Heat wave guidance: when to keep windows closed or use AC

Research by the Society of Automotive Engineers finds that driving with windows open is fuel‑efficient only below about 72 km/h; above that speed the aerodynamic drag makes air‑conditioning the cheaper option, with open windows raising fuel use by roughly 20 % versus a 10 % increase from the AC.

Home‑heat‑wave advice from experts in Croatia and Slovenia recommends keeping windows, doors, blinds and curtains closed during the hottest part of the day (approximately 10 a.m. to 6 p.m.) to limit solar gain. Early‑morning or late‑evening ventilation creates a cross‑draft that brings in cooler air. Reflective or light‑coloured shades can cut solar heat gain by 40‑70 %. When outdoor temperatures fall after sunset, opening windows on opposite sides of the house helps cool the interior.