Light shining through the sides of curtains: 5 causes, 5 solutions
If light is passing around the sides, top, or bottom of your blackout curtain, the problem isn't necessarily the fabric. In 80% of cases, it's a matter of dimensions or installation. A curtain that's too narrow (side overhang less than 15 cm), too short (5 cm above the floor), or hung too low (rod flush with the frame) will let light through even if the fabric blocks 100% of rays.
This guide identifies the 5 sources of light leakage, along with precise solutions and costs for each. The goal: to go from a "nearly blackout" curtain to complete darkness without replacing the curtain if the fabric is suitable.
The 5 Sources of Light Leakage (Diagnosis)
Close your curtain in broad daylight and observe from your bed. Light is entering through one or more of these 5 points:
| Leakage Source | What you see | Cause | Frequency |
|---|---|---|---|
| 1. Sides | Vertical band of light between the curtain and the wall, on each side of the window | Curtain too narrow, insufficient side overhang (less than 15 cm per side) | Very frequent (70% of cases) |
| 2. Bottom | Horizontal band of light at floor level, visible from the bed | Curtain too short, gap of more than 5 cm between the hem and the floor | Frequent (50% of cases) |
| 3. Top | Halo of light above the curtain, between the rod and the ceiling | Rod positioned too low (flush with the frame instead of 10-15 cm above) | Frequent (40% of cases) |
| 4. Center (2 panels) | Vertical line of light in the middle, between the 2 panels | No central overlap (the 2 panels meet edge to edge) | Systematic with 2 panels without overlap |
| 5. Through the fabric | Diffused halo over the entire curtain surface (yellowish or grayish tint when the sun hits) | Fabric not blackout (80-95% blackout instead of 100%), degraded coating, or fabric too thin | Frequent on curtains under 40 euros |
Quick test: In the dark, turn on your phone's flashlight and place it against the curtain fabric (window side). If light passes through the fabric and you see a halo on the room side, the problem is the fabric itself (source 5). If no light passes through the fabric but you see light at the edges, the problem is sizing or installation (sources 1 to 4).
Solution for Each Leakage Source
Leakage N°1: Sides (Most Frequent)
The problem: The curtain does not extend far enough past the window. Light passes between the curtain and the wall on each side. This is the most frequent leak because standard-sized curtains do not provide enough side overhang.
Solutions:
- Ideal solution: Replace with a wider curtain with 15 to 20 cm of overhang on each side. A custom-made curtain to exact dimensions permanently eliminates the problem. Cost: 80 to 350 euros.
- Quick solution: Attach magnetic clips or velcro strips between the edge of the curtain and the wall. The curtain is pressed against the wall and light no longer passes through. Cost: 10 to 20 euros. Limitation: not very aesthetic and the curtain is harder to open/close.
- Intermediate solution: Add a strip of opaque fabric (like black fleece) sewn or clipped to each edge of the curtain. Adds 10 to 15 cm of effective width. Cost: 15 to 30 euros (haberdashery).
Leakage N°2: Bottom
The problem: The curtain stops 5, 10, or 15 cm from the floor. Light passes through the bottom and creates a luminous band visible from the bed.
Solutions:
- Ideal solution: Replace with a longer curtain that skims the floor (2-3 cm gap) or "breaks" slightly on the floor (1-2 cm excess). Cost: replacement with a custom-made curtain, 80 to 350 euros.
- Quick solution: Undo the bottom hem to gain 3 to 5 cm in length (if the existing hem allows). Cost: 0 euros (basic sewing) or 10 to 20 euros (tailor alteration).
- Complementary solution: Add a draft stopper at the base of the window or curtain. Blocks light at floor level. Cost: 5 to 15 euros.
Leakage N°3: Top
The problem: The rod is positioned flush with the window frame (or 2-3 cm above it). Light passes between the top of the curtain and the ceiling.
Solutions:
- Ideal solution: Reposition the rod 10 to 15 cm above the frame, ideally 15 to 20 cm. Cost: 0 euros if you fill the old holes (tube of filler at 3 euros). The existing curtain can be reused if its height is sufficient.
- Maximum solution: Ceiling mounting (curtain track fixed directly under the ceiling). Eliminates all top leakage. Particularly effective in homes without shutters. Cost: 30 to 80 euros for a ceiling track.
- Quick solution: Add a band of opaque fabric (like a valance) above the curtain to fill the gap. Cost: 20 to 40 euros.
Leakage N°4: Center (2 panels)
The problem: The 2 panels meet edge to edge in the center, without overlapping. The slightest opening (1 cm is enough) lets a vertical line of light through, visible from the bed.
The solution: The 2 panels must overlap by 10 to 15 cm in the center when closed. If your current panels are too narrow to overlap, you need wider panels. When ordering, add 10 to 15 cm to the width of each panel for overlap.
If the panels overlap but separate during the night (air movements, drafts): attach a magnetic clip in the center to keep the 2 panels in contact. Cost: 5 to 10 euros.
Leakage N°5: Through the Fabric
The problem: The fabric itself lets light through. Three possible causes:
- The curtain is not a true blackout: it blocks 80 to 95% of light (the term "blackout" is not regulated in France). Solution: replace with a true 100% multi-layer blackout.
- The coating is degraded: after 2 to 4 years of use (10-20 washes), the opaque coating cracks and lets light through micro-holes. Solution: replace the curtain (the coating cannot be repaired).
- The fabric is too light and too thin: a single-layer white curtain of 200 g/m² lets 20 to 40% of light through. Solution: a technical multi-layer curtain (620 g/m²) blocks 100%* of light regardless of color, because the blackout comes from the internal membrane.
To understand the difference between blackout by coating and structural blackout, consult our article thermal blackout curtain vs simple blackout.
Summary Table: Cost of Each Correction
| Leakage Source | Quick Solution | Quick Cost | Permanent Solution | Permanent Cost |
|---|---|---|---|---|
| Sides | Magnetic clips / velcro | 10-20 € | Custom curtain with 15-20 cm overhang | 80-350 € |
| Bottom | Undo hem / draft stopper | 0-15 € | Custom curtain that reaches the floor | 80-350 € |
| Top | Raise the rod | 0-3 € | Ceiling track | 30-80 € |
| Center (2 panels) | Central magnetic clip | 5-10 € | Wider panels (+10-15 cm overlap) | 80-350 € |
| Through the fabric | None (the fabric is the problem) | — | Replace with a 100%* multi-layer | 80-350 € |
Practical advice: if you have leaks on the sides, bottom, AND top (3 cumulative sources), quick solutions (clips, draft stopper, raising rod) cost 15 to 40 euros. But they fix the symptoms, not the cause. A custom-made curtain with the correct dimensions (80-350 euros) eliminates all 3 problems at once and lasts 10 to 15 years. In the long run, it is more economical and more effective.
The Real Problem: The Curtain Was Bought in a Standard Size
Most light leaks come from the same underlying problem: the curtain was bought in a standard size (140x230, 240x260 cm, etc.) and does not match the exact dimensions of the window + rod + floor.
A 140 cm curtain on a 120 cm window extends only 10 cm on each side (instead of 15-20 cm). A 230 cm high curtain on a rod-to-floor configuration of 245 cm leaves a 15 cm gap at the bottom. These few centimeters are invisible in the store but create obvious light leaks once the curtain is installed in the dark.
The custom blackout curtain solves the problem at the source: manufactured to the exact dimensions of your window (width and height to the nearest centimeter), with the recommended overlaps integrated. Zero lateral leaks, zero gaps at the bottom, zero halos at the top. Consult the size guide for correct measurement.
*Data from tests performed under optimal conditions.
Frequently Asked Questions
Why does my blackout curtain let light through on the sides?
Because the curtain is too narrow: it does not extend far enough past the window. To block light on the sides, the curtain must extend 15 to 20 cm beyond the frame on each side. A 140 cm curtain on a 120 cm window only extends 10 cm per side, which is insufficient to prevent light from bypassing the fabric. Definitive solution: a custom-made curtain with the correct overhangs.
How do I know if the problem is the fabric or the installation?
Flashlight test: in the dark, place your phone's flashlight against the fabric on the window side. If light passes through the fabric (halo visible on the room side), the problem is the fabric (not a true 100% blackout or degraded coating). If no light passes through but you see light at the edges, the problem is the sizing or installation.
Does a light-colored blackout curtain let more light through at the edges?
No. Lateral, top, and bottom leaks are dimension problems, not color problems. A white curtain that is too narrow lets light through on the sides exactly like a black curtain that is too narrow. Color only affects the light that passes through the fabric (source 5). For a technical multi-layer curtain (620 g/m²), even white fabric blocks 100%* of light through the fabric.
How to block light between 2 curtain panels?
The 2 panels should overlap by 10 to 15 cm in the center when closed (not just meet edge to edge). If your panels are too narrow to overlap, you need wider panels. In the meantime, a magnetic clip placed in the center keeps the 2 panels in contact and prevents light from passing through. Cost: 5-10 euros.
Should I raise the curtain rod to block light at the top?
Yes, if the rod is less than 10 cm above the window frame. The ideal position is 15 to 20 cm above the frame. Raising the rod is free (you just need to fill the old holes with a tube of filler for 3 euros). For maximum top blackout, ceiling mounting (curtain track fixed under the ceiling) eliminates any space between the curtain and the ceiling.
How much does it cost to correct light leaks?
Quick solutions (palliatives): 0 to 40 euros (raise rod, magnetic clips, undo hem, draft stopper). Permanent solution (custom-made curtain with correct dimensions): 80 to 350 euros. If your current curtain has good fabric (100% blackout confirmed by the flashlight test) but incorrect dimensions, quick solutions may suffice. If the fabric lets light through, replacement is the only solution.
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