Blackout Curtains, Compared on Stated Light Blocking

The most checkable number on a curtain page is also the one most often left undefined. A listing offers 99% light blocking, 100% blackout, or room darkening, and those phrases sit side by side while describing three different levels of certainty.

One measurement, read in two directions

Light blocking and light transmission are the same measurement taken from opposite ends. AATCC TM203 — the standard test method for the light-blocking effect of textiles — is written to spectrophotometrically measure the light blocking properties of window-covering textiles against light passing through them, as seen by a standard observer on the darkened side of a curtain or drape. The two figures are complements: what is reported as blocked plus what is reported as passed through comes to 100.

That is why two panels can be quoted at 98% and 2% and be describing identical cloth. It also sets the limits of the method. TM203 runs in transmission mode and is not recommended for mesh constructions with holes over 3 mm across, so for a voile there is no valid figure to compare.

“Blackout” is a marketing term without a threshold

AATCC publishes the methods and does not publish a pass mark for the word. There is no agreed percentage at which a textile becomes blackout, so each supplier sets its own bar and its own method behind it. That is not a reason to ignore the figure — it is a reason to ask what sits underneath it. Is the figure for the fabric alone or for the finished panel including its lining? Which method was used? Was the sample flat or hung, and measured on one layer or the assembled pair?

The window’s own label is a separate system again. The National Fenestration Rating Council rates windows, doors and skylights for U-factor, visible transmittance, solar heat gain coefficient, condensation resistance and air leakage. The covering is not part of that rating, so the two labels have nothing to say about each other.

Rod pocket against grommet

The difference matters for light rather than for hanging.

A grommet panel is fixed at discrete points. The rings pull each grommet forward and away from the wall, and the panel is often drawn flatter than the same cloth on a pocket, which reduces how much cloth overlaps where the two halves meet. Grommets do let fullness be set on the rod, which matters on an off-centre window.

A rod pocket panel wraps the rod along its full width, and the pocket is a second layer of cloth across the head of the panel — usually why a pocket panel holds a fuller overlap. The cost is a stiff seam band: if the rod is thin for the pocket, the fabric spreads unevenly and the head shows banding instead of uniform folds.

Where the light actually gets in

Side leakage, and it is an installation problem rather than a fabric one. A panel that covers the glass exactly and stops at the frame edge leaves a lit strip on both sides, and no figure on the page changes that. Two things fix it: generous overlap at the centre, and a return onto the wall beyond the frame. Both are placement decisions made before fabric is chosen, which is why the measurements worth taking first come first.

Three specifications worth ordering to

The picks below are blocking tiers rather than products, because the number is what separates them and the number is what a page can misstate.

• If the room has to be genuinely dark. The specification is a stated figure at or above 99 per cent, attributed to a named method, on a lined construction. The limitation is what the page will not say: whether it was measured on the finished panel or on the cloth alone.
• If the bedroom wants darker rather than dark. A stated figure in the low nineties on a single layer, with a dimming sheer behind it, is the honest middle. It darkens a room enough for most sleepers and costs less than a lined pair. Where it fails is early summer, and in shift sleepers, for whom a gap anywhere is a gap.
• If light is arriving around the edges. A placement specification rather than a fabric one: panels overlapping at the centre by several inches, a return onto the wall beyond each side of the frame, and a hem that reaches the floor. Buy the assembly, not the fabric.

Whether panels come singly or as a pair decides whether that overlap is real, and fabric weight decides whether a single layer had the body to block on its own. Both are covered in the window edit.