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Blackout Curtains · Troubleshooting

Blackout Curtain Not Fully Blocking Light: 6 Causes and Fixes

If light is still getting into the room with a blackout curtain closed, the problem is usually sizing or mounting rather than the fabric itself. We cover the 6 most common causes of light leakage and

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How to fix it

Work through these in order, the most common causes come first.

  1. Start With the Basics

    Panel width versus window width: Total closed panel width should be at least 2 to 2.5 times the window's actual width. This is the single most common reason for visible light leakage, more common than any issue with the fabric's actual blocking rate. Rod placement: A rod mounted directly above the window frame, rather than extending 4 to 6 inches past it on each side, leaves side gaps that let in light even with correctly sized panels.

  2. Diagnosing Your Specific Problem

    Light comes in mainly at the sides: This points to undersized panels or a rod mounted too close to the frame. Extend the rod and confirm panel width against the sizing guidance above. Light comes in mainly at the top: This points to a gap between the top of the curtain and the ceiling.

  3. See Also

    How to Choose a Blackout Curtain Best Blackout Curtains for Shift Workers Blackout Curtains Buying Guide

Start With the Basics

Panel width versus window width: Total closed panel width should be at least 2 to 2.5 times the window's actual width.

This is the single most common reason for visible light leakage, more common than any issue with the fabric's actual blocking rate.

Rod placement: A rod mounted directly above the window frame, rather than extending 4 to 6 inches past it on each side, leaves side gaps that let in light even with correctly sized panels.

Color choice: Lighter colors block meaningfully less light than darker colors in the same product line, which can be mistaken for a fabric defect when it is actually an expected performance difference by colorway.

Diagnosing Your Specific Problem

Light comes in mainly at the sides: This points to undersized panels or a rod mounted too close to the frame.

Extend the rod and confirm panel width against the sizing guidance above.

Light comes in mainly at the top: This points to a gap between the top of the curtain and the ceiling.

A ceiling-mounted rod, valance, or cornice board closes this gap.

A faint glow comes through the fabric itself, not around the edges: This points to the fabric's construction rather than sizing.

See Also

How to Choose a Blackout Curtain Best Blackout Curtains for Shift Workers Blackout Curtains Buying Guide Blackout Curtain Fading in Sunlight

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Answers

Frequently asked questions

Why does light still come through the sides of my blackout curtain?

This is the single most common cause of light leakage, and it comes from the panel being narrower than the window rather than the fabric letting light through. If the curtain rod is mounted at the same width as the window frame, closing the panels leaves a gap on each side where light passes straight through. Extending the rod 4 to 6 inches beyond the frame on each side, along with using wider panels, closes most of this gap.

Is a small light gap at the top of the curtain normal?

Some gap at the top is common, particularly with rod pocket or grommet curtains that sit an inch or more below the top of the rod. A ceiling-mounted rod, or a valance or cornice board installed above the curtain, closes this gap for rooms where a truly dark ceiling-to-floor result matters.

Does the color of the curtain affect how much light gets through the fabric itself?

Darker colors block more light than lighter colors in the same fabric construction, typically by 3 to 10 percentage points. A light gray or pastel blackout curtain will show more of a faint glow through the fabric itself, even when properly sized and mounted, compared to the same product in black or navy.

Will adding a blackout liner fix a curtain that isn't dark enough?

Often, yes, for a curtain that is properly sized but still lets in more light than wanted through the fabric itself. A separate blackout liner clipped or sewn behind an existing curtain adds another light-blocking layer without replacing the curtain entirely, and is a lower-cost fix than buying an entirely new set of curtains.

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