Do Blackout & Thermal Curtains Actually Keep Heat Out in Summer?

Quick answer: Yes — blackout and thermal curtains do help keep heat out in summer. Most of a room's summer heat comes in as sunlight through the glass, and closing a curtain over the window blocks and reflects much of it before it becomes heat. The U.S. Department of Energy estimates that closing draperies during the day can reduce heat gain by up to around a third. The catch: it only works if the curtains are light-colored on the window side, fitted tightly, and closed before the room heats up. Here's how to get the most from them.

How curtains block summer heat

In summer, the biggest source of unwanted heat isn't warm air sneaking in — it's radiant heat: sunlight passing through the glass and warming everything it lands on. A curtain placed over that window intercepts the sunlight before it turns into heat inside the room, and a reflective (light-colored) backing bounces some of it back out through the glass.

That's why a curtain's fabric and fit matter more than the label. We tested how much real thermal fabric actually does here: Seriously, Do Thermal Curtains Really Work?

How much do they actually help?

Per the U.S. Department of Energy, medium-colored draperies with a reflective backing can cut summer heat gain by up to roughly 33% when closed during the day. In practice that can mean a noticeably cooler room and less work for your AC — especially on east- and west-facing windows that take direct sun. It won't make a room cold on its own, but as the cheapest, no-install upgrade at the window, it earns its place. For the nuance on when blackout fabric helps most (and when it doesn't), see why blackout curtains sometimes don't reduce heat.

What makes the biggest difference

• Close them before peak heat. Draw curtains on sunny-facing windows in the morning, before the sun hits — not after the room is already warm.
• Light or reflective on the window side. A white or light backing reflects sunlight; a dark fabric facing the glass absorbs it and can re-radiate heat inward.
• Cover the whole window, tightly. Mount the rod high and wide and let panels reach the floor. Gaps at the sides and top let heat (and light) leak in — our light-gap guide closes them.
• Prioritize sun-facing windows. East and west windows gain the most heat; start there.

Blackout vs thermal vs both

Blackout curtains are built to block light, and most quality blackout panels also block a lot of solar heat. Thermal curtains add an insulating layer (often a foam or multi-weave backing) designed specifically to slow heat transfer — useful in both summer and winter. The best summer performers are usually thermal + blackout in one, in a lighter color. Browse thermal curtains and blackout curtains to compare.

Pair with a blind for even more

Two layers beat one. A roller blind close to the glass plus curtains in front traps a pocket of air between them and blocks more solar heat than either alone — and you can schedule a motorized roller blind to close against the afternoon sun automatically. Weighing your options? See Motorized Blinds vs Curtains vs Manual Blinds, or compare coverings head-to-head in thermal curtains vs vertical blinds.

The Deconovo difference

Deconovo thermal and blackout curtains are OEKO-TEX® Standard 100 certified and trusted in 21,000,000+ homes worldwide — designer-quality, sustainably crafted comfort that helps keep summer rooms cooler and winter rooms warmer.

Frequently asked questions

Do blackout curtains keep heat out in summer?
Yes. By blocking and reflecting sunlight at the window, blackout curtains reduce solar heat gain — the U.S. Department of Energy estimates up to about a 33% reduction when closed during the day.

What color curtains are best for keeping heat out?
Light or white on the window-facing side reflects sunlight best. If you want a dark look inside, choose panels with a light or reflective backing.

When should I close curtains to keep a room cool?
Close them in the morning before direct sun hits the window, especially on east- and west-facing windows, and keep them drawn through the hottest part of the day.

Are thermal or blackout curtains better for summer heat?
A thermal-plus-blackout curtain in a lighter color usually performs best, combining an insulating layer with strong sunlight blocking.

Do curtains help lower cooling costs?
They can reduce the heat your AC has to remove, especially on sunny windows. Pairing curtains with a blind improves results further.