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How to Choose Blackout Curtains That Actually Block Light
Look for a 3-pass foam lining, not a dark colour. Width 2× the window. Hang 10–15 cm above the frame and 15–20 cm past each side or light leaks in stripes.
Updated 27 Aug 2026 · 6 min read · how to choose blackout curtains
Lining is the product; colour is vanity
3-pass blackout is foam-foam-foam on the back of the cloth: beige, grey, or black on the lining face, and it blocks. A navy unlined cotton at 200 GSM is a dimout at noon and a glow at 5am. Hold it to a lamp in the shop. If you see the bulb as a disc, it is not blackout.
Thermal blackout often adds a layer that is worth about as much as a 1 tog. It will not replace a summer or winter insert. It can knock a draught off single glazing.
| Claim | What you get | Buy? |
|---|---|---|
| 3-pass blackout | Nearly no disc of streetlight | Yes, for sleep |
| Dimout / room-darkening | 70–90%, glow at edges if poorly hung | Living room |
| Unlined heavy cotton | Mood, not blackout | If you like the cloth, add lining |
| “Blackout” 150 GSM printed poly | Often dimout plus smell | No |
Size is overlap, not the glass
Rail 15–20 cm past each side of the frame. Drop: to 1 cm above the floor, or a 2 cm kiss. Width of cloth: 2× rail for a wave, 1.5× if you accept a flatter hang. Two panels that meet with a 5 cm overlap beat one panel with a centre gap. The hang guide is curtain height.
The rest of the room still has to sleep
Blackout on a 4.5 tog night with percale is a summer cave. Blackout on a 13.5 tog night in a 12C room is winter. Neither curtain fixes a 3/10 mattress you hate — firmness — or a shiny sateen you overheat in. Light, heat, and loft are three purchases. Percale vs sateen is still the sheet one.
Questions, answered
- Will blackout lining make the room too hot?
- It can add a little thermal resistance and will trap solar gain if you leave it closed on a south window all day. Open them when you are not sleeping. It is not double glazing.
- Eyepins vs pole pocket vs rings?
- Rings or a wave track stack clearer of the glass, which is the blackout point. A tight pole pocket sits on the glass and leaks at the top.
- Can I just use a roller blind?
- A cassette blackout blind in a recess with side channels can beat cheap curtains. A £15 roller with 2 cm gaps on three sides will not.
