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Bedroom Curtains Not Blocking Light? What Blackout Really Means

Most light leaks around the edges, not through the fabric. Fix the fit first, then switch to thermal blackout curtains if the fabric itself is the problem.

By Jamilur Rahman Updated November 5, 2026 · 7 min read

Time
15 min
Cost
$50-$150
Difficulty
Easy
A dim bedroom with closed curtains and thin lines of daylight showing around the edges and between the panels
In this guide
  1. Quick answer
  2. Free vs. permanent fix
  3. What you'll need
  4. Why it happens
  5. Try this first (free)
  6. When free fixes aren't enough
  7. Buy the fix
  8. How to read the label
  9. What to skip
  10. FAQ
  11. Related problems

Quick answer

Curtains let light in for two reasons: the fabric is too thin (room darkening is not the same as blackout), or light leaks around the edges, over the top and between the panels. Test the fabric with a phone flashlight. Then hang the rod wider and higher and overlap the panels. If you can see light through the cloth, a thermal blackout curtain has a dense lining that blocks nearly all of it and also helps keep the room cooler.

Free fix vs. permanent fix

Free fix · 15 minutes

Fix the fit and seal the gaps

  • Move the rod higher and wider than the window
  • Overlap the panels at the center
  • Clip or pin gaps shut at the sides
  • Use a phone flashlight to find where light comes through

How long it lasts: Works if your fabric is thick enough. Light still shines through a thin or light-filtering fabric.

Most readers choose Permanent fix
Thermal blackout curtains with a dense lining (illustration)
Illustration

Thermal blackout curtains

A blackout curtain has a dense woven or coated lining that blocks nearly all light passing through the cloth. The thermal layer also slows heat moving through the window. The free fixes seal the edges, but only blackout fabric stops light going through the fabric.

What to get: Thermal blackout curtains with a dense lining

How long it lasts: Years. The lining is built in and does not wear off like a stick-on fix.

Check price on Amazon

As an Amazon Associate I earn from qualifying purchases.

What you'll need

  • Tape measure

    Measure the window and the rod

    You have it

  • Phone flashlight

    Test the fabric and find leaks

    You have it

  • Binder clips or safety pins

    Hold the center overlap and the sides closed

    You have it
  • Blackout curtain liner (illustration)
    Illustration

    Blackout curtain liner

    Hangs behind curtains you already own

    Check price
  • Our pick
    Thermal blackout curtains (illustration)
    Illustration

    Thermal blackout curtains

    Dense lining that blocks light through the fabric

    Check price
  • Wraparound curtain rod (illustration)
    Illustration

    Wraparound curtain rod

    Closes the gap at the sides

    Check price

Why it happens

1

Room darkening is not blackout

Labels like room darkening or light filtering mean the fabric blocks some light. Blackout fabric is made to block nearly all of it. Thin cotton, linen and sheers block little.

2

Light leaks around the edges

Most light comes in around the sides, over the top and between panels, not through the cloth. A rod mounted tight to the window frame leaves gaps.

3

Panels that are too narrow

Panels hung flat across the window leave gaps when closed. Curtains need fullness: the panels should add up to about 1.5 to 2 times the window width.

4

Curtains that are too short

Panels that stop at the sill leave a bright strip under the hem, and street light comes in at night.

5

Light from elsewhere

Door gaps, power lights and phone chargers also glow. Fix the window first, then look for these.

Try this first (free)

Do these in order, starting with the simplest.

  1. Test your fabric

    Tool used: Phone flashlight

    Turn off the room lights and hold the phone flashlight on one side of the curtain while you look from the other. If you see a bright glow, or the weave shows, it is not blackout.

  2. Move the rod wider and higher

    Tool used: Tape measure

    Mount the rod 4 to 6 inches above the window frame, and extend it at least 4 to 6 inches past each side. More is better. The curtain then covers the gaps around the frame.

  3. Hang them full

    Make sure the panels add up to about 1.5 to 2 times the window width. Close them so the center edges overlap by a few inches.

  4. Close the remaining gaps

    Tool used: Binder clips or safety pins

    Pin or clip the center overlap shut. Let the panels reach the floor, or at least cover the sill. For a gap you cannot close, a rolled towel along the sill blocks the rest.

  5. Add a liner behind what you have

    Tool used: Blackout curtain liner

    Hang a blackout liner behind curtains you like instead of replacing them. It only works if the liner is as wide as the curtain.

    Blackout curtain liner (illustration)
    Illustration

    Tool for this step

    Blackout curtain liner · Hangs behind curtains you already own

    Check price on Amazon

  6. Replace the fabric if light still shows

    Tool used: Thermal blackout curtains

    If the flashlight test shows light coming through the cloth, replace it with thermal blackout curtains hung on a wide rod.

    Thermal blackout curtains (illustration)
    Illustration

    Tool for this step

    Thermal blackout curtains · Dense lining that blocks light through the fabric

    Check price on Amazon

When free fixes aren't enough

If the flashlight test shows light through the fabric, no amount of adjusting will darken the room. Thin, light-filtering or room-darkening curtains were never made to be fully dark. Thermal blackout fabric solves that. Blackout cloth still leaks at the sides if the rod sits tight to the window, so keep the fit fixes. If light creeps in along the edges anyway, a wraparound rod that curves back to the wall closes the sides. Lined, closed curtains also reduce heat moving through the glass, which helps in summer and winter.

Buy the fix

Still stuck after the free fixes? These are the products we would look at.

Our picks compared
PickProductFixesWho it's forBuy
Best pick
Thermal blackout curtains with a dense lining (illustration)
Illustration
Thermal blackout curtains with a dense lining
light passing through the curtain fabric itself Anyone who sleeps late, works nights, or has a bright street light or sunrise at the window. Check price
Budget pick
Blackout curtain liner (illustration)
Illustration
Blackout curtain liner
curtains you like that let light through Renters and anyone who loves how their current curtains look. Check price
Upgrade pick
Wraparound curtain rod (illustration)
Illustration
Wraparound curtain rod
light leaking around the sides of the curtain Anyone who has the right fabric but still sees light at the edges. Check price
Best pick
Thermal blackout curtains with a dense lining (illustration)
Illustration

Thermal blackout curtains with a dense lining

Fixes: light passing through the curtain fabric itself

Look for: Sold as blackout, not room darkening or light filtering; A dense triple-weave or coated lining; Wide enough to gather to 1.5 times the window, and long enough to reach the floor; Machine washable.

Who it's for
Anyone who sleeps late, works nights, or has a bright street light or sunrise at the window.
Honest downside
You like waking to daylight. A light-filtering curtain with tie-backs suits you better.
Check price on Amazon
Budget pick
Blackout curtain liner (illustration)
Illustration

Blackout curtain liner

Fixes: curtains you like that let light through

Look for: Wide enough to match your curtain; Hangs from your existing rod with rings or hooks; An opaque coating on one side.

Who it's for
Renters and anyone who loves how their current curtains look.
Honest downside
Your curtains are already heavy and the leaks are at the sides. Fit fixes will help more.
Check price on Amazon
Upgrade pick
Wraparound curtain rod (illustration)
Illustration

Wraparound curtain rod

Fixes: light leaking around the sides of the curtain

Look for: Returns that curve back to the wall on both sides; A strong bracket that holds heavy blackout panels; Long enough to extend past the window on each side.

Who it's for
Anyone who has the right fabric but still sees light at the edges.
Honest downside
You rent and cannot drill into the wall or frame.
Check price on Amazon

As an Amazon Associate I earn from qualifying purchases. We don't show prices because they change, so use the button to see today's price.

How we picked: How we picked: this page is based on window covering and sleep-environment guidance, product specs from curtain makers, and the problems and fixes people report in reviews. We do not test products by hand.

How to read the label

  • Blackout: built to block nearly all light through the cloth.
  • Room darkening: blocks a lot, but the room stays dim, not dark.
  • Light filtering: softens daylight and gives privacy but blocks little.
  • Thermal: has an insulating layer. Many blackout curtains are also thermal.

What to skip

Foil on the glass and thin dark curtains

Foil blocks light but looks rough, can leave residue on the glass and is a hassle to take down. A thin black curtain is not blackout either, since color alone does not stop light. Look for lined, dense fabric.

Frequently asked questions

What is the difference between blackout and room darkening curtains?
Blackout fabric is made to block nearly all light. Room darkening blocks a good part of it, so the room is dim but not dark. Dense weave or a lining, not color, is what makes a fabric blackout.
Should a bedroom have curtains or not?
For most people, yes. They give privacy, block light for sleep and help with temperature. If you want daylight to wake you, use a lighter curtain or tie them back in the morning.
Why do my blackout curtains still let in light?
Light gets in at the sides, top and center. Mount the rod wider and higher, overlap the panels, or add a wraparound rod. Use a flashlight to check the fabric itself is not the leak.
What are the best curtains for a bedroom?
Floor-length, lined or thermal blackout curtains, hung from a wide rod above the window, with panels that overlap at the center. Choose the weight by how dark you want the room.

As an Amazon Associate I earn from qualifying purchases. Affiliate disclosure

Thermal blackout curtains with a dense lining (illustration)
Illustration

Our pick: Thermal blackout curtains with a dense lining

Check price