Can Curtains Make a Room Warmer?

Giovanni Valle is a licensed architect and LEED-accredited professional and is certified by the National Council of Architectural Registration Boards (NCARB). He is the author and managing editor of various digital publications, including BuilderSpace, Your Own Architect, and Interiors Place.

We all know that curtains can help to cool a room by blocking direct sunlight, but one thing we often do not think about is how curtains can also have the reverse effect during the colder months.

Curtains can in fact make a room warmer. The curtain’s color, thickness, material, placement relative to the window, and other factors (not related to the curtains) can influence a room’s temperature.

But how exactly do curtains make a room warmer? I’ll explain the mechanics of how curtains heat your house (or not) in the following sections. Since thermal curtains work best for keeping a room warm (spoiler alert!), I’ll briefly discuss them and how to choose the best curtain for you.

How Do Curtains Affect a House’s Temperature?

When we talk about how curtains affect room temperature, the assumption is that the curtains are closed. Otherwise, the effect is less pronounced.

Curtains can make a difference in room temperature by regulating the heat that comes through your windows as well as trapping the heat that’s already inside the room.

Curtains affect a house’s temperature by reducing the sunlight that comes into a room. If a room already has heat stored within it, curtains can keep some of that heat from going out the windows.

Choosing the Right Curtain Material, Color, and Placement Is Crucial

All curtains can block sunlight, albeit to varying degrees. Their ability to do so depends on:

Color: A dark-colored curtain absorbs more light (which subsequently converts to heat) than a light-colored one. Therefore, the darker the curtain, the warmer the room.
Material: For example, blackout curtains can block as much as 99 percent sunlight due to double-lined and tightly woven fabric.
Placement: Where you place your curtains will determine how effective the curtains are in keeping your room warm or cool. If you put thick, dark curtains in a room that’s already hot, the room will stay warm, which isn’t ideal if you’re trying to cool down the room. But if you’re trying to warm a room, thick, dark curtains will help you block indoor heat from escaping.

Curtains Can Block Some Heat From Entering or Exiting a Room

Imagine if there’s a gap between the window and the curtain. In that case, cold air can get into the room due to convection, where the heat goes up (i.e., out the window) and cold goes down (i.e., through the gap under the curtain and into the room).

Suppose there’s no gap between the curtain and the window. If the temperature outside the window is lower than the one inside, and the room is already warm, the curtain can block cold draughts.

As you can see, even blackout curtains can’t keep all sunlight from coming into a room. They can reflect most of the sunlight outside, but some heat will stay inside the room.

Heavy, Thick Curtains With Insulative Properties Can Warm a Room

The best curtains for heat retention are heavy, thick, and have solid insulative properties. To understand how insulated curtains work, you need to know what conduction and insulation are.

Conduction involves the transfer of heat energy between neighboring molecules, causing those molecules to vibrate faster (i.e., heat up). If a room doesn’t have a curtain, and the temperature outside is colder than the one inside, the heat will move towards the more frigid space.

If you want to prevent heat loss in a room due to conduction, you need to install a material over a window that doesn’t conduct heat well (i.e., a thermal insulator). Insulated curtains trap air inside the room, meaning there’s less opportunity for the hot air to come into contact with the cold air out the window and flow towards it.

Remember, conduction involves the rapid vibration of molecules. The more solid an object is, the more likely it’ll get hot due to conduction. Since gasses (i.e., air) have molecules further apart than solids do, it transfers heat at a slower rate.

We’ve touched on what kinds of curtains are best for heat retention. Let’s expand on those below.

What Kind of Curtains Can Keep Heat Inside?

As I said, some curtains are better at keeping a room warm than others. If you want a curtain for keeping the heat in (instead of blocking heat from outside), there’s only one choice.

Thermal curtains can keep heat inside. They work primarily as insulators, but they can also have other properties. Thermal curtains are different from blackout or solar curtains, which prevent sunlight from coming into a room.

How to Choose the Best Thermal Curtains

Before searching for the best thermal curtains available online, you must first know what to look for. Below are pointers on choosing the thermal curtain that works best for you.

You can choose the best thermal curtains by doing the following:

Pick the right size and length for your window.
Check the R-value.
Consider getting extra features.

Let me explain these pointers further.

Pick the Right Size and Length for Your Window

Grab a measuring tape, and find out the length and width of your window. Then, measure the distance between the window’s highest point and the floor. A good curtain is long enough to reach the floor and is at least twice the window’s width.

Check the R-Value

Any thermal curtain worth its salt should come with a specification regarding its R-value. The R-value measures the effectiveness of a curtain as an insulator. In other words, the higher the R-value is, the better its insulative capabilities are.

Consider Getting Extra Features

Although insulation is a thermal curtain’s primary function, it’s not the only function you should consider.

For example, some thermal curtains double as blackout curtains, retaining heat and blocking sunlight. You can also find thermal curtains with triple-layer fabric and soundproof materials.

Conclusion

Curtains can make a room warmer, provided the right variables are in place. The curtains must have strong insulative properties, dark color, and a fit that prevents heat from escaping through convection and conduction.