My living room has one big window that felt like a cold spot all winter. I could feel the draft standing three feet away. I hung a set of thermal curtains in November, and my January heating bill came in a third lower than the same month the year before. The curtains cost about what one month of that difference was worth.

How thermal curtains work

Windows are the weak point in most rooms. Heat escapes through the glass, and cold radiates in. Thermal curtains add layers of insulation between the room and the glass. The best ones have a thick lining, often multiple layers, that traps air and blocks the cold from radiating inward.

What to look for

• Thick lining, not just dark fabric. A dark curtain is not automatically thermal. Check the backing. The thermal ones feel heavier and usually have a distinct lining layer.
• Full coverage. The curtain needs to be wider than the window and long enough to reach the floor or the sill, so cold air cannot flow around the edges. Mine overlap the window frame by several inches on each side.
• Rod pocket or grommet style. Both work. Grommets slide easier, rod pockets seal tighter at the top. I chose a rod pocket because less gap at the top.

How I use them

The curtains go open during sunny winter days so the sun helps warm the room, and closed as soon as the sun drops. That little routine matters: thermal curtains keep heat in, but they also block sunlight, so you want the sun when it is there.

I hung mine on a double rod with a sheer curtain behind the thermal one. During the day I can close the sheer for privacy without losing the light, and at night the heavy layer goes across. It looks intentional instead of like a blanket stapled to the wall.

If you rent or hate drilling, a tension rod inside the frame works for many windows, and draft stoppers along the bottom handle the gap under the curtain.

The room feels warmer and quieter too, since the heavy fabric deadens street noise. My heating bill did the talking, but the comfort sold me.