· Michael Blair
Blackout fabric blocks light through the material. It does nothing about the gap at the sides, top and bottom, which is where nearly all the light in a supposedly dark room comes from.
Edges, not fabric
An inside-mounted blackout blind has a clearance gap down each side by necessity, and that gap admits a surprising amount of light.
Side channels that the blind runs in are the solution, and they are what separates genuine blackout from room darkening.
What actually works
Blackout roller or cellular shades with side channels, or an outside mount with generous overlap, ideally combined with lined curtains on a wraparound track.
Layering is what gets a room genuinely dark — one product rarely does it alone.
- Side channels are the difference between darkening and blackout
- Outside mount with generous overlap if channels are not possible
- Wraparound curtain track removes the side gap
- Address light spill at the top too
- Check for light from inside the room — standby LEDs
The top gap
Light above the headrail is the second source and is often overlooked. A fascia or a valance closes it, and an outside mount avoids it.
People fix the sides, remain frustrated, and never look up.
Light from inside
Standby LEDs on electronics, a smoke alarm indicator, and light under the door add up in a room that is otherwise properly dark.
It is worth checking with everything closed and the lights off before concluding the window treatment has failed.