Neither "blackout" nor "room darkening" is a legally defined term, which is worth knowing before comparing two labels. In general use, blackout means the fabric passes essentially no light — 99 to 100 percent blocked — and room darkening means a dense fabric that blocks around 90 to 95 percent, leaving a dim room rather than a dark one.
That difference sounds decisive. In a real room it is usually the least important variable.
Why blackout curtains disappoint
Hold a blackout panel up to a window and it blocks the light completely. Hang it on a rod and the room is still noticeably bright, and the natural conclusion is that the fabric was oversold.
It usually was not. Light is entering around the panel: down the sides where the curtain meets the wall, over the top above the rod, and under the hem. In a standard rod installation those gaps let in far more light than the fabric ever does.
Fixing the gaps
Mount wide and high. The rod should extend 15 to 20 cm past the window frame on each side, so the body of the panel covers wall rather than stopping at the glass. Mount it 15 to 20 cm above the frame, or at the ceiling. This costs nothing beyond drilling in a different place, makes the biggest single difference, and has the side effect of making the window look considerably larger.
Wrap the ends to the wall. A curved track, a bay-window rail or simple return brackets bring the panel edge into contact with the wall and eliminate the side leak entirely.
Overlap the panels by at least 10 cm in the middle, and hang them long enough to touch the floor.
Who actually needs full blackout
For most bedrooms, room darkening is enough, and there is a reasonable argument that a little morning light is good for waking. Full blackout earns its place where light is genuinely disruptive: shift work, small children, an east-facing bedroom in summer, or a street light directly outside.
The energy question
Blackout curtains do reduce heat transfer, and the effect is real if modest. A dense panel with a bonded coating creates a still air layer against the glass, which cuts summer solar gain and winter conduction loss.
The larger effect is in summer, and it depends entirely on closing them during the day — most of the heat in a bedroom at night arrived through the window as sunlight hours earlier and was stored in the walls and furniture. Blocking it before it arrives is far easier than removing it afterwards.
Getting a room genuinely dark
If you need proper darkness, layering is the answer rather than a better single panel: a blackout blind fitted inside the window recess, with blackout curtains mounted wide and high outside it. The blind handles the glass, the curtains handle the perimeter, and between them there is no path for light.
The short version
- Blackout blocks 99-100% through the fabric; room darkening 90-95%.
- Neither term is regulated, so read the description rather than the label.
- Most disappointment is light leak at the edges, not the fabric.
- Mount 15-20 cm wider and higher than the window. Free, and the biggest gain.
- Return brackets or a wrap-around track kill the side leak.
- For true darkness: a blind inside the recess plus curtains outside it.
Measuring for panels wide enough to do this properly is covered in the curtain measuring guide, and how high to hang curtains covers the mounting height in detail.
