When summer temperatures rise, the instinct is to throw open every window in the house. In fact, you might just get the idea from your neighbours, especially if you see endless windows open all the way down your street.

Fresh air feels like the obvious answer. However, opening your windows at the wrong time can actually make your home hotter, not cooler.

In this guide from the Lee Jackson team, we’ll explain exactly when to open windows in hot weather, when to keep them shut and what actually makes a difference when outdoor temperatures are extreme.

The Short Answer

Opening windows during a heatwave can provide relief, but only if you open the windows exactly the right time of day.

If the outside air is cooler than the air inside your home, then go ahead. If it is, open the windows. If it is not, keep them shut.

The problem during a heatwave is that outdoor air can stay warmer than indoor air well into the night. In that situation, open windows simply let the heat in rather than letting it out.

When To Open Windows During A Heatwave

There are two windows of opportunity during a heatwave when opening windows genuinely helps:

Early Morning

Overnight temperatures tend to drop, so by early morning, the air outside is often cooler than the air that has built up indoors. Opening windows from around 5am to 8am allows cooler air to move through the house before the day heats up. Close them again once the outside temperature starts to climb.

Late Evening

Once the sun sets and outside temperatures begin to fall below indoor levels, windows can go back open. This allows the heat that has built up in your walls, ceilings and floors to dissipate overnight. During a prolonged heatwave this matters, because buildings absorb heat over several days and the indoor temperature can creep higher each day if that stored heat has nowhere to go.

When To Keep Windows Shut During A Heatwave

During the hottest part of the day, typically between 10am and 8pm in a British heatwave, keeping windows closed helps preserve the cooler air already inside your home. Paired with closed blinds or curtains on south and west-facing windows, this can reduce indoor temperatures noticeably.

This approach works because your home acts as a thermal buffer. Once the indoor air heats up it is difficult to cool down, so preventing heat from entering in the first place is more effective than trying to remove it once it is inside.

Understanding Which Direction Your Windows Face

Not all windows heat up equally during a heatwave. The direction a window faces determines how much direct sunlight it receives throughout the day, which has a significant effect on how quickly a room warms up and how much heat enters through the glass.

South Facing Windows

South facing windows receive direct sunlight for the longest period of the day. In summer they are in the sun from late morning through to mid-afternoon, making them the single biggest source of solar heat gain in most UK homes. Keeping the blinds or curtains on south-facing windows closed during daylight hours has a bigger impact on indoor temperature than any other window in the house.

West Facing Windows

West facing windows receive direct afternoon and evening sun, which is often the most problematic for bedrooms. The sun is lower in the sky during the afternoon, meaning it penetrates further into a room and heats surfaces more intensely. Rooms with west facing windows tend to peak in temperature in the early evening, exactly when you want to be cooling down for sleep.

East Facing Windows

East facing windows catch the morning sun but are in shade by mid-morning. In a heatwave, these are less problematic during the day and are often the safest to open in the early morning ventilation window, since they face away from the afternoon heat.

North Facing Windows

North facing windows receive little or no direct sunlight in summer. They are the coolest windows in the house and the best candidates for opening during the day if ventilation is needed, since they are unlikely to bring in direct solar heat.

If you are unsure which direction your windows face, a basic compass app on a smartphone gives a reliable reading. Stand at the window looking outward and check the direction you are facing.

The Limits of Open Windows in Extreme Heat

Window management works reasonably well in a short sharp heatwave where temperatures drop significantly overnight. It becomes less effective in prolonged heat events because:

Overnight temperatures may not drop enough to provide meaningful cooling.
Buildings absorb heat over several consecutive hot days, pushing internal temperatures progressively higher.
In urban areas, the urban heat island effect keeps night-time temperatures elevated compared to rural surroundings.
Older properties with solid walls absorb and retain more heat than newer builds.

In these situations, open windows alone are not sufficient to maintain a comfortable or safe indoor temperature.

What Air Conditioning Does Differently

The key difference between open windows and air conditioning is that air conditioning actively removes heat from indoor air rather than simply moving air around. A split system air conditioner absorbs heat from inside your home and transfers it outside, reducing the actual temperature of the air indoors regardless of what is happening outside.

This means air conditioning works reliably during a heatwave even when outdoor temperatures are at their peak. It also removes humidity from the air, which matters because high humidity makes heat feel significantly more oppressive and makes it harder for the body to regulate temperature through sweating.

Is Air Conditioning Worth It For A UK Home?

With UK summers becoming hotter and longer, home air conditioning is no longer the unusual investment it once was. The UK Health Security Agency issues heat health alerts and recommends keeping living spaces below 26 degrees where possible. For bedrooms in particular, sleep quality deteriorates significantly above this threshold.

Modern split system air conditioners are more energy-efficient than older models and can also function as heat pumps in winter, providing year-round value. Running costs depend on unit size and usage, but are broadly comparable to running a tumble dryer.

For households with young children, elderly occupants or individuals with health conditions that are exacerbated by heat, the case for air conditioning is particularly strong. The NHS advises that high indoor temperatures pose a genuine risk to these groups.

Practical Tips for Keeping Your Home Cooler Without AC

If you do not have air conditioning, combining several approaches gives better results than relying on any single method:

Open windows only when outside air is cooler than inside air, typically early morning and late evening.
Close blinds and curtains on south and west-facing windows during the day to block direct solar gain.
Use internal fans to circulate cooler air already inside rather than drawing hot air in from outside.
Avoid using heat-generating appliances such as ovens and tumble dryers during the hottest part of the day.
Create cross-ventilation by opening windows on opposite sides of the house during cooler periods, allowing air to move through rather than stagnate.
Move to lower floors during the day if possible, as heat rises and upper floors tend to be significantly warmer.

UK Heatwave FAQs

Need help keeping cool in the heat? We’ve answered some common questions below.

Or, for queries about cooling your home with air conditioning, please give us a call on 0800 949 9070.

Should You Open Windows At Night During A Heatwave?

Yes, as long as the outdoor temperature has dropped below the indoor temperature. Check both temperatures before opening them. If the outside is still warmer, keep them shut and try again later in the night or early morning.

Is It Better To Open Windows Or Use A Fan In A Heatwave?

It depends on the outdoor temperature. During the hottest part of the day, it is better to keep windows shut and use a fan to circulate the cooler air inside. In the evening, when the outside cools down, opening windows combined with a fan to move air through the house is more effective.

Can Open Windows Replace Air Conditioning In A Heatwave?

In a mild warm spell, window management may be enough. In a serious heatwave, particularly one lasting more than a few days, open windows cannot remove heat actively and are limited by whatever temperature the outside air reaches overnight. Air conditioning is the only reliable way to maintain a controlled indoor temperature regardless of external conditions.

What Temperature Is Too Hot Indoors?

Obviously, the definition of when it is ‘too hot’ is somewhat subjective. However, the UK Health Security Agency recommends keeping the main living area below 26 degrees and bedrooms below 24 degrees where possible. Sustained indoor temperatures above these levels can affect sleep, concentration and physical health, particularly in vulnerable individuals.

Need Help Cooling Your Home This Summer?

Lee Jackson installs and services air conditioning systems for homes and businesses across the East Midlands.

Whether you want a single-room solution or a multi-room system, our engineers can advise on the right setup for your property and usage.

Get in touch with our team to discuss your requirements and arrange a free site survey.