How do I stop mould in my Tokyo apartment while I'm away for months?

What Tokyo's year does to a closed room

The Japan Meteorological Agency's monthly normals for central Tokyo (1991–2020) show the pattern clearly. Average relative humidity is around 51 to 52 percent in January and February, then climbs: 68 percent in May, 75 percent in June, 76 percent in July, 74 percent in August and 75 percent in September, before falling to 64 percent in November. Average temperatures in July and August are 25.7 and 26.9 degrees Celsius.

The Tokyo Metropolitan Government's housing health guidelines describe the conditions mould prefers as a temperature of roughly 20 to 35 degrees Celsius and humidity of 70 percent or more, with food in the form of skin oils, soap residue, wallpaper paste and condensation. The same guidelines suggest 40 to 60 percent as the target range for indoor humidity. In other words, an unventilated Tokyo apartment between June and September matches the description almost exactly.

A Tokyo Metropolitan Government survey (fiscal 2016) found that 33.7 percent of households had seen mould at home in the previous three years. Among those households, 79.7 percent reported it in the bathroom, and more than 20 percent each in the built-in closet, the washroom and the bedroom. These are the places to think about first.

Wood, tatami, washi and urushi: what each needs

Solid wood takes up and releases moisture, so it swells in a humid summer and shrinks when winter air and heating dry it out. Furniture made for Japanese conditions usually allows for this movement in its joints and panels. A piece built for a drier climate may not, and drawers that ran smoothly in March can stick in August.

Tatami (woven rush mats) is one of the surfaces the Tokyo guidelines name for mould. Their advice, if mould appears, is to spray disinfecting alcohol, brush along the weave with a brush dipped in alcohol, wipe, and dry the mat well. Washi (handmade Japanese paper) shades are paper; keep them away from windows where condensation forms and dust them before you leave.

Urushi lacquer has a different weakness. The Wajima Museum of Urushi Art notes that urushi deteriorates under ultraviolet light, so lacquer pieces should not sit for long in direct sun. In an empty home with the curtains open, a tray on a sunny sideboard can fade over a few years.

Choosing pieces that tolerate an empty home

Warm minimalism suits a part-time home for a practical reason: fewer pieces mean fewer surfaces for dust and fewer closed corners where damp air stays still. When you choose, a few points make a real difference.

• Raised legs on sofas, beds and cabinets, so air can move underneath. A bed base sitting flat on the floor traps moisture.
• A gap of a few centimetres between furniture and outside walls, where condensation is most likely.
• Natural oil or lacquer finishes that the maker can advise on, and a written care note for each piece.
• Upholstery in fabrics that can be removed and cleaned, and mattresses that can be stood up or aired.
• Open shelving or slatted doors for linen storage, instead of sealed cupboards packed full.

Before you leave: a short checklist

Most of the work happens in the last hour of a visit. The aim is to leave nothing wet, nothing sealed and nothing pressed together.

• Run the bathroom fan and leave the bath empty, wiped and dry, with the door open if the layout allows.
• Open closet, wardrobe and cupboard doors a little. Pull futon and bedding out of the built-in closet or store them in breathable covers.
• Set the air conditioner's dry mode or a dehumidifier on a timer. If you use a dehumidifier, choose one that drains continuously or switches off safely when the tank is full.
• Wipe window frames and the drainage channel at the base of the sash, which the Tokyo guidelines recommend keeping clear so condensation can run outside.
• Close curtains or blinds on sunny windows to protect lacquer, textiles and wood from ultraviolet light.

Who checks the home while you're away

In a Japanese condominium, the management company contracted by the owners' association looks after common areas such as the exterior, corridors and lifts. It does not usually look inside your unit. For that, owners often ask a relative or acquaintance in Japan, or pay a home-checking service to visit, open windows, run water and report with photos.

The Ministry of Land, Infrastructure, Transport and Tourism's brochure for foreign unit owners (March 2026) explains a new option: from April 2026, owners who live outside Japan may appoint a domestic manager with an address in Japan to vote at general meetings and pay management fees on their behalf. That role covers your duties to the building; it is separate from checking the inside of the home, so agree on each in writing.

Whoever visits, give them a short list: open the windows for a while, run the taps and the bath fan, check the closet and the back of the bed, and photograph anything that looks dark or smells musty.

Signs to look for when you return

A musty smell on opening the door is the first sign. Then check the places the Tokyo survey named: the bathroom ceiling and grout, the built-in closet, the washroom and the bedroom, including the underside of the mattress and the back of furniture against outside walls.

Small spots on hard surfaces can usually be cleaned. The Tokyo guidelines advise opening windows, wearing rubber gloves and a mask, and not using a vacuum cleaner directly on mould, which spreads the spores. They also warn that most household mould removers are chlorine-based and must never be mixed with acidic cleaners. If mould has spread into walls or has come back in the same place, ask the management company, because the cause may be a leak or condensation in the building itself.