Where the Water Sits Tells You Which Problem You Have

The short answer: how to stop condensation on windows depends entirely on which side of the glass the water is on, and that is the question almost nobody asks first. Water on the inside is a humidity and ventilation problem, and it is the fixable one: get your indoor relative humidity down to 45 to 55 percent and it stops. Water on the outside is not a problem at all, and in fact means your windows are insulating extremely well. Water trapped between two panes means the sealed unit has failed, and no amount of ventilation, heating or dehumidifying will ever touch it. Two of those three states cannot be fixed by any of the advice you will find online, which is why so much of it seems not to work.

Disclosure: this post contains affiliate links, and as an Amazon Associate I earn from qualifying purchases. If you buy through them I may earn a small commission at no extra cost to you. I only recommend things I would use in my own home.

Three kinds of condensation, three different answers

Almost every guide on this treats condensation as one problem with a list of solutions attached. It is three problems that produce the same visible symptom, and they need completely different responses. Before trying anything, put your finger on the glass and work out which one you have.

Inside the glass: humidity, and the fixable one

If you can wipe the water away with your hand, it is on the room side and it is condensation from your own indoor air. Warm air holds more moisture than cold air, so when air in the room meets the cold surface of the glass it cools rapidly and gives up that moisture as droplets.

This is a ventilation and humidity issue rather than a window fault, which is worth stating because people frequently conclude their windows have failed. Well insulated, airtight homes are actually more prone to it, since the moisture your household produces has fewer routes out.

Everything in the rest of this article addresses this version, because it is the one you can do something about.

Outside the glass: your windows are working

Water on the outer face of the glass confuses people and it is genuinely good news. It forms when the outer pane becomes colder than the surrounding air, typically overnight after a clear cold night, and dew settles on it exactly as it settles on a car roof or a lawn.

The reason the outer pane is that cold is that almost no heat is escaping from inside to warm it. In other words, exterior condensation is a sign that your glazing is performing extremely well. It burns off within an hour or two of sunlight and there is nothing to fix.

The only thing worth knowing is that shade reduces it, so trees or shrubs near a window mean less morning dew on the glass. That is a landscaping observation rather than a repair.

Between the panes: the unit has failed

If the misting is inside the sealed unit, meaning you cannot wipe it from either side, the window itself has broken. Double glazing works because the gap between panes is sealed and usually gas filled, and once that seal fails, humid air gets in and condenses in a space you cannot reach.

This is the case where all the standard advice is wasted effort. Ventilating the room, running a dehumidifier, opening the curtains and heating differently make no difference whatsoever, because the moisture is not in your room. The unit needs replacing, and often only the glass rather than the whole frame, which is considerably cheaper than people fear.

So the diagnosis takes ten seconds and it decides everything. Wipeable from inside means read on. Wipeable from outside means congratulations. Not wipeable at all means call a glazier and ignore every other tip you have read.

Work out your fix in 3 taps

Where the water is, which room, and what your household does. Those three decide whether this is a habit change, a purchase, or a phone call.

Where the moisture actually comes from

Interior condensation is an arithmetic problem: your household puts water into the air and the air has to give it up somewhere. Knowing the quantities tells you which habit is worth changing, because they differ by an order of magnitude.

The top two rows carry most of the problem in most homes, and the first is the one worth acting on immediately because it is the largest single figure and the easiest to change. Five pints per load is a bathtub’s worth over a fortnight of washing, and putting the airer in a hallway distributes it through the entire house.

The last row deserves attention before anything else. If your condensation is severe, persistent and does not respond to any of this, check for a leaking roof, a burst pipe in a wall or under the floor, or damp coming up through the structure. A live water source produces more moisture than any household activity, and no amount of ventilation will outpace it.

Ventilation, heat and the curtain rule

Once you know where the moisture comes from, three levers control whether it ends up on your glass. Two are obvious and one is the opposite of what most people do.

Ventilation, which matters more than volume suggests

Interior condensation is fundamentally a ventilation deficit rather than a window fault, and a very small amount of continuous airflow makes a disproportionate difference to morning condensation.

Trickle vents are the cheapest version and the most commonly forgotten. They are the small closable slots in the top of a window frame, and a remarkable number of homes have them permanently shut because somebody closed them one cold winter and nobody reopened them. Check yours tonight.

Beyond that: extractor fans in the kitchen and bathroom running longer than feels necessary, internal doors left ajar so moisture can disperse rather than concentrating, and if the house has a mechanical ventilation or heat recovery unit, making sure it is actually switched on. Modern airtight construction depends on those units, and they are frequently turned off for being noisy.

Steady heat beats intermittent heat

Condensation forms because of a temperature difference between air and glass, so the more the room temperature swings, the more often that difference becomes extreme. A bedroom that is warm at midnight and cold at five in the morning produces more condensation than one held at a stable, moderate temperature.

That is not an argument for heating your bedroom hard. Cold rooms encourage condensation because warm humid air from your breath hits cold surfaces and cools rapidly, so a gently and consistently heated room is better than either extreme. Consistency is the lever rather than temperature.

Open the curtains, which sounds wrong

This is the counterintuitive one and it is worth trying tonight because it costs nothing. Closed curtains and blinds trap cool air against the glass and block air movement across the surface, so cold air pools at the sill and the moisture that condenses there has nowhere to disperse.

Leaving curtains open, or at least not tucking them onto the windowsill, allows air to circulate across the glass and noticeably reduces morning condensation. Several window manufacturers make the same point independently, which is unusual agreement in this subject.

If you want the curtains closed for privacy or warmth, the compromise is to keep them clear of the sill so air can move up behind them, and to open them first thing rather than mid-morning.

One thing worth attempting only after the above: upgrading the glazing. Higher performing glass keeps the internal surface warmer, which reduces the temperature difference that triggers condensation in the first place. It works, and it is an expensive answer to a problem that ventilation usually solves for nothing, so it belongs last on the list rather than first.

Buy this first: a hygrometer

This whole subject has one target number, 45 to 55 percent relative humidity, and almost nobody with a condensation problem knows what their reading actually is. That is a strange situation given how cheap it is to find out, and it is why this is the first purchase rather than a dehumidifier.

The specification that matters is accuracy of plus or minus two to three percent relative humidity, which is well within what you need to distinguish 48 percent from 65. It refreshes every ten seconds, so you can watch the number climb while a shower runs, which is more persuasive than any article about ventilation.

The useful feature for this particular job is the face icon, which shows dry, comfortable or wet at a glance. You do not need to interpret a number every morning, only to notice when it has moved into the wrong band. It sits on a shelf, sticks to a magnetic surface or hangs on a hook, so it can live in the room that actually has the problem.

Worth it or skip: Worth it before anything else, because it tells you whether you have a humidity problem at all and whether the changes you make are working. Skip it only if your condensation is between the panes, in which case the sealed unit has failed and your indoor humidity is irrelevant.

Check the digital hygrometer on Amazon

Two more, once you know your number

A three pack, because humidity varies by room. This is the version I would actually buy. A bathroom at 70 percent and a living room at 45 need different responses, and a single meter moved around gives you snapshots rather than a picture. Put one in the worst room, one in a room with no problem, and one wherever you dry laundry, and within a week you know exactly where the moisture is being produced. See the three pack →

A quiet dehumidifier, if ventilation is not enough. Sometimes the moisture is unavoidable, in a flat where laundry has to dry indoors or a bedroom with no trickle vents. This one covers 1,000 square feet, which suits a bedroom or bathroom rather than a whole house, and the specification that matters for a bedroom is that it runs quietly enough to sleep through. It has a continuous drain option, so it does not need emptying every morning. See the 8 pint dehumidifier →

The same moisture that fogs your windows ends up in fabric. I go through it in what to do when a room smells musty, and the cushion version is in drying foam properly.

What does not work, and why it is still recommended

Several fixes appear on every list and range from useless to actively counterproductive. Knowing why saves you buying them.

Silica gel and moisture absorbers

Small tubs of moisture-absorbing crystals placed on the windowsill are recommended as an affordable alternative to a dehumidifier, and the honest description is that they are affordable and much less effective.

The physics is simple: a tub holding a few hundred millilitres cannot compete with a household producing several litres of water vapour a day. In a wardrobe or a small enclosed cupboard they genuinely help, because the volume of air is tiny. On a bedroom windowsill they absorb a trivial fraction of what two sleeping adults produce.

They are not a scam and they are not a solution. Use them where the space is enclosed and small.

Anti-condensation sprays and washing-up liquid on the glass

Both work by the same mechanism, which is worth understanding because it explains the limitation. A surfactant film makes water spread into a thin invisible layer rather than beading into visible droplets.

So the water is still there. You have not removed any moisture from the room, you have made it less visible on the glass. That is genuinely useful on a bathroom mirror you want to see into, and it does nothing for the underlying problem, including the mould risk on the frames and sills where the water still ends up.

Wiping it up every morning

This one is better than its reputation and it is not a fix. Water removed with a cloth or a window vacuum is water that cannot run onto the sill and soak into the frame, which is where mould starts, so wiping genuinely reduces the damage.

What it does not do is address why the water arrives. If you have been wiping for three winters, the wiping is working exactly as well as it ever will and the humidity is still above 60 percent.

Replacing the windows, when the water is on the inside

This is the expensive mistake and it happens regularly. Interior condensation is a ventilation issue rather than a window fault, and modern airtight glazing can make it worse rather than better by removing the draughts that were quietly ventilating the room.

Higher performing glass does help, by keeping the internal surface warmer so the temperature difference is smaller. But it is a partial improvement at considerable cost to a problem that trickle vents and an extractor fan usually solve for nothing. Try the free things first and measure the result.

Tried things and it has not worked? 2 taps

The quiz above is for diagnosing. This one is for when you have already changed something and the windows are still wet.

People also ask about window condensation

How do you stop condensation on windows overnight?

Three changes do most of the work in a bedroom, which is where overnight condensation is worst. Leave the door ajar so moisture disperses into the rest of the house rather than staying in the room with you. Open a trickle vent or the window a fraction if it is safe, since even a small amount of continuous airflow makes a disproportionate difference. And keep the room gently and steadily heated rather than letting the temperature swing, because condensation forms on the temperature difference. Two adults asleep release roughly a litre of water vapour overnight, so the moisture has to go somewhere.

Why is there condensation on the outside of my windows?

Because your windows are working extremely well, which is the opposite of what most people assume. Exterior condensation forms when the outer pane becomes colder than the surrounding air, typically after a clear cold night, and dew settles on it the same way it settles on a lawn or a car roof. The outer pane is that cold precisely because very little heat is escaping through the glass to warm it. It clears within an hour or two of sunlight and there is nothing to fix. Shade from trees near a window reduces it, which is an observation rather than a repair.

What humidity level stops window condensation?

Between 45 and 55 percent relative humidity, with below 50 percent being where overnight condensation largely stops. Most homes with a condensation problem sit above 60 percent and nobody in them knows, because almost nobody measures. A hygrometer costs very little and turns the whole subject from guesswork into a reading, which also tells you whether the changes you make are working. Buy more than one or move it around, since humidity varies considerably between a bathroom and a living room.

Should I leave curtains open to stop condensation?

Yes, and it is the counterintuitive tip worth trying tonight because it costs nothing. Closed curtains trap cool air against the glass and block air moving across the surface, so cold air pools at the sill and the moisture that condenses has nowhere to disperse. Leaving them open, or at least clear of the windowsill rather than tucked onto it, noticeably reduces morning condensation. Several window manufacturers make this point independently, which is unusual agreement in this subject.

Window condensation FAQ

Is window condensation actually harmful?

The water on the glass is not, and where it ends up is. It runs onto the sill and into the frame, and sustained damp there grows mould and mildew and can rot timber frames or damage seals over years. That is why wiping is worth doing even though it fixes nothing: water removed from the glass is water that never soaks into anything.

Do moisture absorber tubs work?

In a wardrobe or a small enclosed cupboard, genuinely. On a bedroom windowsill, barely. A tub holding a few hundred millilitres cannot compete with a household producing several litres of water vapour a day, and two sleeping adults alone produce about a litre. They are an affordable and much less effective alternative to a dehumidifier, which is roughly how the honest sources describe them.

Does an anti-condensation spray work?

It changes how the water looks rather than removing it. A surfactant film makes moisture spread into a thin invisible layer instead of beading into droplets, so the glass appears clear while the same water is still on it. Useful on a bathroom mirror. Useless against the mould risk on the frames and sills, where the water still ends up.

Will new windows fix it?

Only partly, and only if the water is on the inside. Higher performing glass keeps the internal surface warmer, which reduces the temperature difference that triggers condensation, so it does help. But interior condensation is fundamentally a ventilation issue, and modern airtight glazing can make it worse by removing the draughts that were quietly ventilating the room. Try trickle vents and extractor fans first and measure the result before spending thousands.

Why is only one room affected?

Because either the moisture is produced there or the room is colder than the rest. Bedrooms get it from breathing overnight in a closed room, bathrooms and kitchens from short intense bursts, and utility rooms from laundry. A room that is closed off and unheated will also get it from moisture produced elsewhere that condenses in the coldest place available. Measuring two rooms at once usually identifies which of the two is happening.

What if nothing helps at all?

Then look for a water source rather than a ventilation solution, because no amount of airflow outpaces a live leak. Check for a leaking roof, a burst pipe within a wall or under the floor, and rising damp through the structure. Also check that a vented tumble dryer actually vents outside and that the hose is not split, since a disconnected dryer puts the entire water content of every load into the room.

The condensation cheat sheet

Free printable

The Cozy Home Starter Checklist

A home that manages its own moisture feels better to live in before you can explain why, and most of it comes down to small habits rather than equipment. This printable walks you through room by room, catching the things that quietly go wrong.

Get the free Cozy Home Starter Checklist

Keep reading

Household moisture does not stop at the glass. These cover where else it ends up, and what to do about it.

• Two Numbers Decide This, and Nobody Mentions Either
  The wettest object in the house. Yes it goes in the machine, and the gauge decides whether it survives.
• The One Thing Almost Every Guide Tells You to Do Wrong
  Where bathroom humidity shows up first, and why public health guidance says not to scrub it off.
• Sunlight Saves One Mattress and Destroys Another
  The 48-hour mould clock, why sunlight ruins latex, and the honest point at which a mattress is finished rather than damp.
• Four Different Things, and Two of Them Ruin a Machine
  Protectors, pads, encasements and toppers, and why heat cracks the waterproof layer that stops all of this happening.
• Submerging It Is a Drying Question, Not a Washing One
  Half the guides say submerge and half say never, and the answer depends entirely on whether you can dry it afterwards.
• Hotels Wash the Layer That Touches You. Nothing Else
  Two adults release about a litre of moisture a night, and where it ends up decides how often the bedding needs washing.
• Washing Foam Is Easy. Drying It Is Where Sofas Die
  The same problem in a cushion, including the bag and vacuum method that pulls water out of a foam core.
• Laundry Room Smells Musty? Find the Source First
  If a room smells before the windows fog, this is where the moisture is hiding.

Touch the glass, then measure the room

Most of the frustration with this subject comes from treating three different problems as one. Water on the inside is your own air giving up moisture on a cold surface, and it responds to ventilation. Water on the outside means your glazing is performing so well that almost no heat is reaching the outer pane, which is a compliment rather than a fault. Water between the panes means the sealed unit has broken, and every tip about curtains, vents and dehumidifiers is wasted effort because the moisture is not in your room.

So touch the glass first. It takes ten seconds and it decides whether you have an afternoon of habit changes ahead of you or a phone call to a glazier.

If it is the interior kind, the numbers do the rest. Aim for 45 to 55 percent relative humidity and measure it, because most homes with this problem sit above 60 and nobody knows. Then work on the largest sources rather than ventilating harder: laundry dried indoors is almost five pints a load, two sleeping adults are a litre a night, and a dryer that does not really vent outside is the whole wash. Open the trickle vents, leave the curtains off the sill, and run the extractor fan for longer than feels necessary.

I wiped a bedroom window every morning for two winters before buying a five pound meter and discovering the room sat at 68 percent. The fix turned out to be the airer in the hallway.