10 rules for keeping rooms cool in summer: From shading to air conditioning

Modern people spend more time indoors than outside. Activity levels and productivity are directly linked to the amount of natural daylight available throughout the day, which is why glazed areas continue to increase. However, more glazing also means a greater impact on the indoor climate: improved thermal insulation in winter but a higher risk of overheating in summer.

Specialists at the Austrian institute Holzforschung Austria studied windows, solar shading and ventilation as a single integrated system, focusing on providing sufficient daylight and a comfortable indoor climate throughout the year without air conditioning. The study was carried out by modelling a reference apartment in Vienna based on an unfavourable climate scenario for urban conditions projected for 2050.

What temperature is comfortable in summer?

Thermal comfort is subjective and varies from person to person, but specialists rely on well-established reference values.

Ukrainian sanitary standards for workplaces recommend an indoor temperature of 22–24 °C in summer (for light-duty work), with an upper limit of 28 °C. As temperatures approach 28 °C, productivity declines noticeably.

Germany's Workplace Directive sets three threshold values:

• ˙ Above 26 °C, where direct sunlight enters the room, window shading is mandatory.
• ˙ Above 26 °C despite shading, additional measures are required, including night-time ventilation, reducing internal heat gains and relaxing dress code requirements.
• ˙ Above 30 °C, effective cooling measures are mandatory.

The World Health Organization (WHO) recommends keeping indoor temperatures below 32 °C during the day and below 24 °C at night. For infants, people aged over 60 and those with chronic health conditions, these values represent minimum safety requirements rather than comfort targets.

10 rules for keeping rooms cool

Following these rules helps achieve the best results in reducing heating and cooling costs while ensuring good access to daylight.

1. External shading is far better than internal shading. Internal shading is better than no shading at all

External shading provides the greatest reduction in indoor temperature during summer. Where external shading is not possible — for example, because of heritage conservation requirements — internal shading is still better than none, particularly for large windows.

The colour of the shading affects more than just aesthetics:

• Dark shading = less daylight indoors, better glare protection.
• Light shading = more daylight indoors, reduced glare protection.

2. Shading is necessary for windows facing all orientations, including north

A calculation based on the sun's position for Vienna showed that, from May to September, solar radiation on north-facing façades amounts to around 60% of that on south-facing façades. This means there is a genuine risk of overheating even in rooms with north-facing windows.

For comparison, according to profileSOLAR.com, during summer each kilowatt of installed solar capacity generates an average of 6.42 kWh per day in Vienna, 6.45 kWh in Dnipro, and 6.50 kWh in Kyiv. Solar gains in Ukraine are therefore no lower than in Austria.

3. IGU Ug-value ≤ 1.0 W/m²K

The Ug-value of external windows should not exceed 1.0 W/m²K and should be as low as possible. The Ug-value indicates heat loss through the insulating glass unit (IGU): the lower the value, the better the thermal insulation performance.

4. Solar factor g ≥ 0.5

g-values below 0.5 should be avoided because they reduce daylight utilisation and solar heat gains during the heating season. Under the conditions of the Holzforschung Austria study, the use of solar control glass is not recommended.

Important context: this recommendation applies to the specific modelling conditions used in the study — a temperate climate with an emphasis on daylight and a balance between winter and summer performance. Different parameters may be appropriate for buildings used mainly in summer or for highly exposed sunny façades.

The total solar energy transmittance of glazing, also known as the solar factor (g-value), determines how much solar radiation the insulating glass unit (IGU) transmits. The higher the g-value, the more solar energy enters the room.

5. At least 50% of windows should be openable

To ensure sufficient night-time ventilation, at least 50% of the windows should be openable.

Cross-ventilation is the most effective way to replace indoor air quickly.

6 (debatable). There is no single answer to when windows should be opened for ventilation

The traditional advice is to open windows only when the outdoor temperature is lower than the indoor temperature. This is the recommendation of Holzforschung Austria, the WHO and the Umweltbundesamt. The reasoning is straightforward: opening a window when it is hotter outside simply allows hot air to enter the building.

However, this principle has become the subject of considerable debate among experts.

Why the debate arose

The disagreement is not about temperature but about indoor air quality. In 2024, Swiss meteorologist Jörg Kachelmann described the advice to "keep windows closed during the day" as "active euthanasia". In interviews with WDR and posts on X, he argued that humidity builds up and indoor air quality deteriorates in sealed rooms. For older people and those with chronic health conditions, this may be more dangerous than warm outdoor air. "Trust your instincts before you feel seriously unwell," he advises.

Christof Asbach, President of the Gesellschaft für Aerosolforschung (Society for Aerosol Research), supports the need for ventilation for a different reason: where several people occupy a room, concentrations of CO₂ and airborne pathogens increase, posing a health risk regardless of temperature.

Environmental chemist Kerstin Effer from Verbraucherzentrale NRW adds that at elevated temperatures, older finishing materials and furniture can emit hazardous substances, including PCBs, PAHs and wood preservatives. In such cases, ventilation becomes essential.

Dr Alina Herrmann of the Cologne and Heidelberg University Hospitals takes a more balanced view: priority should be given to the temperature difference. If it is 38 °C outdoors and 29 °C indoors, windows should remain closed. However, if the temperature difference is minimal, ventilation is advisable.

Following an enquiry from WDR, the Bundesverband der Verbraucherzentralen decided to revise its official recommendations, making them more nuanced.

What determines whether ventilation is advisable

In a detailed response to WDR, experts from the Umweltbundesamt acknowledged that the right time to ventilate depends on several factors simultaneously.

• ▶ Temperature difference. This is the primary guideline. If it is cooler outside, ventilate. If it is hotter, keep windows closed and shaded. However, temperature is only one factor.
• ▶ Number of occupants. The more people in a room, the faster CO₂ levels and airborne pathogens build up. Hans-Guido Mücke (Umweltbundesamt) notes that two people in a large house can go longer without ventilation than one person in a small room.
• ▶ Indoor pollution sources. Candles, smoking, emissions from older furniture or interior finishes may make indoor air quality worse than outdoor air, even during hot weather.
• ▶ Window orientation. Windows exposed to direct sunlight should remain closed and shaded. Windows on the shaded side of the building may be opened even during the day if the temperature difference is small.
• ▶ Outdoor pollution sources. A busy road, industrial facility or nearby airport may justify reducing ventilation.

Practical conclusion

Ventilate intensively in the early morning and at night: this is the most effective way to cool the building and refresh the indoor air.

During the day, assess the situation. If it is significantly hotter outside, keep windows closed and shaded. However, if the air becomes stuffy and several people are in the room, a short period of intensive ventilation is preferable to leaving windows permanently tilted open or using a small ventilation opening, as it refreshes the air more quickly while introducing less heat into the room.

Keeping windows in the tilt position during the day is the least effective option: the slow air exchange does not adequately improve air quality but continuously allows warm air to enter the room.

6a. Fans: when they help and when they do not

A fan does not cool the air in a room. Instead, it creates airflow that accelerates the evaporation of sweat, making the body feel cooler. The actual room temperature remains unchanged or may even increase slightly because of the heat generated by the motor.

Up to what temperature is a fan effective?

The WHO recommends using fans only when the air temperature is below 40 °C. Above this threshold, a fan delivers hot air to the body faster than it can cool itself through perspiration.

Dr Alina Herrmann (Cologne and Heidelberg University Hospitals) suggests a lower threshold of 36 °C. According to medical evidence, fans provide reliable cooling up to this temperature. Above it, the effect becomes neutral or even negative.

Where and how to position a fan

• ˙ At night with windows open: placing a fan near a window accelerates night-time ventilation.
• ˙ During the day with windows closed: direct the airflow towards people rather than walls, as airflow across the skin provides the cooling effect.
• ˙ In front of a bowl of ice: directing a fan across cold water or ice can reduce the air temperature slightly — a simple technique for extremely hot days.

Using a fan together with air conditioning

The WHO recommends using air conditioning together with a fan. Setting the air conditioner to 27 °C while using a fan provides a perceived temperature of around 23 °C. This improves energy efficiency without reducing thermal comfort.

What a fan cannot replace

Without external shading (Rule 1) and proper ventilation (Rule 6), a fan simply circulates hot air. The most effective combination is shaded windows, a building cooled overnight, and fan-assisted air movement during the day.

7. Provide night-time ventilation

Night-time ventilation should be provided to remove the heat accumulated during the day.

If night ventilation is not possible because of external conditions (for example, excessive noise), the ventilation system should provide the highest possible level of cooling air exchange during the night.

8. Additional shading is required even where an overhang is provided

Even where there is a balcony, canopy or similar architectural feature, additional movable external shading is still required. Even with a 2 m overhang, the cooling demand is reduced only slightly.

Important: overhangs deeper than 1 m noticeably reduce daylight availability and this should be taken into account during the design stage.

9. Consider window size

Large glazed areas improve daylight availability but require well-designed and properly operated external shading. Without it, large windows become a major source of overheating during summer.

10. Consider the wind resistance of shading systems

Depending on the location and installation conditions, the wind resistance of the solar shading system should be taken into account when selecting a solution.

In Ukraine, the wind resistance classification of roller shutters, external blinds and awnings is determined in accordance with DSTU EN 13561:2018, "External blinds — Performance requirements including safety".

How to use air conditioning together with passive cooling

Air conditioning has become a standard solution for Ukrainian homes and offices, particularly after several consecutive hot summers. However, even if an air conditioner is installed, most of the basic principles still apply.

An air conditioner works most efficiently when the other measures are already in place. External shading reduces its cooling load. Night-time ventilation shortens operating hours. The correct temperature setting and the use of a fan reduce energy consumption. Without these measures, an air conditioner simply becomes an expensive way of compensating for avoidable heat gains.

What temperature should you set?

According to medical recommendations from various countries, the temperature difference between indoors and outdoors should generally not exceed around 5–8°C (with an absolute maximum of 10–12°C) to avoid heat stress and unnecessary electricity consumption.

A common mistake is setting the thermostat to the lowest possible temperature in the hope of cooling the room more quickly. In reality, an air conditioner cools at a fixed rate, while a large temperature difference between outdoors and indoors increases the risk of catching a cold and raises energy consumption.

The WHO recommends using an air conditioner together with a fan. Air movement creates an additional cooling effect equivalent to around 3–4°C, allowing the air conditioner to consume less energy.

External shading remains essential

Without external shading, solar radiation continuously heats the room, forcing the air conditioner to operate at maximum capacity. This results in higher electricity bills and faster wear of the equipment.

Researchers at Holzforschung Austria have demonstrated that external shading systems—including roller shutters, external blinds and awnings—are the most effective way to reduce solar heat gains. When an air conditioner is used, this translates directly into lower electricity consumption.

The rule is simple: install external shading first, then use the air conditioner. External shading reduces the amount of work the air conditioner has to do—not the other way round.

Ventilation is necessary even with air conditioning

Most domestic air conditioners are recirculating systems: they cool the same indoor air rather than supplying fresh outdoor air. Without ventilation, CO₂ concentrations increase, while odours and indoor pollutants accumulate.

According to Christoph Asbach (Gesellschaft für Aerosolforschung), regular air exchange is essential regardless of whether an air conditioner is installed, particularly when several people occupy the room.

A practical approach is to ventilate intensively at night or early in the morning to cool the building and refresh the indoor air without using the air conditioner. During the day, keep windows closed and shaded while the air conditioner maintains the indoor temperature. If your air conditioner includes a fresh-air ventilation mode, use it.

Air conditioner maintenance

Dirty filters are the most common reason why an air conditioner struggles to cope during the hottest days. A clogged filter can reduce cooling efficiency by 15–25% while placing additional strain on the compressor.

• ˙ Clean or replace the filter once a month during the cooling season.
• ˙ Have the unit professionally serviced once a year before the start of the cooling season.

A dirty filter can also become a source of bacteria and mould spores released into the indoor air.

How to reduce electricity costs

With unstable electricity supplies and rising energy tariffs in Ukraine, using an air conditioner efficiently is not only a matter of comfort but also of reducing operating costs.

• ˙ External shading can reduce the cooling load on an air conditioner by 30–50%.
• ˙ Night-time cooling through open windows reduces the number of hours the air conditioner needs to operate during the day.
• ˙ Using an air conditioner together with a fan helps save energy without sacrificing comfort.
• ˙ Set the timer to switch the air conditioner off at night if sufficient night-time ventilation is available.

Basements and semi-basements: a different approach to ventilation

Basements and semi-basements behave differently from ordinary rooms during hot weather. Intuitive decisions such as "I'll open the windows because it's hot" or "I'll keep everything closed to retain the cool air" can both create problems.

Why basements are a special case

Basement walls remain considerably cooler than the outside air, even during periods of extreme heat. When warm, humid summer air enters the basement, it comes into contact with these cool surfaces, causing moisture to condense. Water droplets form on walls, mould develops, and musty odours appear. This can happen even on a dry, hot day if the outdoor air contains sufficient moisture—as it typically does in Ukraine during summer, when daytime relative humidity usually ranges from 40% to 70%.

The German Environment Agency (Umweltbundesamt) warns about this risk: warm summer air entering a basement condenses on cool surfaces and creates conditions favourable for mould growth.

When should you ventilate a basement?

The logic is the opposite of that for ordinary rooms. A basement should be ventilated at night or early in the morning, when the outdoor air is not only cooler but also significantly drier.

A practical rule: ventilate the basement between 22:00 and 08:00, when the outdoor temperature falls below 20°C. During the day, keep windows and doors closed.

If you have a hygrometer, ventilate the basement only when the outdoor relative humidity is lower than the humidity inside the basement to ensure that ventilation improves rather than worsens the situation.

Can ventilation be avoided altogether?

No. In a tightly sealed basement, carbon dioxide, radon (particularly in certain geological areas), odours and pollutants emitted from building materials accumulate. A minimum level of ventilation remains necessary—but only at the appropriate time of day.

Basements used as shelters during wartime

Many basements are used as shelters, and prolonged occupancy by large numbers of people can rapidly increase both CO₂ concentrations and humidity.

Under such conditions, ventilation becomes a matter of safety. If circumstances permit, brief periods of intensive ventilation through available openings are essential even during the day. In such situations, indoor air quality takes priority over the risk of condensation.

If the shelter has a mechanical ventilation system, it should operate continuously. If no such system is available, at least an extract fan should be used.

Semi-basement flats

Walls below ground level remain cool, so the risk of condensation persists, particularly in corners and on external walls. If condensation appears on walls or window sills after daytime ventilation, switch to ventilating only at night or early in the morning.

In brief: rules for basements and semi-basements

• ˙ Ventilate at night and in the morning rather than during the day.
• ˙ Pay attention to outdoor humidity, not just temperature.
• ˙ Do not stop ventilating completely—a minimum level of air exchange is always necessary.
• ˙ In occupied spaces, indoor air quality takes priority over the risk of condensation.

Conclusion: a system rather than individual measures

All of these measures work best when applied together. External shading reduces solar heat gains. Ventilation at the right time cools the building and refreshes the indoor air. A fan maintains comfort throughout the day. Where available, an air conditioner complements the system—but it cannot replace any of the other elements.

In the Ukrainian context, the reliability of electricity supply adds another important consideration. The more effective the passive cooling strategy (external shading combined with ventilation), the lower the dependence on active cooling and the lower the electricity costs when the power grid is unstable.

This article is based on the study Coole Fenster — Fenster mit Beschattung im Spannungsfeld sommerliche Überwärmung, Tageslicht, winterlicher Wärmeschutz in der Klimakrise by C. Leh, V. Stiegler, R. Wolffhardt, S. Vavrik-Kirchsteiger, J. Bachinger, H. Ferk, P. Schober and B. Nusser (Holzforschung Austria), as well as WHO guidance on heatwaves and materials published by GLASWELT and WDR featuring Alina Herrmann, Hans-Guido Mücke, Christoph Asbach, Kerstin Effer and Jörg Kachelmann.