
4. Practical Q&A (Troubleshooting Foreign Lifestyle Habits)
[Summary] Answers practical questions regarding humidity increases from indoor drying and how to deal with cold drafts in winter.
Q. In winter, cold wind comes in through the air supply vents and makes the room cold. Can I seal them with tape?
A. You must absolutely never seal them. Sealing the air supply vents causes the indoor air to stagnate, and your windows will be soaking wet with condensation overnight. If you feel cold, the correct approach is to slightly narrow the opening of the vent or raise the temperature setting on your air conditioner or heater.
Q. I am not used to hanging laundry outside, so I dry everything indoors. Will this cause mold?
A. In Japan’s climate, constantly drying clothes indoors in the living room is one of the biggest causes of mold outbreaks, as the massive amount of moisture released from wet clothes is absorbed by the wallpaper. If the property is equipped with a “Bathroom Dryer (浴室乾燥機),” instruct them to strictly use it with the door closed. If there is no dryer, purchasing a dehumidifier is strongly recommended.
Conclusion: Eliminate Risks of Climate Differences Through “Prior Announcement”
The importance of the “24-hour ventilation system” is not intuitively understood by foreigners from dry regions. Cases where they turn off the switch without malice, resulting in restoration disputes costing millions of yen, are endless. It is a crucial task for HR managers to logically explain during the move-in orientation that “Japanese apartments are like sealed plastic cases” and to mandate proper ventilation.