Value Village ／ Everyday know-how
This answer is a draft. It stays unpublished until legal review and source verification are complete.
Wake up in a Japanese flat in January and the windows are streaming. Condensation is the winter counterpart of the rainy season's damp — the moisture is the same, it just has somewhere to land — and it feeds the mould that appears on window frames and curtains by spring.
| Cause | Detail | Fix |
|---|---|---|
| Single glazing and aluminium frames | Standard in most homes built before ~2010; the frame conducts cold; the glass surface can drop below 5°C while the room is 20°C | Inner window (uchimado) or double glazing — national subsidies covered up to half the cost in 2023–25; insulating film or bubble sheet as the cheap version |
| Indoor humidity from living | A family adds several litres of water vapour a day (breathing, cooking, bathing, indoor laundry) | Ventilate (two windows, 5–10 min, twice a day); bathroom fan on; dehumidifier for laundry |
| Unvented combustion heaters | Kerosene and gas fan heaters release about 1 L of water per litre of fuel | Ventilate hourly (also for CO); or switch to air-conditioner/heat pump |
| Furniture against exterior walls | Cold, still air behind it condenses | Leave 5–10 cm gap; check behind the wardrobe in February |
| Curtains | Trap moist air against cold glass; mould on the hem by spring | Open curtains in the morning; wash them in spring |
| The water itself | — | Wipe each morning with a squeegee or cloth; condensation-absorbing tape on the frame bottom; keep the sill dry |
[Verify] Subsidy figures with MLIT/METI/MoE joint window-renovation programme; insulation statistics with MLIT.
It is the mould condition being met every night. Wipe it, yes, but also ventilate and move the wardrobe off the outside wall.
Warmer air holds more water and the glass stays cold; condensation often gets worse. Reducing humidity and insulating the glass is the fix.
They add about a litre of water vapour per litre of fuel (and CO2 and some CO). The dry feeling is warmth, not low humidity. Ventilate hourly.
Insulating film, bubble sheet, thick curtains kept open by day, absorbent tape, furniture off the wall, a dehumidifier — all removable. Ask the landlord about inner windows; some accept them.
Windows are the biggest heat-loss path and the state has been paying half the cost. For owners, it is the highest-return home upgrade in Japan; it also ends the streaming windows.
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https://window-renovation2025.env.go.jp/
https://www.mlit.go.jp/jutakukentiku/house/jutakukentiku_house_tk4_000103.html
https://www.hokeniryo.metro.tokyo.lg.jp/kankyo/kankyo_eisei/jyuukankyo/indoor/kabi.html
⚠️ Editorial note: the source URLs above were identified by search. Our working environment cannot reach go.jp domains, so the contents have not been checked against the text. Verify every source before publication and record who checked it and when.
What we share here are ideas and everyday practices passed down among people living in Japan, together with general information drawn from public sources. They are not official recommendations, and they are not professional instruction. Practices vary between households and regions.
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