The 19:45 Rule: How Thermal Curtains Cut Heating Bills

With the start of spring, many people are reluctant to turn down the heating even as temperatures outdoors become milder. Once darkness falls, homes cool down and the thermostat switches on again. A remarkably simple routine at around 19:45 can make a difference here – without a new heating system, smart-home gadgets or major alterations.

Why checking the time suddenly matters

In March, daylight lasts longer, the sun remains higher for more of the day and warms indoor spaces. Windows act rather like a magnifying glass: they admit the sun’s rays and warm the air inside, but the glass also releases that heat back outside later on.

“Closing the curtains at the right time in the evening considerably slows heat loss through the windows and saves energy.”

In many areas, the sun sets at roughly 19:45. From that point onwards, the effect reverses: windows become colder and the heat stored indoors escapes. Leaving curtains open then effectively means “heating” the outdoors.

The trick is to let the sun do the work during the day, then seal the room at the right time in the evening. This keeps free daytime warmth indoors for longer, meaning the heating comes on later – or may not need to come back on at all.

How much can this really save on heating bills?

Windows are considered one of the biggest weak points in a building’s thermal envelope. Even modern glazing transfers considerably more heat outside than a well-insulated wall. Curtains are not insulation in the strict sense, but they create an additional layer of air.

That air layer has measurable benefits:

• The surface temperature of the window falls more slowly.
• Cold air dropping down from the window is reduced.
• The room feels warmer, even when the thermometer reading barely rises.

A perceived increase of just one to two degrees in “comfort temperature” may be enough to lower the thermostat slightly. Every degree reduced saves around six per cent of heating energy on average. Over a full month, that can quickly amount to several dozen pounds, and considerably more in a poorly insulated house.

The key difference: ordinary curtains versus thermal curtains

Not every fabric hung in front of a window serves the same purpose. Thin decorative curtains may look attractive, but retain very little heat. Purpose-made thermal curtains, by contrast, are designed to keep warmth inside or prevent summer heat from entering.

Type of curtain	Typical material	Thermal protection effect
Lightweight decorative curtain	Thin cotton, voile	Barely noticeable
Heavy fabric curtain	Thick cotton or polyester blend	Noticeable reduction in draughts
Thermal curtain	Multi-layered, often with fleece or a thermal layer	Significantly less heat loss

Thermal curtains are generally made with several layers, sometimes including a coating or lining of fleece or wool. They provide a stronger visual and thermal seal around the window area, which is particularly noticeable with older windows that offer poor insulation.

Everyday costs and savings potential

A high-quality pair of thermal curtains can easily cost close to £100. That may seem expensive initially, but it becomes more reasonable when their lifespan is considered.

• Lifespan: several years, often substantially longer than a heating system operates unchanged
• Use: useful every day of the year – for warmth in winter and protection from heat in summer
• Savings: fewer heating hours and less use of air-conditioning units

In many cases, this additional barrier is enough to avoid using the heating in the bedroom altogether at night. Anyone who maintains this throughout an entire heating season gradually recovers the investment through the kilowatt-hours saved.

How the 19:45 rule works in everyday life

The benefit depends on routine. Waiting to close the curtains until the room already feels chilly wastes some of the potential. A fixed approach around sunset works better.

• Open curtains fully during the day so that as much sunlight as possible enters the room.
• Keep an eye on the time in the early evening – in March, this is often around 19:45.
• Close all curtains and, where fitted, shutters no later than dusk.
• If necessary, turn the heating down slightly and check whether the comfort temperature remains the same.

People living in a flat without shutters benefit especially from thick curtains. In homes with modern triple glazing, the impact is smaller but still noticeable, particularly in rooms with large window areas.

Summer bonus: keeping heat out rather than cooling it away

Thermal curtains are not solely a winter solution. Many models work almost in reverse during summer by keeping heat outdoors. The principle is similar, but the timing is shifted.

“Air the room briefly in the morning, then close the curtains – this keeps the midday heat outside for considerably longer.”

Anyone who airs their home thoroughly early in the day while the air is cool, then closes the windows and curtains, prevents walls and furniture from heating up too much during the day. This reduces the need for fans or portable air-conditioning units. Here too, the curtain has an indirect effect on the electricity bill.

Common mistakes that cancel out the effect

Many households have curtains but use them incorrectly from an energy perspective. A few common pitfalls are easy to avoid:

• Hiding radiators behind floor-length curtains – the heat remains trapped between the curtain and the window.
• Leaving curtains open after it is already dark and cold outside.
• Using short, narrow curtains that do not cover the entire window area.
• Keeping windows permanently tilted open while expecting thermal protection.

Ideally, a curtain should cover the entire window and overlap the frame slightly, reach to just above the windowsill, and hang freely in front of the radiator without completely covering it.

How to estimate your own savings effect

Those who are curious can begin with a simple comparison: on two similarly cold evenings, leave the curtains open once and close them consistently at sunset on the other occasion. An inexpensive room thermometer or smart thermostat can show how quickly the temperature drops.

If the temperature falls more slowly with the curtains shut, the next step is to turn the heating down a little further. If the comfort temperature remains unchanged, the direct impact on energy consumption becomes clear – especially in rooms used frequently in the evening, such as the living room or children’s bedroom.

Why March is the ideal time to make the change

The period around the start of spring is perfect for getting used to this new routine. Conditions outside are becoming milder, while indoors even light thermal protection is often enough to avoid running the heating at full output. Anyone who adopts the 19:45 rule now will benefit automatically next autumn.

The combination of longer daylight, an ongoing heating season and moderate outdoor temperatures turns a seemingly ordinary action into an effective way to counter rising energy prices. A pair of thermal curtains and a glance at the clock are all it takes to turn solar warmth into a genuine reduction on the next bill.