Do Shutters Really Cut Heat Loss by 62%?

The heating goes on in Scotland around now, and with it comes the annual wave of window covering advertising built around a single number. Shutters that cut heat loss by 40%. Blinds that save 50%. This year we have seen 62%, alongside a promise of a 30% cut in household heating bills.

We sell shutters. We would love all of that to be true.

Some of it is. Shutters genuinely do reduce heat loss through a window, and there is proper laboratory evidence for it. But the gap between what has been measured and what is being advertised has become wide enough that a homeowner has no realistic way of telling the two apart. So here is the measured figure, where it came from, and three checks you can run on any claim before it costs you anything.

The number that has actually been measured

In 2023 the British Blind and Shutter Association commissioned Energy House Labs at the University of Salford to test what window coverings really do to heat loss. You can read the report yourself, and we would encourage you to, because everything below comes out of it. Energy House is a full-scale terraced house built inside an environmental chamber, where the weather can be held steady at anything from minus 12 to plus 30 degrees. It is about as close to a controlled experiment as this industry gets.

Eight products were tested against the same window: a modern sealed unit, 4mm glass, 16mm cavity, 4mm low emissivity glass, argon filled, in a PVCu frame. The bare glazing measured a centre pane U-value of 1.52 W/m²K. Here is what happened when each covering was closed.

Product tested	U-value W/m²K	Change
Bare glazing (baseline)	1.52	none
Roller blind, free hanging	1.32	13% better
Zip blind, standard fabric	1.17	23% better
External roller blind	1.20	21% better
Honeycomb blind, free hanging, foil insert	1.09	28% better
Honeycomb blind, fitted to the glass	1.00	34% better
Interior shutter	1.06	30% better
Zip blind, metallised fabric	0.96	37% better

An interior shutter came out at a 30% reduction. That is a genuinely useful result, and it is the honest ceiling for what anyone can currently prove about a shutter on a modern double glazed window in this country.

You will also see 28% and 33% quoted from this same study, and it is worth knowing why. The test panel performed slightly worse than its specification, so the researchers published a second, corrected set of figures alongside the measured ones. They were explicit about which to use, writing that “priority should be given to the ‘measured’ U-values”. The 30% is the measured one.

Three other things in that table are worth sitting with.

A shutter was not the best performer. A zip blind with a metallised fabric beat it, and so did a honeycomb blind fitted tight to the glass.

Every one of those figures is a centre pane measurement, and the report lists “how centre pane U-values relate to whole window heat loss” as a question still to be answered. It also carries a footnote that ought to appear beside every percentage in this industry and almost never does: the results “apply to the combination of blind and glazing tested”.

And the measurement has an error bar. The report notes that uncertainty in this test method is typically between 14 and 28%. So the shutter figure is around 30%, give or take, from a study its own authors labelled an Initial report. As far as we can find, no final version has been published since. A number quoted to the nearest whole percent, from a test nobody will show you, should worry you more than a number with an honest range around it.

Check one: better than what?

This is the check that does most of the work.

“62% better” is not a sentence until you know what it is being compared with. Better than an uncovered window is one claim. Better than a competitor’s shutter is a completely different claim, and it can be technically accurate while telling you almost nothing about your own house.

If a percentage appears without a stated comparison, it is not a measurement. It is a shape that looks like one.

Ask the question plainly: sixty two per cent better than what, on what glazing, at what outside temperature?

Check two: does the R-value line up?

Some marketing quotes an R-value improvement alongside a heat loss figure. R-value is resistance to heat flow, and it is the reciprocal of the U-value we have been using. That relationship lets you check the two numbers against each other with a calculator.

If a product improves R-value by 44%, the heat flowing through falls to 1 divided by 1.44, which is 0.694. That is a 30.6% reduction in heat loss, not 62%.

So if a page tells you the same product delivers a 44% R-value gain and a 62% cut in heat loss, both against the same comparison, those two numbers cannot both be right. One of them is measuring something other than what it appears to measure, and you are entitled to ask which.

This is not an obscure test. It takes ten seconds and it catches a surprising amount.

Check three: heat loss through the window, or the heating bill?

These get used interchangeably in advertising, and they are not the same thing at all.

Reducing heat loss at the glass by 30% is a real, measured, worthwhile improvement. It does not reduce your heating bill by 30%, because your windows are only one of the routes heat leaves your house.

The government’s own Buyer’s Guide: Choosing the best windows and glazing for your home puts it plainly: “18% of heat loss occurs through windows”. The same factsheet series puts around a third through the walls of an uninsulated home and up to a quarter through the roof. So the windows are the smaller share, and improving that smaller share by 30% cannot possibly take 30% off the whole bill. The arithmetic simply does not reach.

Any claim that converts a glazing percentage straight into a bill percentage has skipped a step that nobody has published the working for.

Watch out too for the word “guaranteed” sitting in the same sentence as “up to”. A guarantee is an absolute promise. “Up to” is the opposite of one. Putting both in one clause is a red flag, and it is precisely the sort of wording the advertising codes exist to catch.

Check four: can you see the report?

A certificate is not a report. A certificate is a single page with a logo and a headline number on it. A report tells you the test dates, the glazing specification, the temperatures, what the comparison product was, and what the researchers themselves said the results did and did not show.

The Salford work is published in full, and you can download it here. Anyone can read it, including the section where the researchers list what they have not yet tested. Single glazing is on that list.

That matters, because single glazed windows are where the biggest gains genuinely are. An old sash window loses far more heat than a modern sealed unit, so a shutter closed over one will do proportionally more good. We think the effect is real. We are simply not going to put a percentage on it, because as far as we can find, nobody has measured it in a UK laboratory and published the result.

If a supplier will not send you the underlying report, you have learned something.

What we tell our own customers

Shutters help. On a modern double glazed window, the measured figure at the glass is 30 per cent, with the test method’s own uncertainty around it. On older single glazing the effect is larger, though we cannot yet tell you by how much. Solid panel shutters do more than louvred ones, because a closed louvre blade still leaves a joint line.

Three conditions apply, and they are the ones the advertising tends to leave out.

The shutter has to fit properly. The insulating effect comes from trapping a still layer of air between the panel and the glass, and a badly fitted shutter with gaps around it lets that air circulate and carries the benefit away with it. This is one of the reasons we measure and fit everything ourselves.

The shutter has to be closed. An open shutter has a U-value of nothing at all. The gain arrives on winter evenings when the louvres are shut, and disappears the moment they are not.

And it works both ways round. The same layer that holds heat in during a Scottish January keeps solar gain out during a July heatwave, which for many of our customers with glazed extensions and garden rooms turns out to matter more than the winter benefit.

The short version

• The only published UK laboratory test of an interior shutter puts it at 30 per cent less heat loss at the glass, on modern double glazing, and you can read it in full.
• A shutter was not the best performer in that test. Two blind types beat it.
• Percentages without a stated comparison are not measurements.
• A 44 per cent R-value gain is a 30.6 per cent heat loss reduction, not 62 per cent. Check the arithmetic.
• Heat loss through a window and a household heating bill are different quantities. Government figures put windows at 18 per cent of a home’s heat loss.
• If nobody will show you the report, treat the certificate as marketing.

None of this means you should not buy shutters for warmth. We fit them for exactly that reason across central Scotland every winter. It means you should buy them on a number somebody can show you the working for.

If you would like to talk through what shutters would realistically do for a particular room, arrange a free home appointment or ask us for a brochure. We will tell you where they help, and where a different product would do more.

Frequently asked questions

Do shutters reduce heat loss? Yes. Independent testing by Energy House Labs at the University of Salford, commissioned by the British Blind and Shutter Association, measured an interior shutter reducing centre pane heat loss by 30 per cent on a modern argon filled double glazed unit. The shutter works by trapping a still layer of air between the panel and the glass. The full report is published by the BBSA and anyone can download it.

How much can shutters save on heating bills? Nobody has published a reliable figure, and you should be cautious of anyone who offers one. Government figures put windows at 18 per cent of a home’s heat loss, against around a third through the walls of an uninsulated home. Improving the smaller share by 30 per cent cannot take 30 per cent off the whole bill.

Are shutters better than blinds for insulation? Not always. In the Salford testing a zip blind with a metallised fabric reduced heat loss by 37 per cent and a honeycomb blind fitted tight to the glass reduced it by 34 per cent, both ahead of the 30 per cent recorded for an interior shutter. Shutters offer other advantages, including privacy, light control and durability, but on measured thermal performance alone they are not automatically the leader.

Do shutters work on single glazed windows? The benefit should be larger on single glazing, because an uncovered single glazed window loses far more heat to begin with. The Salford study did not test single glazing and listed it as an area for further research, so we do not quote a percentage for it.

What should I ask a supplier who quotes an energy saving percentage? Ask what the figure is compared with, what glazing was tested, whether it refers to heat loss at the window or to a heating bill, and whether they will send you the full test report rather than a certificate. If the R-value and heat loss figures are both quoted, check that they agree with each other.

Does the shutter have to be closed to insulate? Yes. The insulating layer only exists when the panels and louvres are closed, so the benefit arrives on winter evenings and overnight. A well fitted shutter with minimal gaps around the frame holds that layer of still air far better than a loose fitting one.