Nobody who has ever owned a set of vinyl blinds would be surprised to hear that west-facing windows are vinyl blinds' biggest enemy. If you notice your blinds warping, sagging, or slats bending after every summer, your blackout curtains may be the problem. "Melting" is a bit of a stretch, but what is happening is called heat deflection. PVC blinds start to soften and change their shape at 130°F, and that's the temp blinds start to "melt". This heat is deflected and trapped by the blackout curtains, and the dead airspace between the curtains and window tends to be at that temperature.
Why blackout curtains trap heat against your windows
Curtains trap a lot of heat, and that's a problem. Sunlight goes through the window from the visible light and short-wave side. Then, the dark color of your curtains absorbs this heat and converts it to long-wave infrared. The glass blocks long-wave infrared, so the heat gets trapped instead of released. This is the primary mechanism that leads to high temperatures.
The air pocket between the curtain and window is where things are getting dangerous. Due to a lack of airflow and high heat retention, the temperature can climb 30 to 40 degrees more than the temperature of your room. On a summer afternoon where it is 95 degrees, the temperature behind your curtains can be more than 130 degrees, which is enough to melt vinyl.
Some setups make this worse. Dark curtains trap more heat. Curtains that are very close to the window trap more heat as close off more gap for heat to escape. Blackout curtains with dark rubber or foam backing are worse as they trap heat more than other curtains.
Regarding modern windows, If your home has Low-E (low emissivity) glass, the situation is a little different. Low-E glass are great for keeping a home cooler as they reflect infrared energy which increases glass thermal efficiency, However, the situation is still not ideal. With curtains close to the glass, in rest, heat can still build up around them and trap, this is worse on the afternoon with the sun on the glass, this is worse on the west side. Older homes with standard clear glass or single-pane windows experience this problem more severely.
Signs your windows are overheating before the blinds begin to warp
Touch your curtains in the late afternoon on a sunny day. If the fabric is hot, not just warm, that heat is coming from somewhere. The curtain has been absorbing energy as it radiates to the trapped air.
After the sun sets, open the windows and feel the air that comes out. If you feel a hot pocket of air, then the windows are hot and are running at a temperature that is higher than the temperature of the room.
Check your vinyl window blinds. Look for the waves and ripples on the slats. They are a sign of early warping. The bottom slats are the first to warp because the top of the pocket of trapped air is where the heat rises to and then it radiates down. The vinyl may also soften and sticky so that the blinds do not tilt easily.
Look at the double pane window seals. The heating and cooling cycles are stressing the seals. If there is condensation on the inside of the panes, then that means that the seal is heat stressed.
The curtains in summer and winter have opposite effects
This is the seasonal trap many homeowners fall into. The curtains feel like a smart investment in the winter because they easily insulate against the air that is cold near the glass and they radiate the heat into the room. The heating bill then drops because the radiate heat is coming from the solar energy that is passing through the window.
When summer comes, everything changes.
The heat absorption in January becomes a problem in July. During the summer the sun is stronger and hits your windows for longer. Instead of a nice heat, your adding heat to a room your spending money to air condition.
In addition, the winter sun comes in low through the south windows. The summer sun hits the west windows at a higher angle and blasts them in the afternoon. If you picked the curtains based on winter, you probably didn't think about which windows would be worst in summer.
How does reflective backing eliminate heat buildup?
From a physics point of view, the solution is simple. Do not let the heat build in the first place.
White and reflective curtain backing bounce light that hits them back to the window and do not absorb the light. According to the U.S. Department of Energy, medium-colored draperies with white-plastic backings can reduce heat gains by 33%. Silver or metallic thermal backing reflects even more. In contrast, a black backing absorbs more than 90% of light and converts it to heat.
This explains why two curtains that both advertise to be "blackout" can have radically different performances in the summer. One features a white thermal backing that keeps the fabric cool to the touch. The other has a dark rubber coating that turns the curtain into a radiator.
When it comes to heat, the face color of your curtain is much less important than the backing color. A white backing will always be cooler than a black backing. A navy curtain with white backing will be cooler than a beige curtain with black backing. Always pay attention to the window side of the fabric, not just the room side.
When looking for curtains, keep an eye out for "3-pass blackout lining" - that is an industry term that gives you a specific thing to search for. Also check for "thermal backing," "reflective coating," or specific mentions of white or silver backing materials. "Blackout" by itself tells you nothing about how the curtains perform when it comes to heating.
100% Blackout
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Energy SavingWhat to do if you already have curtains that trap heat
One way to solve the problem is to replace all of the curtains, but if the curtains you have are otherwise in good condition, there are other ways to solve the problem that are less expensive. These solutions work whether you own your home or are renting.
Making airflow behind the curtains is free, and it is often the most effective solution for renters who cannot modify the windows. Heat rises, and if your curtain is sealed at the top and bottom, then you are creating an oven. Make sure the top of the curtain is open enough to allow heat to escape, and the bottom should sit a couple of inches above the sill to keep a gap for air to vent. Hanging curtains a few inches away from the wall also helps air to circulate around the edges.
Add a reflective liner behind existing curtains. White blackout liner panels, which provide a reflective backing to your curtains, are sold separately and hang on the same rod as your decorative curtains. They'll leave your curtains intact, are cheaper than a full replacement, and work in rentals since you're just adding to an existing rod.
Consider window film carefully. Window film can be a good option as it protects your blinds, even when the curtains are open, by stopping solar heat before it enters the room. However, there's an important caution: do not apply standard tint or automotive film to double-pane windows. Because the outer pane stays cooler, the inner pane absorbs heat and expands, which can crack or cause a thermal stress fracture and blow the seal. Most US homes have double-pane windows. Use films rated for insulated glass units (IGU) or dual-pane windows, and consider professional installation. If you're renting, window film may not be permitted under your lease.
Options for renters. If modifying windows or putting up exterior coverings isn't possible, stick with airflow methods. Reflective window coverings on tension rods are reusable and free with respect to your lease. Some renters hang a white bedsheet for a cheap, temporary, and reflective layer behind their curtains during the hottest months. Positioning curtains with command hooks further away from the wall helps air circulation.
Homeowner exterior solutions. Awnings and exterior solar screens stop heat before it enters the window. This is the best solution for severe cases because it means the heat isn't getting in the home at all, though it requires outside ownership.
Going forward with blinds protection
For windows where the blinds have already warped, the blinds are due for a replacement. If it's a chronic hot window, warping will continue with vinyl blinds. Faux wood blinds with composite cores are better for heat. Aluminum blinds are the best option for problematic windows, though they have a different aesthetic.
The fundamental problem is not with the curtains or the blinds. The blinds close all the way and so do the curtains. From the curtain's perspective, it's all good. They're not broken. The problem is of a more… fundamental nature. It is the entrapment of solar energy. The laws of physics dictate that if solar energy is trapped in an environment and the environment is of a limited volume, that energy will raise the temperature of that environment. Heat can build up faster than it dissipates even with modern Low-E windows. Just take a look at the glass and the backings of the blinds. In the early afternoons, when the blinds start to warp, and the back of the curtains start to sweat; feel the curtains and backings. It can be a lot cheaper to put a panel on the back and make some airflow than to replace blinds that have warped.
