Why Outside Mount Cellular Shades Are the Secret to Better Blackout (and How to Measure)

Most homeowners default to installing window treatments inside the window frame. It feels like the standard choice, offering a flush, built-in look that showcases the molding. However, window depth is rarely as cooperative as we’d like it to be. When you are dealing with shallow frames, crank handles that get in the way, or simply a desire for total darkness, outside mount cellular shades become not just an alternative, but the superior option. By mounting the shades on the trim or the wall surface above the window, you bypass the constraints of the casing entirely.

Choosing an outside mount isn't just a technical fix; it changes the entire aesthetic and function of the room. It allows for complete coverage, eliminating those annoying slivers of light that sneak in around the edges of inside-mounted blinds. For honeycomb structures specifically, this mounting style protects the fabric from getting crushed against window latches and provides a cleaner, more substantial visual anchor for the room.

A Lesson Learned the Hard Way

I learned the value of this mounting style during a renovation of a 1920s bungalow a few years back. The windows were charming, original double-hungs, but the actual depth of the frame was barely an inch. I initially tried to force an inside mount because I wanted to see the vintage trim. It was a disaster. The headrail protruded awkwardly into the room, and the bottom rail got hung up on the sash lock every time I tried to lower it. The result was a blind that looked like an afterthought.

I switched to honeycomb shades outside mount, attaching them directly to the face of the trim. The difference was immediate. The shade operated smoothly without hitting the glass or the hardware, and because the honeycomb fabric is naturally insulating, having it overlap the frame actually improved the draft protection. It taught me that fighting your architecture is a losing battle; sometimes you have to build over it to get the functionality you need.

The Blackout Advantage

If your primary goal is room darkening, outside mount cellular blinds are arguably the most effective solution on the market. With an inside mount, there is always a mandatory deduction in width—usually about 1/8 to 1/4 of an inch—to ensure the fabric doesn't scrape the sides of the window frame. That gap is where sunlight intrudes, creating a halo effect that can wake light sleepers.

Mounting the treatment outside the frame eliminates this gap entirely. By extending the width of the shade beyond the opening, you create a seal against the wall or trim. This is particularly effective with honeycomb fabrics, which are already opaque. When you overlap the window opening by two or three inches on each side, light has no direct path to enter the room. This setup turns a standard bedroom into a true sleep sanctuary.

Visualizing the Setup

A common hesitation involves aesthetics. People worry that mounting blinds on the wall will look bulky or hide too much of the window. If you look at outside mount cellular shades images, you will see that the profile of the headrail is surprisingly slim. Unlike heavy wood blinds or roman shades which stack thickly at the top, cellular shades compress into a very tight stack height. When raised, they virtually disappear, allowing you to expose the full view.

Reviewing photos of outside mount cellular shades can also give you ideas on how to manipulate the perception of window size. If you have small windows, mounting the shade several inches higher than the actual frame and extending it wider makes the window appear significantly larger. It is a classic interior design trick that adds grandeur to a room without the cost of structural renovation. When browsing design galleries or cellular shades outside mount pictures, pay attention to where the headrail sits relative to the ceiling; often, higher is better.

Measuring for the Perfect Overlap

Getting the measurements right for an outside mount is less stressful than an inside mount because you don't need millimeter-perfect precision to fit a recess, but you do need a strategy for coverage. The standard rule is to measure the width of the area you want to cover, not just the glass.

If you have nice trim you want to cover, measure from the outer edge of the left trim to the outer edge of the right trim. If you are mounting on the wall to cover a window with no trim (or ugly trim), add at least three inches to the total width of the window opening. This gives you 1.5 inches of overlap on each side, which is the sweet spot for privacy and light control. For the height, measure from where you want the headrail to sit down to the sill (or below the sill if there is no protruding ledge). Always add at least three inches above the window opening for the mounting brackets to ensure they have a solid surface to bite into, usually the header beam behind the drywall.

Dealing with Obstructions

One technical detail often overlooked with honeycomb blinds outside mount is projection. If you are mounting on the wall above the window, but the window has a thick sill or a crank handle that sticks out, the shade might hit it as it lowers. This causes the fabric to bulge, which ruins the look and damages the pleats over time.

The solution is simpler than it seems: spacer blocks or extension brackets. These are small plastic or metal pieces that push the headrail further out from the wall. Before ordering, measure how far your window handle or sill protrudes. If it sticks out two inches, and your shade’s headrail is only 1.5 inches deep, you will need spacer blocks to clear the obstruction. Most manufacturers offer these free of charge or for a nominal fee, but you have to know to ask for them.

Installation Nuances

Installing outside mount cellular shades is generally forgiving. You aren't fighting tight corners inside a frame. However, leveling is critical. Window frames in older houses are rarely perfectly level. If you align your brackets perfectly with the top of the window trim, your shade might hang crooked.

Use a long spirit level to mark your bracket holes rather than measuring up from the window frame. If the ceiling is sloped or the floor is uneven, the eye will naturally be drawn to the shade. Ensuring the headrail is perfectly level acts as a visual correction for the rest of the room. Once the brackets are up, the headrail snaps in, and the honeycomb fabric cascades down to cover whatever imperfections lie beneath.

The Verdict on Versatility

While inside mounts have their place, the outside mount offers a level of forgiveness and utility that is hard to beat. It solves the problem of shallow depth, fixes light leakage issues, and allows you to control the visual proportions of your windows. Whether you are trying to hide damaged casing or simply want the best possible blackout experience for a nursery or media room, shifting the mounting point to the wall or trim is a smart move. Just remember to account for your overlap and check for protruding handles before you order.

Frequently Asked Questions

Do outside mount shades look bulky?
Not usually. Cellular shades have a very low profile and tight stack height compared to wood blinds or shutters. If you mount them high enough, they essentially disappear when fully raised, keeping the window area looking open and uncluttered.

How much wider should the shade be than the window?
For optimal light control and privacy, aim for an overlap of at least 1.5 to 2 inches on each side (3 to 4 inches total added to the window width). If you are mounting directly onto the trim, simply match the width of the trim from edge to edge.

Can I install outside mount shades on a door?
Yes, outside mount is the standard for french doors or patio doors with glass inserts. You will likely need "hold-down brackets" at the bottom to keep the shade from swinging around when the door opens and closes.