January 6, 2026
By the HalfKey team
Tokyo apartment heating and mold, the resident rules
Tokyo buildings are not heated as a system. Each room runs its own aircon, sometimes a kerosene heater, often a kotatsu. The same closed-window habits that keep heat in also grow mold on the closet wall. Both problems share one fix: airflow.
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Tokyo apartments are not heated as a building. They are heated room by room, by whatever appliance the landlord installed. The hallway is not heated. The bathroom is not heated. The bedroom is heated only when you turn the aircon on, and only in the room where the aircon hangs.
This is the first rule new residents miss. There is no thermostat in the wall. There is no boiler in the basement. The walls between you and the next apartment are concrete. Concrete sheds heat fast once the aircon stops. When you turn the aircon off and go to sleep, the room loses 5 to 8 degrees Celsius by morning in January.
The second rule: the same sealed-window habits that keep heat in also grow mold. Plan for both at the same time.
What the aircon actually does in winter
The wall unit you see in every Tokyo apartment is an エアコン (eakon — air conditioner, but in Japanese the word covers heating mode too). It is a heat pump. In summer it moves heat out. In winter it moves heat in. The remote has a sun icon for heat and a snowflake for cool. Pressing the wrong one in February is the most common confusion in our move-in messages.
A 6畳 (rokujō — six-tatami room, about 10m²) unit in a 1K draws roughly 600 to 900 watts in heating mode. It peaks near 1,400W when the outdoor coil is iced over. At ¥31 per kWh on 従量電灯B (jūryō dentō B — TEPCO's standard residential plan), eight hours of heating costs ¥75 to ¥220 a day. The range depends on outdoor temperature and your thermostat setting. A 1LDK with two units running doubles that. Plan ¥4,000 to ¥7,000 of January electricity above your summer baseline.
The unit struggles below 5°C outdoor. You will hear it cycle into a defrost mode every 30 to 45 minutes, blowing room-temperature air for two minutes before resuming heat. This is normal. It is not broken.
Set the target to 22°C, not 28°C, and turn it off when you leave the room. Use the 暖房 (danbō — heating mode) setting, not 自動 (jidō — auto), because auto often overshoots and drives the bill up. Aim the louvres downward; warm air rises on its own. If the room still feels cold at 22°C, the issue is usually a leaking window seal, not a weak unit. Tape the seal with foam draught strip from any Cainz home center, ¥600 a roll.
Kerosene heaters and the building rule
Some older apartments and shared houses still have a 石油ストーブ (sekiyu sutōbu — kerosene heater). A 5L tank runs roughly 8 hours on the lowest setting. A tank costs about ¥900 at the Eneos stand on Kannana-dōri or any gas station selling 灯油 (tōyu — kerosene).
Building rules vary, and you must check yours before lighting one. Most newer マンション (manshon — mid-rise concrete apartment) prohibit kerosene heaters entirely. The rule is in the 入居のしおり (nyūkyo no shiori — the move-in handbook the landlord gives you). Older wood アパート (apāto — low-rise wood apartment) often allow them, but require you to crack a window 2cm every hour to vent CO. The hokenjō (public health office) and your building manager treat unvented combustion as a fire-and-CO risk, not a comfort preference. Ignore the rule and your renter's insurance lapses.
The kotatsu economy
A こたつ (kotatsu — low table with a heating element underneath and a quilt over the frame) draws 200 to 600 watts. That is a quarter to a third of an aircon. You sit at it, you sleep at it, you eat at it. The rest of the room stays at 12°C all winter. Many residents in older buildings run the kotatsu plus a small bathroom space heater. They leave the aircon off until the room drops below 14°C.
A new kotatsu at Nitori on Meguro-dōri runs ¥9,800 to ¥24,800 for the full set (table, heater, quilt, mat). Used ones move on Mercari for ¥3,000 to ¥6,000. Furnished mid-term apartments sometimes include one; sometimes the operator has stored it for the season. Ask before you buy.
Why mold appears in mid-term stays
Tokyo is humid. Average July humidity at the Otemachi weather station is 76 percent. The rainy season (tsuyu) runs mid-June to mid-July. It pushes overnight indoor humidity past 85 percent in any room with the windows shut. Concrete walls in a manshon stay 2 to 4°C cooler than room air. Moisture condenses on the inside of the exterior wall. Closet doors stay closed. Shower steam vents into the hallway when you forget to close the bathroom door. The futon sits on the same tatami all week.
Three weeks in, a black ring appears on the wall behind the bed. Or a fuzzy patch shows up on the leather of a shoe you have not worn. That is カビ (kabi — mold). It is not a sign the building is dirty. It is a sign airflow stopped.
A mid-term tenant grows mold faster than a long-term resident for one reason. The long-term resident has already arranged the room so the futon stays off the wall and the dehumidifier sits in the right corner. You arrived with a suitcase and pushed the bed against whatever wall fit. That is often the exterior wall, which is where condensation collects.
Concrete prevention, in order
Run the bathroom 換気扇 (kankisen — exhaust fan) for 30 minutes after every shower. Most Tokyo bathrooms have the fan switch on the wall outside the door; check both wall plates if you cannot find it.
Set the aircon to 除湿 (jōshitsu — dehumidify) mode, not cooling, on humid days under 28°C. The icon is usually a water-drop. It pulls moisture without dropping the temperature 5 degrees.
Crack closet doors 5cm whenever you leave the apartment. Air must move through the closet, or the wall behind your hanging clothes grows mold first.
Air the futon on the balcony rail twice a month from May through September. 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 the actual work.
Buy a small dehumidifier box for each closet. パナソニック (Panasonic) sells the basic granule type called 水とりぞうさん at any Welcia drugstore, ¥250 for a three-pack. The granules turn to liquid as they pull moisture; replace when full, usually every 4 to 6 weeks in summer.
If mold appears anyway, spray it with カビキラー (Kabi Killer — a sodium-hypochlorite mold cleaner, ¥400 at Welcia or Sundrug). Wait 15 minutes. Wipe with a dry cloth. Wear gloves; the spray bleaches fabric on contact. Throw the empty can into your ward's recycling correctly: 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, because the rule is not the same one block over.
What to do tonight
Turn the aircon to 22°C heat. Open the closet doors 5cm. Run the bathroom fan for 30 minutes after your shower. If your building allows a kerosene heater and you want one, read the move-in handbook section on ventilation before you light it. If the futon has been on the floor for two weeks, hang it on the balcony rail tomorrow morning. Pull the bed 10cm away from any exterior wall before you sleep.
Walk the room once a week with your hand on the wall behind the bed and inside the closet. Cool and dry is fine. Cool and damp is the warning. If you feel damp, run the aircon on jōshitsu for two hours and crack the closet doors wide.
The rules are dull. The cost of skipping them is a moldy wall and a ¥40,000 cleaning charge at move-out.