Tokyo monthly stay in tsuyu: humidity, mold, operator gap

The first sign is a faint sweet smell on the futon. The second is a dark spot on the inside of the closet door, low and round, the size of a 500-yen coin. By the time you notice the smell, the spot is two weeks old. This is 梅雨 (tsuyu — the East Asian rainy season, mid-June to mid-July in Tokyo). It is not heavy rain. The actual operational problem is humidity.

Tokyo's outdoor humidity averages 78 percent in June and 76 percent in July at the Otemachi weather station. Indoor humidity in a sealed manshon runs 73 to 78 percent on any rainy day with the windows shut. Concrete walls hold 22 to 24°C while the air outside is 26 to 28°C. The temperature gap drives condensation on the inside of any exterior wall. The same gap soaks the closet wall behind closed doors and the futon left flat on the tatami for three nights.

Operators rarely brief midterm guests on this. They assume locals know.

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City, ward, building: who controls the humidity

Tokyo runs three layers of humidity rules, each owning a different part of the problem.

The city rule is the climate. The 気象庁 (Kishōchō — Japan Meteorological Agency, JMA) declares the tsuyu open day, usually June 7 to 11. It declares the tsuyu close day, usually July 18 to 22. Track both at jma.go.jp/bosai/forecast. The window is six weeks. Plan your defenses for the full window, not the five worst days.

The ward rule is the trash schedule. Mold-soaked items become 燃えるごみ (moeru gomi — burnable trash) once you can no longer save them. Wet futon covers, fabric shoe inserts, woven kitchen mats. Setagaya runs burnables Tuesday and Friday before 8am. Shibuya runs Tuesday and Friday at the same hour. Bunkyō runs Tuesday, Thursday, Saturday. A wet futon cover the size of a single bedsheet is one full bag, and the bag must close. Check your ward's gomi calendar before you set the bag on the curb.

The building rule sits on the 入居のしおり (nyūkyo no shiori — move-in handbook). Most modern manshon mandate the bathroom 換気扇 (kankisen — exhaust fan) stay on overnight from June 1 to July 31. Some operators print the rule in English. Most do not. Read the handbook section labelled 換気 (kankin — ventilation) before the first weekend in June.

Why mold grows faster on a midterm guest

A long-term resident has already arranged the room. The dehumidifier sits in the corner that holds the most damp. The bed is 10cm off the exterior wall. The futon rotates onto the balcony rail twice a month. The closet door cracks open every morning when they leave for work.

You arrived with a suitcase three weeks ago. The bed sits flush against whatever wall fit. The closet door has been closed since you hung your jacket. The futon has not moved off the floor. You leave the apartment in the morning and shut every window because the forecast said rain.

Mold grows in the order those decisions cost you. The futon goes first, because you sweat into it and it cannot breathe. The closet wall goes second, because the door blocks airflow against an exterior surface. The leather and cotton you packed go third — boots, a belt, the canvas bag you brought for groceries. Eight to ten days into a steady tsuyu week is the normal blooming time. Three weeks unattended is a wall covered in dark speckles.

This is not a sign the building is dirty. It is a sign airflow stopped. The fix is mechanical, not chemical.

The 10am to 2pm airing window

Tsuyu does not rain continuously. It rains in 3 to 8 hour blocks broken by humid but dry periods. JMA's hourly forecast at jma.go.jp/bosai/forecast/#area_type=class20s names the dry hours by day. Most days, 10am to 2pm carries the lowest humidity reading of the 24-hour cycle. Outdoor humidity often drops to 62 to 68 percent in this window.

Open the windows during the dry block, not after the rain stops. The post-rain air is saturated and pulls moisture into the apartment, the opposite of what you want.

Open two windows on opposite walls so air crosses the unit. A single open window does not move air; it just leaks the temperature gradient. Set a 20-minute timer the first time. Most rooms exchange a full air volume in 12 to 15 minutes at a 1m/s breeze. After 20 minutes, close everything and turn the aircon to 除湿 (jōshitsu — dehumidify mode). The aircon's drop icon, not the snowflake, is the mode you want.

The building manager's assumption is that you do this every dry day for the six-week window. Most midterm guests do it once, on a Sunday, after the apartment already smells.

Renting a 除湿機 from Don Quijote or Bic Camera

A standalone 除湿機 (joshitsuki — dehumidifier, the appliance, not the aircon mode) pulls 5 to 8 litres of water a day from a sealed 1K. Welcia and Daiso sell desiccant boxes for closets, but those handle a closet, not a room. For a bedroom that runs 28°C and 78 percent humidity at 11pm, you want the appliance.

The rental path is faster than the buy path for a 60- to 90-day stay. Don Quijote's レンタル (rentaru — rental) counter at the Roppongi and Shibuya MEGA stores rents a Mitsubishi MJ-M120SX for ¥3,800 a week. The Panasonic F-YC120HMX rents at the same price. They take a passport and a credit card; no Japanese guarantor is required. Bic Camera runs a similar service through the Akasaka and Yūrakuchō stores at ¥4,200 to ¥4,800 a week. The Bic units include a hose attachment. You run the drain into the bath instead of emptying the 4L tank twice a day. For a six-week tsuyu, ¥22,800 to ¥28,800 is the all-in rental.

Buying instead runs ¥18,000 to ¥34,000 for the same models at any Yamada Denki on Aoyama-dōri. The crossover point against rental is roughly the eighth week. If your stay is 60 days or shorter, rent. If it is 90 days and you fly through August into September with another humid month behind tsuyu, the buy math leans the other way. Most operators do not provide a dehumidifier; halfkey lists which units include one in the unit page's amenities filter, and about 30 percent do.

Run the unit in the bedroom from 8pm to 8am. Move it to the living-kitchen area during the day. A single unit on a rotation pulls the apartment from 78 percent down to 55 to 60 percent by morning. That is the line where mold growth slows.

The kankisen rule the manager wishes you knew

Most Tokyo bathrooms built after 1995 vent through a ceiling-mounted 換気扇 with a wall switch on the corridor side, not inside the bathroom. Switch it on at move-in and leave it on through the entire tsuyu. The fan draws roughly 18 watts; running it 24 hours adds about ¥440 to the monthly electricity bill.

Why this matters: shower steam vents up the stack and out the building roof when the fan is on. With the fan off, the steam migrates into the corridor, then the closet on the next wall, then the bedroom. A closed bathroom door slows but does not stop this. The fan is the only thing that pushes the moisture out of the apartment instead of around it.

Some buildings switch off the kitchen rangehood overnight to save on common-area airflow noise. Check the kitchen vent at midnight; if you do not feel a slight pull at the hood, you are circulating moisture rather than removing it. The fix is to leave the bathroom fan on. The kitchen-vent issue is the building's, and complaining solves nothing during tsuyu.

Crack the closet doors 5cm whenever you leave. Air must move through the closet, or the wall behind your hanging clothes condenses first. This pairs with the fan running, because the fan creates the slight negative pressure that pulls air through the gap.

When mold is already there

If a black ring appears on the wall, do not wipe it dry. Wiping spreads the spores into the air and seeds new patches. Spray it with カビキラー (Kabikirā — Kao's sodium-hypochlorite mold cleaner, ¥420 at Welcia or Sundrug). Wait 15 minutes. Wipe with a damp cloth, then a dry one. The cleaner bleaches fabric on contact; wear gloves and an apron, and lift any clothing or curtain near the wall before you spray.

For the futon, hang it on the balcony rail on the next dry forecast day, between 10am and 2pm. Beat it once with a 布団叩き (futon tataki — woven plastic paddle, ¥400 at any Don Quijote). The sun and the airflow do most of the work. If the futon already smells, take it to a 布団クリーニング (futon kuriiningu — futon cleaning service). The chain Lemonade Cleaning runs same-week pickup-and-return for ¥3,500 to ¥6,000 per futon. Price depends on size. Lemonade has counters at most Tokyo wards' shopping arcades and a phone-order line in basic English.

For leather goods that have already grown white fuzz, wipe with a cloth dampened in 70 percent isopropyl alcohol. Do not use the chlorine-based cleaner; it strips leather color. Air the item out of the closet for 48 hours before storing again. If the canvas grocery bag has a deep mildew smell, throw it in the burnables. Replace at any Don Quijote for ¥600.

Throw the empty Kabikirā can out by ward. Setagaya wants pressurized cans rinsed and punctured, dropped at the resource cart on Tuesday. Shibuya takes them whole on Wednesday in the metals bag. Minato collects them at the building stockyard with a separate label. Check your ward's gomi calendar; the rule is not the same one block over.

The futon airing rotation, mapped

A futon shed of moisture takes four hours of direct airflow on a dry day. Tsuyu rarely gives you a full dry day, but it gives you windows.

Hang the futon on the balcony rail when the JMA hourly forecast shows three consecutive hours below 70 percent humidity. Most weeks during tsuyu deliver one or two such windows. Set a phone alarm for 11am every day; check the forecast in 30 seconds. If the window is there, hang the futon before noon. Bring it in by 4pm. The airflow does the work; the sun is a small bonus.

If the balcony faces a busy road, hang the futon on the inside of the balcony rail rather than the outside. Tokyo air-quality readings at the Hibiya and Setagaya stations show particulate concentrations 30 to 40 percent higher between 7am and 9am during commuter hours. The 11am to 2pm window is also the cleanest air window of the day.

A futon airer (物干しスタンド — monohoshi sutando) inside the apartment is the second-best option for blocked balcony days. Stand the futon on the airer during the dry hours with the bathroom fan on and the dehumidifier running. The airflow is half what the balcony delivers, but it is better than the futon flat on the tatami for another 24 hours.

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Walk the apartment this Sunday

Pull the bed 10cm away from any exterior wall. Find the bathroom 換気扇 switch on the corridor side and leave it on for the next six weeks. Open the closet doors 5cm. If the doors swing shut on their own, clip them open with the cloth pegs sold at Daiso for ¥110.

Read the move-in handbook section labelled 換気 before next Sunday. If your building requires the bathroom fan on overnight or restricts the daytime airing window, that section tells you. Operators who do not print the rule in English usually accept a Google Translate paraphrase if you email and ask.

Check the JMA hourly forecast tonight at jma.go.jp/bosai/forecast and find tomorrow's lowest-humidity window. If it falls between 10am and 2pm, plan to open two opposite windows for 20 minutes during that block. Set the aircon to jōshitsu mode for the rest of the day.

Walk to the Don Quijote on Roppongi-dōri or the Bic Camera on Akasaka-dōri this week and ask for the joshitsuki rental counter. Bring your passport. The unit will fit in a taxi or one Yamato 80-size box for delivery the next day.

The rules are dull. The cost of skipping them is a moldy wall, a ruined futon, and the cleaning charge waiting at move-out.