The Best Time of Day to Close Curtains to Keep Rooms Cool

The ability to keep your indoor space cool during the summer heat often boils down to small, thoughtful choices. One of the most simple and most efficient strategies is to know precisely when you should close your curtains. Although it might seem intuitive to close your curtains when the room is warm However, studies and guidelines from a university suggests that timing matters higher than people believe.

Why Timing Matters More Than You Think


Curtains play an important role in delaying the solar heat gain. This is one of the major sources of heat in the indoors in summer. If sunlight hits the window, it does more than simply brighten the room. It also brings the radiant heat which is taken up by furniture, walls as well as flooring. When that heat gets inside the structure, it is much more difficult to eliminate.

A guideline issued by guidance from the University of Colorado Boulder in the United States emphasizes this exact aspect in their summer cooling tips. In their publication for keeping cool in summer, the blinds and curtains need to be closed and left closed until temperatures fall in the evening. This recommendation demonstrates a fundamental idea in the field of building science. It is significantly more effective to stop warmth from entering than attempt to cool an area when it is already warm up.

The Science Behind Closing Curtains Early

A 2026 article on energy efficiency, that draws on findings from both The National Renewable Energy Laboratory and ASHRAE highlights the significance of taking early action. Both of these institutions, which are based within the United States, are well-known for their research in buildings and energy systems. They have found that light or reflective window covers can cut down cooling demands between 15 and 22 percent when shut prior to peak exposure particularly on windows facing west.

The most striking aspect of this study is the fact the fact that timing can have a significant influence. Closing curtains when sunlight has already saturated a room decreases the effectiveness of curtains substantially. After that, the majority of the warmth has been stored and absorbed into the indoors and is difficult to reverse.

How Sunlight Affects Different Rooms

The temperature of every room is not in the same way. For many houses the west and south-facing windows play the largest role in contributing to the build-up of heat in afternoons. They are exposed to the greatest amount of light in the heat of the day. This can be leading to indoor temperatures rising fast.

Practically, this implies that waiting until noon or even later to close the curtains could be too tardy. The area may already be significantly warmer. It is important to know when the sun will start hitting the windows directly, and then be ready to act prior to that.

This is especially important for houses and apartments that rely upon passive cooling, not permanent air cooling. In the absence of mechanical cooling that can counteract the heat, blocking solar gain becomes important.

A Simple Daily Routine That Works

An organized daily schedule will make a significant difference when it comes to maintaining a cooler indoor space. Early in the morning the temperature is still lower and the sun is not as than intense, opening the curtains could allow natural light to enter without lots of warmth. As the day goes on and the sun begins to rise, your focus will change to block sunlight prior to it reaching its peak intensity.

The closing of curtains in the late morning or when direct sunlight starts to enter the space, can help prevent building up heat in the hot time of the day. The ability to keep them shut through the day will keep the room fairly stable in the temperature. At night when outdoor air is more cool than indoor air, opening the windows and curtains can assist to let out any heat that is accumulated.

This is a method that's in accordance with Boulder's university of Boulder's advice that recommends keeping curtains shut until cooling at night commences.

Expert Insight and Real-World Relevance

Curtains' efficiency to control indoor temperatures does not only apply to cooling in summer. Many experts in the field of energy highlight the role they play in regulating the flow of heat throughout the year. In particular, heating expert Nicholas Auckland has noted that closing the curtains is among the most efficient strategies to preserve warmth during winter. Although his remarks are focused on winter weather but it also demonstrates a larger idea: curtains work as barriers to control the flow of heat into and out of an area.

In summer, the same principle is applied in reverse. Instead of allowing heat to stay inside the curtains keep undesirable heat from entering. They are a useful tool that can be used for enhancing the comfort of your home and decreasing the need for air cooling.

The Practical Takeaway

The best timing to shut curtains is prior to direct sunlight reaching the windows. This is usually in the morning, especially during the summer heat. If a room isn't warm suggests that the heat has already escaped and retreated to the in the indoors. If you act early and keep curtains shut during the most scorching time of day, you will be able to significantly lower the temperature inside and increase comfort.

The simple practice, backed by The University of Colorado Boulder and research that is linked to National Renewable Energy Laboratory and ASHRAE shows how even minor changes to daily habits could lead to significant improvements on energy efficiency and comfort.