The curtain trick that keeps small rooms warmer without blocking natural light, according to insulation testers

The first winter in my flat, the smallest room was the one that made me dread bedtime. It had a big, pretty window and a radiator underneath, the classic set‑up estate agents call “light‑filled” and insulation testers call “a heat leak with curtains”. By 9 p.m., the walls felt cold to the touch and there was a slice of air by the window that might as well have been coming straight off the North Sea.

I did the usual things: thicker duvet, draught excluder at the door, turning the thermostat up “just for tonight”. Heavy curtains felt like the obvious answer, but every sample I held up turned that little room from bright to bunker. Cutting the light wasn’t an option. So I did nothing, and shivered.

Then a friend who works as an insulation tester came round with a thermal camera. He pointed it at the window, watched the colours swirl, and said something that stuck.

“You don’t need a darker curtain. You need a slower breeze.”

What fixed that room in a single afternoon wasn’t a new boiler or triple glazing. It was a see‑through curtain, hung in a very particular way, that behaved less like decoration and more like an invisible blanket.

The see‑through curtain that acts like insulation

We tend to think of curtains as one of two things: heavy and warm but gloomy, or light and pretty but useless against the cold. The testers who spend their days scanning homes with thermal cameras will tell you there’s a quiet third category.

A dense, floor‑length sheer or voile, hung snugly around the window, can cut the cold edge in a small room without dimming the daylight. It doesn’t “insulate” in the same way as a wall, but it does something surprisingly powerful: it slows the loop of cold air spilling off the glass and spreading across the floor.

In pictures, it looks almost too simple. A pale, tightly woven voile, fitted from near the ceiling to just kissing the sill or floor, wider than the window so it returns to the wall at each side. Not a frilly net half‑way down the glass, not a decorative panel floating mid‑air, but a full‑height soft barrier.

On a thermal camera, that soft barrier shows up as a calmer, more even band of colour. Instead of bright blue streaks of cold dropping from the frame, there’s a quieter gradient. Insulation testers like this trick because it costs very little, takes an hour to fit, and doesn’t ask you to live in the dark all winter.

The trick is to use a “day curtain” as a controlled air gap, not just as something you forget to dust.

What’s actually happening behind that fabric

If you’ve ever felt a mysterious “curtain of cold” by a window, you’ve already met the problem. Warm air in the room hits the cold glass, cools, gets heavier and drops. More warm air rushes in to replace it. That rolling loop becomes a permanent, invisible draught.

Bare glass makes that loop fast and noticeable. Heavy curtains help a bit, but in small rooms they often create a pocket of warm air trapped between radiator and fabric, while the rest of the room stays stubbornly chilly. Insulation testers see this all the time: radiators blasting away at the back of a curtain, while the bed two metres away sits in a cold pool.

A dense sheer hung correctly changes the game. It creates a thin, still layer of air between fabric and glass. The cold air dropping off the window slows down as it meets the curtain, mixes more gently with room air and doesn’t rush along the floor in the same dramatic way. You feel less of a “slice” and more of a gradual gradient.

In tests on awkward small bedrooms and box‑room offices, assessors commonly see:

• 1–2°C higher air temperature near the window once a full‑length sheer is installed.
• Radiators cycling less often because the thermostat isn’t constantly chasing a cold pocket.
• Fewer complaints of “cold feet but hot head” – that classic sign of strong convection loops.

You still lose some heat through the glass, of course. But you lose less comfort, which is what your body actually notices.

How insulation testers hang curtains when they’re off‑duty

People who test insulation for a living tend to be quietly fussy about their own curtains. The set‑up they return to again and again looks like this:

• A slim, ceiling‑fixed track or pole, starting a few centimetres from the ceiling.
• A sheer or voile that is at least 20–30 cm wider than the window on each side.
• Fabric that reaches the sill at minimum, ideally the floor, with a gentle break.
• A second track or pole further into the room for heavier night‑time curtains, if you want them.

By day, the sheer is fully closed, giving privacy and that crucial air layer, while the heavier curtains are stacked right off the glass on either side so every drop of light can come in. By night, the heavier curtains close over the sheer for extra warmth.

A few details matter more than you’d think:

• Coverage at the top: Gaps above the pole are escape routes for warm air. A ceiling‑fixed track, or one mounted high with the curtain almost touching the ceiling, works best.
• Returns at the sides: Let the fabric overlap the wall by a hand’s width. This stops cold air sneaking round the edges.
• Length: Voiles that stop half‑way down the window look neat but do little for comfort. Full‑height fabric calms the cold at floor level, where your feet live.

Insulation testers sometimes use the phrase “soft secondary glazing” for this set‑up. It’s not glass, but it does mimic the idea of trapping a calmer layer of air next to the window.

Choosing the right “warm but bright” fabric

Not every sheer will do the job equally well. The ones insulation testers recommend share a few traits:

• Tightly woven, slightly weighty: Think of the hotel‑style voiles that hang in a soft vertical line, not the lacy net that puffs at every breath of air.
• Light colour: White or pale neutral fabrics bounce daylight around the room, making even small spaces feel bigger.
• Matte rather than shiny: Shimmering fabrics can create glare; a matte finish gives a softer, more even light.

You don’t need specialist “thermal voiles” for this to work, although some people like them. A simple, good‑quality polyester or cotton‑blend voile, cut generously, will often do the job.

If you already have heavy curtains you love, you don’t need to replace them. You’re adding a working layer behind them. Think of it as giving your window a base layer under its jumper.

Why this matters more in small rooms

Large living rooms can absorb a bit of discomfort. A cold spot by the patio doors is annoying, but you can move your chair. In a small bedroom or study, there’s nowhere to escape to. The bed is against the outside wall, the desk is under the window, and every draught feels personal.

Heavy curtains that cover radiators can make this worse. In a box room, closing thick drapes at 4 p.m. might trap most of the heat on the window side of the fabric. The room you’re actually sitting in stays lukewarm while the glass and curtain have a sauna together.

By contrast, a full‑height sheer:

• Lets the radiator heat the room, not just the window.
• Softens the cold edge along the wall, making the whole space feel more even.
• Protects your sense of daylight, so you don’t end up living with the lights on at noon.

For renters, there’s another bonus: it’s reversible. A slim track fixed into the ceiling or just above the frame is usually an easier conversation with a landlord than major glazing upgrades.

How to try the curtain trick in a single afternoon

You don’t need to turn your room into a building‑physics experiment. A straightforward, light‑touch approach works.

1. Measure generously

   • Width: window plus at least 20 cm each side.
     
   • Height: from just below the ceiling to the floor (or sill if floor‑length truly won’t work).

2. Fit a simple track or pole as high as you can
   Ceiling‑fixed tracks are ideal, but a high wall‑mounted pole close to the ceiling still helps. Aim to minimise the gap where warm air can shoot over the top.

3. Choose a dense, pale voile
   Look for words like “privacy voile”, “sheer panel” or “day curtain” with photos that show a firm vertical drape, not something flimsy and net‑like.

4. Hang so it just brushes the floor or sill
   If it’s floor‑length, a tiny break on the floor is better than a hovering hem that lets cold air pour underneath.

5. Do a quick draught test
   On a breezy day, stand by the closed sheer with a thin tissue or a small candle. You should feel and see less movement near your legs than you did with the window bare.

You can add your existing thicker curtains on a second pole in front, or leave the sheer as your main dressing if you prefer a clean look.

Small tweaks that quietly boost the effect

Insulation testers who use this trick at home tend to stack a few other small habits around it:

• Close at dusk, open at dawn
  Let the winter sun warm the room when it’s out by pulling any heavy curtains right off the glass. Once it’s dark, close them to trap some of that warmth in.

• Avoid blocking heat sources
  If your radiator is under the window, keep heavy curtains from puddling on top of it. Either shorten them to the sill or use tie‑backs until night‑time.

• Mind the trickle vents
  If your window has built‑in vents at the top, don’t smother them completely. Fresh air matters. You can angle the track a little forward so air can still move.

Here’s a quick picture of what “good enough” looks like:

Element	What to aim for	Why it helps
Height	As close to the ceiling as practical	Reduces warm air escaping over the top
Width	Window + 20–30 cm each side	Blocks side draughts
Fabric density	Tightly woven, soft drape	Creates a calmer air layer

Things to watch out for

No house trick is perfect, and this one has a few caveats.

If your windows are very prone to condensation, pinning a cold, damp voile directly against the glass can encourage mould. Leave a little breathing space between fabric and pane, and pull the voile back fully now and then to let the frame dry. Washing it regularly also helps.

If you use unflued gas heaters or dry clothes in that small room, moisture will be higher anyway. In those cases, this curtain trick is best paired with:

• Short, sharp bursts of ventilation (windows on latch for 10–15 minutes).
• A small dehumidifier if you can stretch to one.

And if you feel strong draughts rather than just a general coldness, you may have gaps around the frame that need proper sealing. A curtain can soften the symptom, but it can’t replace a sealant gun.

A quick guide you’ll want to pass on

What surprised me most about that small room wasn’t how cold it had been, but how quickly it stopped feeling like a fridge once the new sheer went up. The daylight barely changed. The mood did.

The insulation tester who first waved his thermal camera at my window summed it up neatly later:

“You’re not trying to turn glass into a wall. You’re just asking the cold air to calm down.”

For many small rooms, that’s enough. A simple, see‑through curtain, hung with a bit of intent, can turn the sharp bite by the window into a background murmur – and let you keep the brightness that made you love the room in the first place.

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FAQ:

• Will a sheer curtain really make a noticeable difference to warmth?
  In insulation tests on small, leaky rooms, a full‑length dense voile typically lifts the temperature near the window by 1–2°C and, more importantly, removes the sharp “cold edge” that makes rooms feel draughty.
• Does this work with double‑glazed windows, or only single glazing?
  It helps with both. Single glazing shows the biggest change, but even modern double glazing can create uncomfortable convection loops that a sheer will calm.
• Do I still need heavy curtains if I use this trick?
  Not necessarily. The sheer alone improves comfort by day. Heavier curtains over the top at night will add extra insulation, but the core benefit comes from the well‑fitted day curtain.
• Won’t floor‑length curtains make my small room feel smaller?
  Surprisingly, pale, full‑height fabric often makes rooms feel taller and more intentional. Because they spread daylight evenly, the overall effect is usually softer and more spacious, not cramped.
• Is this better than window film or secondary glazing?
  Film and proper secondary glazing can cut heat loss more, but they’re less flexible and harder to remove in rentals. A dense sheer is a low‑cost, reversible step that improves comfort straight away and works well alongside those upgrades if you add them later.