What do people get wrong about opening windows and ventilating a home in Japan?

This answer is a draft. It stays unpublished until legal review and source verification are complete.

Opening windows morning and evening is a household reflex in Japan, reinforced during 2020 by public guidance that gave it numbers. Visitors often assume it is about fresh air; it is at least as much about moisture and mould.

The guidance

• The 2003 requirement for 24-hour mechanical ventilation followed cases of "sick house syndrome" from formaldehyde in tightly built homes. The small vents in walls and the bathroom fan that never turns off are that system — do not switch it off or tape the vents.
• Ventilating in the morning clears overnight moisture (breathing, futon); in the evening it clears cooking moisture.

[Verify] Guidance figures with the MHLW 2020 documents and the Building Standards Act provisions on mechanical ventilation.

Misunderstanding 1｜One window is enough

Air needs an inlet and an outlet. One open window mainly lets air stand. Diagonal pairs — windows on opposite walls — move it through.

Misunderstanding 2｜Air conditioning ventilates

Most home air conditioners recirculate indoor air and do not exchange it with outside. They control temperature and (when dehumidifying) moisture, not freshness.

Misunderstanding 3｜Do not ventilate in the rainy season — it lets humidity in

On a rainy day, outdoor humidity is high, but indoor humidity from laundry and bathing is often higher still. Short ventilation plus a dehumidifier or air-conditioner dry mode is the usual combination. Long open windows on rainy days do more harm.

Misunderstanding 4｜The little wall vents are draughts to be blocked

They are the intake for the mandatory 24-hour ventilation. Blocking them in a tightly built flat pushes humidity up and condensation onto north-facing walls — which is where the mould starts.

Misunderstanding 5｜Winter ventilation just wastes heat

Ten minutes with two windows costs little heat compared with the moisture problem it solves: condensation on windows and in wardrobes. Households ventilate in the warmest part of the day in winter.

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Sources

• Ministry of Health, Labour and Welfare — Ventilation guidance for homes and offices (2020)

https://www.mhlw.go.jp/stf/seisakunitsuite/bunya/0000121431_00094.html

• Ministry of Land, Infrastructure, Transport and Tourism — Building Standards Act: mechanical ventilation and sick house measures (2003)

https://www.mlit.go.jp/jutakukentiku/build/sickhouse.html

• Tokyo Metropolitan Government Bureau of Public Health — Preventing mould at home

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.

About this article

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.

This is general information. Results vary from person to person and depend on your own conditions and surroundings.

Please decide for yourself, using your own judgement, whether any of this suits your situation. We cannot accept responsibility for any damage, loss or injury arising from following this article.

Always follow the instructions on any product you use, and never mix cleaning products. If you rent your home, check with your landlord before making any changes.

Sources

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