Who is responsible for mold repair costs in a rental apartment?

If mold on wallpaper or inside closets is caused by insufficient ventilation or neglect of condensation—constituting a breach of the tenant's 'duty of care' (duty to manage the property with the care of a good manager)—it is highly likely that the tenant will be held responsible for the repair costs as part of the 'restoration to original condition' (genjo kaifuku) upon moving out. Practicing good mold prevention during the rainy season is a vital form of self-protection to save on future move-out costs.

Which is cheaper: Air conditioner 'Dry' mode or 'Cooling' mode?

It depends on the dehumidification method. 'Weak Cooling Dehumidification' (which cools and removes moisture) is generally cheaper to operate than standard Cooling mode. Conversely, 'Reheat Dehumidification' (which cools then reheats the air before returning it to the room) is more expensive. It is best to use 'Dry' mode during chilly, damp, rainy days when you don't want to lower the room temperature, and 'Cooling' mode on hot mid-summer days.

What is the most effective way to prevent the damp, musty odor of indoor-dried laundry?

To prevent the growth of odor-causing bacteria (Moraxella), the key is to dry your laundry within 5 hours. To achieve this, dry clothes in the center of the room where there is good airflow, use an electric fan or air circulator to blow air directly onto the laundry, and use the 'arch hanging' method—placing longer items on the outside—to create a clear path for air to pass through.

Staying Comfortable from Rainy Season to Summer: Dehumidification and Mold Prevention for Rental Apartments

Q: Why is it necessary to keep the bathroom exhaust fan running 24 hours a day?

The bathroom is the most mold-prone area in any home. Running the exhaust fan for just a few hours is insufficient to fully remove moisture, creating an environment where mold spores thrive. Keeping it on 24/7 ensures the bathroom stays consistently dry. With electricity costs ranging from only 50 to 150 yen per month, it is the most effective and affordable preventative measure compared to the effort and expense of professional mold remediation.

"It’s June, and the room feels damp already..." or "I’m worried about that musty smell on my laundry." Do you have these concerns? In reality, if you neglect moisture and mold prevention in your rental apartment, you risk being charged high restoration costs when you move out. From daily ventilation habits and proper use of bathroom exhaust fans to the smart use of your AC’s "Cooling" vs. "Dry" modes (which directly impacts your electricity bill), our real estate professionals break down everything you need to know.

Why is mold prevention in rental apartments so important from the rainy season through summer?

Japan’s summers are exceptionally humid by global standards, and high-density concrete apartments tend to trap moisture easily. Mold thrives in just a few days when three conditions are met: humidity over 70%, temperatures between 20–30°C, and a food source (dust or human skin cells). Learn the right measures to prevent unnecessary move-out repair disputes and maintain a clean, comfortable, and high-quality living environment.

1. Don't lose money! The risk of move-out costs caused by mold neglect

Under a lease agreement, tenants are obligated to manage their rented property with the care of a good manager, a concept known as "Duty of Care" (zenkan chui gimu).

For example, if wallpaper peels because you failed to wipe off condensation around windows, or if the inside of a closet becomes moldy due to trapped moisture, this is considered a lack of proper maintenance by the tenant. It is highly likely that you will be charged the full out-of-pocket costs for replacing wallpaper or repairs when you move out.

Preventing mold isn't just about daily comfort—it is a smart way to protect your security deposit (Shikikin) and keep your future moving costs to a minimum.

【The Line for Restoration】
Repairs due to normal wear and tear (natural deterioration over time) are the landlord's responsibility. However, the Ministry of Land, Infrastructure, Transport and Tourism guidelines clearly state that mold growth is often viewed as a result of the tenant failing to perform regular cleaning and ventilation.

2. Keep moisture and mold away! 5 tips for humidity control

Simple changes to your daily routine can dramatically improve air circulation throughout your room and significantly reduce mold growth. Here are five tips you can start implementing today.

1 Keep the bathroom exhaust fan running "24/7"

The bathroom is the most common source of mold. Many people turn it off after a few hours, but this leaves moisture behind, allowing mold to thrive. Always leave your bathroom exhaust fan on 24/7.

💡 Pro Tip: You might worry about the electricity bill, but for standard bathroom fans, running it all month costs only about 50 to 150 yen. Considering the costs of professional cleaning or mold-removal agents, this is the most cost-effective and best possible prevention measure.

2 Leave a "5cm gap" between furniture and walls

Are your beds, bookshelves, closets, or refrigerators pushed right against the wall? If air escape routes are blocked, condensation builds up in the gaps, leading to mold growth on your wallpaper.

By placing furniture about 5cm away from walls, you create a path for air, preventing trapped moisture behind your furniture.

💡 Pro Tip: Before the rainy season, check for dust in those gaps. Dust is a perfect "food source" for mold. Regular cleaning in these gaps directly helps prevent mold.

3 Use "slatted mats" (sunoko) and moisture absorbers in closets

Closets are airtight and humid, making them the most common spots for moldy clothes. Do not place storage cases or futons directly on the floor or against walls; use a slatted mat to elevate them.

Additionally, place moisture-absorbing packets in the four corners of the bottom of the closet to efficiently soak up excess humidity.

💡 Pro Tip: Instead of sealing closet doors shut, try leaving them open just a few centimeters. On clear, dry, sunny days, open the doors fully and point a fan into the closet to cycle the air.

4 "Center drying" and "Air circulators" are the gold standard for indoor laundry

When it rains, you can't hang laundry outside, and the moisture from your clothes (which can be several liters) is released directly into your room. To prevent musty odors and humidity pockets, change how you dry your clothes.

Hanging them on curtain rods or near walls can cause mold to grow on the curtains themselves. Always dry laundry in the middle of a well-ventilated room and use a fan or circulator to blow air directly on them to ensure they dry quickly (within 5 hours).

5 Smart use of your AC’s "Dehumidification" vs. "Cooling" modes

The air conditioner is essential for controlling humidity during the rainy season. Cooling and Dry modes have different mechanisms; using them correctly allows you to reduce humidity while maintaining a comfortable room temperature. (See the comparison table below for details).

💡 Pro Tip: There are two types of dehumidification: "Weak Cooling Dehumidification" and "Reheat Dehumidification." Checking your AC's manual or model number beforehand is recommended, as it affects your electricity bill and room temperature.

AC Feature Comparison: Cooling vs. Weak Cooling Dehumidification vs. Reheat Dehumidification

💡 Watch for the difference in AC "Dehumidification" methods:

"Dry" mode can either be 'Weak Cooling Dehumidification' (which returns the cooled air to the room, making it slightly cooler) or 'Reheat Dehumidification' (which heats the air before returning it, keeping the temperature stable).

For those rainy days when you want to remove moisture but avoid feeling chilly, 'Reheat Dehumidification' is very convenient, though it consumes more energy and may raise your electricity bill.

Mode	Purpose	Approx. Electricity Cost	Recommended Use
❄️ Cooling	Quickly lower room temperature	Standard	Hot, muggy summer days over 30°C
💧 Weak Cooling Dry	Prioritize reducing room humidity	Cheapest (Lower than cooling)	Days that feel muggy but not overly hot
🔥 Reheat Dry	Reduce humidity without lowering temperature	Most Expensive (Higher than cooling)	Rainy season days that are damp but chilly

Helpful Humidity & Mold Prevention Data

Here is a summary of common questions and data regarding cost-effectiveness in mold prevention.

Cost of Bathroom Ventilation

Running the fan 24/7 costs only about 50 to 150 yen per month. It is significantly cheaper than dealing with mold remediation costs later.

Mold Neglect & Restoration

If charged upon move-out for mold removal or wallpaper replacement, costs can range from tens of thousands to hundreds of thousands of yen.

Frequently Asked Questions (FAQ)

Q. Why is it necessary to keep the bathroom exhaust fan running 24 hours a day?

A. Bathrooms trap the most moisture. Running it for just a few hours doesn't remove all the humidity. Keeping it on 24/7 to ensure the room stays dry is the cheapest and most powerful way to prevent mold.

Q. Who pays for mold repair costs in a rental apartment?

A. If mold was caused by a failure to ventilate or wipe up condensation, it is considered a breach of the tenant's management duty, and you risk being liable for the out-of-pocket repair costs (e.g., replacing wallpaper) when you move out.

Q. Which is cheaper: AC Dehumidification or Cooling?

A. It depends on the dehumidification type. Generally, "Weak Cooling Dehumidification" is cheaper than Cooling. Conversely, "Reheat Dehumidification" costs more due to the energy required to heat the air back up.

Q. What is the most effective way to prevent the damp smell of indoor-dried laundry?

A. Hang laundry in the center of the room and use a circulator or fan to blow air directly on the clothes, drying them quickly (within 5 hours). Reducing the time they stay damp blocks the growth of odor-causing bacteria.

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