How to Blackout Windows

Sometimes, even with blackout window treatments, a room can still be brighter than expected.

The reason is simple: blacking out a window is not just about choosing an opaque fabric. Light can pass directly through a window covering, but it can also enter around the sides, top, or bottom, or through gaps in the window treatment.

If you're trying to figure out how to blackout windows, this guide will help you address the problem through blackout materials, mounting methods, accurate sizing, and better coverage.

For most windows, effective light control comes from combining the right material with accurate measurements, an appropriate mounting method, and enough coverage around the window.

Here are five practical ways to block light from windows, reduce light gaps, and create a darker room.

If a significant amount of sunlight passes directly through your existing window covering, switching to Blackout Shades is one of the most straightforward solutions.

Blackout shades use opaque materials designed to prevent light from passing directly through the shade.

Blackout is not limited to one particular style of window treatment. Different types of shades can use blackout materials, including:

Blackout Roman shades combine light-blocking fabric with the soft texture and structured folds of a Roman shade. They are particularly well suited to bedrooms where both light control and interior design matter.

Blackout cellular shades have a clean, streamlined appearance, while their honeycomb construction can also help reduce heat transfer through the window.

Blackout roller shades have a compact, simple profile that works particularly well in modern and minimalist interiors.

Whichever style you choose, it is important to understand one distinction:

Blackout fabric does not automatically mean a completely blacked-out window.

The fabric addresses light passing through the material. If light is entering around the shade, you also need to consider coverage and installation.

If the center of the shade is already dark but bright lines remain along the sides, take a closer look at how the shade is mounted.

An Inside Mount positions the shade within the window opening. It provides a clean, fitted appearance, but some clearance around the shade is usually necessary for proper operation.

Those small spaces can allow light into the room.

An Outside Mount positions the shade beyond the window opening, allowing it to extend past the edges of the window and provide greater coverage at the top and sides.

If reducing light from the sides of the shade is a priority and the window structure allows it, a properly sized outside-mounted blackout shade can provide more coverage.

Outside mounting does not automatically guarantee complete blackout, however. The amount of overlap required depends on the window, shade design, measurements, and installation conditions.

Curtains can also help block light from windows.

They can be used on their own or as a second layer over shades or blinds. One of their main advantages is the amount of coverage they can provide.

Curtains can extend beyond both sides of the window and from above the opening toward the floor. With appropriate sizing and placement, this additional fabric can cover areas where light might otherwise enter.

For better light control, consider:

• whether the fabric provides enough opacity;
• whether the curtain panels are wide enough;
• how far they extend beyond the sides of the window;
• whether there is a significant gap above the curtains;
• and whether light enters between the panels when they are closed.

Simply choosing curtains labeled “blackout” does not guarantee a completely dark room.

The fabric determines whether light can pass through the curtain, while the coverage determines whether light can travel around it.

For a long-term blackout solution, accurate sizing can be just as important as the blackout fabric itself.

Ready-Made Shades are generally available in predetermined sizes, while Custom Shades can be made according to the dimensions of a particular window and installed as either an Inside Mount or Outside Mount depending on the window structure.

For an outside mount, the shade can also be sized to provide additional coverage around the window opening.

Custom sizing does not mean eliminating every light gap. Inside-mounted shades still need enough clearance to operate properly.

The real advantage is that custom sizing can help:

Reduce unnecessary gaps caused by poor fit and allow the window treatment and mounting method to better match the actual window.

For unusually sized windows, or windows where ready-made shades do not provide enough coverage, custom Blackout Roman Shades, Blackout Cellular Shades, or Blackout Roller Shades can make it easier to create a more appropriate fit.

For bedrooms that need light control every day, properly fitted custom shades can also make it easier to switch between blackout and natural daylight as needed.

Some windows have a very specific problem:

The center of the shade is dark, but narrow lines of light remain visible along the sides.

If you're trying to figure out how to stop light coming in from the sides of blinds or shades, focus on the perimeter of the window treatment rather than simply increasing the opacity of the fabric.

An Outside Mount and additional overlap can help reduce some edge light.

For situations where tighter light control is required, specialized blackout systems may use Side Channels or Side Tracks to make it more difficult for light to enter along the sides of the shade.

These features are not standard on every blackout window treatment, however, and they are not necessary for every bedroom.

For most residential windows, a sensible order is:

Accurate measurements → appropriate mounting method → sufficient coverage → specialized light-gap solutions if still necessary.

If you need to control light in a bedroom or another room every day, permanent Blackout Shades are generally more convenient than temporary blackout materials.

The main differences between shade styles are appearance, construction, and operation.

Blackout Roman Shades

A good choice for bedrooms where you want strong light control together with the softer texture and folded appearance of fabric.

Blackout Cellular Shades

A clean and practical option with honeycomb construction that can also help reduce heat transfer through the window.

Blackout Roller Shades

A compact option that works particularly well in modern and minimalist spaces.

Once you have chosen the shade style, consider the actual window dimensions, Inside Mount or Outside Mount installation, and the amount of coverage needed around the opening.

If blackout is the priority, choosing an appropriate blackout material and then matching the sizing and installation to the window is usually more useful than searching for one universally “best” blackout shade.

One of the most common mistakes is focusing entirely on Blackout Fabric while ignoring light around the window.

Even if the material itself is completely opaque, poor sizing, insufficient coverage, or an unsuitable mounting method can leave visible lines of light around the shade.

Another common mistake is assuming that Inside Mount is always the best choice.

Inside mounting usually provides a cleaner appearance, but if minimizing edge light is the main priority, the additional coverage provided by an Outside Mount can sometimes be more useful.

For better blackout performance, consider the complete combination:

Fabric + sizing + mounting method + overall window coverage.

There is one more question worth considering:

Do you actually need the room to be as dark as possible?

Some people may find that a very strong blackout setup makes a room unnecessarily dark during the day, especially when they frequently move between privacy, natural daylight, and full light control.

For night-shift workers who sleep during the day, people who are regularly disturbed by early-morning sunlight, or homeowners creating a home theater environment, stronger blackout performance may be exactly what is needed.

For other rooms and lifestyles, however, darker is not automatically better.

If you simply want to reduce bedroom brightness while retaining some sense of natural light, Room-Darkening Shades may be enough.

If your main goal is to soften harsh sunlight and reduce glare while keeping the room bright, Light-Filtering Shades may be more appropriate.

In simple terms:

Light Filtering → Softens natural daylight

Room Darkening → Significantly reduces room brightness

Blackout → Provides the strongest light blocking through the shade material

If you're still unsure which level of light control you need, read our Blackout vs. Room Darkening guide to compare the two before choosing your window treatment.

What is the best way to blackout windows?

First determine whether light is passing through the window covering or entering around it. For a long-term solution, use an opaque Blackout Shade with appropriate sizing and coverage. If significant edge light remains, consider additional overlap, curtains, or specialized light-gap solutions where necessary.

How can I stop light coming in from the sides of blinds or shades?

First determine whether the gaps are caused by poor sizing or by the normal clearance required for an Inside Mount. An Outside Mount can sometimes reduce side light by providing additional overlap. For more demanding blackout applications, specialized systems may also use Side Channels.

Can blackout shades block all light?

Blackout Fabric can prevent light from passing directly through the material, but light may still enter around the edges after installation. Completely opaque fabric and a completely dark room are not the same thing.

How can I blackout windows without curtains?

Blackout Roman Shades, Blackout Cellular Shades, and Blackout Roller Shades can all provide strong light control without traditional curtains. Final performance also depends on measurements, mounting style, and edge coverage.

Is an outside mount always better for blackout?

Not always. Outside mounting can provide greater overlap around the window opening and may help reduce edge light on some windows. The best mounting method still depends on the window structure, available mounting space, shade design, and desired appearance.

Can blackout shades block sunlight?

Yes. Blackout materials are designed to prevent sunlight from passing directly through the shade. If sunlight remains visible around the sides or top, additional coverage or better control of edge gaps may be needed.