Do thermal curtains save energy?

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Heat leaves a room through a window three ways. Conduction through the glass itself. Convection — room air touching cold glass, cooling, becoming denser, and sliding down onto the floor as a cold current, pulling more warm air in behind it. And radiation from warm surfaces in the room toward the cold pane.

A thermal curtain addresses all three, but unequally. It adds a still-air layer that slows conduction a little. A reflective backing cuts radiation. Its biggest single contribution is interrupting the convection loop — and that is precisely the part that depends on how you hang it, not on what fabric you bought.

1. 1How well it's sealed at the top. An open gap above the curtain lets the convection loop run freely — warm air over the top, down the glass, out at the hem. A pelmet or a board closes it, and this single change roughly doubles what the same curtain achieves.
2. 2Whether it reaches the floor. A curtain stopping at the sill leaves the loop an exit. Floor length closes the bottom.
3. 3Your glazing. On single glazing the window is losing a lot and a curtain intercepts a meaningful share. On modern double or triple glazing there is much less to intercept, so the same curtain does proportionally less.
4. 4Whether you actually close them. A curtain open all night does nothing. This sounds obvious and it is the largest real-world variable — most of the theoretical benefit is lost to habit.

Treat any specific percentage in a product listing as marketing rather than measurement. A figure like "reduces heat loss by X%" usually comes from a laboratory test of the fabric in a sealed frame — not a curtain hanging in a real room with a gap at the top and a foot of air at the bottom.

What is defensible: thermal curtains measurably slow heat loss through glazing; sealed installations perform substantially better than loose ones; and the comfort improvement next to the window is obvious to anyone who sits there. What is not defensible: a dollar figure, a percentage off your bill, or a payback period.

Curtains over a radiator. A floor-length curtain hanging in front of a radiator under a window traps the heat behind the fabric and sends it straight up the cold glass. This is the most common way a thermal curtain actively costs you money. Use a sill-length curtain there, or a shade, or tuck the curtain behind the radiator's top edge.

Treating a leak as a cold surface. If air is actually moving, that is a gap in the frame and no curtain will fix it. Test with a candle flame or an incense stick on a windy day, and seal what you find. How to stop window drafts.

Closing them in winter daylight. A window in direct winter sun is delivering free heat. Leave those open until the sun is off the glass, then close everything. See summer vs winter curtain setup.

The panels that do this best are in best thermal curtains; the better-insulating alternative is in best insulated cellular shades.