Yes — blackout curtains do help keep heat out, but how well depends on the fabric weight, whether there's a lining, and how the curtains are hung. A thin blackout panel blocks light but does little for heat. A heavyweight curtain with proper coverage can noticeably reduce solar heat gain and keep a room cooler in summer. Here's what's actually happening and how to get the most out of it.
How blackout curtains block heat
Heat enters a room through windows in two main ways: radiant solar heat (sunlight passing through the glass) and conducted heat (warmth moving through the glass itself). Blackout curtains address the first one directly.
When sunlight hits a curtain before it hits the glass, the fabric absorbs or reflects that energy instead of letting it into the room. The denser the fabric — measured in GSM (grams per square meter) — the more heat it can absorb before any gets through. A 270 GSM blackout curtain stops light and significantly reduces solar gain. A 420 GSM blackout velvet does both, plus creates an insulating air pocket between the curtain and the window that slows conducted heat as well.
The air gap matters: Leave an inch or two of space between the curtain and the window glass. This trapped air acts as a buffer layer, slowing heat transfer in both directions — keeping heat out in summer and keeping warmth in during winter.
Do blackout curtains keep heat out in summer?
Yes, especially on south- and west-facing windows that get direct afternoon sun. Drawing blackout curtains before the sun hits those windows — not after the room is already warm — is the key move. Once heat is already inside the room, curtains trap it as much as they block it.
The practical effect is meaningful. A room with heavy blackout curtains on a west-facing window will feel noticeably cooler by late afternoon compared to a room with thin or no curtains. You'll run the AC less, or not at all on mild days.
For the best summer performance:
- Choose a curtain with a lighter-colored lining — white or cream linings reflect more solar radiation than dark ones
- Hang the rod as wide as the window frame (or wider) so curtains cover the full glass when drawn
- Close curtains before the sun hits that wall, not after
Do blackout curtains keep heat in during winter?
Yes — and this is where fabric weight makes an even bigger difference. Heavier curtains (350 GSM and up) create a more effective insulating barrier between the cold glass and the room. This is sometimes called "thermal performance," and it's distinct from light blocking.
Natural wool fabrics are particularly good at this. Wool's fiber structure traps air naturally, making it inherently insulating without any special lining or treatment. Synthetic blackout fabrics work too, especially with a dense weave or triple-pass construction.
Dawn Wool Curtains — 340 GSM, 55% wool / 45% acrylic blend. Confirmed thermal insulation and noise-reducing properties. Light-blocking at 50–80% (unlined), with blackout lining option. A natural choice for rooms where you want genuine warmth retention. Shop Dawn Wool →
Blackout vs. thermal curtains: is there a difference?
Blackout curtains are defined by light blocking — they're made to block light, and heat reduction is a bonus effect of the dense fabric. Thermal curtains are specifically designed for insulation, often with a foam or flannel interlining that reduces heat transfer more aggressively than a standard blackout fabric would.
In practice, many curtains are both. A heavyweight blackout velvet with a lining functions as a thermal curtain too. The distinction mainly matters when comparing a thin blackout panel (good at blocking light, modest heat performance) against a purpose-built thermal curtain (prioritizes insulation, light blocking secondary).
If your main goal is heat control, look at fabric weight and GSM rather than just "blackout" in the product name.
By the way, add a lining for a thicker and better thermal effect.
Which fabrics perform best for heat?
| Fabric type | GSM range | Heat blocking | Best for |
|---|---|---|---|
| Lightweight blackout | 200–280 GSM | Moderate — good light block, limited insulation | Light control first, heat control secondary |
| Mid-weight blackout | 280–380 GSM | Good — solid insulating layer | Bedrooms, living rooms year-round |
| Heavyweight velvet / chenille | 380–540 GSM | Very good — dense pile traps air effectively | Rooms with high heat gain or cold winters |
| Wool blend | 300–400 GSM | Excellent — natural fiber insulation | Year-round thermal performance, noise reduction |
Elma Soft Blackout Velvet Curtains ⭐ — 420 GSM velvet, 90–95% light blocking, confirmed thermal and noise-reducing properties. One of Curtarra's best sellers for rooms that need both real darkness and year-round comfort. Shop Elma Velvet →
What limits how much heat blackout curtains can block
Even the best curtain can only do so much if it's poorly hung. The two biggest issues:
Light gaps = heat gaps. Any gap around the edges of the curtain — at the top, sides, or between panels — lets convection move warm air in (or cold air out). The curtain works as an insulator only where it actually covers the window.
Rod placement. A rod mounted too close to the window frame, or not extending far enough past the edges, means the curtain can't seal the window properly when drawn. Mount the rod as close to the ceiling as practical, and extend it at least 4–6 inches past the window frame on each side.
Custom sizing matters here too. A curtain cut to your exact window dimensions can be hung to minimize gaps, which a standard-sized panel often can't.
Frequently asked questions
Do blackout curtains keep a room cooler in summer?
Yes, particularly on windows that receive direct sun. Drawing heavy blackout curtains before afternoon sun hits a west-facing window can reduce room temperature meaningfully compared to having no curtains or thin ones. The effect is strongest with heavier fabrics (300 GSM and up) and curtains that fully cover the window.
Do blackout curtains work the same as thermal curtains?
Not exactly. Blackout curtains are designed to block light; heat insulation is a secondary benefit. Thermal curtains are designed specifically for insulation. Many curtains overlap — a heavyweight blackout fabric functions thermally too — but a thin blackout panel won't perform like a dedicated thermal curtain. Fabric weight is the better indicator than labeling.
Will blackout curtains reduce my energy bill?
They can contribute. The US Department of Energy notes that window treatments can reduce heat gain through windows significantly, with the effect depending on fabric type, color, and how the curtain is hung. The savings are real but modest — more noticeable on single-pane windows or in rooms with a lot of glass.
Do blackout curtains keep cold out in winter?
Yes. A heavy curtain creates an insulating air pocket between the cold glass and the room interior. Wool or heavyweight synthetic fabrics work especially well. Keeping curtains closed on cold nights and opening them on sunny days (to let solar warmth in) is the most effective seasonal approach.
What color blackout curtain is best for blocking heat?
The lining color matters more than the face color. A white or light-colored lining reflects more solar radiation outward. The face color you see from inside the room can be any color you like — it doesn't significantly affect heat performance from the interior side.
If you want genuine heat blocking along with light control, fabric weight is the number to watch. Custom sizing means your curtains can be hung to fully cover the window — no gaps at the edges where heat sneaks through.
