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?

Yes. 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.

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 how to fix each one.

Step-by-Step Guide

Fix 1: Check for side light gaps from undersized panels

Measure the window's width and compare it to the total width of the closed panels. If the total is less than roughly 2 to 2.5 times the window's width, the panels are likely too narrow to fully overlap the frame. Either replace with wider panels or add a third panel to close the gap.

Fix 2: Extend the curtain rod beyond the window frame

Mount the rod so it extends 4 to 6 inches beyond each side of the window frame, rather than sitting directly above it. This allows the panels to close well past the frame's edges, closing the side gaps that are the most common source of light leakage even with properly sized panels.

Fix 3: Close the gap at the top of the curtain

If light comes in above the curtain rod, consider a ceiling-mounted rod, or add a valance, cornice board, or a strip of adhesive-backed blackout fabric across the top gap. This is most noticeable in rooms with high ceilings or a rod mounted well below the ceiling line.

Fix 4: Switch to a darker color in the same product line

If the panels are correctly sized and mounted but still show a faint glow through the fabric, a darker colorway of the same product typically blocks meaningfully more light. This is a simple fix that does not require changing brands or sizes.

Fix 5: Add a blackout liner behind the existing curtain

A separate blackout liner, either clipped to the same rod behind the curtain or sewn onto the back of the panel, adds another light-blocking layer. This is a cost-effective fix for curtains that are the right size and color but still let in more light than needed.

Fix 6: Check for light bleeding through worn or thin sections of fabric

Older curtains, particularly ones that have gone through many hot-dryer cycles, can develop thinner or cracked sections in a laminated backing where more light passes through than when new. Inspect the fabric against a bright window for uneven light spots, which indicate backing degradation rather than a sizing issue, and consider replacement if the wear is widespread.

A blackout curtain that still lets in a visible strip of daylight is one of the most common complaints in this category, and it is usually fixable without buying an entirely new set of curtains.

Most cases come down to sizing and mounting rather than the fabric itself. A panel that is narrower than the window, or a rod mounted flush with the frame instead of extending past it, will leak light at the sides regardless of how good the fabric's blocking percentage is on paper.

The fixes below start with the most common and easiest to check, then move toward less common causes like fabric wear.

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. A woven-only fabric without a laminated backing will show more of this glow than a laminated Thermaback-style panel, especially in lighter colors.

The curtain used to be dark enough but has gotten lighter over time: This points to wear on the laminated backing, often from repeated hot-dryer use, rather than a sizing or mounting issue.

Quick Diagnosis

Symptom	Most Likely Cause	Fix
Light at the sides	Undersized panels or rod too close to frame	Extend rod, widen panels
Light at the top	Gap above curtain, no valance	Ceiling mount or add valance/cornice
Faint glow through fabric	Non-laminated construction or light color	Switch to laminated backing or darker color
Getting lighter over years	Backing wear from hot drying	Wash cold, hang dry, or replace
Overall dim but not dark	Correctly sized but base fabric limit reached	Add a blackout liner