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Window Condensation and Mould in Japanese Apartments in Winter: How to Stop It

Condensation (結露) forms when warm, humid indoor air touches a cold window. Many Japanese apartments have single-pane glass in aluminium frames, so windows run with water on winter mornings, and the damp frames and curtains grow mould. To reduce it: wipe the water off each morning, air the room for a few minutes twice a day, keep indoor humidity around 40–60%, avoid unvented heaters that release moisture, and add absorbent tape (結露吸水テープ) and insulating film (断熱シート) to the glass.

Why it happens

  • Single-pane glass and aluminium frames get very cold, and the air touching them can't hold its moisture.
  • Cooking, showers, drying laundry indoors and breathing all add moisture to a small room.
  • Kerosene and gas fan heaters without a flue release water vapour as they burn, which adds to condensation.
  • Closed curtains trap cold, humid air against the glass overnight.

Daily habits that help

  • Wipe the glass and the bottom of the frame every morning with a cloth or a squeegee (結露取りワイパー).
  • Open two windows for 5–10 minutes twice a day to swap humid air for dry outside air.
  • Run the kitchen and bathroom fans while cooking and after showers.
  • Keep furniture a few centimetres off exterior walls so air can circulate.
  • Wash curtains that touch wet glass before mould sets in.

Things you can add

Absorbent tape stuck along the bottom of the glass soaks up drips before they reach the frame. Insulating film or bubble-wrap sheets on the glass keep its surface warmer and reduce condensation; most peel off cleanly, which matters in a rental.

A dehumidifier helps if the room stays damp. In winter, a desiccant type (デシカント式) works better in cold rooms than a compressor type (コンプレッサー式), though it uses more electricity; a hybrid type (ハイブリッド式) does both.

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FAQ

Is window condensation normal in Japan?
Yes, it's very common in winter, especially with single-pane windows. Daily wiping and ventilation keep it from turning into mould.
Which dehumidifier is best for winter in Japan?
A desiccant (デシカント式) or hybrid (ハイブリッド式) model, because compressor models lose power in cold rooms.
Will insulating film damage my rental windows?
Most window films are designed to peel off, but check the product says it can be removed (はがせる) and avoid wired or specially coated glass if the instructions say so.