“Drivers urged to put washing up liquid in cars” – why you SHOULDN’T do it

Right now, the internet is alive with tips from local and some major news outlets with the “amazing” advice to rub washing up liquid on your car windscreen, so as to avoid condensation.

But, please don’t – it’s a badly thought out solution, and here’s why…

Why you shouldn’t do this

Unlike those articles, I’m going to get to the point quickly.

Yes, there is a scientific basis for the washing-up liquid tip, but it’s important to understand how it works and its limitations. The technique is not about preventing condensation (the formation of water droplets) entirely, but about changing how the water collects on the glass.

Here’s the science: Why it works (kind-of)

Washing-up liquid contains surfactants (surface-active agents). When applied to glass, these surfactants perform two key functions:

1. Reduce Surface Tension: Surfactants significantly lower the surface tension of the water droplets that form on the glass.
2. Hydrophilic Layer: They create a temporary, thin, film or barrier on the glass that is more hydrophilic (water-attracting) than the bare glass itself.

Instead of water condensing into tiny, separate, light-scattering beads (which cause the fogging/misty effect), the water forms a thin, continuous sheet or film that is much more transparent. This effect is known as an anti-fogging or sheeting action. The resulting sheet of water is easier to see through and is also easier to wipe away.

Why the technique isn’t a permanent fix

While the surfactant action helps manage the water that does condense, it has significant drawbacks that explain why it isn’t a permanent or complete solution:

1. It doesn’t eliminate moisture: The washing-up liquid does not remove the moisture from the air inside the car (which is the root cause of condensation). It only changes the physical appearance of the resulting condensed water. The air is still humid, and condensation is still occurring.
2. Streaking and visibility: If you don’t buff the residue away perfectly, the film left behind can cause streaks or a haze, especially when dry or when light shines through it (like headlights), which can severely and dangerously obstruct your vision while driving. Considering this tip is about improving your driving vision, this is a serious side effect.
3. It’s a temporary effect: The soap film is not durable. It is easily worn away by wiping, or it will eventually wash or degrade, meaning you have to reapply it regularly.
4. Dirt attraction: The residue can attract and trap dust and dirt, which can build up quickly on the inside of the windscreen, creating a dirty film that is even worse for visibility than simple condensation.

What should you do?

The most effective, long-term solutions still involve reducing the humidity inside the car (e.g. using the air conditioning, ensuring no wet items are left inside, and checking for leaks) or using purpose-made anti-fog products designed to be streak-free and longer-lasting.

Where did this advice come from?

Although written differently, and by different people, they were mainly posted in just the last few days from local outlets all sharing an almost identical website. Dig a little further and you see they’re all part of Reach PLC (Aka the Mirror Group – and this is why some national publications are posting the same content).

Those local sites are all referencing an article from The Express, published just 2 days ago. That article lists no sources but makes liberal uses of expressions such as “according to experts”, without saying who those experts are.

The author is Claire Schofield, who I reached out to on Twitter for further details but, as yet, has not responded.

However, The Express seems pretty obsessed on this subject and only last year shared how a NASA scientist recommended you do it. Spoiler: he advises you to blast the windscreen with warm air from the heater. You know, the thing that’s there to do that exact job. Quite amazing.

Yet, it’s hard to see where this advice has suddenly sprung from. Previous to this flurry of re-posting over the last few days, it’s not really come up. Last year, The Mirror shared that using washing-up liquid on windows in the house can help with compensation. Otherwise, you have to go back to 2021 where a tip of mixing water, vinegar and washing up liquid in a spray bottle and using it for this purpose was evident on a few websites. By diluting it down, some of the side effects of the method will be reduced, and is probably better than the “pure washing up liquid” method now being suggested.

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So, in summary, don’t do it. Indeed, “this one amazing tip” type of advice, so common these days, is often to be viewed suspiciously. It’s either stupidly obvious and, if it’s not, there’s probably a good reason one (it’s simply not true).