10 golden rules to avoid overheating rooms in summer without air conditioning

Modern humans spend more time indoors than outdoors. Human activity and productivity directly depend on the amount of natural light throughout the day. Therefore, glazing areas are being increased to enhance natural light intake and reduce artificial lighting costs. However, larger window areas have heightened their impact on heat retention in winter and overheating in summer.

Specialists from the Austrian wood research society Holzforschung Austria considered windows and sun protection holistically. Typically, attention is given either to summer overheating protection or winter heat loss prevention. However, these approaches impose conflicting requirements on windows. Therefore, windows, shading types, and ventilation were analyzed with the aim of providing daylight and creating a comfortable indoor climate year-round, preferably without using air conditioning.

The research identified recommendations for window manufacturers, architects, and users.

10 Golden Rules for Cool Rooms

Following these rules allows for optimal results in reducing heating and cooling costs while ensuring good daylight accessibility.

1. External shading is much better than internal shading. Internal shading is better than no shading

External shading minimizes indoor temperature increases during summer.
If external shading is impossible, for example, due to heritage preservation, internal shading is better than no shading, especially for large windows.

The color of the shading should be chosen not only for visual reasons, as it affects daylight intake and glare protection.

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

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

Calculations based on Vienna's solar position show that from May to September, solar radiation energy on the north side still reaches about 60% of the energy on the south side. This means that without shading, there is a risk of overheating in summer in rooms with north-facing windows.

For comparison:
According to profile SOLAR.com, in Vienna, each kilowatt of installed solar power generates an average of 6.42 kWh per day in summer. In Dnipro, located at a similar latitude as Vienna, this figure is 6.45 kWh, and in Kyiv, it is 6.50 kWh.

3. Thermal transmittance coefficient of glazing Ug ≤ 1.0 W/m²K

The thermal transmittance coefficient of external window glazing (Ug) should not exceed 1 W/m²K and should be as low as possible.

Ug indicates heat loss through the glazing. The lower the Ug value, the less heat passes through the glass. Hence, the lower the U value, the better the insulation properties of the glazing and the window as a whole.

4. Solar factor ≥ 0.5

Solar factor values for glazing below 0.5 should be avoided as they impair daylight utilization and result in solar energy loss during the heating season. Under the conditions of this study, the use of solar protection glass is not recommended.

The total solar energy transmittance coefficient of glazing, or solar factor, determines how much solar light passes through the glazing. The higher the g-factor, the more solar light enters the room.

5. At least 50% of windows should be openable

To ensure adequate nighttime ventilation, at least 50% of windows should be openable.

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

6. Open windows for ventilation only when the outdoor temperature is lower than indoor temperature

Windows should only be opened for ventilation when the outdoor temperature drops below the indoor temperature. Additionally, windows should remain open when the indoor temperature matches the outdoor temperature to cool elements inside the room, such as walls and furniture.

If ventilation conditions exist but shading systems block ventilation, sun protection should not be used (lowered).

7. Ensure nighttime ventilation

Nighttime ventilation is necessary to remove heat accumulated during the day. During hot days, windows should remain closed during the day.

If nighttime ventilation is impossible due to external factors (e.g., high noise levels), the ventilation system should provide the most effective cooling air exchange at night.

8. Additional shading is needed even if an overhang is present

Even with a balcony, canopy, etc., additional external movable shading is necessary. Even with a 2-meter-deep overhang, the cooling need is only slightly reduced.

It should be noted that overhangs deeper than 1 meter significantly negatively affect daylight intake.

9. Consider window size

Large glazing areas positively impact daylight intake but require well-planned and properly used external shading to make windows suitable for summer.

10. Consider shading placement for wind resistance

Depending on the location and installation conditions, the wind resistance of the sun protection system should be considered when choosing the system.

In Ukraine, the wind load levels of roller shutters, external blinds, and awnings are defined according to DSTU EN 13561:2018 "External blinds. Performance requirements and safety requirements."

What is a comfortable room temperature in summer?

The perception of a comfortable indoor climate is individual for each person. However, to rely on specific numbers, sanitary norms are often used.

In Ukrainian sanitary norms for the indoor climate of industrial premises during the summer, the recommended air temperature is 22-24 °C (for light work). The upper limit is 28 °C. However, if the office temperature approaches 28 °C in summer, workers' productivity decreases.

According to the German Workplace Directive on employee protection:

• ▶ If sunlight penetration through transparent components raises indoor temperature above 26 °C, these components must be equipped with sun shading systems.
• ▶ If, despite the sun protection system, indoor temperature exceeds 26 °C, additional measures must be taken.
  Possible measures include effective sun protection control, nighttime ventilation, reducing internal heat loads, and morning ventilation or relaxing dress code rules.
• ▶ If indoor air temperature exceeds 30 °C, effective measures must be taken.

The research was conducted by modeling a reference apartment in Vienna based on an unfavorable climate data forecast for the city center in 2050. Four target variables were evaluated: daylight, heating and cooling needs, and thermal comfort.

Source:
Study "Coole Fenster - Fenster mit Beschattung im Spannungsfeld sommerliche Überwärmung, Tageslicht, winterlicher Wärmeschutz in der Klimakrise" — C. Leh, V. Stiegler, R. Wolffhardt, S. Vavrik-Kirchsteiger, J. Bachinger, H. Ferk, P. Schober, B. Nusser