Why Do Blackout Shades Let Light In Around the Edges?

You close your new blackout shades, the fabric looks completely opaque—and yet you still see thin lines of daylight around the sides.

Does that mean the shades are not really blackout?

Not necessarily. One of the most common misunderstandings about blackout window treatments is the difference between blackout fabric and whole-window blackout performance.

A blackout fabric can prevent light from passing directly through the material while small amounts of light still enter around the edges, top, or bottom of the window treatment.

In this guide, we'll explain why blackout shades can still have light gaps, which window treatments provide better room darkening, and what you can do to reduce edge light in bedrooms, nurseries, media rooms, and other light-sensitive spaces.

Blackout Fabric vs. Whole-Window Blackout

The first thing to understand is that blackout describes the fabric or material, not necessarily the entire window opening.

A blackout shade typically uses an opaque fabric designed to block most or all visible light from passing directly through the material. However, the shade itself usually needs to be slightly smaller than the window opening so it can operate smoothly.

That installation clearance can create narrow spaces where daylight enters.

In other words:

• Blackout fabric: prevents light from passing through the fabric.
• Whole-window blackout: minimizes light entering through both the fabric and the gaps around the window treatment.

These are not always the same thing.

Why Do Blackout Shades Have Light Gaps Around the Edges?

Several factors can cause light to appear around blackout shades even when the fabric itself is fully opaque.

1. Inside-Mount Shades Need Installation Clearance

Inside-mount shades are installed within the window frame. For the shade to raise, lower, and fit properly, there normally needs to be a small amount of clearance between the shade and the sides of the window opening.

This clearance is especially noticeable during bright daylight because even a very narrow gap can create a visible line of light.

The effect is often more noticeable in:

• Bedrooms with dark interiors
• Nurseries
• Home theaters
• East-facing rooms in the morning
• West-facing rooms in the late afternoon

2. Roller Shade Fabric Is Narrower Than the Overall Shade

Side light gaps are particularly common with blackout roller shades.

A roller shade includes more than just fabric. The roller tube, brackets, cassette, operating mechanism, and mounting hardware all require space. Because of this, the fabric itself is usually slightly narrower than the full width of the shade system.

When installed inside the window frame, this can leave a narrow light gap on each side.

This does not necessarily mean the blackout fabric is defective. It is often a normal result of the roller shade's mechanical design.

3. Windows Are Not Always Perfectly Square

Another common source of edge light is the window opening itself.

Many windows—especially in older homes—are not perfectly square. The width at the top of the window may be slightly different from the width in the middle or at the bottom.

Even small variations can create uneven gaps once a shade is installed.

This is why accurate measurements at multiple points are important when ordering custom window shades.

4. Window Trim and Hardware Can Affect the Fit

Handles, locks, cranks, trim, or uneven mounting surfaces can prevent the shade from sitting as close to the glass or window frame as expected.

The farther the blackout material sits from the window, the easier it is for sunlight to enter around the edges.

5. Bright Sunlight Makes Small Gaps Look Larger

A gap that looks insignificant at night can become very noticeable when strong sunlight hits the window directly.

This is particularly common with south- and west-facing windows.

The shade may still dramatically darken the room, even though bright lines remain visible around the perimeter.

Do Blackout Cellular Shades Have Side Light Gaps?

Yes, blackout cellular shades can still have small gaps around the edges when installed as an inside mount.

However, cellular shades are often a strong choice for bedrooms because the shade fabric usually extends relatively close to the sides of the window opening.

Their honeycomb construction can also provide additional insulation by creating air pockets between the room and the window.

When paired with blackout fabric, cellular shades can provide:

• Strong room-darkening performance
• Improved daytime privacy
• Better nighttime privacy
• Reduced glare
• Additional insulation around the window

Still, if your goal is an almost completely dark room, the small installation gap around an inside-mounted cellular shade should be considered.

Do Blackout Roller Shades Block All Light?

Blackout roller shade fabric can block light through the material very effectively, but roller shades are one of the window treatments where side gaps can be easiest to notice.

This happens because the fabric must remain narrower than the mounting hardware so the shade can roll smoothly without rubbing against the window frame.

A cassette or fabric-wrapped headrail can help reduce light entering from the top while also creating a cleaner, more finished appearance.

However, standard roller shades may still allow some light along the sides.

For many homeowners, this level of room darkening is more than sufficient for bedrooms, guest rooms, offices, and media spaces.

Are Blackout Curtains Better for Eliminating Light Gaps?

Curtains work differently from inside-mounted shades because they can extend beyond the actual window opening.

When blackout curtains are installed several inches above the window and extend well past both sides of the frame, they can cover more of the areas where light typically enters.

For stronger room darkening, curtains should generally:

• Extend beyond both sides of the window
• Be mounted above the top of the window frame
• Reach close to the floor or windowsill
• Have enough fullness to prevent stretched fabric gaps

Blackout curtains can therefore be an excellent option for reducing perimeter light, especially when combined with another blackout window treatment.

How to Reduce Light Gaps Around Blackout Shades

If maximum room darkening is important, there are several ways to reduce the amount of light entering around your shades.

1. Measure the Window Carefully

Accurate measurements are one of the most important factors in minimizing light gaps.

For an inside mount, measure the window width at the top, middle, and bottom. Measure the height in multiple locations as well.

Also check the usable mounting depth and look for handles, locks, or other hardware that could interfere with the shade.

Custom-made shades generally provide a closer fit than standard ready-made sizes because the product is made specifically for the dimensions of your window.

2. Consider an Outside Mount

An outside-mounted shade is installed over the window opening instead of inside it.

Because the shade can extend beyond the edges of the window frame, an outside mount can cover more of the areas where daylight normally enters.

For stronger room darkening, the shade should extend beyond the window opening on all sides where practical.

This approach can be especially useful for bedrooms, nurseries, and media rooms.

3. Choose a Shade With a Cassette

With roller shades, a cassette can help cover the roller tube and reduce light entering from the top of the window.

A cassette also gives the shade a more finished appearance and can make the installation look more integrated with the window.

4. Layer Shades With Blackout Curtains

One of the most effective ways to create a darker room is to combine blackout shades with blackout curtains.

The shade blocks light through the main window area, while the curtains overlap the sides and top of the opening.

This layered setup can also improve:

• Privacy
• Insulation
• Sound absorption
• Interior style

For bedrooms and nurseries where darkness is especially important, combining two window treatments can provide better results than relying on a single product.

5. Use Side Channels for Maximum Light Control

For specialized applications where extremely high levels of darkness are required, side-channel systems can cover the gaps between the shade fabric and the window frame.

These systems are more commonly used in dedicated home theaters, photography rooms, or other environments where controlling nearly all incoming light is essential.

Inside Mount vs. Outside Mount for Blackout Shades

Both mounting styles have advantages, but they perform differently when it comes to edge light.

Which Blackout Window Treatment Is Best for a Bedroom?

The best choice depends on how dark you want the room to be and the style you prefer.

Blackout cellular shades are an excellent option if you want a clean appearance, strong room darkening, insulation, and cordless operation.

Blackout roller shades provide a simple, modern appearance and are easy to operate. Adding a cassette can further improve top light control and create a more finished look.

Blackout curtains can cover a larger area around the window and are especially useful for reducing light around the perimeter.

For sleepers who are particularly sensitive to light, combining a blackout shade with curtains is often the most effective approach.

Are Small Light Gaps Normal With Blackout Shades?

Yes. Small amounts of light around the sides of an inside-mounted blackout shade are generally normal.

Blackout refers primarily to the light-blocking performance of the material itself. Because window shades require operating clearance and mounting hardware, achieving a completely sealed window with a standard shade alone is difficult.

What matters is choosing the right product and installation style for your expectations.

If your goal is simply to make a bedroom significantly darker, a properly measured blackout cellular shade or blackout roller shade may be sufficient.

If your goal is near-total darkness, consider an outside mount, wider overlap, layered curtains, or a specialized light-blocking system.

Frequently Asked Questions

Why does my blackout blind have light around the sides?

Most inside-mounted blackout shades need a small amount of space between the product and the window frame so they can operate properly. This clearance can allow narrow lines of daylight to enter around the sides.

Does blackout mean 100% darkness?

Not necessarily. Blackout usually means that the material prevents light from passing directly through it. Light may still enter around the perimeter of the window treatment.

Which shades have the smallest light gaps?

The answer depends on the window, measurements, mounting style, and product construction. Custom-sized cellular shades can provide a close inside-mount fit, while outside-mounted shades can overlap the window opening to reduce perimeter light.

Are blackout cellular shades better than blackout roller shades?

Both can provide excellent room darkening. Cellular shades also offer insulating honeycomb construction, while roller shades provide a clean, minimal appearance. Roller shades may show slightly more noticeable side gaps because the fabric needs clearance from the operating hardware.

How can I make my bedroom completely dark?

For stronger room darkening, use accurately measured blackout shades, consider an outside mount, and layer the shade with blackout curtains that extend beyond the top and sides of the window.

Final Thoughts

Seeing a small amount of light around a blackout shade does not automatically mean the blackout material is failing.

The key difference is between blocking light through the fabric and blocking light around the entire window opening.

Blackout cellular shades, blackout roller shades, and blackout curtains can all dramatically reduce incoming light, but the final result depends on the window dimensions, mounting method, product design, and installation.

Choosing a custom-sized window treatment and understanding how edge gaps work before ordering can help you create a darker, more comfortable room—and avoid unrealistic expectations about what a standard blackout shade can achieve.

Explore custom blackout and room-darkening window treatments at Curtaincustom to find the right solution for your bedroom, nursery, office, or media room.