How to fix curtain light gaps

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If the rod is roughly the width of the window, the panels have nowhere to overlap and light runs straight down both edges. The fix is a rod 8 to 12 inches wider than the window, so each panel extends 4 to 6 inches onto the wall beyond the frame.

Then add returns. A return bracket holds the rod away from the wall and bends the end of the rod back to it, so the panel wraps round and touches the wall instead of ending in mid-air. This is the single most effective thing you can do, and return brackets cost very little. Wrap-around and "C"-shaped rods do the same job in one piece.

If you cannot move the rod — a rental, a fixed track — the workable substitutes are magnetic edge strips (on a steel frame), a strip of hook-and-loop tape holding the panel edge to the wall, or simply tucking the panel edge behind a piece of furniture placed against the wall. Inelegant, but it works.

Light coming over the header hits the ceiling and scatters through the whole room, which is why a top gap feels worse than its size suggests. Grommet headers are the worst offenders: the panel hangs an inch or so off the wall and there are open gaps between each grommet.

• Raise the rod closer to the ceiling so there is less wall above the curtain for light to travel down.
• Fit a pelmet or a board across the top. A painted board fixed above the rod, projecting a few inches, closes the path completely and also helps thermally by stopping the convection loop.
• Use a ceiling-mounted track instead of a wall rod — no gap above by definition.
• Switch the header to back tab or rod pocket, both of which hold the panel much closer to the wall than grommets do.

Two panels that meet edge to edge will separate as they settle, and a vertical slit of daylight appears down the center. They need to overlap by 3 to 4 inches, which means more total panel width than most people buy — aim for 2 to 2.5 times the window width combined.

Cheap fixes if you do not want another panel: a couple of magnetic curtain clips holding the overlap closed, a long bulldog clip at eye height, or a center-overlap track where one carrier crosses the other. Any of the three costs less than a curtain.

Panels that stop at the sill leave light bouncing off the wall below and up into the room — less obvious than a side gap but it lifts the whole room's brightness. Floor-length panels, or a half-inch off the floor, close it. Remember the drop is measured from the rod, so a rod mounted 6 inches above the frame needs panels 6 inches longer than you first estimated. See the curtain length guide.

If the light is an even glow across the whole panel rather than a line at an edge, the fabric is passing light and no amount of mounting will fix it. Hold a section up to a bright lamp: a true blackout weave shows nothing. If yours glows, it is room-darkening fabric and you want a genuine blackout panel — best blackout curtains — or a liner behind what you have.