Some room darkening curtains come with the liner built-in. This makes them heavy, and expensive. And, unfortunately if you are trying to be stylish, the blackout curtains are limited in selection. If you are looking for cute curtains to match the colors in your room, you may not be able to find lined curtains that match.
So, what do you do?
It's called a "tension rod". You can buy curtain liners, or blackout curtains that are thick, and preferably a light color to reflect the light and heat away from the window. Get a small tension rod that can fit inside the window frame, behind where the normal curtain hangs. This allows you to go with a less-expensive, generic room darkening curtain, while still maintaining the stylish look of your regular window treatment.
This is an inexpensive way to do two things: darken and cool the room, and keep your room looking pretty with matching curtains.Christy - Room Darkening Curtains info from http://roomdarkeningcurtains.net