Every winter, there’s that one neighbor. It’s 25°F (about -4°C) outside, everyone else has pulled their curtains tight and cranked the thermostat, and she throws hers wide open around mid-morning like it’s the middle of July.
If you’ve ever driven past and thought she’s going to freeze in there, you had it backwards. She’s paying less to heat her home than you are. And she’s doing it on purpose.
She’s not forgetful. She’s not cold-blooded. She hasn’t got a space heater hidden behind those open panels. She’s figured out something about winter windows that most people get exactly wrong, and it’s quietly costing the rest of the block real money every heating season.
Almost nobody uses their curtains the way the government tells them to
Start with a number that will make you look sideways at your own living room. The U.S. Department of Energy says 75% of American households leave their window coverings in the same spot all day, every day. People pick a look they like, and that’s where the curtains stay: usually closed in winter, because closed feels warmer.
That one habit is quietly one of the most expensive things happening inside the average home in January. The DOE’s own tips page is blunt about it: curtains and blinds are supposed to be moved. Not once a week. Twice a day, on sunny days, and only on the side of the house that gets the most sun.
Windows leak. Every homeowner knows that part. The DOE puts the loss at around 30% of a home’s heating energy escaping through its windows. What almost no one talks about is the other direction. On a clear winter day, those same windows can pull in a serious chunk of free warmth, if you let them.
The number that changes how you look at your own windows
The clearest real-world math on this comes from a passive-solar home walkthrough by John Kraemer, an energy consultant who tracked his own house through a full winter.
His math, on a clear day near the winter solstice: 275 square feet (about 25.5 m²) of south-facing glass, with all the shades open, can bring in roughly 16,000 watts of solar heat. That’s the same as ten electric space heaters running at once, for free.
Even on a fully cloudy winter day, the sun’s strength drops about 5x. That same set of windows still delivers around 3,200 watts, or two space heaters’ worth of free warmth on a gray day.
Now flip the picture. Those same 275 square feet of south-facing glass, with the curtains yanked tight from sunrise to sunset because it “feels warmer.” All that free heat sits on the outside of the fabric and bounces right back out into the yard. The furnace covers the gap. Your gas bill covers the gap.
That’s the wrong-time-of-day mistake in one line. A closed curtain on a sunny winter window at noon isn’t insulating your home. It’s blocking a space heater the sun is trying to hand you.
Why the trick only works on one side of your house
This move only pays off on the right windows, and it’s a matter of how the sun moves in winter. In summer, it climbs high overhead, so south-facing windows get shaded by the roof and don’t take a straight hit. In winter, it sits low in the southern sky and drives right through south-facing glass for most of the daylight.
That’s why the DOE tips page says to open coverings on the south-facing windows on sunny winter days, not everywhere. North-facing windows barely see any direct winter sun, so those can stay closed all day. East and west windows get a slice of morning or afternoon sun. But south is where the free heat lives.
So if you’re the person who throws every curtain in the house open on a January morning, you’re not winning at anything. You’re just chilling down rooms that never got sun in the first place. She only opens the curtains on the sunny side.
The other half of what she’s doing, that nobody sees
Now the part that closes the loop, and it’s the reason the “always-closed” crowd isn’t fully wrong, just half-right.
The moment the sun drops, that same glass turns into a cold panel pulling heat right back out of your house. This is where curtains and shades actually earn their reputation as insulation. The DOE reports that snugly fitted cellular shades (the honeycomb-style ones) can cut heat loss through windows by 40% or more, which works out to roughly 10% off your total heating bill for the season. Even regular heavy drapes, pulled at dusk, trim nighttime heat loss by about 10% on their own.
So the neighbor who “keeps opening her curtains” is really doing a two-part daily move:
- Open the south-facing coverings by mid-morning so the sun can dump warmth into the house all day
- Close every covering in the house at sunset so the heat she just banked doesn’t leak back out through cold glass overnight
That’s the whole trick. Not a gadget. Not a smart blind. Not a $600 window film install. Two 30-second walk-throughs a day, on the right windows, at the right times.
The people paying the highest winter heating bills are usually doing the exact opposite: coverings shut in the daytime (blocking free heat) and sometimes left open at night (letting bought heat leak straight out through the coldest surface in the room).
Want to copy her exactly? The whole routine
Do this on any sunny winter day. It costs nothing.
- Mid-morning, once the sun’s fully up: open the curtains, blinds, or shades on every south-facing window. Pull them all the way clear of the glass, not just partway. Skip any window that’s blocked by a tree or a neighbor’s roof, it won’t be getting real gain anyway.
- Leave north-facing coverings closed all day. Those don’t get direct winter sun, so closed cuts heat loss without giving up any free warmth.
- East windows: open in the morning, close by early afternoon. They stop getting direct sun once it swings south.
- West windows: open by early afternoon, close at dusk. They catch the late sun right when the day starts cooling off.
- At sunset, close everything. Every window, every direction. The job flips from catching heat to holding on to it. Pull the curtains fully across the glass with no gaps at the sides or bottom. Cellular or honeycomb shades earn their price right here.
- On heavily overcast days, you can leave the south coverings closed if you want. You’ll miss a little free gain, but it’s a smaller number when the sky’s gray.
The whole thing takes under a minute, twice a day. Nothing to buy. On the sunny stretches of winter, this one habit can meaningfully cut what your furnace has to do.
Sources: (1) U.S. Department of Energy, Energy Efficient Window Attachments; (2) U.S. Department of Energy, Fall and Winter Energy Saving Tips; (3) John Saves Energy, Passive Solar Heating case study; (4) Green Building Advisor, A Quantitative Look at Solar Heat Gain.
