We Paid Two Labs to Test Our Thermal Curtains. Here's What We Learned

Most thermal curtain claims online are just claims. "Cuts heat loss by up to 50%." "Slash your heating bills." Rarely is there a test report behind them. So we sent our own fabrics and linings to two independent European laboratories and asked them to measure what really happens. Some of the results surprised us.

Who did the testing

We did not test these ourselves. We paid for the tests and the labs ran them, which is the only way the numbers mean anything.

• Winter: STFI, Germany. The Saxon Textile Research Institute, an accredited lab, put each material in a "heat box" (DIN EN ISO 8990). A real double glazed window sits between a 21°C warm side and a 6°C cold side, and the lab measures how much power it takes to keep the warm side at 21°C, with and without the curtain.
• Summer: Sonnergy France. A specialist solar materials lab measured how much light, UV and heat each material reflects and lets through (EN 410, EN 14500), then calculated how much of the sun's heat gets into the room (EN 13363-1).

Finding 1: the fabric hardly matters. The lining does.

We expected the heavier fabrics to win. They didn't. Every fabric and lining we tested on its own, from a dimout to a blackout to our Werringbone soundproof curtain, cut heat loss through the window by almost exactly the same amount.

STFI reports P2022 2412 (January 2023) and P2022 2412.1 (June 2023). Results within the margin of error, so treat 36 to 41% as one group, not a ranking.

You can feel the difference too. With a curtain and lining closed, the air next to the glass was around 6°C colder than with no curtain. That is cold being held at the window instead of drifting into the room as a draught.

Finding 2: in summer, the window side does the work

Summer is a different problem. It is about reflecting sunshine back out before it turns into heat. Here the white backing on our blackout linings and blackout fabric did the job, bouncing back 64 to 70% of sunlight.

Sonnergy France report 35/2022, June 2022. On single glazing the best result rises to around two thirds of the sun's heat kept out.

All four blackout materials also blocked 100% of light and 100% of UV, the part of sunlight that fades floors and furniture. Because the reflective side faces the window, the colour of the curtain on the room side makes no difference to summer performance.

Finding 3: blackout is not warmer than dimout in winter

A common assumption. In the heat box, our dimout and blackout fabrics scored the same (32%). Where blackout wins is summer, thanks to that reflective white back.

Finding 4: soundproof does not mean giving up warmth

Our Werringbone soundproof curtain scored 31% on its own and around 38% with a lining, in line with everything else. Quiet and warm are not a trade-off.

The honest bit: what it does to your heating bill

This is where a lot of marketing falls apart, so we worked it through properly. The lab percentages are for heat lost through the window only. Windows are only part of your home, so a 40% result is not a 40% cut in your bill.

Using the Ofgem price cap from 1 October 2026, a typical UK winter, curtains closed for half the heating hours, and the lab result knocked down by a quarter for real-world gaps around the edges, here is our estimate per 1.5 x 1.5 m window per year:

An estimate, not a promise.

So no, thermal curtains will not cut your heating bill by 30%, and be wary of anyone who says they will. What people actually notice is comfort: no cold draught falling off the window on a winter evening, and a bedroom that does not bake on a summer afternoon. One side effect worth knowing: because the glass stays colder, you may see a little more condensation on winter mornings. Open the curtains and let the room breathe.

How to get closer to the lab result at home

In the lab the curtain edges were sealed. At home, air escapes round the sides and over the top. These five things close that gap:

1. Go wider. Fit the track well past each side of the window so the curtain overlaps the wall.
2. Go higher. Fit above the window or on the ceiling so less air escapes over the top.
3. Go longer. To the sill or the floor, so cold air does not spill out underneath.
4. Add a lining. The single biggest winter upgrade in our tests.
5. Use a strong track. Lined curtains are heavy. A metal heavy-duty track keeps them closing fully.

And close them at the right time: at dusk in winter, and before the sun hits the window in summer.

Which set-up should you choose?

• Best value: a blackout fabric with a white back, no lining. 32% winter, around 50% summer, 100% blackout. Browse blackout curtain fabrics
• Warmest: blackout fabric plus Light Out lining. 40% winter.
• Quiet and warm: Werringbone soundproof curtain with Acoustic Thermal Blackout lining. Around 38% winter, 48% summer.
• Any fabric you love: your choice of face fabric with one of our blackout linings, such as Light Out or Drapewell. 36 to 41% winter.

Ask for the report

Whoever you buy from, ask to see the test report behind the claim. Every figure in this post comes from the labs' own reports, and we publish every page of them, along with the full results tables and test methods, on our thermal curtains page.