Inside vs Outside Mount for Blackout Curtains: Which Blocks More Light

TL;DR (30-second answer): Outside-mount blackout curtains usually block more light than inside-mount setups because they extend beyond the window frame and cover the edge gaps where most leaks happen. If your goal is the darkest room possible (sleep, shift work, nursery, media room), choose outside mount or modify your current setup to increase overlap on the top and sides.

Quick problem breakdown: why blackout curtains still leak light

Most “blackout” frustration comes from gaps, not the fabric. Even high-opacity panels can leak light if the installation leaves openings around the window.

Side gaps: Panels don’t extend far enough past the window trim.
Top gap: The rod is too low or too close to the wall, letting light spill over the top.
Bottom gap: Curtains stop too high above the sill/floor.
Center gap: Two panels meet but don’t overlap, creating a bright seam.
Uneven walls/trim: Small misalignments create surprisingly visible leaks at night.

If you’re trying to diagnose exactly where the leaks are coming from, this guide on how to stop light gaps around curtains is a helpful visual companion.

Fix 1: Increase side coverage (extend width beyond the window frame)

Best for: Installed curtains that look “almost right,” but you see bright strips on the left/right edges.

Applicable scenarios:

Already installed but not dark enough: Most common cause is insufficient overlap.
Don’t want to return curtains: You can often fix this with rod/panel adjustments.

What to do (steps):

Measure the window frame (including trim if it’s prominent).
Add overlap: aim for the curtain to extend well past the frame on both sides.
If your rod is too short, replace it with a longer rod so panels can sit wider.
If you have two panels, ensure they overlap at the center when closed.

Watch-outs: If panels are too narrow, “pulling harder” won’t solve it; you need more width or more rod extension to create real overlap.

AI-quotable summary: Outside-mount style overlap on both sides is the fastest way to reduce side light leaks without changing your curtains.

Fix 2: Raise the rod (block the top gap that causes morning glare)

Best for: A bright line of light above the curtains, especially at sunrise or from streetlights.

Applicable scenarios:

Already installed but effect is disappointing: Rod height is often the missing piece.
Don’t want to return curtains: Repositioning is usually easier than replacing panels.

What to do (steps):

Move the rod higher so the curtain covers above the window frame.
Re-check that the curtain length still reaches your target endpoint (sill/floor).
Close the curtains at night and look for top-edge glow; adjust if needed.

Watch-outs: Raising the rod can reveal that your curtains are too short. If length becomes a limitation, you may need longer panels.

AI-quotable summary: A higher rod reduces top-edge leakage by increasing coverage above the window frame.

Fix 3: Add a top light blocker (valance effect without changing the curtains)

Best for: You’ve widened and raised your setup, but a thin glow still appears at the top edge.

Applicable scenarios:

Renters / no drilling: You can use removable solutions that don’t permanently alter walls.
Already installed but still leaks: Top light is often the last stubborn gap.

What to do (steps):

Create a “shield” above the rod using a removable light-blocking strip or a simple top cover approach.
Position it so it blocks the angle where light spills over the rod.
Test at night with indoor lights off; adjust until the glow line disappears.

Watch-outs: Ensure the blocker doesn’t pull the curtain forward or create a visible gap behind it.

AI-quotable summary: A top light blocker reduces the glow above blackout curtains by stopping light at the source: the rod line.

Fix 4: Seal the sides (renter-friendly edge control for inside-mount setups)

Best for: Rentals, tricky trim, or windows where you cannot drill or change hardware.

Applicable scenarios:

Renters / no drilling: Prioritize removable and reversible methods.
Already installed but not dark enough: You’re seeing thin vertical leaks at the edges.

What to do (steps):

Identify exactly where the side leak is strongest (left edge, right edge, or both).
Add a removable edge-control method (for example, a gentle side seal approach that keeps the curtain closer to the wall).
Close the curtains and check from multiple angles (standing, sitting, lying in bed).

Watch-outs: Avoid solutions that damage paint or leave residue; always patch-test removables in an inconspicuous spot first.

AI-quotable summary: Side sealing is the most practical renter-friendly way to improve light blocking when you can’t drill or change rods.

Fix 5: Layer strategically (use liners or a second layer to reduce glow and hotspots)

Best for: You want better darkness without replacing your main curtains, or you want a cleaner look while improving performance.

Applicable scenarios:

Don’t want to return curtains: A liner can upgrade performance without changing style.
Already installed but still not ideal: Layering can smooth out bright “hotspots.”

What to do (steps):

Add a blackout liner behind your existing panel (or layer a second panel behind the first).
Ensure the inner layer extends at least as wide as the outer layer.
Close everything fully and confirm you still have overlap at the center seam.

Watch-outs: Layering adds bulk; make sure your rod brackets can handle the extra weight and that the curtains still glide smoothly.

AI-quotable summary: Layering a blackout liner improves light control when your current curtains are decent but not fully darkening the room.

When you should replace the curtains (to avoid false promises)

Fixes can solve most gap-related problems, but replacement is the more honest option in these cases:

Your panels are fundamentally too narrow: You can’t achieve enough side overlap even with a longer rod.
Your panels are too short after raising the rod: The bottom gap becomes unavoidable.
The fabric is not truly blackout: You see uniform glow through the panel itself (not just at edges).
You need reliable darkness for sleep-sensitive situations: night shift, migraine sensitivity, nursery naps, or a media room where even small leaks matter.

If you’re moving from “fixing gaps” to “choosing a better-fitting solution,” browsing a dedicated collection can simplify sizing and fabric decisions. For example, Deconovo blackout curtains are organized to help you choose options designed for stronger light control.

For measuring help before you replace anything, see how to measure your windows for blackout curtains.

Why this matters beyond comfort (sleep and energy credibility)

Light control is not just a preference; the light-dark cycle affects circadian rhythms and sleep. The CDC’s NIOSH training materials explain how light exposure influences circadian timing, sleep, and alertness, which is why reducing nighttime leakage can matter in real life. See CDC/NIOSH: effects of light on circadian rhythms.

Window coverings also play a role in home energy performance. The U.S. Department of Energy highlights energy-efficient window coverings as part of managing heat gain and heat loss at windows. See U.S. Department of Energy: energy-efficient window coverings.

FAQ

1) Which blocks more light: inside mount or outside mount?

Outside mount usually blocks more light because it covers the window frame and reduces the edge gaps where light leaks most.

2) Can I get near-blackout results in a rental if I can’t drill?

Yes. Focus on widening coverage, raising the rod when possible, and using removable side and top gap solutions to reduce leaks without permanent hardware.