Many homeowners ask a simple question that affects sleep and comfort level: should blackout cellular shades be installed as inside or outside mount? The right answer depends on window frame depth, how you want to block light, trim style, and how the room is actually used. Remember a few rules of thumb, and choosing isn’t that hard—at least that’s what I think.
Key Takeaways
- Inside mount looks like it’s built-in and can protect the window trim. But without side channels, it easily lets in thin streaks of light—and this really affects light blocking.
- Outside mount covers more of the window frame, so it usually blocks light better and hides flaws in the frame. But you need to calculate the overlap width carefully, otherwise it’ll look messy.
- If the window frame is deep and square, installing inside mount with side channels gets close to total blackout. For shallow or uneven frames, outside mount works better for darkening the room.
- Measuring is super important. Getting the width, height, and jamb depth right determines if it fits properly, which hardware to choose, and how well it blocks light.
- Bedrooms and media rooms work best with outside mount, or inside mount with light-blocking accessories—people around me who’ve installed them this way have given good feedback.
Should Blackout Cellular Shades Be Inside Or Outside Mount?
What an Inside Mount Is
Inside mount means installing the headrail within the window frame or jambs. The shade sits inside the window opening, so the trim and casing are still visible. With blackout cellular fabric, the pleats absorb light, and the frame acts like a natural “little pocket” to trap light—I think this is pretty convenient.
This installation works great if the window opening is deep, square, and free of obstacles. Also, if you already have curtains or sheer panels at home, it pairs well with them.
Inside Mount: Advantages
- It has a clean look, like it’s built-in, and highlights the original window trim—it just looks nice.
- The shade stays inside the frame, so it’s better protected and less likely to get bumped.
- It’s easy to layer with curtains or cornices because the part that sticks out is really small.
Plus, inside mount blends better in modern-style rooms. It also doesn’t collect as much dust on the sides—this saves hassle when cleaning.
If you want to check out different sizes and fabrics in one place, many buyers start with a curated collection of blackout cellular shades—I think this saves time, no need to look around everywhere.
Inside Mount: Limitations and Fit Requirements
- Thin streaks of light might leak in from the left and right sides, and if the headrail is small, light will also leak from the top—this is an issue a lot of people I’ve talked to have mentioned.
- The window frame needs to be relatively square. If the frame is slightly slanted or curved, there will be uneven gaps after installation.
- Depth is crucial. Many headrails need at least 1.5 inches of depth, and to fit completely flush, you’ll need around 2 to 2.5 inches.
Manufacturers will deduct a little from the width you order to make the shade slide smoothly. So for inside mount, the deduction is usually around 1/8 to 3/8 inch—don’t think this small deduction doesn’t matter; it could cause the shade to get stuck. If you’re renting and not allowed to drill holes, a no-screw design can solve the problem. For example, if the jamb is straight and the depth is sufficient, you can use classic no‑drill perfect‑fit cellular shades for a snug inside fit.
What an Outside Mount Is
Outside mount means installing the headrail above the window opening or on the wall. The shade covers all sides of the window frame. This installation is like a blackout curtain panel, but it stacks neatly and insulates like honeycomb cells—it’s more practical than a regular curtain panel, I think.
The fabric covers the window casing, so it can hide uneven plaster lines and old, uneven frames—this is really good for older houses.
Outside Mount: Advantages
- It covers more on the sides and top, so it blocks light more effectively—this is way more obvious than inside mount.
- It’s suitable for shallow windows, French doors, and tilt-in sashes (the ones with little space).
- Installing the headrail a bit higher and wider can make small windows look bigger—this creates a better visual sense of space.
For wide windows, a wider headrail can cover uneven areas and still look neat. In many cases, this installation also prevents that halo of light around the frame at night—no more glare disturbing your sleep.
Outside Mount: Limitations and Install Considerations
- For many sizes, using more fabric means the price will be a bit higher—but it’s worth it for better light blocking.
- The shade might stick out beyond the window casing, changing the original outline—you should think about how it will look before installing.
- If you miscalculate the overlap width, light will still leak from the sides—you need to measure this carefully.
Installers usually aim for at least 1.5 to 3 inches of overlap on each side, and 2 to 4 inches of overlap at the top. For very wide windows, choose a product designed for broad openings, such as cordless extra‑wide blackout honeycomb shades, which stack neatly while covering more of the wall.
Light Blocking Performance: Inside vs Outside
Both installations can darken a room, but they work differently. Inside mount relies on accurate installation plus accessories to block side light. Outside mount relies on covering more area.
| Mount | Typical Light Control | Notes |
|---|---|---|
| Inside | With side channels, light blocking is very good—even close to total blackout. A friend of mine installed it this way, and it’s as dark as night during the day | Suitable for deep, square frames |
| Outside | With 2 to 3 inches of overlap on each side, light blocking is excellent—hardly any light leaks through | Hides frame flaws and uneven jambs |
Energy savings matter too. The U.S. Department of Energy says well-fitted window coverings can reduce heat loss, block drafts and radiant heat transfer, and make the room more comfortable. This benefit pairs well with blackout fabric—great for both good sleep and reducing glare. For specific tips, check out the DOE’s guidance on window coverings and energy.
Managing Gaps: Side Channels, Returns, and Valances
True total blackout isn’t just about the fabric—it needs a complete set of parts. Three details are key—miss one, and light might leak:
- Side channels: U-shaped tracks block the gap where cords and fabric move. This accessory is most useful for improving light blocking with inside mount.
- Returns or winglets: Small L-shaped pieces installed on the headrail or cornice block light from the top and sides.
- Valances and cassettes: A front cover hides the roller or headrail, reducing light leaks from the top. Though often used for roller shades, the same idea works for cellular shade hardware—I’ve seen this combination before, and it looks nice.
Also, painting the inside of the frame a dark color reduces reflection. On the flip side, painting the jambs bright white will amplify any leaked light—a lot of people don’t notice this.
Privacy, Aesthetics, and Room Use Cases
Different rooms have different privacy needs, so your choice between inside and outside mount should change too. You need to first look at the light during the day and night, think about your decor goals, and then match them to the window’s shape to find the right fit—that’s what I always advise people to do.
Bedrooms, Nurseries, and Day Sleepers
Even a little light can disrupt sleep in bedrooms. Studies show that light exposure before and during sleep can disrupt melatonin and circadian rhythms—so good light blocking in bedrooms is extra important. To learn more about this effect, check out Harvard Health’s overview on blue light at night.
That’s why many people who value sleep either choose outside mount with plenty of overlap or inside mount with side channels. Top-down bottom-up shades balance privacy and natural light, but how well they block light still depends on how tightly the edges are sealed—don’t overlook this.
If you want daytime privacy without losing that clean look, a top-down option like classic top down bottom up cellular shades is flexible and neat—I think this works well for people who stay at home during the day.
Media Rooms and Home Offices
Screens make even small light leaks more noticeable. Outside mount usually works better for reducing glare, especially for windows facing east or west—this is really obvious when the sun is strong in the morning or afternoon. But inside mount with side channels can also achieve similar light blocking while keeping the window trim visible. In offices, the neat look of inside mount pairs well with built-in furniture and bookshelves.
Measuring and Fit Guidelines
Accurate measurements are the foundation. A small measurement error can lead to light leaks or the shade rubbing against something later. But if you follow the steps, it’s actually not hard—I’ve taught people, and as long as you follow the steps, you can measure correctly.
Inside Mount: Depth, Squareness, and Clearances
- Measure the width at the top, middle, and bottom. Record the smallest number—otherwise, the shade won’t fit.
- Measure the height at three points too. Record the largest number—don’t let it be too short later.
- Check the depth. Many headrails need at least 1.5 inches of depth, and around 2 to 2.5 inches for a fully flush fit.
- Make sure the frame is square. If the diagonals differ significantly, there will definitely be gaps on the sides after installation—you’ll be able to see them with your eyes.
Also, check for locks, cranks, or tilt latches—these might hit the fabric or bottom rail. For renters or people who move frequently, if the dimensions fit, a drill-free or perfect-fit option can protect the window frame from damage.
For a visual guide on measuring, consult the step-by-step tutorial how to measure windows for shades.
Outside Mount: Overlap, Frame Obstacles, and Trim
- Add 1.5 to 3 inches to each side for overlap. If the wall is wide enough, add even more—this ensures better light blocking.
- Add 2 to 4 inches above the window opening to block top light leaks and secure the brackets to solid material—otherwise, they might fall.
- Decide where the bottom of the shade will end. Stopping at the windowsill looks neat; extending below it can block light under the sill.
Window trim varies in thickness, so make sure the part of the shade that sticks out won’t hit the trim. If the wall is uneven, use spacers or shims to level the headrail—otherwise, it will be crooked, look ugly, and leak light.
Installation Tips to Maximize Blackout
A neat installation plus the right accessories usually gets the light blocking to a “good sleep” level—I’ve installed this a few times, and it always works.
Mounting Height, Overlap Strategy, and Light-Sealing Add‑Ons
1) First, decide the installation height. For outside mount, install the headrail high enough to cover the top edge of the frame and the trim’s crown—otherwise, light will leak from the top.
2) Plan the overlap width. Use painter’s tape to mark the left and right boundaries to preview the look. Add more overlap where the light is strongest—don’t skip this step.
3) Add light-sealing accessories. For inside mount, install side channels tightly along the path the fabric moves. For either mount, consider adding a valance or small pieces to block top light leaks.
4) Test before fully tightening the screws. Slide the shade up and down, then check for light leaks at night. A small adjustment to the position can fix that final halo of light—this step is often the key.
Also, if you plan to repaint, use a dark or matte paint for the inside of the frame. This small change can significantly reduce reflection—I’ve tried it, and it really works.
Budget, Maintenance, and Warranty Considerations
Price depends on size, fabric, and accessories. Outside mount usually uses more fabric to cover the frame, so material costs will be a bit higher—but it’s worth it for better light blocking. Inside mount may need side channels for total blackout, which also adds a bit to the accessory cost. However, for most standard windows, installation time is similar, so labor costs don’t differ much.
Maintenance is simple. Use a vacuum’s soft brush to remove dust from the pleats. A hair dryer on low cool setting can smooth out small wrinkles from shipping—don’t use hot air, as it can damage the fabric. For wide windows, occasionally check the brackets to prevent sagging, which would cause light leaks. Also, thermal fabrics keep the room comfortable year-round, so many homeowners choose insulated options like 1.5‑inch heat‑insulated cellular blinds, which are great for drafty walls.
Warranty terms vary by manufacturer and seller, so buyers should check the coverage for cords, hardware, and fabric. Fast after-sales support is also important—if the headrail bends or the bracket warps, light will leak, and it will be a hassle if no one helps fix it.
Custom vs Ready‑Made and Long‑Term Care
Custom sizes fit irregular windows and reduce side light leaks from the start. Ready-made shades work for standard sizes, but they may leave larger gaps when used as inside mount—keep this in mind. Handle the shade gently during daily use to maintain the pleats’ shape and keep the rails aligned.
If you might move in the future, outside mount shades can be adjusted for installation in a new space—it’s hassle-free. On the other hand, inside mount shades are sized for one specific window, so they may not work in a new place—you need to think long-term about this.
Conclusion
Choosing between inside and outside mount isn’t about style—it’s about deciding how to block light based on the window’s shape. Deep, square frames work well with inside mount plus side channels for near-total blackout and a built-in look. Shallow, uneven, or extremely bright windows are better with outside mount and plenty of overlap for the darkest results. With accurate measurements, the right hardware, and suitable fabric, both installations can make the room comfortable and prevent glare on screens—that’s what I believe.
Frequently Asked Questions
Q: Which mount blocks more light for most windows?
A: Outside mount usually blocks more light because it covers the wall and window frame. Inside mount can match this performance if side channels and a top cover are used—I’ve seen both installations work well.
Q: How much overlap is needed for outside mount?
A: Many installers aim for 1.5 to 3 inches of overlap on each side and 2 to 4 inches at the top. For windows with strong light, wider overlap reduces side light leaks.
Q: Do top-down bottom-up shades reduce blackout performance?
A: They offer flexibility, but there may be a small gap in the middle. However, with quality rails and side channels, light leakage is minimal—hardly enough to affect sleep.
Q: What if the window is shallow?
A: Outside mount is safer. It won’t hit handles or tilt latches, and it still ensures good light blocking—I always advise this for shallow windows.
Q: Are cordless cellular shades safe for nurseries?
A: Yes. The cordless design has no dangling cords, so there’s no risk of strangulation. Many blackout fabrics also help babies or shift workers sleep well during the day—this is really practical.