You step out of the shower and the mirror has vanished behind a grey haze. You wipe a stripe with your palm. It clears for four seconds, then clouds over again. Bathroom mirrors fog because warm, damp air meets cooler glass, and the moisture in that air condenses into thousands of tiny droplets. Two approaches dominate the fix: build heat into the mirror so condensation never forms, or treat the glass so droplets cannot settle in the first place. Both work. They cost different amounts, suit different bathrooms, and ask different things of you.
How each option actually works
Heated mirrors
A demister pad is bonded to the back of the glass, essentially a flat, low-power heating element. Switch it on and the glass warms a degree or two above the surrounding air. Condensation needs a cold surface to form on, so the mirror stays clear while you shower.
Coverage matters more than most people expect. Many demister pads sit behind the centre of the mirror only, which is fine for a modest bathroom cabinet but can leave the outer edges misting over after a long, hot shower. If you have a wide mirror and a small, poorly ventilated room, look for one with a pad that covers most of the glass.
Anti-fog sprays and coatings
These work on surface tension. Instead of beading into droplets, water spreads across the treated glass as a thin, transparent film, so you can still see through it. The effect comes from a coating, and coatings wear off. A well-applied spray might last a week or two on a lightly used mirror, and rather less if the mirror is cleaned often.
Upfront cost: what you are really paying for
If you are replacing a mirror anyway, the price difference between a plain model and a heated one is often modest. The real cost appears when you already have a mirror you like and want to keep it. A retrofit demister pad is cheap on its own, but you will usually need a permanent power supply, which means an electrician, and that is where the money goes.
Anti-fog spray is the opposite: very little to buy, nothing to install, but you keep buying it. If you are unsure whether you need a heated mirror, a bottle of spray is a sensible experiment rather than a commitment. Give it a month and see whether you mind reapplying it.
Installation and wiring
Sprays involve no installation at all, but the application method decides whether they work. Clean the glass properly, let it dry, apply a thin and even coat, then buff it off with a dry, lint-free cloth. A heavy, wet coating is the usual reason people end up with smeary mirrors.
Heated mirrors are a small electrical job, and in the UK mains work in a bathroom is covered by building regulations, with specific zones around baths and showers. Use a qualified electrician rather than running a cable yourself. In practice the work usually involves:
- Choosing a supply — often a fused spur from the lighting circuit, sometimes a dedicated switched supply.
- Routing the cable neatly, usually chased into the wall or hidden behind tiles or a cabinet.
- Fitting a switch, pull cord or timer that is safe to use with wet hands.
- Checking the mirror's IP rating suits the zone it is being fitted in.
- Sealing the edges properly so steam cannot reach the pad.
If you cannot alter the wiring — a rented flat, a tiled wall you are not prepared to open up, or a mirror position with no practical route to power — a spray or an anti-fog cloth is effectively your only option.
Switch the demister on when you turn the shower on, not when you step out. The glass needs a few minutes to climb above the dew point, and a cold mirror will fog briefly no matter how good the pad is.
Running costs
A demister pad draws very little. It is closer to a small light bulb than a hairdryer, and if you run it for twenty minutes a day the annual electricity cost is small change in most households. You can work it out from the rating plate on the mirror: take the wattage, divide by a thousand, multiply by the hours you use it each day, then multiply by your unit rate from your energy bill.
What pushes the cost up is leaving it on. A mirror with no timer that gets switched on in the morning and forgotten until bedtime uses many times more than one that runs for twenty minutes. A timer, a pull cord with a built-in delay, or simply getting into the habit will keep it negligible.
Sprays cost pennies per application but the cost recurs forever, and heavy users get through a bottle quickly. Over several years a heated mirror usually comes out ahead on running costs, once you have absorbed the installation. That is a long payback, so treat it as a convenience decision rather than a financial one.
Real-world effectiveness
Heated mirrors are consistent. They do not need reapplying, they do not streak, and they work the same way on a January morning as in July. Their weakness is warm-up time and very steamy rooms: if your bathroom has no working extractor fan, a long shower can still beat a small pad, particularly around the edges of the glass.
Sprays are immediate and can be impressive on a clean, cool mirror. Their weaknesses are the opposite. They fade, they need a dry buff to avoid smears, and ammonia-based or abrasive cleaners will strip the coating and undo your work. Cheap products can struggle in a room that fills with steam.
Whichever you choose, deal with the ventilation as well. An extractor fan that actually shifts air, a window cracked open, or the door left ajar while you shower removes the moisture at source and makes both options perform noticeably better.
Which one suits your bathroom
- Replacing the mirror or renovating: a heated mirror with a timer, fitted by an electrician, is the tidiest solution. Do it while the wall is open.
- Keeping an existing mirror: an anti-fog spray costs far less than a retrofit pad plus wiring, especially if the cable route is awkward.
- Rented property or no spare circuit: spray, cloth or an anti-fog film. No drilling, no electrician, nothing to undo at the end of the tenancy.
- A busy family bathroom used several times each morning: heated, with a timer. Nobody wants to reapply a coating before the school run.
- A cold, single-glazed or poorly ventilated room: heated, plus a serious look at the extractor fan.
- Mirrors in more than one room: sprays travel. Demister pads do not.
Looking after whichever you choose
With a heated mirror, avoid harsh, abrasive cleaners on the glass edges and never chip or lift the seal at the back. Pads are sealed units and generally last for years; if yours stops clearing in the middle but not at the edges, or stops working altogether, check the switch and the supply before assuming the pad has failed, and check the warranty before you replace anything.
With sprays, keep to mild glass cleaner, a soft cloth and a microfibre buffing cloth kept separately for the final polish. Store bottles upright with the cap tight, and do not spray onto a hot mirror straight after a shower — apply it when the glass is cool and dry and it will last much longer.
A practical way to decide
- Fix the ventilation first. If steam clears quickly, both options improve and you may find a spray is enough.
- If you are keeping your mirror, buy one spray and use it properly for a month. If the reapplying does not bother you, you have your answer.
- If you are replacing the mirror, or already have an electrician in for other work, price a heated model with a timer and factor in the fitting.
- Either way, keep a squeegee within reach. Ten seconds of wiping handles the worst of the steam and makes any coating last longer.
For any mains wiring in a bathroom, or if you are unsure which zone your mirror sits in, get a registered electrician to advise — it is not a job worth improvising.
Photo: Vitaly Gariev / Pexels
