As temperatures climb across the UK, homeowners are looking for ways to keep their houses cool without relying on expensive air conditioning units that sit idle for most of the year. A method known as window stacking has gained attention as a potential solution for managing indoor temperatures, though it requires a specific approach to be effective.
Window stacking involves layering different types of window coverings, usually three in total, to create a combined system that blocks heat, controls light, and improves comfort. The method relies on a heat-blocking layer, a light-diffusing layer, and a heat-control layer working together, rather than a single blind or curtain.
According to Anthony Threlfall, head of marketing at Everest, the technique provides much greater control over the temperature and airflow in a room throughout the day. The layers can be adjusted to manage heat gain and reduce the need for mechanical cooling systems.
The mechanics of airflow
To make the system work, the arrangement of windows and their position matters. Anton Neike, cooling and energy expert and CEO at Taupia, explains that the stack effect requires two sets of openings. One should be on the ground floor, ideally on the cooler, shaded side of the home. The other should be on an upper floor to let hot air escape.
For best results, the bottom sash of the lower windows should be opened to let cool air in, while the top sash of the upper windows lets warm air out. If a home has two floors, opening first-floor windows on the windward side and second-floor windows on the opposite side is recommended.
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Choosing the right materials
The fabrics used in the setup must be chosen carefully to avoid trapping heat. Debbie Leigh, design manager at interior textiles company ILIV, advises against relying on heavy fabrics alone during hot weather, as these can actually trap heat rather than reflect or diffuse it.
She suggests starting with a sheer or voile fabric close to the glass to diffuse strong sunlight without blocking natural light completely. This should be followed by a well-fitted blind, such as a blackout or thermal roller blind, which can be pulled down during the hottest parts of the day. The final layer can be curtains, which offer insulation when temperatures drop in the evening.
Timing and flexibility are key
Experts warn that the timing of window operation can determine whether the strategy works or backfires. Opening windows during the hottest part of the day allows hot air to enter, which is a common mistake. The stack effect only works when the air outside is cooler than the air inside, so it is best used in the early morning and late evening after sunset.
Over-layering can also reduce airflow around the window, making rooms feel stuffy if everything is kept closed all the time. Ross Evans, co-founder of ShuttersUp, notes that choosing dark materials that absorb heat rather than reflect it can also reduce effectiveness. He adds that overcrowding the window with too many layers that become difficult to use will not help.
The effectiveness of the setup depends on flexibility. Treatments must be adjustable so that homeowners can manage both heat and airflow. Window treatments with adjustable louvres, for example, allow heat to be kept out during peak sunshine and cooler air to be welcomed in when temperatures drop.
