Do thermal curtains really keep a room warmer? Close them and see

Pull the curtains on a cold evening and the room feels cosier. Part of that is just the feeling of being shut in. But part of it is real: a window is the thinnest, leakiest part of your wall, and a curtain can slow the heat escaping through it. How much it helps depends less on the word "thermal" on the label than on a few things you can see and change.

Why a window loses so much heat

Glass is thin and a good conductor, so warmth passes through it far faster than through a wall. Builders describe this with a U-value: the watts that leak through one square metre of window for every degree of difference between inside and outside. Lower is better. Single glazing is roughly 5 to 6, while double glazing falls somewhere between about 1.2 and 3.7 depending on its age and build.

The room side of the glass also gets cold, which is why you can feel a chill when you sit next to a window, even with the heating on.

What a curtain actually does

A curtain adds two things: the fabric itself, and a thin layer of still air trapped between the fabric and the glass. Still air is a surprisingly good insulator, but only while it stays still. If cold air can slip in at the top, sink down the glass and spill out under the hem, it circulates like a slow conveyor belt and carries your warmth away.

That is why fit matters as much as fabric. In a 2019 lab test by Consumer NZ, floor-length curtains beat sill-length ones, gaps at the top and sides let cold air circulate, and heavy lined curtains kept in more heat than thermal-backed ones. Cellular (honeycomb) blinds did best of all, which fits the same idea: they trap air close to the glass.

What the model shows

Play with it for a minute and three things stand out. First, the same curtain does far more for an old single-glazed window than for modern double glazing, because there is simply more heat to hold back. Second, going from "gaps at the top and sides" to "sealed to the frame" often saves more than going from a plain lined curtain to a heavy one. Third, the glass behind a closed curtain gets colder, which is where condensation starts.

That last point is worth a habit rather than a purchase. Close curtains at dusk, and open them in the morning, especially on sunny days when the sun is free heat. A damp room with tightly closed curtains and cold glass is how mould spots form on frames and sills, so keep some ventilation going.

Measure before you buy

A thermal curtain only works if it is big enough to cover the window properly. Measure these before you shop:

• Window width, frame to frame. The rod or track should extend roughly 10 to 15 cm beyond each side so the curtain overlaps the wall, not just the glass.
• Height from the rod position to the floor. Rod about 10 to 15 cm above the frame, or close to the ceiling; for warmth, let the hem fall to within about 1 cm of the floor.
• What sits below the window. If there is a radiator under it, measure the sill depth and the gap to the radiator. Long curtains hanging in front of a radiator trap its heat behind them and can be a fire risk, so end sill-length there, or hang them clear of the radiator.

Try the free options first

You may not need new curtains at all. Hang the ones you have closer to the glass and nearer the floor, add a second layer, or sew or clip in a lining. A charity shop or second-hand listing is a good source of heavy, interlined curtains that were made to last. A draught-proofing strip around the frame helps too, because it stops the cold air before it reaches the curtain.

What to look for

Across budgets, a short checklist covers most of what matters:

• Lower budget (around €25–45 per panel): look for "lined" or "insulating" in the description, and check the size is wide enough that the curtain can hang in soft folds rather than stretched flat. Enough drop to reach the floor matters more than a fancy label.
• Middle (around €45–80): a heavier fabric or a thicker lining, and a width that lets you overlap the wall on both sides.
• Higher budget: interlined or made-to-measure curtains, plus a rod or pelmet that keeps the top closed and curtains that return to the wall at the sides.

Whatever you choose, the measurements above decide whether it works as well as the model suggests.