You bought blackout shades, lowered them on the first night, and there it is. A thin, glowing line of light running down both sides of the window.
The fabric isn't faulty. It's doing exactly what "100% blackout" promises. The light isn't coming through the shade; it's sneaking around it.
Most of the time, that comes down to one choice on the order page: inside mount or outside mount. It's one of the questions we hear most at Beasen Home, so let's settle it properly.
The Short Answer
Outside mount blocks more light. The shade sits on the wall and overlaps the window opening, so there's no open edge for light to slip past.
Inside mount looks cleaner, but it always leaves a small gap on each side. With the right fabric and a few add-ons, though, an inside mount can get surprisingly close to full darkness.
The rest of this guide explains why, and helps you pick the right mount for each room in your home.
What's the Difference Between Inside and Outside Mount?
Inside mount means the shade is installed inside the window frame, the recessed box around the glass. The headrail tucks into the top of that opening, and the fabric drops within it.
Outside mount means the shade is installed on the wall or trim above the window. It's made wider and taller than the opening, so it covers the glass plus a few inches of wall around it.
Same shade, same fabric. The only thing that changes is where it hangs, and that one detail decides how dark your room can get.
Why Inside Mount Shades Always Leak a Little Light
A shade inside a frame can't touch the frame. It needs a little breathing room on each side so it can roll up and down without scraping.
On top of that, the fabric is always slightly narrower than the headrail, because the brackets and motor sit at the ends of the roller. On our roller shades, for example, the visible fabric is about 13/16 inch narrower than the width you order.
Split that across two sides and you get a slim strip on the left and right. On a sunny morning, that strip glows. It's not a defect; every brand's inside mount shades have it.
Light can also find its way in at two other spots:
- The top, where daylight glows above the roller.
- The bottom, when the sill isn't perfectly level and the bottom bar can't sit flat against it.
Why Outside Mount Gets the Room Darker
An outside mount shade isn't squeezed into an opening, so it can be made bigger than the window. That extra fabric overlaps the frame on every side.
Light that slips around the edge of the glass hits the back of the shade instead of reaching your room. That overlap is the whole secret.
How much overlap you need depends on the style. Ourwindow measuring guide recommends adding 3 to 6 inches in total to the width and height for roller and zebra shades, and 5 to 8 inches for Roman shades. More overlap means a darker room, as long as your wall has space for it.
Inside Mount vs Outside Mount at a Glance
|
Inside Mount |
Outside Mount |
|
|
Light blocking |
Very good, with thin side gaps |
Best, with little to no edge leak |
|
Look |
Clean, built-in, shows off window trim |
Bolder; covers trim and can make windows look bigger |
|
What you need |
Enough frame depth for the headrail |
Flat wall space above the window |
|
Measuring |
Must be precise |
More forgiving |
|
Handles and cranks |
Can get in the way |
Shade hangs in front of them |
|
Best rooms |
Living rooms, kitchens, home offices |
Bedrooms, nurseries, media rooms |
When Inside Mount Is the Better Choice
Outside mount wins on darkness, but darkness isn't the goal in every room. Inside mount makes more sense when:
- You love your window trim. Inside mount keeps moldings on show and gives a tidy, custom-fitted look.
- A little glow is fine. In a living room or office, a thin edge of light keeps the space feeling bright rather than closed off.
- There's no wall space. Windows that sit tight against a ceiling, cabinet, or corner often leave no room for an outside headrail.
- Your frames are deep. A deep recess hides the headrail neatly, so nothing sticks out into the room.
Before you order, check the depth. As a general rule, we suggest going outside mount if your frame is less than 2.4 inches deep, or if a lever or crank sits where the shade would hang.
When Outside Mount Is the Better Choice
Go outside mount whenever "as dark as possible" is the top priority:
- Bedrooms and nurseries, where an early sunrise or a street lamp can wake you (or the baby) too soon.
- Media rooms, where a single streak of light can wash out the screen.
- Night-shift sleepers, who need midday to feel like midnight.
- Shallow or out-of-square frames, where an inside mount won't fit or will show uneven gaps.
- Windows with handles, cranks, or alarm sensors, since the shade simply hangs in front of them.
This matters more than it sounds in sleeping spaces. TheSleep Foundation explains that light at night can slow the body's melatonin production and interrupt sleep cycles, so even a thin strip of morning sun can cost you rest.
Already Have Inside Mount Shades? Here's How to Close the Gaps
Maybe you love the inside mount look, or your windows leave you no choice. Either way, you don't have to live with a glowing outline.
- Add side light blockers. Our Smart Light Blockers for Shades are slim tracks that run along the window edges and cover the side gaps, darkening a room by up to 97%. They attach with magnetic strips, so you can remove them to tilt windows or take out screens. They work with existing blackout shades, but they aren't suitable for stucco walls.
- Choose a shade style with smaller gaps. Not every shade leaks the same amount. Our cellular shade fabric is only about 3/16 inch narrower than the headrail, versus about 13/16 inch on roller shades, so a blackout cellular shade naturally sits tighter in the frame.
- Layer a curtain over the shade. A blackout drapery hung wide outside the frame catches whatever slips past the shade. Our Motorized 90% Blackout Drapery Curtains pair well over an inside mount roller, and both can close together on one schedule.
- Pick a cassette valance if it's offered. A cassette encloses the roller at the top, which helps cut the glow above the shade.
Does the Shade Fabric Matter as Much as the Mount?
Yes, and it's the part most guides skip. The mount controls the edges, but the fabric controls everything in between.
An outside mount with a 70% light filtering fabric will still feel bright. An inside mount with true 100% blackout fabric and light blockers will feel far darker. If you're unsure which level you need, our guide onblackout blinds vs room-darkening shades explains the difference simply.
For the darkest result, start with a 100% blackout fabric like these:
- Smart Motorized Roller Shades 100% Blackout Essential: simple, slim, and easy to install either way.
- Smart Motorized 100% Blackout Roman Shades – Strate: a softer, fabric-folded look that works beautifully as an outside mount in bedrooms.
- Smart Motorized 100% Blackout Zebra Shades 95mm Slate: adjustable bands for daytime, full darkness when closed.
You can browse every fabric and color in our Smart Motorized Roller Shades collection, too.
A Simple 3-Question Check for Every Room
Stand in the room on a bright morning and ask yourself:
- Does this room need to be truly dark? If yes, go outside mount, or inside mount with light blockers.
- Is the frame at least 2.4 inches deep, with nothing in the way? If not, outside mount is the easy answer.
- Do I want the trim to show? If yes, and darkness isn't critical, inside mount will look best.
Many homes end up with both: outside mount in the bedrooms and inside mount in the living spaces. That's completely normal.
Before you commit to a color, order ourblackout swatch samples and hold them up in your own light. And if you'd rather not drill a single hole yourself, ourinstallation service can connect you with a licensed pro.
Our Take
If darkness is the goal, outside mount wins, plain and simple. If looks lead the way, go inside mount and close most of the gap with light blockers or a layered curtain.
Whichever you choose, every Beasen shade is motorized. Set your blackout shades to close at sunset and open at your wake-up time throughApple HomeKit, Alexa, or Google Assistant, and you'll never wake up to a stray beam of sunlight again.
Frequently Asked Questions
1) Will a 100% blackout shade make my room pitch black with an inside mount?
Not completely. The fabric itself blocks all light, but the small gaps at the sides still let some through. Adding light blockers or layering a blackout curtain gets you much closer to full darkness.
2) How big are the side gaps on an inside mount shade?
It depends on the style. On our roller shades, expect a gap of just under half an inch on each side. Cellular shades sit much tighter, with only a sliver of space on each side.
3) Can I switch my inside mount shade to an outside mount later?
Usually not with the same shade. An inside mount shade is made to fit inside the opening, so it would be too small to overlap the frame once moved to the wall. You'd need a new shade ordered at the larger outside mount size.
4) Do outside mount shades look bulky?
Not when they're sized well. Mounting a few inches above and beyond the frame can actually make a window look taller and wider, and a cassette valance gives the top a clean, finished edge.
5) Do light blockers work with motorized shades?
Yes. Light blockers attach to the window frame, not to the shade, so they don't get in the way of the motor. The shade moves freely while the tracks cover the gaps at its sides.
