Tension rods can hold shower or window curtains and are designed to be extended to the required width. A spring within the rod allows you to compress the rod as needed so that it can be inserted between two walls. When the rod is released, the spring creates pressure and tension within the rod causing the rod to press firmly against the wall. These rods are frequently used for shower curtains to prevent using screws in shower walls.
Valances and Cafe Curtains
Valance and cafe-style curtains are usually thin, lightweight curtains that stay in one position most of the time. Both are typically lightweight enough to be used with a tension curtain rod. The window opening needs to be constructed of a secure material. If the inner window wall is soft or crumbly, it will not be a good location for the rod to press into.
Sheer and Lace Curtains
Sheer curtains are curtains made from a thin fabric that allows light to pass through. These fabrics tend to be light enough for a tension rod. Be careful with longer length rods (82 inches or more), as the rods appear to be less able to carry loads the wider they are extended. You might also need to use fewer sheer panels across a window than you would if using sturdier rod configurations.
Tab-top curtains are single curtain panels with tabs sewn at the top for the curtain rod. Often this style of curtain panel is made from inexpensive and relatively light material. However, this style of curtain may be on the borderline of the amount of weight a tension rod can hold. If you are using the tension rod in a single window that is about 36 inches wide, you can probably use two lightweight tab-top curtains on the rod. If your window is more than 50 inches wide, those same two curtain panels may be too heavy for the rod.
Lined curtains and drapes are heavy. In almost all cases this type of drape will be too heavy for a tension rod. Traditional, heavy drapes can pull screws out of plaster and drywall. This is aggravated by the action of moving the drapes by the homeowner. When drapes are moved, different stresses are placed on the rods and hardware. For a tension rod, these extra stresses could mean the rod would be displaced quickly.