With the energy price cap up 4% from 1 October 2026, to £1,723 a year for a typical household (Uswitch), “thermal blinds” are everywhere this autumn. The honest answer is that a good blind, well fitted and used at the right times, makes a window a bit warmer in winter and a lot cooler in summer sun. It won’t transform your bills, and anyone who tells you exactly how much it will save is guessing.

How a blind holds heat

Glass is the weakest part of most walls: heat inside the room passes through it much faster than through the wall around it. A closed blind doesn’t insulate like loft insulation, but it does two useful things:

  • It traps a layer of still air between the fabric and the glass. Still air slows heat transfer; the thicker and better sealed that layer, the more it helps.
  • It reflects some heat back into the room if the side facing the window has a metallised or reflective coating.

The weak point is the edges. Air cooled by the glass sinks and spills out of the gaps at the bottom and sides, pulling warm room air in at the top. A blind with big gaps around it is mostly decoration as far as heat is concerned.

What the evidence says

Independent testing of blinds and heat loss is rare. The clearest UK result we found comes from the University of Salford’s Energy House 2.0, a climate-controlled lab where full-size homes are tested. Researchers tested several roller blinds for Harris Parts, a blind manufacturer (so this was paid work for a company with a product to sell, which we think you should know). Every blind they tested reduced heat loss compared with standard double glazing alone. The best one, with a flexible metallised layer, cut the window’s U-value by 15% (Salford Energy House). The U-value is how fast heat passes through, so lower is better.

What that does and doesn’t tell you:

  • It’s one window in a lab, not a measured heating bill. Windows are only one of the ways a house loses heat, alongside walls, roof, floor and draughts, so a 15% better window is a much smaller saving on the whole house.
  • It’s the best blind in the test. Ordinary fabrics did less.
  • It assumes the blind is closed. A blind that’s open all evening saves nothing.

You’ll see bigger figures in retailer adverts and search results: percentages of heat loss “cut”, or a number of pounds saved a year. We haven’t found independent measurements behind them, so we don’t repeat them.

What blinds can’t do

  • Stop draughts. If you can feel air moving around the frame, a blind just hides the problem. Draught-proofing the window is cheaper and does more.
  • Make single glazing behave like double glazing. A blind helps most where the glass is worst, but secondary glazing or new windows are in a different league.
  • Work while open. Heat is lost through glass all day; the blind only helps for the hours it’s down.

Which blinds hold the most heat

From most to least effective, other things being equal:

  1. Honeycomb (cellular) blinds. The fabric forms rows of hollow cells that hold extra pockets of air. They’re designed for this job and usually sit in a frame or side channels with small gaps.
  2. Thermal or lined roman blinds. A thick face fabric plus a thermal or blackout lining is one of the warmest fabric options, especially fitted outside the recess with a generous overlap. See the roman blinds guide.
  3. Thermal-backed roller blinds. Rollers with a coated, reflective or “thermal” back. They’re cheap and simple, but fitted inside a recess they leave a strip down each side (see the roller blinds guide).
  4. Plain rollers, venetians and verticals. They help a little, but slats and louvres leak air through every gap, so don’t buy them for warmth.

The fit matters as much as the fabric. In order of how well they seal:

  • Perfect-fit or side-channel systems clip into the window frame itself, so the fabric sits close to the glass with almost no edge gap. They’re popular on uPVC windows.
  • Outside the recess with overlap. Extend the blind past the window opening on each side and below the sill, so cold air has further to travel to escape.
  • Inside the recess. This is the neatest look but leaves side gaps. Measure tightly (the measuring guide explains the deductions) and choose a fabric with a thermal back to make up for it.

Using them well

  • Close them at dusk in winter, and on cold, cloudy days in rooms you aren’t using.
  • Open them in the day when the sun is out, especially on south-facing windows. Winter sun through glass is free heat.
  • Mind the radiator. If there’s a radiator under the window, a blind that stops at the sill (inside the recess, or a short outside fit) keeps the radiator’s heat in the room. Long blinds or curtains hanging over a radiator send that heat up behind them, straight to the glass.
  • Watch for condensation. A closed blind keeps room warmth off the glass, so the glass gets colder and condensation can form on it overnight. Open the blinds in the morning, ventilate and wipe the glass dry. If the frames or walls are getting damp or mouldy, that’s a ventilation or damp problem a blind won’t fix, and fabric blinds will mark. The bathroom blinds guide covers fabrics that cope better with moisture.

Blackout blinds and summer heat

In summer the same idea works the other way. Sun through glass heats a room the way it heats a parked car. Which?’s scientific adviser recommends keeping blackout blinds fully closed on sun-facing windows during the hottest part of the day to stop that heat building up (Which?, August 2026). A blind with a white or reflective back facing the window reflects more of the sun than a dark one.

Blackout and thermal aren’t the same thing, though. Blackout means the fabric blocks light. Many blackout fabrics are also thick or coated, so they help with heat, but read the product description: a thin blackout roller with big edge gaps does little for warmth. For bedrooms where sleep matters too, the which blind for which window chooser and the child-safe blinds guide (cordless options for children’s rooms) are worth a look.

Is it worth paying for fitting?

A thermal roller or roman on an ordinary window is a normal DIY job. The fitting walkthrough applies unchanged. A fitter earns their money on the setups that seal best but are fiddly to get right: frame-fitted systems on uPVC (wrong screws or positions can damage the frame or void its guarantee), honeycomb blinds in large or awkward windows, and a run of blinds across a bay window. The DIY vs professional comparison and cost guide help you weigh it up.

Sources

We’ve linked the sources above so you can check them yourself (all checked October 2026):

Frequently asked questions

Do thermal blinds actually keep heat in?

Yes, a little. A closed blind traps a layer of still air against the glass, and that slows heat escaping. In a University of Salford lab test run for a blind manufacturer, the best roller blind cut the window's heat loss rate (its U-value) by 15%. That's for the window, not the whole house, so your heating bill falls by much less than that.

How much will thermal blinds save on my heating bill?

Nobody can honestly give you a figure for your home. It depends on your glazing, how many windows you have, how well the blinds fit and when you close them. Retailer figures like a fixed percentage or pounds a year are marketing estimates, not independent measurements. Treat blinds as a modest extra, after draught-proofing and glazing.

Is a blackout blind the same as a thermal blind?

No. Blackout is about blocking light; thermal is about holding heat. Many blackout fabrics are thick or coated, so they help with heat too, but a thin blackout roller with big side gaps won't do much. Look for a thermal or reflective backing, or a lined roman, if warmth is the goal.

Should I close blinds in summer to keep the house cool?

Yes, on sun-facing windows during the hottest part of the day. Which?'s scientific adviser recommends keeping blackout blinds fully closed on sun-facing windows at peak daylight hours to stop solar heat building up inside.