Blackout blinds

Fabric is about sixty per cent of the job

People buy a blackout blind, install it, and are surprised to still be awake at 5:20 in June. The fabric did its job. The problem is that every roller blind must be slightly narrower than the opening it sits in, or it would bind on the way up, and that clearance is a slot of daylight down each side. At dawn, with the sun low and directly on that wall, the slot is bright enough to read by.

So the products that actually darken a room are the ones that address the perimeter: side channels that the fabric runs inside, a cassette that closes the gap at the head, and enough overlap at the sill. None of that is exotic. It is just the part that gets left off a quote to make the number look better.

What we check at the measure

The reveal depth decides everything. Channels need somewhere to land, and a shallow reveal — common in condos and in anything with a deep sill trim — may not have it. In that case the honest options are an outside mount that overlaps the whole opening by a good margin, or accepting a visible edge line. We will tell you which one your window is, and if the answer is that you cannot get true blackout in that opening, you will hear that too.

We also look at where the light is actually coming from. An east bedroom with a streetlight outside has two different problems and they are not solved by the same thing: sunrise is a perimeter problem, and a streetlight at head height is often a top-of-blind problem.

Blackout without the equipment look

Blackout is a lining, not an aesthetic. If you want the room dark but you do not want the room to look like a hotel, the lining goes behind something else:

• Roman shades with a blackout liner keep fold and texture on the face.
• Cellular shades with a foil liner block the light and slow the heat loss off the glass at the same time, which is worth having on a north wall in February.
• Drapery with a blackout lining and the returns wrapped to the wall handles the perimeter by geometry rather than by hardware.

Each of those trades a little darkness for a lot of softness. Which one is right depends on whether the room is for sleeping or for living, and we would rather ask that question than assume it.

Children’s rooms

Cordless is our default in any room a child sleeps in, and there is no upcharge for it. A spring mechanism or a motor replaces the chain entirely, so there is nothing hanging at a height a child can reach. Where an existing blind in the house still has a cord, a tensioner fixed to the wall is the minimum.

Made to your opening

Every panel is cut to the measurement taken in your home, not rounded to the nearest stock width and trimmed on site. On a blackout blind that difference is not cosmetic — a blind that is a quarter-inch narrow gives you a quarter-inch of daylight down each side for the next decade.

The measure is free, it takes under an hour, and it costs you nothing if you decide the answer is a stock size from a big-box store. Sometimes it is.

Blackout, room-darkening, dimout: the words are not interchangeable

Three terms get used as if they were one, and the gap between them is where most disappointment comes from.

Blackout describes a fabric that passes no light at all. Hold it up to a bare bulb and you see nothing through it — not a glow, not a hot spot. It gets there by being either a tight weave with an acrylic foam backing, or a fabric with a thin vinyl layer laminated inside it.

Room-darkening passes some light and is honest about it. A dense weave with no backing will look opaque on the roll and still glow at the window at noon. It is a perfectly good product for a living room. It is not what you want over a bed.

Dimout is a marketing word with no fixed meaning. When you see it, ask which of the first two it is.

There is a test you can do with a sample in your hand, and we bring samples so you can do it: put it flat against the window in daylight and look at it from 30cm away, in a room with the lights off. Blackout fabric goes the colour of the fabric. Anything else goes the colour of the sky.

The important sentence, though, is that none of this describes the blind. A genuine blackout fabric in an assembly with a 5mm gap down each side gives you a dark room with two bright lines in it. The fabric spec and the room result are different claims, and only one of them is worth paying for.

The room that has to go dark in the middle of the day

Night shift is a different brief from a bedroom, and it is worth saying so because the usual advice is written for sunrise.

At three in the afternoon the sun is high and strong, the whole wall is lit, and every weakness in the perimeter shows at once — not just the side gaps, but the sliver above a top-mounted roller and the line where the fabric stops short of the sill. Sunrise forgives more than midday does, because at dawn the light is coming in almost horizontally and a decent overlap at the sides catches most of it.

For a daytime sleeper the answer is usually the full assembly rather than a better fabric: channels down both sides, a cassette closing the head, and the blind running past the sill rather than to it. If the reveal cannot take channels, an outside mount with a generous overlap on all four sides gets closer than most people expect, and it costs less than the hardware.

We would rather set that expectation at the measure than after installation. A room can be made properly dark. It just takes the perimeter seriously, and the quote has to say so.

Side channels: what they cost and what they buy

If the perimeter is where a blackout wins or loses, side channels are the piece of hardware that settles it, and they are almost never offered unless you ask.

A channel is a slim aluminium section fixed to the reveal or the wall on each side of the blind. The edge of the fabric runs inside it, so there is no line of sight from the room to the glass at the sides. On a coated blackout in a properly channelled opening, the difference between the blind down and the lights off is close to genuine darkness rather than “dark enough”.

What they cost you is threefold, and all three are worth knowing before you commit.

Depth. Channels need the blind to sit far enough back that the fabric can enter them, so on a shallow reveal the whole assembly moves onto the wall face — which works, and looks like equipment rather than like a blind fitted to a window.

Friction. A fabric running in a channel does not free-fall. The blind is slower and needs a more deliberate pull, and a spring mechanism has to be tensioned for it. On a window operated many times a day that is a real change in feel.

Permanence. Channels are screwed to the reveal. Removing them later leaves fixings in the trim. In a rental or on original woodwork worth keeping, that alone can be the deciding fact.

Where they are unambiguously right: a bedroom for somebody who sleeps during the day, a nursery, a room with a projector, and any window where the alternative would be blackout curtains over the top of a blind that never quite worked.

Where they are usually not worth it: an ordinary bedroom used at night. The gain over a well-fitted outside-mounted blackout with a generous overlap is small, and the loss in feel and reversibility is not.

The gap at the top, which nobody mentions

Everyone talks about the sides. The head of the blind leaks more light than the sides do on most installations, and it is the one people are surprised by.

The mechanism has to sit above the fabric, so there is always a strip between the top of the cloth and the top of the opening. On an inside mount that strip is inside the reveal, above the glass, and light coming through the window travels down behind the blind and out into the room over the top edge — which is why a blackout can look perfect from the front and still put a soft glow on the ceiling.

Three fixes, in ascending order of effort.

Mount higher. The cheapest and most effective. Fixing the blind above the opening rather than inside it puts the head of the blind above the head of the glass, so there is no window behind the strip at all. This is why so many genuinely dark rooms have blinds that look slightly oversized.

A cassette. Wrapping the tube closes the path down behind the fabric. It needs depth, and on an inside mount it needs depth you may not have.

A pelmet or a deep fascia. Effectively a board across the front of the head. It is what a curtain pelmet has done for a century, and it works for the same reason.

If a room has to be properly dark, the order of operations is: mount high, overlap generously at the sides, then consider channels — not the reverse. Most rooms never need the third step if the first two are done deliberately.

Blackout in a place you cannot drill

Rentals, heritage trim, and steel frames all rule out the usual fixings, and this comes up often enough to be worth stating.

Perfect-fit frames. For uPVC double glazing with a rubber gasket, a slim frame clips into the bead around the glass and the blind lives inside it, touching the glass. No screws anywhere. Because the fabric sits inside a frame on all four sides, the light control is very good — better than a conventional inside mount — and it comes out when you leave, leaving nothing behind.

Tension fittings inside the reveal. Only sensible on narrow windows, and only where the reveal is square and sound. On an old opening they slip.

Free-standing over the recess. A blind on a batten fixed to a pelmet board that is itself held by removable fixings. Fiddly, and occasionally the only answer on protected woodwork.

What we would not do is fix a channel to original mouldings on a house where the trim is the point, and we will say so rather than take the order.

What it does to heat, in both directions

Nobody buys a blackout blind for thermal reasons and almost everybody notices the effect.

In July, on a west-facing window, a light-coloured blackout blind lowered before the sun comes around reflects a real fraction of the heat back through the glass before it reaches the room. A dark blackout blind absorbs it instead and re-emits much of it inward — the room still gets the heat, just later and more gently. Colour is doing that, not opacity, which is why the darkest fabric is not automatically the coolest room.

In February the effect runs the other way and comes with a caveat worth knowing. A well-fitted blind over a cold window creates a still pocket of air against the glass, which is genuinely useful insulation. It also means the glass runs colder than it otherwise would, and on an older window that is already prone to it, you can get more condensation on the pane, not less. If you have a window that streams in winter, say so at the measure — the answer is usually to leave a deliberate gap at the top, and it is easier to build that in than to retrofit it.

Last updated 11 August 2026.