You may have seen the advertisements where thermal curtains are said to save you money by insulating your home in the winter and keeping the heat out in the summer. If you go to any home improvement site, you can find people who think these are the best thing since sliced bread and others who think they are just overpriced fabric. In this article, we hope to find out the truth and see what thermal curtains can do.
First, what can thermal curtains do? more importantly, what can they not do? Let's look at a few scenarios.
First things first, thermal curtains do not create heat or cold, rather, they create a barrier against the transfer of heat through your windows in the winter. You will not find your windows freezing outside and your curtains magically heating the inside of your cosy room. If you know how to use them properly, they can be a valuable asset to your home.
Imagine a window. In winter, your window allows heat to escape in and cold to come in. In summer, they do the opposite. Windows are porous, meaning they allow the transfer of air. If you add a thick and woven curtain, you stop the transfer of air.
Do they really make that much of a difference?
When installed correctly, thermal curtains are estimated to reduce heat loss at a rate of 25% in the winter. The U.S. Department of Energy made that estimate. As for summer, heat gain can be reduced by 33% in sun-exposed windows when medium curtains with a white reflective backing are installed. While this is all valuable information, this data does have to be placed in context. For example, Cellular (honeycomb) shades) can reduce heat loss by 40% and have a more diverse temperature-control feature. Compared to those, thermal curtains have less to offer, but they do help.
In the fall and winter, thermals help heat loss at a rate of 25% and make winter more manageable. For a cost of 30–60 dollars, thermals block drafts, making the room less drafty.
They are effective at reducing radiant heat transfer. During peak sunlight hours, closed thermal curtains on south and west-facing windows can decrease the temperature of a room. However, not many people recognize the importance of curtain color. If a curtain is dark and lacks a reflective backing, it will absorb solar radiation and re-emit that heat into the room. A curtain for summer cooling needs a white or light-colored backing that reflects solar heat out the window. It's fine if the fabric on the side of the window is dark, but the side facing the window needs to be light or reflective.
They provide some sound dampening. This isn't their primary purpose, but the thick fabric of the curtain absorbs some noise.
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Curtains can't insulate. If your home loses a lot of heat through the walls or ceilings, then window treatments are just a small part of the solution.
They don't do window upgrades. Curtains can't compete with double or triple-pane windows with low-E coatings.
They don't work when they are open. This is obvious, but it is essentail to consider. If you want to allow light into the room, your thermal curtains are not working during that time.
What causes such differences in results?
People's reviews often present a paradox. Certain people express experience with amazing changes, claiming that some rooms went from frigid to cozy. In contrast, others spent a considerable amount and reported that nothing changed. Both groups are likely to be telling the truth. In most cases, the difference just comes down to a few factors.
The type of windows you have makes a huge difference. Old, single pane windows will see a drastic change if thermal curtains are added, because those windows are bad enough at insulation that any improvement is noticeable. On the other hand, if you already have decent double pane windows, curtains will make practically no change.
Plenty of people get this part wrong. Curtains work by creating a layer of still air between themselves and the window. Any air that moves freely is not insulated, so if there's a gap at the sides, top, or bottom, the still air layer is compromised.
Curtain installation mistakes include:
- Not extending the curtain over the window frame. They need to extend 3-4 inches over the frame to cover air gaps.
- Hanging the curtain too high. A gap at the top is a major warm air escape route, while cold air can dip through the gap. A cornice or valance can cover this.
- Grommet-style curtains that leave gaps at the rod. Avoid grommet tops (the ones with big metal rings). They are popular and cheap, but those rings are literally holes that let trapped heat escape at the top. Rod pocket or back-tab styles sit closer to the wall and seal much better.
The DIY fix that most people don't know about: Velcro strips or magnetic tape can pin the sides of the curtain to the wall or window frame. Stick one side to the edge of the curtain, and the other side to the wall. This transforms a mediocre curtain into one that actually creates an air pocket seal. Instead of a 10% improvement, you'll get a 25% improvement.
But the easiest fix is buying a wraparound curtain rod (also called a French return rod). These curve directly into the wall, closing the side gaps automatically without any tape or fuss. They run $15-25 at most retailers and are a better long-term solution than adhesives.
Important note for homes that have radiators or baseboard heaters: If, like many homes in the Northeast and Midwest, you have heat units under your windows, then floor length curtains present a serious dilemma. The curtain traps the heat behind it, sending warmth to the cold window instead of into your room. You're insulating your room from its own heat source.
With electric baseboard heaters, there's an added risk of fire. You need to keep curtains at least 12 inches away from the electric heating elements. For homes with heat under the window, your curtains should stop at the windowsill, or you should tuck them behind the radiator, or use a heat deflector to keep warming air into the room.
Results will vary due to local climate and how your home is built. Someone in Minnesota with drafty windows from the 50s will certainly notice thermal curtains more than someone with modern homes in San Diego. Baseline conditions influence the extent to which the improvement can be seen.
A note on fabric stiffness
Watch out for what might be called the "stiffness factor." Old-school thermal curtains use a stiff foam coating (3-pass blackout lining) that insulates well but hangs like a plastic shower curtain. The coating resists the natural tendency of woven fabric to drape softly against walls and floors, which actually breaks the air seal you're trying to create.
Newer "triple weave" fabrics have three layers woven together rather than coated on. They drape more like normal curtains but offer slightly less insulation. If you buy the stiff coated kind, you may need to add weights to the hem so they hang straight and seal against the wall. Otherwise, the bottom flares out and defeats the purpose.
When thermal curtains make sense (and when they don't)
In some contexts thermal curtains are worth it. They are not universally good or universally bad. They're an option to consider some problems.
They are worth it especially when:
You rent and cannot modify windows. Curtains are insulation upgrades you can take with you so it is at least better than nothing. Cellular shades are more effective but they can be expensive and custom ordered, which typically runs $100-200 per window. At $30-60 thermal curtains offer the best ROI for renters.
You have older windows with single panes and do not have budget to replace them. They wont be able to match the new window performance, but they will reduce drafts and heat loss at a fraction of the cost.
You have problem windows. There might be a room with a large west facing window that turns into an oven every summer afternoon. White backing thermal curtains on that window can make a real difference.
You want layered protection. Here's what the pros know: thermal curtains paired with cellular shades offer the best insulation. The shades provide the primary thermal barrier; the curtains add an additional layer and seal the light leaks around the edges of the shades. If you live in an area with extreme temperatures, this combo beats either option alone.
They probably aren't worth it when:
You have new modern double or triple pane windows. The incremental improvements you'll see will be small and potentially unnoticeable.
You leave curtains open most of the day. If thermal curtains aren't being drawn, you're just losing insulation.
You're trying to solve a heating issue in the entire home. If every room feels cold, it's likely an issue with the HVAC, insulation, or air sealing; not the window treatments.
You have heating elements directly under your windows and want floor-length curtains. The curtain will work against your heating system.
Worth trying, with realistic expectations
Thermal curtains work. They provide measurable insulation benefits that research confirms. But they're not magic, and they're not the most effective window insulation available. They reduce heat transfer by roughly 25%, which is helpful but modest.
If you decide to try them, focus on these things. First, get the right backing. For year-round use, choose curtains with white or reflective backing, not just thick fabric. Second, install them properly: wide enough to overlap the frame, long enough to reach the floor (unless you have under-window heating), mounted close to the ceiling. A wraparound rod eliminates side gaps without DIY tape solutions. Third, avoid grommet-top curtains if insulation is your goal, since the metal rings create gaps at the rod where heat escapes. Rod pocket or back-tab styles seal better.
The people who report thermal curtains don't work usually have installation issues they don't realize. The curtains look fine, but they're not actually creating a sealed air pocket. Get the installation right, and you'll likely join the camp that considers them a worthwhile investment.
