Should I open windows during a heatwave?

The reason is that a UK home is built to hold warmth, not shed it. Most were designed to retain heat, not release it, which makes them prone to overheating during heatwaves³. Opening a window in the middle of a hot afternoon therefore does not cool the building; it replaces cooler indoor air with hotter outdoor air, and the fabric of the home absorbs that heat.

Ventilation still matters, and it is the cheapest cooling a household has. A well insulated home is usually easier to keep cool, with ventilation by opening windows at night and using extractor fans and vents⁴. The skill is timing rather than whether to ventilate at all.

Why open windows can make your home hotter

A window is a hole in the building envelope, and in summer it works in both directions. When outdoor air is hotter than indoor air, an open window is a route for heat to enter, and the thermal mass of walls, floors and furniture then stores it. That stored heat is released slowly through the evening, which is why a home that has been aired all afternoon can still be uncomfortably warm at midnight.

The design of UK housing makes this worse. Most homes were designed to retain heat, not release it, making them prone to overheating during heatwaves³. The same insulation that keeps winter warmth in keeps summer heat in, and a well insulated home is usually easier to keep cool only if it is ventilated at the right time, by opening windows at night and using extractor fans and vents⁴.

Several factors influence how much heat a property needs, including property size, number of external walls, wall thickness, insulation levels, type of glazing, number of people in the home and daily habits⁷. The same list explains how a home behaves in a heatwave: a top-floor flat with a large south-facing window and several occupants will overheat faster than a shaded ground-floor house with thick walls.

There is a wider point about summer comfort and energy independence. Improving homes so they stay warm in winter and cool in summer, investing in ventilation, shading and passive cooling, and ensuring energy is affordable for essential needs including cooling are all part of what the National Energy Action policy position sets out⁸. A household that can manage heat with ventilation and shading is less dependent on a powered cooling appliance and the electricity it draws.

When opening windows actually helps: night and early morning

Night ventilation is the part of the day that does the real work. Official guidance on traditional buildings states that all windows lose heat at night, except in the very warmest weather, and that in all circumstances windows and rooflights should retain their full capacity to ventilate rooms⁶. That second point matters for anyone renovating: ventilation capacity is not a feature to be designed out.

For bedrooms specifically, the advice is to open the windows a few hours before bedtime, releasing hot air that rises upwards in the house⁹. Upper floors collect the day's accumulated heat, so a top-floor bedroom benefits most from an early evening airing rather than a last-minute one.

There is a limit to how much night air helps. Air conditioning use during a heatwave can raise night-time temperatures by 1 to 1.5C in the surrounding area, a figure reported from Phoenix, Arizona, during a heatwave¹⁰. That is a non-UK example, and it describes the urban heat effect of widespread air conditioning rather than a UK condition, but it illustrates that mechanical cooling shifts heat outdoors rather than removing it.

In practice, a household should expect the indoor temperature to fall overnight only if the outdoor air is genuinely cooler and air can move through the building. A still, humid night with windows open on one side of the house will achieve little.

Cross-ventilation: how to cool a house with windows and doors

Cross-ventilation means giving air a route in one side of a home and out the other, so that it moves through rather than sitting in a room. Door undercuts provide airflow between rooms, which helps to provide cross-flow ventilation and reduces the build-up of moist air in rooms¹¹. Internal doors left open, or trimmed at the base, are part of the cooling system.

Window type matters. Double hung sash windows can be very effective for ventilating a room when both top and bottom are open in hot weather, because the two openings sit at different heights and set up a stack effect¹². Tilt and turn windows have a different advantage: as they can be locked partially open, they are a secure way to allow some fresh air into your home¹².

The building fabric supports this. Solid wall insulation helps create a more even temperature in the home, helps prevent condensation on walls and ceilings, and can reduce the amount of heat building up inside during summer hot spells¹³. Double glazing works by trapping air between two panes of glass, creating an insulating barrier that reduces heat loss, noise and condensation, and installing it can cut heat loss through windows by half¹⁴. Combining loft insulation, external wall insulation and new window glazing can reduce a home's heat transfer coefficient by approximately 60%¹⁵.

The trade-off is that a well-sealed home ventilates less by accident. Draught stripping for windows and doors is treated as an energy-saving material in the legislation that sets out VAT relief for such work, which shows how far the push to seal homes has gone¹⁶. Sealing is right for winter and needs a deliberate summer counterpart.

Shading and ventilation together: blinds, curtains and shutters

Shading and ventilation are not alternatives; they do different jobs at different times. Shading keeps solar heat from entering during the day, and ventilation removes the heat that has accumulated once the air outside cools.

Where shading is fitted matters more than how much. Internal blinds and shutters help to some extent, but by that point the heat is already inside the home, between the window and the shading device⁵. It is much more effective to shade windows from the outside, as inside shading still lets most heat through the glass¹⁷. A government-commissioned study found shading devices, especially external shutters, to be the most effective means of keeping heat out⁵. For windows facing south through to west, awnings, blinds or shutters are the recommended remedial action to protect from solar heat gain¹⁸.

Curtains have a seasonal double role. Heavy curtains can make a real difference to how much heat is lost during the winter, and in the summer blinds can help keep rooms cooler, so a fan is not needed¹⁹. Thick or thermal lined curtains help keep draughts at bay, particularly with single glazing, but the same guidance says to open curtains during the day to let in sunshine and warmth, which is winter advice and should be reversed in a heatwave²⁰.

Ventilation, security and air quality during hot weather

Open windows raise two questions that have nothing to do with temperature: whether the opening is secure, and what the incoming air carries. On security, tilt and turn windows can be locked partially open, which is described as a secure way to allow some fresh air into the home¹². That is a property of the window rather than a general reassurance, and ground-floor openings remain a household judgement.

On air quality, the evidence runs both ways. Poor ventilation can trap moisture and everyday pollutants indoors, making conditions like asthma and allergies worse over time²³. Against that, an open window admits whatever is outside, and no source in this area distinguishes between outdoor pollen and indoor moisture as competing risks. Mechanical heat recovery ventilation provides a constant supply of clean, fresh air which contributes to a healthy, comfortable living environment, with additional benefits and efficiencies from combining heat recovery with ventilation²⁴. That route cools less than an open window but filters and tempers the air.

There are safety reasons to ventilate that override comfort. Gas emergency advice is to ventilate the property by opening doors and windows to allow fresh air to circulate²⁵. High temperatures can be dangerous to your health, which is why hot weather preparedness matters in the first place¹.

For households with vulnerable occupants, the practical advice is to ensure those with underlying health conditions have medication, to plan activities for cooler times such as morning or evening, and to keep those cared for out of the sun at the hottest time of day⁹. Bedroom windows are particularly susceptible to condensation, since moisture from breath and body heat can accumulate in the room, which is a further argument for airing bedrooms before sleep²⁶.

What this means for a household's energy independence

Ventilation and shading are the two cooling measures a household can operate without buying anything or drawing a watt. That matters for independence in a direct sense: a home cooled by night air and external shading is not exposed to electricity prices, to a supplier, or to the grid at the moment of peak summer demand. It also matters for resilience, since a passive approach keeps working in a power cut.

The dependence that remains is on the building itself. A home with small windows, no cross-flow and no external shading cannot be ventilated into comfort, and the options then are mechanical. A portable air conditioning unit needs to be properly vented to the outside using a special fitting for the window, and a vent simply put through an open window will let heat back in¹⁷. That is a reminder that powered cooling is only as good as its installation.

There is also a health dimension that runs through the year. Summer fuel poverty is a recognised problem, and the same homes that are cold in winter overheat in summer because most were designed to retain heat, not release it³. Improving homes so they stay warm in winter and cool in summer, with investment in ventilation, shading and passive cooling, is the policy answer set out by the National Energy Action⁸. For an individual household, the equivalent is a routine: shade and close by day, ventilate by night, and keep the air moving through the whole home rather than one room.