This 30-second curtain test shows if your radiators are wasting up to a third of their heat

You top up the gas, nudge the thermostat, and still the room feels cooler than it should. The boiler groans away in the background while you pull on another jumper and wonder where the warmth is disappearing to. Often, the answer is hanging right in front of you: your curtains.

One tiny change in how your curtains sit over a radiator can decide whether that heat ends up in the room or bleeding straight into the window. A 30‑second check on a chilly evening can tell you if you are losing up to a third of the warmth you pay for. No tools. No app. Just your hand and a bit of fabric.

The 30-second curtain test

Wait until the heating has been on for at least 20–30 minutes and the room should, in theory, feel cosy. Stand by a window where a radiator sits underneath. Close the door behind you so you are testing this room, not the whole house.

Now slide your hand down behind the curtain, between the fabric and the window, without touching the radiator itself. If the air rising there feels noticeably hotter than the air in front of the curtain, your radiator is warming the glass and the cold air by the window instead of the people in the room. That gap is acting like a chimney, taking heat where you do not need it.

Step back out and feel in front of the curtain, then over the top of it. If you notice a strong flow of warm air trapped between fabric and glass, rather than spilling into the room, your setup is wasting a serious slice of your radiator’s output. Long curtains that drape over the top of the radiator, pelmets that box heat in, and blinds that sit too close to the panel all make the problem worse.

“If your hand feels like it is in a hot pocket behind the curtain, that is your money going up the window,” says an energy adviser who runs home visits in the Midlands.

Why curtains can steal a third of your radiator’s warmth

Radiators do most of their work by moving air. Cold air sinks by the window, the panel warms it, and that air rises and rolls into the room. When a thick curtain hangs in front, the radiator is suddenly heating a narrow column of air trapped behind fabric, pointed straight at the coldest surface in the room: the glass.

Studies and energy trials suggest that badly placed curtains can divert 20–30% of the useful heat away from the room. The radiator is still using the same gas or electricity, but less of that warmth actually reaches your sofa or desk. In older homes with single glazing or draughty frames, the effect is even sharper, because that warmed air dumps its energy straight into the window and outside air.

There is a twist. Curtains themselves can be brilliant insulation when used properly. Closed at night, they form a barrier that slows heat escaping through glass. The trick is to let the radiator breathe into the room first, then let the curtain do the insulating. That is what the 30‑second test reveals: whether the fabric is acting as a partner or a thief.

How to fix a wasteful window in minutes

You do not need to buy designer drapes or rip out radiators. Small adjustments in height, width, and gaps make a surprising difference. Work window by window, using the test as your guide.

Start with the obvious clash: curtains draped directly over the top of the radiator. If you can, hook them back slightly with a simple tieback so warm air can escape under and around, not just behind. Even pulling them a few centimetres clear of the panel edge can change the airflow. If the curtain puddles on the windowsill in front of the radiator, train it to sit either above the sill or behind it, not sealing the gap.

For blinds that sit almost flush against a radiator, stop them just above the top of the panel in winter evenings rather than fully down, then close curtains over them. This leaves a slot for warmth to roll out while the fabric still traps cold air close to the glass. If your curtains are extra-long, a cheap heat‑deflecting shelf or even a narrow board rested on the radiator brackets (never fully covering the panel) can kick air into the room instead of up the window.

Simple habits help too. Open curtains fully during the day to let sunlight warm the room, then close them after dusk to keep that warmth in, making sure they do not smother the radiator. Close doors so the heat you release into the room does not sprint off down the hall or upstairs, forcing the boiler to work harder to maintain temperature.

Quick checks and fixes

• Run the 30‑second test on every radiator under a window.
• If the air behind the curtain is much hotter, pull the fabric clear of the panel edge.
• Use basic tiebacks, hooks, or even a neatly placed chair to hold curtains off the radiator.
• Stop blinds just above the radiator top and let curtains cover the rest.
• Consider a small shelf above the radiator to nudge rising air into the room.

Small adjustments, real savings

The numbers sound abstract until you pair them with your bill. If a typical radiator in a living room feeds £30–£40 worth of heat into that space each month in winter, losing a quarter of its output to the window could mean £7–£10 quietly disappearing. Multiply that by two or three radiators under long curtains, and you are looking at savings that pay for a few simple hooks or tiebacks within weeks.

It is not only about money. Rooms that heat evenly feel more comfortable at a slightly lower thermostat setting. Once you stop throwing warmth at the glass, you may find you can nudge the thermostat down by half a degree without noticing, or delay the heating coming on by fifteen minutes in the evening. The boiler cycles a little less, the house feels a little calmer, and you are not huddled under a blanket wondering why the radiator is “useless”.

Think of the curtain test as one of those quiet winter habits that adds up alongside draught‑proofing and sensible timer settings. You do it once, make a few adjustments, and the benefit rolls on every time the heating clicks into life. No gadget, no subscription, just better use of what you already own.

Curtain situation	What it does	What to do instead
Curtain covers radiator completely	Traps heat behind fabric, warms glass	Tie back or shorten so panel can breathe
Blind drops to radiator top	Blocks warm air from rolling out	Stop blind above radiator; use curtains for privacy
Heavy curtain, big gap at top	Heat races up to cold window	Add a shelf or deflector to push air into room

FAQ:

• Does this really make a noticeable difference? Yes. If your radiators sit under windows and your curtains block them, freeing the airflow can reclaim a chunk of lost heat. Many households notice rooms warming faster and staying comfortable on a slightly lower thermostat.
• Should I tuck curtains behind the radiator instead? If there is safe space and the fabric does not touch hot pipes or valves, tucking behind the panel so it hangs between radiator and wall can help. The key is to keep a clear path for warm air into the room.
• Are radiator shelves or deflectors worth it? They can be, especially under deep sills or long curtains. A simple shelf above the radiator helps direct heat into the room and reduces how much warmth hits the window.
• What if I rent and cannot change fittings? Use temporary tiebacks, adhesive hooks, or furniture placement to keep fabric off the radiator. You can also adjust blind height in winter evenings without making any permanent changes.
• Do I still need curtains if I have double glazing? Yes. Even with modern windows, glass loses more heat than an insulated wall. Curtains that are properly positioned help reduce that loss, as long as they do not smother the radiator underneath.