Blackout curtains are often marketed as the solution for better sleep and lower energy bills. But do they actually keep heat out? After testing twelve different blackout fabrics (including top sellers and new-generation materials) the results showed that most “blackout” curtains don’t block heat effectively at all.
Here’s what we learned, why some fabrics trap heat instead of reflecting it, and what sets Sleepout blackout fabric apart as the only blackout material proven to block light and reduce heat without releasing harmful chemicals.
How Blackout Curtains Actually Work
A blackout curtain’s primary job is to block incoming light. This is usually achieved through dense fabrics or light-blocking layers that prevent sunlight from passing through.
But when it comes to temperature control, a curtain’s ability to manage radiant heat depends on more than just thickness. The weave, backing, and surface color all play major roles in how much heat gets reflected versus absorbed.
In general, a lighter-colored backing reflects sunlight more effectively and helps maintain a cooler indoor environment. Darker materials, on the other hand, absorb heat—making a space warmer, especially when sunlight directly hits the fabric.
Why Some “Blackout” Curtains Trap Heat Instead of Blocking It
Many curtains marketed as “blackout” aren’t truly blackout at all. Most are made with woven fabric that darkens a room but still allows light (and heat) to pass through. Adding a proper blackout layer is more costly, so many brands skip it or use cheaper woven fibers instead.
Even when blackout brands add a coating, it’s often made with PFAs or formaldehyde-based compounds. These chemicals help create opacity but can off-gas when exposed to sunlight and heat, contributing to poor indoor air quality. Over time, that buildup of heat and emissions can make a room both warmer and more stuffy.
So yes, blackout curtains do help with heat, but most curtains that do not use a light backing, or 100% blackout layer actually trap heat.
Sleepout is the first and only brand to make blackout curtains without PFAs and completely free from formaldehyde, while still being 100% blackout to block every bit of light that touches the fabric to keep your room cooler.
What to Look For in a Curtain That Actually Keeps Heat Out
When evaluating blackout curtains for thermal performance, three main factors make the difference:
1. Light-Colored Backing
A light backing helps bounce sunlight away rather than absorbing it, keeping the room cooler.
2. True Blackout Fabric Construction
A true blackout curtain uses an opaque barrier layer, not just a dense weave. This prevents both light and heat transfer through the fabric.
3. Safe, Non-Toxic Materials
Materials free from PFAs and formaldehyde won’t degrade or release fumes when heated by the sun—making them safer, cleaner, and more effective in the long term.
Sleepout® Fabric was designed to meet all of these requirements. The truth is, very few curtains do—because it’s cheaper and easier to skip the materials and layers that make a real difference in heat control.
The Role of Certification in Fabric Safety and Performance
The UL GREENGUARD Gold certification is one of the most rigorous third-party standards for indoor air quality. It ensures products meet strict limits for chemical emissions, even under heat exposure.
Currently, Sleepout blackout fabric is the only GREENGUARD Gold Certified blackout fabric in the world. This means it’s been independently tested and verified to not off-gas harmful chemicals—a critical factor when choosing blackout curtains for bedrooms, nurseries, or any enclosed space.
Because the Sleepout fabric is formaldehyde-free, PFAs-free, and naturally thermally insulating, it performs differently from typical blackout options. When sunlight hits it, the material remains stable and helps prevent heat buildup rather than contributing to it.
How Sleepout’s Design Supports Thermal Comfort
Each Sleepout curtain uses a light-colored that helps bounce sunlight away before it can warm the glass or fabric. Combined with a dense, triple-weave construction, this design forms a natural thermal barrier that can keep rooms several degrees cooler on sunny days.
The same insulating effect works in winter too, reducing heat loss through windows and improving energy efficiency year-round.
Unlike other brands that rely on marketing claims, Sleepout backs its performance with a measurable standard: every curtain blocks 100% of light that touches it, ensuring consistent results for both darkness and heat management.
The Bottom Line
No, not all blackout curtains keep heat out. When choosing curtains for both darkness and temperature control, look for:
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A light-colored backing
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A true blackout barrier layer
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PFAs-free and formaldehyde-free materials
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Verified GREENGUARD Gold certification
Curtains that combine these features don’t just block light, they stabilize room temperature, protect indoor air quality, and make your space more comfortable year-round.
Sleepout’s GREENGUARD Gold Certified blackout fabric shows what’s possible when curtains are engineered for both safety and performance: no off-gassing, no heat buildup, and complete light control.
