Why it works

Why opening windows can make your room hotter

Short answer

An open window does not automatically cool a room. If the air outside is hotter, ventilation can replace cooler indoor air with hotter air. Open up when outside becomes cooler instead.

Use the temperature difference

During the hottest part of the day, shade the glass and keep hotter outdoor air out. In the evening or early morning, open safe windows when the outdoor air is cooler than the indoor air.

Create a route

Air exchange is stronger when air can enter on one side and leave on another. With a single window, an outward-facing fan near the opening can help remove warm indoor air when cooler replacement air can enter.

Important exceptions

Wind, pollution, noise, security, window safety and indoor moisture can change what is practical. Air conditioning also works best with windows closed.

Related guides

How to cool a room without air conditioning →How to use a fan effectively →How to sleep during a heatwave →How fans cool you →

Sources & evidence

Reviewed 2026-08-21

  1. UK Health Security Agency: Beat the heat: keep cool at home checklist ↗

    Supports: Window timing, shading, ventilation, reducing internal heat and identifying higher-risk homes.

    official guidance · Accessed 2026-08-21