Condensation on window

What Causes Foggy Windows and How to Prevent It

Do your windows have a foggy appearance? Fortunately, as unsightly as it may be, window-fogging is a common phenomenon, and there are several ways of counteracting it, if not avoiding it altogether.

Why are My Windows Fogging Up?

In order to prevent windows from fogging up, you need to figure out what’s causing them to fog up in the first place.

Windows fog for several different reasons. All of them, however, can be reduced by the same phenomenon: airborne water vapour settling on a cold surface and condensing into tiny droplets.

These droplets diffuse light that passes through them, and thereby produce the ‘fogged’ effect.

Since glass is invariably among the coldest surfaces in the home, it’s here where you’ll first notice this fogging effect.

Windows fogging on the inside

Condensation on the inside of a window is caused by excessive humidity inside the home. You’ll notice this most often in winter, when the difference in temperature between the exterior and interior of the property is most pronounced. If you’re releasing a lot of moisture into the air by cooking or showering, then the problem is going to be even more apparent – which is why kitchens and bathrooms tend to be more affected than other rooms.

Windows fogging between panes

Double-glazed windows are formed of two glass panels, between which is sandwiched a layer of inert gas, typically argon. This gas is kept in place by air-tight seals running around the edges of the window. Should this seal start a leak, the gas will be able to escape.

This is usually noticeable when water-droplets appear the interior of the window, where they’ll condense. Manufacturers normally ship their double-glazed window with drying agents on the inside, which remove any moisture trapped during manufacture. If there’s a leak, however, these drying agents begin to lose their efficacy.

What Keeps Windows from Fogging Up?

There are two main ways in which you can prevent windows from fogging up.

  1. Raise the temperature of the glass so that water can’t settle.
  2. Reduce the amount of moisture in the air.

Since we want our windows to be as energy-efficient as possible, and there’s nothing we can do to alter the humidity outdoors, we should resign ourselves to the fact that fog on the outside of your window is a natural and largely unavoidable phenomenon. Fog on the inside of your windows, on the other hand, can often be corrected through proper ventilation.

A new window will be more efficient at keeping out drafts than an old one – and this can interfere with a building’s ability to properly ‘breathe’. This is why many new windows come with some form of ventilation built-in – most commonly, ‘trickle vents’, which constantly let in a steady stream of fresh air in the home (and let moist air out).

Other rooms may benefit from extractor fans – namely the kitchen and bathroom.

When you’re boiling pasta, crank the extractor to its maximum setting; it’ll suck up all the steam you’re creating. This will prevent water vapour from spreading throughout the room.

Similarly, an extractor fan near your shower will reduce moisture in your bathroom.

If your windows are fogging up even in environments where you aren’t creating moisture, you might consider investing in a dehumidifier.

Excessive airborne moisture doesn’t just cause your windows to fog up – it’ll also accelerate rot and other structural problems. This means you need to treat it seriously, even if you don’t need to see out of the window in your bathroom!

Do Foggy Windows Need Replacing?

If your windows are fogging up, then you might wonder whether your windows need replacing. If fog is forming between the panels of a double-glazed window, the answer is almost definitely yes. If this has happened, your window’s seal has been compromised and the unit will only continue to deteriorate over time.

You may be able to live with the problem initially, but eventually you’re going to need to get the windows replaced.

sliding window

What Are Trickle Vents For (and do you need them?)

Over the past few decades, considerable advancements have been made in the art of draught-exclusion.  Modern windows are airtight, which is great – in terms of energy-efficiency.

Unfortunately, while we don’t want draughts to get into our home, an entirely air-tight interior is bad news for moisture and air quality levels.  After all, there’s no point in having an air-tight window if you can’t see out of it because it’s covered in condensation!

For this reason, window-manufacturers began to build little cold air vents in the frames of their windows, which allow a small amount of air to pass from one side of the window to the other.  These are known as trickle-vents.

window with trickle vent

Do you have to have trickle vents?

The short answer is – it depends.

It was once the case that building regulations stipulated a ‘no worse than existing’ clause for trickle vents. This basically meant that if you were replacing a window with a trickle vent, you’d need to bring in another window with a trickle vent.

This rule went pretty much unchallenged until 2006, when the Department for Communities and Local Government (they’re the people in charge of building regulations) decided that all new windows should come with trickle ventilation.  The fenestration industry, much of which still manufactures and sells trickle-free windows, was displeased. After some discussion, the rules changed. What was once considered mandatory is now merely advisable.

This means that homeowners have a little more freedom to make their own decisions about the sort of windows they’d like installed.  The company carrying out the installation is sure to have an opinion on the property’s ventilation requirements, and it’s usually best to follow their advice.

What size trickle vents do you need?

If you’re replacing windows with trickle vents, the ventilators should be “no smaller in geometric open area than the existing ventilators“.

If this isn’t known, “habitable rooms should have trickle ventilator of 5000mm2 equivalent area and wet rooms should have 2500mm2 equivalent area“.

What are the alternatives to trickle vents?

There’s a very simple alternative to fitting windows with trickle vents – opening a window. It’s a good idea to open all the windows in the house for a few minutes every day (yes – even in winter!) to allow moist air to escape, and dry air to replace it.

This is especially important in the bathroom and kitchen.

Can I fit trickle vents into my existing window?

If you’ve got a problem with condensation in an older window, it is sometimes possible to install a trickle vent.  That said, fitting a vent takes considerable time and skill. In many cases, the best solution would simply be to get the window replaced.

Looking for new windows for your home? Browse our sliding sash or casement windows or find out about our handmade bespoke windows.

condensation on window

Why Is There Condensation Between My Window Panes?

A steamed-up window is quite a common sight in modern homes – particularly if it’s cold outside and you’ve got the heating cranked up.  As the airborne moisture comes into contact with a cold window pane it condenses. This is normal (although you may want to take steps to reduce the amount of moisture in your home). Condensation that forms between the panes, however, is evidence of something a great deal more concerning.

How does double glazing work?

Double glazing makes a significant difference to the energy-efficiency of a building.  By sandwiching a layer of air between two panes of glass, we can slow the transfer of heat from one side to the other considerably.  When an inert gas like argon or xenon is used inside the cavity, the flow of heat is reduced even further.

In order to deal with any moisture that might get trapped between the panes during sealing, window manufacturers would use silica balls; the sort that you find shipped alongside electronic equipment.  These balls act as a desiccant; they’ll absorb small amounts of moisture, and prevent condensation from forming.

So what does all this mean for windows in which condensation has formed?  Simply put, it means that the seal which safeguards the interior of the window has broken, and airborne moisture from outside has found its way in.  The more condensation inside the window, the greater the degree to which the seals have failed.  This doesn’t just, of course, mean that you’ll need to put up with more condensation blocking your view of the outside world; it also means that your window will be far less energy-efficient.

Do I need to replace the window?

If your window’s still under the manufacturer’s guarantee, you’ll want to get it replaced straight away.  The window has failed, and they’re obliged to correct it.

If your guarantee has expired, you have a decision to make.  A double-glazed window with a broken seal will still function better than a single-paned window, but it won’t be anywhere near as insulating as a window with an intact seal.  Repairing the seal is difficult and expensive. It requires an engineer to identify where the leak is, suck the gas from the window, and refill it.  If there are multiple tiny leaks spread across the edge of the glass, a repair would be near-impossible.

Ultimately, the window will need to be replaced.

Looking for new windows for your home? Browse our sliding sash or casement windows or find out about our handmade bespoke windows.

water condensation

Why Does My Double Glazing Get Condensation On the Inside?

Is there condensation on the inside of your windows? Have you noticed that your windows are wet on the inside in the morning?

This can be a worrying find (understandably) but the good news is that the phenomenon actually indicates that your windows are working as they should. In fact, you’re as likely to get condensation on new windows as old windows, and new windows may even increase condensation, since they should reduce draughts in the home.

That said, condensation on the inside of windows is something we should try to reduce, since it can damage window frames. It’s also indicative of a bigger problem – excessive moisture in the home.

Why do you get condensation on the inside of a window?

One of the main advantages of double glazing is its ability to prevent heat from moving from one side of the glass to the other.  It does this by sandwiching a vacuum (or a layer of inert gas) between two sheets of glass.  When heat energy builds on one side, it has trouble passing through this inner layer, and so instead is mostly conducted back into the room.

This will prevent heat escaping, and reduce your energy bills,  but in doing so it’ll create a considerable difference in heat from one side of the glass to the other.  It’s this difference that creates the conditions for condensation.

Cold air contains less energy than warm air, and it’s less capable of keeping water vapour in its gaseous form.  This means that the air inside your home will be damper than the air outside.  When this air hits a cold surface, like a window, it will lose the energy necessary to hold onto the airborne water, and so that water will be deposited.  Over time, this effect causes the build-up of water condensing on the glass.

Chances are, you will notice this happening primarily in  three rooms of the house:

  • The kitchen.
  • The bathroom.
  • The bedroom.

Cooking, showering and bathing all create substantial moisture which unless you keep the rooms very well ventilated, will settle on the windows. This probably isn’t surprising. What is puzzling is why we get condensation on bedroom windows, since we’re probably not cooking or showering in there.

The most likely reason is that during our waking hours we typically move around the house but at night we’re confined to one small space for 8 hours or more, often with the door closed. This causes all the water we lose over those 8 hours to build up. We also tend to keep bedrooms a little cooler than the rest of the house, which mean the surface of the windows will be colder, and water will be more likely to condense when it hits it.

What’s bad about condensation?

Condensation can reduce the lifespan of a window, as it’ll encourage the growth of mould which can damage the frame.  It can also be taken as evidence that the humidity inside your home is excessive – which can be a health concern.

How do you stop condensation on windows?

To prevent condensation settling on windows we need to take steps to reduce the amount of condensation in the home generally.  Ideally, we want humidity levels to be at around 50%.  In order to achieve this, we might try the following:


Aim to open windows for about 20 minutes daily (yes, even in winter!) This will allow the damp air to escape and (unless it’s especially humid outside) dry air to replace it.

Installing trickle vents can help reduce condensation too.

Use extractor fans

If you’re cooking or showering, an extractor fan will remove damp air before it can settle on surfaces and cause problems.

Dry laundry outside

Drying laundry inside significantly increases the amount of water vapour in the air.  The water has to go somewhere, after all.

If drying laundry outside isn’t an option, ensure the space around your laundry is as ventilated as possible.

Manage other humidity sources

When you’re taking steps to guard against excess humidity, you’ll want to consider the things that can contribute to it.  One of the most obvious causes are the living beings which inhabit a house – each of which, be they humans, dogs, cats or guinea pigs, will emit water vapour over the course of the day.  Paradoxically, you might also encounter an uptick in humidity when you return from a spell away – as an empty house will be unheated, and therefore prone to absorbing moisture, which will be released when the heat rises again.

Invest in a dehumidifer

If the steps above don’t remedy the problem, or at least don’t sufficiently reduce condensation in the home, you may want to invest in a dehumidifier, which  will extract moisture from the air.

Do double glazed windows stop condensation?

While condensation can be worse on single glazed windows (due to the internal surface of the window being much colder than the internal surface of a double glazed window) replacing single glazed windows with double glazing is not enough to eliminate the problem. The reason being is that although the inside of your new windows will be warmer, they will simultaneously eliminate draughts. This will reduce ventilation, and contribute to the build-up of moisture.

Acute causes of condensation

If your condensation has appeared almost overnight, then you might be wondering what could possibly have caused it.  There are several potential short-term contributing factors.


When the weather outside gets cold, the chances of condensation occurring increase substantially.  At the start of winter, it’s therefore worth taking preventative steps before the condensation has a chance to develop.

New windows

Newer double-glazed windows are able to do their job much more effectively than old ones, so don’t be surprised to see more condensation after getting new windows installed.

Underfloor heating

Radiators help warm air to rise and circulate, so if you’ve swapped out your radiators for underfloor heating, you might see an uptick in condensation on your windows.

What about condensation between the panes?

So far, we’ve been talking about condensation on the inside of the glass – but what happens when vapour gets in between the window panes?  This means that the window has sprung a leak, which has allowed the water vapour to get in.  The inert gases inside a modern double-glazed window contribute enormously to its ability to retain heat, so if yours is showing this symptom, it’s probably time to replace your windows.

For more information, take a look at our blog post on why condensation forms between window panes.

Looking for new windows for your home? Browse our sliding sash or casement windows or find out about our handmade bespoke windows.

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