When June arrives in Tokyo, concrete rental apartments quickly fill with dampness. Leaving for work with all windows locked and returning late to a stuffy, humid room is a familiar experience. For those living alone in Japan for the first time, it is easy to assume that humidity is just an inevitable part of the rainy season (Tsuyu) to be ignored. However, neglecting moisture can lead to a costly surprise when moving out. Black mold creeping behind wallpapers or ruined clothes in your closet are usually classified as tenant negligence, meaning the restoration costs will be deducted from your deposit. For residents who secured budget-friendly housing, perhaps during the summer off-season room search when landlord negotiations are easier, managing this humidity is your first line of defense. Below, we share practical ventilation strategies for airtight concrete buildings and the truth about AC “cooling” vs. “dehumidifying” electricity bills. If you are struggling with apartment issues or searching for a new home, connect with us for a free English consultation on LINE anytime!
Why Mold Prevention is Essential from the Rainy Season through Summer
Japan's humid rainy season and hot summers create the perfect breeding ground for mold. When room temperatures range from 20°C to 30°C, humidity exceeds 70%, and household dust or dirt is present, mold can multiply and become visible in just a few days. In highly airtight concrete apartments common across Tokyo, trapped moisture cannot escape naturally. Taking preventive measures before the rainy season gets heavy is the key to avoiding landlord disputes and unexpected move-out fees.
1. Who Pays for Mold Damages? The Tenant's Duty of Care (Zenkan Chui Gimu)
Leaving mold unattended in a rental property is usually ruled as tenant negligence. You could face steep wallpaper replacement charges when moving out. Simple daily ventilation is your best defense to protect your security deposit.
When signing a lease in Japan, you will likely encounter the legal term “Zenkan Chui Gimu” (善管注意義務), which translates to the “tenant's duty of care.” This is a legal obligation requiring tenants to maintain and use the rental property with reasonable care and cleanliness.
“I saw condensation on the windows but didn't wipe it off, and now the wallpaper is peeling and moldy.”
“I packed my closet full of boxes without any dehumidifiers, and the back panel is covered in black mold.”
These are classic examples where the tenant is held responsible for failing to perform basic maintenance. While natural wear and tear (such as wallpaper yellowing from sunlight over the years) is covered by the landlord, mold caused by poor ventilation falls entirely on the tenant.
To avoid receiving a bill for tens of thousands—or even hundreds of thousands—of yen for wallpaper replacement when you vacate, daily prevention is essential. Knowing how to prevent restoration disputes when moving out (such as taking photos on your move-in day) is another crucial way to keep your deposit safe. If your current landlord is claiming unreasonable fees, our team can help you navigate negotiations.
【The Line of Restoration Responsibility】
According to the guidelines of the Ministry of Land, Infrastructure, Transport and Tourism (MLIT), natural wear and tear over time is the landlord's responsibility. However, ignoring condensation or mold is considered “insufficient cleaning and management by the tenant,” meaning the tenant must pay for the repairs.
2. Why concrete (RC/SRC) apartments trap humidity
Reinforced concrete (RC) buildings offer excellent insulation, but they easily trap moisture inside. Newly built or recently constructed properties (within 2-3 years) release moisture from the concrete itself, making 24-hour ventilation systems vital.
RC (Reinforced Concrete) and SRC (Steel Reinforced Concrete) apartments are very common in Tokyo. While they provide fantastic soundproofing and keep your AC running efficiently, their high airtightness means that once moisture gets inside, it has nowhere to go.
A common pitfall is the “newly built apartment” trap. In apartments built within the last 2 to 3 years, the concrete foundation is still curing and slowly releasing moisture into the rooms.
Thinking “My apartment is brand new, so it won't mold” is a major misconception. In fact, newly built concrete buildings require extra humidity management during their first few years.
Fortunately, the basics of moisture control are remarkably simple:
- Keep the 24-hour ventilation system ON: Do not close the air vents on the walls or turn off the system. Even in highly airtight apartments, keeping it running ensures that all the air in the room is naturally replaced every two hours.
- Place your dehumidifier in the center of the room: Shoving a dehumidifier flat against a wall blocks proper airflow and reduces its efficiency. The trick is to place it as close to the center of the room as possible, or in a spot with active air circulation.
3. 5 practical steps to protect your room from humidity today
Keep bathroom fans running continuously, leave space behind furniture, avoid leaving futons on tatami mats, use a circulator fan for indoor laundry, and choose the correct AC mode to eliminate mold growth.
1
Keep the bathroom exhaust fan running 24/7
Leaving the bathroom fan on continuously keeps the entire space dry and prevents mold from taking root. It costs only around 100 yen per month, making it highly economical.
Do you turn off your bathroom fan two or three hours after taking a bath? Doing so leaves excess moisture trapped, allowing mold to flourish overnight. The most effective strategy is to keep the bathroom exhaust fan running 24 hours a day, 365 days a year.
💡 Bilingual Agent Insider Tip: Many foreign tenants turn the fan off to save on utilities, but the power consumption of these fans is extremely low. Running it continuously costs only 50 to 150 yen a month. Compared to the hassle of scrubbing mold or paying for professional cleaning services later, this is the cheapest insurance policy you can buy.
2
Leave a gap of at least 5cm between furniture and walls
Placing furniture slightly away from walls creates ventilation paths, preventing condensation and mold on the wallpaper.
Pushing beds, bookshelves, or storage units flat against walls prevents air circulation in those narrow gaps. Condensation in winter and humidity in summer get trapped there. A common mistake is moving a wardrobe after a few months only to discover the wall behind is covered in black mold. When arranging your room, keep furniture about three fingers wide (approx. 5cm) away from the wall.
💡 Bilingual Agent Insider Tip: Accumulated dust behind furniture absorbs moisture and accelerates mold growth. Taking a few minutes to sweep or wipe behind your furniture before the rainy season starts will significantly lower the chances of mold developing.
3
Moisture control for closets, wardrobes, and Tatami rooms
Use wooden slatted mats (Sunoko) in closets, and limit storage to 80% capacity to let air flow. Never leave futons directly on Tatami mats, and avoid wet wiping.
Closets and wardrobes are closed spaces where clothes and bedding absorb moisture, making them highly vulnerable to mold. Instead of placing items directly on the floor, lay down wooden slatted mats (called Sunoko in Japanese) to create air channels. Avoid packing your closets to the brim; keeping them at around 80% capacity allows air to circulate.
If your Tokyo apartment has a traditional Japanese room with Tatami mats, you must take extra precautions. Tatami absorbs moisture well, but it also traps it. Never leave your futon laid out directly on Tatami mats all day. The warmth and sweat from a night's sleep will get trapped in the mat, causing mold to grow in no time. Using a foldable Sunoko bed frame or folding and storing your futon immediately after waking up is the standard practice.
💡 Bilingual Agent Insider Tip: Wiping Tatami mats with a wet cloth to clean them is actually counterproductive as it introduces extra moisture. Clean Tatami by wiping with a dry cloth or using a vacuum cleaner along the direction of the straw weave. If you spot mold, do not use water; instead, spray it with rubbing alcohol (ethanol) and gently wipe it away with a dry cloth.
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4
Indoor drying rule: Hang in the center and use a circulator fan
Hang clothes in the center of the room away from walls and blow air directly with a circulator fan. This helps clothes dry in under 5 hours, suppressing mold and odor-causing bacteria.
On rainy days, it is tempting to hang laundry on curtain rails or door frames. However, this dramatically raises the room's humidity and can lead to mold growing directly on curtains and window frames.
When drying clothes indoors, hang them in the center of the room where air flows best, and point a circulator fan or regular electric fan directly at the clothes to speed up drying.
5
Learn to choose between AC Cooling and Dehumidifying
Use cooling on hot summer days and dehumidifying (cool dehumidify or re-heat dehumidify) during sticky rainy days. This keeps humidity comfortable while saving on electricity.
Even if you set your AC to 28°C in cooling mode, the room might still feel uncomfortably humid. This is because cooling mode prioritizes dropping the temperature and stops dehumidifying once the target temperature is reached.
To tackle high humidity, you need to understand your AC's dehumidifying functions. Switch to cooling during hot midsummer days, and use dehumidifying when it is not too hot but damp and sticky. Understanding how your AC works can keep your bills down. You can also read our Tokyo utility setup and saving guide to avoid bill spikes. Negotiating minor repair works or appliance upgrades with landlords is also something Sorai Tokyo can help with.
💡 Bilingual Agent Insider Tip: Dehumidifying modes generally consist of “Cool Dehumidifying” (弱冷房除湿 - Jaku-Reibo Joshitsu), which lowers room temperature slightly, and “Re-heat Dehumidifying” (再熱除湿 - Sainetsu Joshitsu), which keeps room temperature constant. Re-heat dehumidifying uses more electricity than cooling, so it is best to choose based on the actual indoor temperature.
Comparing AC Modes: Cooling vs. Cool Dehumidifying vs. Re-heat Dehumidifying
Each mode serves a distinct purpose and carries a different electricity cost. Choose based on your room's temperature and humidity for the most cost-effective comfort.
💡 Understanding Japan's Two AC Dehumidifying Systems:
Dehumidifying operates in two ways: “Cool Dehumidifying” (弱冷房除湿 - Jaku-Reibo Joshitsu), which returns the cooled air as-is (making the room slightly cooler), and “Re-heat Dehumidifying” (再熱除湿 - Sainetsu Joshitsu), which heats the air back up before returning it (keeping the room temperature constant).
When it is damp but too cold to run regular AC, Re-heat Dehumidifying keeps the room exceptionally comfortable without freezing you out. However, because it heats the air, it does consume slightly more electricity than standard cooling mode.
| Operation Mode |
Primary Goal |
Estimated Electricity Cost |
Best Suited For |
| ❄️ Cooling (冷房) |
Lower room temperature |
Standard |
Hot summer days exceeding 30°C or when the room is generally hot |
| 💧 Cool Dehumidifying (弱冷房除湿) |
Prioritize lowering humidity |
Low (Cheaper than standard cooling) |
Sticky, humid days when the actual temperature is not excessively high |
| 🔥 Re-heat Dehumidifying (再熱除湿) |
Lower humidity without lowering room temperature |
High (Slightly higher than cooling) |
Chilly, damp days during the rainy season when you want to remove moisture but avoid cold air |
4. Prevention cost vs. Repair cost: Which is more economical?
Compared to the tiny monthly cost of running a bathroom fan (approx. 100 yen), wallpaper restoration costs when vacating are extremely high (ranging from tens to hundreds of thousands of yen). Consistent daily prevention is the smart choice.
Here is a comparison of daily prevention costs versus repair fees if mold is left to grow unchecked.
Bathroom Exhaust Fan Cost
Running the bathroom exhaust fan 24 hours a day for a full month costs around 100 yen. Considering it keeps bathroom mold to a minimum, it is the most affordable insurance policy available.
Unresolved Mold Restoration Fees
If wallpaper or flooring gets damaged due to a lack of tenant care, you will likely face wallpaper replacement charges when vacating, costing anywhere from 30,000 yen to over 150,000 yen. It is also the primary reason tenants do not get their security deposit back.
Frequently Asked Questions (FAQ)
Q. Why should I run the bathroom exhaust fan 24 hours a day?
A. Bathrooms are the most mold-prone areas in concrete apartments. Running the fan for only a few hours after a shower is insufficient to pull all the humidity out, allowing mold to breed overnight. Keeping it running 24/7 ensures the bathroom stays dry. Since it only costs about 100 yen a month, it is the most reliable and affordable prevention method.
Q. Who pays for mold remediation in a rental apartment in Japan?
A. Mold caused by poor daily ventilation or leaving window condensation wiped-free is typically ruled as a violation of the tenant's 'duty of care' (Zenkan Chui Gimu). Consequently, tenants are usually required to pay for wallpaper or surface replacements out of their own pockets when moving out. Regular humidity control is key to protecting your deposit.
Q. Is it cheaper to run AC cooling or dehumidifying mode?
A. It depends on the dehumidifying system. 'Cool Dehumidifying,' which returns the cooled air directly, is generally cheaper than standard cooling. However, 'Re-heat Dehumidifying,' which warms the air before blowing it back into the room, is more expensive than cooling. Switch modes based on both indoor temperature and moisture levels.
Q. What is the best way to prevent that damp smell from indoor laundry drying?
A. To prevent odor-causing bacteria (Moraxella) from multiplying, drying laundry in under 5 hours is the golden rule. Hang your laundry in the center of the room where air flows best, and point a circulator fan directly at it to drastically reduce drying time.
Q. Why is mold more common in concrete (RC) apartments?
A. Concrete apartments are highly airtight compared to wooden houses, preventing indoor moisture from escaping easily. In addition, concrete structures built within the last 2 to 3 years release moisture from the curing concrete, elevating humidity. Keeping your 24-hour ventilation system turned on is essential.
Q. How do I care for traditional Tatami mats to prevent mold?
A. The most critical rule is never leaving your futon laid out directly on Tatami mats all day. Use slatted Sunoko bed frames or dehumidifier pads, and air out your futon regularly. When cleaning, avoid wet wiping to prevent moisture absorption; vacuum along the straw weave or wipe with a dry cloth instead.
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Written by: Sorai Tokyo Editorial Team (Supervised by Licensed Real Estate Agents)
We are a dedicated real estate support team helping foreigners find apartments and settle into Tokyo. From negotiating upfront costs and rent to assisting with guarantor screening, we provide tenant-first support. Reach out to us via our official LINE for quick assistance and a stress-free start to your new life in Japan.
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