Stop the Light Leaks: Why Outside Mount Is the Professional Choice for True Blackout

Achieving a truly dark room is often more complex than simply purchasing a product labeled "blackout." Many homeowners invest in high-quality fabrics only to find that streetlights or early morning sun still intrude into the bedroom or media room. The issue rarely lies with the fabric itself but rather with the installation method. While inside mounts offer a streamlined look, they are inherently prone to the "halo effect"—slivers of light escaping around the edges. For those prioritizing light control, outside mount blackout shades are frequently the superior technical solution.

Understanding the Light Gap Problem

To understand why mounting placement matters, one must look at the mechanics of a window frame. An inside mount sits within the recess of the window. However, manufacturers must deduct a small fraction of an inch from the blind's width to ensure the hardware fits and operates without scraping the jamb. This deduction, while necessary for operation, creates a gap on the left and right sides.

In a standard living area, this gap is negligible. In a nursery or a home theater, however, that sliver of light can be surprisingly intense. Outside mount blackout blinds bypass this constraint entirely. By attaching the treatment to the wall surface or the molding surrounding the window, the fabric can overlap the opening, effectively sealing off light leaks.

When to Choose Outside Mount Over Inside Mount

While the primary driver for this installation style is light control, there are structural reasons to opt for an outside mount. Many older homes feature shallow window casings that lack the depth required to house the headrail of a modern blind. If a window frame depth is less than two inches, an inside mount might protrude awkwardly into the room.

Furthermore, windows with obstructions such as crank handles, alarm sensors, or heavy glass molding often make inside mounting impossible. In these scenarios, room darkening shades outside mount configurations allow the treatment to clear these obstacles smoothly, ensuring the shade hangs flat and operates correctly.

Strategic Measuring for Maximum Coverage

The effectiveness of an outside mount relies entirely on the measurements taken. Unlike inside mounts, where you measure the exact opening, outside mounts require a strategy regarding "overlap." Standard industry recommendations suggest adding at least two to three inches of width to each side of the window frame (four to six inches total). This overlap is critical.

If the blind is only the exact width of the window trim, light will bleed through the side at an angle. By extending the width, you force the light to travel a longer, more obstructed path, which usually stops it before it enters the room. Similarly, mounting the headrail several inches above the top of the window frame helps minimize light leakage from the top while simultaneously creating the optical illusion of taller ceilings and larger windows.

Personal Experience: The Nursery Challenge

I encountered a specific case recently that highlights the necessity of this approach. A client was struggling with a toddler who refused to nap during the day. The room had beautiful, thick molding, and the client initially insisted on inside mount solar shades to show off the woodwork. However, the window faced directly south. Even with blackout fabric, the light gaps on the inside mount acted like laser beams in the afternoon sun, illuminating the room enough to keep the child awake.

We switched tactics and installed outside mount blackout shades. I specified a measurement that extended four inches past the molding on both sides and mounted the headrail four inches above the frame. The difference was night and day—literally. The fabric acted as a seal against the wall. The room became pitch black at 2:00 PM, and the nap time struggles ceased immediately. It demonstrated that functionality sometimes has to take precedence over the desire to display window trim, especially in sleeping quarters.

Selecting the Right Style for Outside Mounting

Not all blind styles translate well to an outside mount. Because the hardware is fully visible, the aesthetic impact of the headrail becomes a significant factor. Roller shades, for example, can look utilitarian if the roll is exposed. When selecting outside mount blackout blinds, it is advisable to choose options that include a cassette or a valance. This housing hides the fabric roll and the brackets, providing a finished, furniture-like appearance.

Cellular (honeycomb) shades are another excellent option for this application. Their construction offers high insulation value, and the headrails are typically low-profile. When mounted outside, the side-to-side density of the cellular fabric does an exceptional job of trapping light against the wall, provided the overlap is sufficient.

Addressing the "Side View" Concern

A common hesitation regarding outside mounts is the side view. Since the blind projects from the wall, you can see behind the shade if you are standing at an angle. This exposes the brackets and the gap between the shade and the wall.

To mitigate this, professional installers often utilize "return" pieces. If you are using wood or faux wood blinds, a return is a small piece of valance material that wraps around the side of the headrail, touching the wall to hide the hardware. For fabric shades, the solution is often positioning. By mounting the shade high enough and wide enough, the angle of view into the gap is minimized. Additionally, ensuring the shade unrolls from the back of the roller (closer to the wall) rather than the front (waterfall style) significantly reduces the distance between the fabric and the wall, tightening the light seal.

Installation Considerations: Anchors and Spacers

Installing room darkening shades outside mount requires verifying the substrate you are drilling into. Since you are likely drilling into the drywall above or beside the window, rather than the solid wood of the jamb, using proper drywall anchors is non-negotiable if you cannot locate a stud. The leverage exerted by pulling a heavy blackout shade can rip standard screws out of drywall over time.

Furthermore, if the window has protruding molding, you may need spacer blocks. These are small plastic or wood shims placed behind the mounting brackets to push the shade further out from the wall. While this clears the molding, it does increase the light gap. In such cases, the width overlap mentioned earlier becomes even more vital to compensate for the increased distance from the wall.

The Final Verdict on Light Control

While inside mounts offer a sleek, architectural fit, they are rarely the correct choice for total light exclusion. By embracing the outside mount, homeowners gain control over light gaps and can accommodate difficult window shapes. The key lies in generous measurements—specifically the width overlap—and selecting a headrail style that complements the room's decor while hiding the mechanical components. For those sensitive to light or heat, the functional benefits of an outside mount far outweigh the aesthetic compromise of covering the window trim.