Why experts advise closing the curtains from 7:45pm onwards

A seemingly minor move at the window can end up saving a surprising amount of money and heating energy.

As daytime temperatures rise outdoors, many homes still feel noticeably chilly by evening. A common reaction is to turn the heating back up-then question why the direct debit has crept so high. Energy specialists point to a low-effort habit that costs nothing: close your curtains in good time in the evening, particularly from mid‑March, when the sun doesn’t set until around 19:45.

Why looking at your windows can make or break your heating costs

Windows are typically the weakest point in a building’s thermal envelope. Even high-spec triple glazing generally insulates less effectively than a solid external wall. That’s exactly why this tip works: if you let sunlight warm your rooms during the day and then reliably draw blinds and curtains at night, you effectively create a “second wall” that slows heat escaping.

Curtains act like an extra insulating air layer, reducing heat loss through the glass and keeping warmth in the room for longer.

UK energy-saving advocate Martin Lewis has highlighted this for years. His routine is straightforward: keep curtains and blinds fully open in daylight so the sun can enter freely. As soon as the sun has gone, close everything. Because sunset shifts through the seasons, this “switch-over” point lands at roughly 19:45 in spring, from around mid‑March.

19:45 from mid‑March: why the timing is so effective for curtains and heating

From the second half of March, the days lengthen noticeably. In practical terms, that gives your living spaces more hours to gather free solar warmth. Shortly after sunset, however, the balance flips: the window starts losing more heat to the outside than it gains from any remaining daylight.

Leave curtains open too long and you are, quite literally, heating the street. Draw them and you trap a portion of the warmth your home has collected throughout the day.

• By evening, sunshine has warmed furniture, floors and walls.
• Those surfaces release heat gradually back into the room air.
• Closed curtains limit how quickly that heat escapes through the glass.
• As a result, the boiler or radiators can run later-or at a lower setting.

Studies looking at thermal curtains indicate that identical rooms can differ by several degrees depending on whether insulating curtains are used. If you live in an older property with ageing windows, you’re likely to notice the benefit even more.

How much difference do thick curtains actually make?

Not every fabric helps in the same way. A light voile may look attractive and provide privacy, but it does little for insulation. For heat retention, the material and construction matter far more.

The denser, heavier and more layered the curtain, the stronger the insulating effect-ideally creating a still air pocket between fabric and window.

Best curtain choices for a warmer evening at home

• Heavy cotton or velvet curtains: a traditional “warmth buffer”, especially common in period properties.
• Thermal curtains with a specialist backing: designed to reflect heat back into the room.
• Curtains with fleece or thick lining: create a noticeable insulating layer at the window.
• Blind plus curtain together: combining both tends to boost results, particularly across wide glazing.

Fit matters as much as fabric. Curtains need to cover the whole window rather than sitting decoratively at the sides. If you use a track or pole, position it so the curtain hangs close to the wall or frame to reduce drafts-while avoiding blocking the radiator.

Everyday tweaks that amplify the curtain trick

People regularly share this method in forums and on social media, with many saying they can turn the heating down earlier in the evening once they become consistent. Others pair it with a few small add-ons that help the room hold temperature.

Measure	Effect in the room
Close curtains consistently from 19:45	Temperature drops more slowly; comfort lasts longer
Keep radiators clear of fabric and furniture	Heat circulates better; less energy required
Replace worn window seals	Noticeably less draught; curtain effect improves
Open everything by day and let sun in	Free solar warmth; heating can pause at times

With energy prices still high, many households try to make each kilowatt-hour work harder. A common approach is lowering the thermostat by 1–2 °C and offsetting that with smarter ventilation and deliberate curtain use. The logic is simple: each degree lower can save roughly 6% on heating energy-if you also slow heat losses, you stay comfortable for longer.

What the physics says about curtains, glass and heat loss

At windows, three processes happen at once: heat conduction through the glass, heat radiating to colder surfaces, and air movement (convection) along the pane. A well-fitting, dense curtain influences all three.

Fabric intercepts the cold air that sinks down the glass, reduces air circulation near the window, and reflects some radiant heat back into the room.

This creates a relatively still layer of air in front of the window-air insulates well as long as it isn’t being stirred up. Think of it as a simplified, domestic version of adding another insulating layer: you reduce the flow of heat to the outside.

When curtains are better left open (or adjusted)

There are a few situations where you need to be careful. If a radiator sits directly under the window, curtains must not wrap around it. Otherwise the heating ends up warming the space behind the curtain, while the rest of the room gains far less benefit.

If that describes your layout, consider these alternatives:

• Shorter curtains that finish above the radiator.
• Side curtains for coverage plus a close-fitting roller blind at the glass.
• Thermal window film on the pane as an additional layer.

Also be aware of condensation. When indoor and outdoor temperatures are far apart, moisture can collect on the glass if air cannot circulate behind tightly closed curtains. Opening curtains briefly in the morning and wiping any damp areas-especially in bedrooms-can help prevent mould issues.

How to get the best results in your own flat or house

If you’re unsure whether it’s worth investing in thicker curtains, try a couple of quick checks. On a cool evening, place your hand near the window surface, then feel the air directly behind a closed curtain. The difference often makes the case immediately.

You can also run a simple comparison with a basic thermometer on the windowsill: measure with curtains open, then repeat with them closed. Within 1–2 hours you’ll usually see how much the temperature drop is slowed.

The payoff tends to be greatest in rooms you use most in the evening-living rooms, children’s bedrooms and home offices. In those spaces, every degree that doesn’t leak out through the glass adds up to a lower energy demand and a noticeably cosier feel when you’re on the sofa after mid‑March, while the outside air cools again.

Two extra upgrades that work well alongside closing curtains at 19:45

If you want to push the benefits further without major work, focus on drafts and control. First, check for cold air around frames and sills: inexpensive draught-proofing strips can reduce infiltration and make the insulating “air pocket” behind curtains more stable.

Second, consider how your heating is scheduled. A simple programmable thermostat (or smarter TRVs on radiators) can align heat output with real use-so when you close curtains at 19:45, your system doesn’t keep overcompensating for losses you’ve already reduced.