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Condensation on Windows: Causes, Fixes and When to Call a Builder

Geometric illustration of window glass and droplets

Water on a window is not one problem but three, and each has a different cure. Droplets on the room side mean humid indoor air is meeting cold glass. Droplets on the outside are often normal. Mist trapped between the panes means the sealed unit has failed and ventilation will not fix it. Work out which one you have, and you will know whether you need a dehumidifier, a new habit, a repaint or a glazier.

We are a building firm in Hereford, so we see what happens after condensation has been left for a few winters: blackened sealant, peeling paint, stained reveals and soft timber. This guide covers the causes and the fixes you can do yourself. It also covers the point where it becomes a job for a tradesperson, including the bathroom paint question many people have.

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TL;DR

  • Condensation inside is moisture from breathing, cooking, washing and drying clothes meeting a cold surface. You control it through moisture, temperature and ventilation together, not one alone.
  • Condensation outside is frequently a sign of efficient glazing on a cool, still morning, not a fault.
  • Mist between the panes is a failed sealed unit. Replace the unit; wiping, heating and extractor fans will not help.
  • Mould-resistant paint is not a moisture repair. Fix the cause first, then redecorate.
  • Tighter new windows can make condensation worse at first unless background ventilation is kept open.

Table of Contents

Where is the water? Start with a quick diagnosis

Before buying anything, look at where the moisture sits. Wipe a pane and see which side comes clean.

What you see Likely cause Usual remedy
Droplets on the inside of the glass, worst in the morning Humid room air meeting cold glass Cut moisture, ventilate, keep surfaces warmer
Droplets on the outside, clear still morning Efficient glazing staying colder than the outdoor air Usually none; keep drainage and seals in good order
Fog or streaks trapped between the panes Failed edge seal in the double-glazed unit Replace the sealed unit
Black spots on sealant, reveals or nearby walls Repeated condensation feeding mould Remove moisture source, clean, dry, then redecorate

This split matters because most advice online assumes the first row. If you have the third row, no amount of airing the room will help.

Condensation on the inside of windows

This is the common one. GOV.UK’s existing home ventilation guide lists showering, washing dishes and clothes, cooking and breathing as the everyday sources of moisture in a home. When that warm, damp air reaches a surface colder than its dew point, it turns to water. Windows are usually the coldest surface in the room, so they show it first.

Council guidance tends to frame it as three factors working together: moisture produced indoors, a cold surface, and poor ventilation. Ashfield District Council describes exactly this trio. It is worst in winter, and around windows, corners and poorly ventilated spaces.

Indoor moisture, cold surfaces and poor ventilation

That is useful, because it tells you there are three levers. You do not need to pull all three hard, but one on its own often disappoints.

Lever one: make less moisture, or catch it at the source

  • Use the extractor fan in the kitchen while you cook and for a while afterwards. Keep the kitchen door closed so steam does not drift through the house.
  • Do the same in the bathroom. Bathroom walls and ceilings can take a couple of hours to dry after bathing, according to Wolverhampton’s damp and condensation booklet, so leave the fan running and the door shut after you finish.
  • Dry washing outdoors where you can. If you must dry it inside, do it in a room with a door shut and a window ajar or an extractor running, not draped over radiators in the living room.
  • Put lids on pans.
  • Wipe the glass and sills in the morning, then wring out the cloth rather than leaving the water to evaporate back into the room.

Lever two: keep the glass and rooms warmer

Heating helps because warmer glass is less likely to reach the dew point. Warwick District Council advises keeping main living areas at 18 to 21°C in colder weather while you are in. The same council is clear that heating alone may only reduce condensation temporarily. If the moisture has nowhere to go, it simply turns up on another cold surface, such as a wall behind a wardrobe.

A steady, moderate temperature beats short blasts of high heat. Cold bedrooms with the door shut and the occupants breathing all night are a classic problem, and the reason the pane is wet when you wake.

Lever three: let air move

Ventilation is the exchange of stale, moist indoor air for fresh outdoor air. The practical points from the GOV.UK guidance:

  • Trickle vents in the window frame should stay open and unblocked, even in winter.
  • Extractor fan filters should be cleaned or replaced according to the manufacturer’s instructions, usually annually.
  • Ventilate bedrooms after you wake.

You do not need to leave windows wide open all day. North Lincolnshire Council’s guidance treats a short, purposeful burst of ventilation as a standard measure, and the research we checked puts it at roughly half an hour to an hour rather than all day. Short and thorough beats long and half-hearted, and it costs far less heat.

What about a dehumidifier?

A dehumidifier can pull moisture out of a room that has no extraction and no easy way to get any. It is a reasonable stopgap in a flat with no outside wall for a fan. Treat it as support for the three levers above, not a replacement. It will not mend a failed sealed unit or a leaking gutter.

Condensation on the outside of windows

People often panic about this, and usually should not. Modern low-emissivity, gas-filled, well-insulated glazing loses very little heat to the outdoors. That means the outer pane stays closer to the outside temperature, and on a cool, calm night it can fall below the dew point of the outdoor air. Natural Resources Canada explains that features such as low-e coatings, inert gas, improved spacer bars and insulated frames reduce heat loss but can increase early-morning condensation on the exterior.

The Fenestration and Glazing Industry Alliance draws the same line between comfort and diagnosis: better insulated glazing usually reduces condensation on the inside, while very efficient glass can show some on the outside in certain weather. Exterior droplets are not proof of poor installation.

It is worth checking only if:

  • water is getting past the frame or showing on the wall or reveal inside,
  • the outer sealant or drainage slots are cracked, blocked or missing,
  • the droplets never clear even in dry, sunny weather.

In that case a site visit is sensible, and it is usually a seal, sealant or gutter problem rather than the glass.

Condensation between the panes

This one is different. If you see fog, a milky haze or water marks between the two layers of glass, the sealed unit has failed. The edge seal has let moist air in, and the drying agent in the spacer is saturated. As one UK repair guide puts it, this is a failed seal, not broken glass.

Milky haze trapped between double glazing panes

Two questions come up constantly.

Will a hair dryer clear it? It may shift surface moisture on the room side, but it cannot restore the seal or remove the moisture trapped inside the unit. Natural Resources Canada advises replacing the insulating glass unit when the seal is broken.

What does it cost? Published 2026 UK trade guides give indicative figures of roughly £40 to £80 per pane for de-misting or resealing and £150 to £350 per pane for a replacement sealed unit; the same guide stresses that size, glass specification, access and frame condition change the total. Those are not our prices, and we do not publish prices for jobs: each one is written up after a visit.

When the frame is sound, replacing only the glass unit is normally cheaper than a whole new window. When the frame itself is warped, rotten or no longer secure, A&E Window Doctor’s repair-or-replace guidance points toward replacing the whole window. We would not quote for whole-window replacement until someone has actually looked at the frame, the unit and the seal.

Why new windows can make it worse for a while

This surprises many homeowners. Old, leaky windows let moist air escape through gaps. Fit tight UPVC windows and that uncontrolled leakage disappears. The moisture stays in the house, and the cold glass gets the blame.

The GOV.UK guidance explains that sealing cracks and replacing leaky windows removes uncontrolled air leakage, so deliberate background ventilation becomes important. It says replacement windows should normally include background ventilation, such as an open, unblocked trickle vent, unless it is shown that useful ventilation was not reduced. Blocking the vents to keep heat in defeats the moisture control the window was designed with.

So the right question before replacing glazing is not “which window stops condensation?” but “how will the new windows and the ventilation work together?” Ask any installer, us included, to show you where the background ventilation will be and how the kitchen and bathroom are extracted.

Bathroom paint, mould and what paint can and cannot do

Many people reach this topic through a bathroom search: which is the best paint for bathrooms, and does mould-resistant paint solve things? Here is the honest answer.

Bathrooms are the wettest rooms in the house and the most likely to show black spotting on ceilings and window reveals. A paint made for damp rooms is a sensible choice there, for two reasons: it gives you a wipeable surface, and some include a fungicide in the film. But paint is a finish, not a moisture repair.

Local-authority guidance is consistent on this. North Lincolnshire Council says to remove excess mould carefully rather than dry-brushing it, treat the surface with an appropriate fungicidal wash, let it dry, and only then redecorate with a suitable fungicidal coating. If it keeps coming back, look at ventilation, heating, leaks and cold bridges. Mould needs moisture, food, a suitable temperature and oxygen; the lasting cure is to take away the moisture and dry the construction.

In practice, the order for a bathroom or bedroom is:

  1. Fix the cause: working extractor, trickle vent open, leaks traced, heating adequate.
  2. Clean the mould off safely and let the surface dry fully.
  3. Repair damaged plaster or sealant. Blown plaster and stained patches need skimming, not just another coat.
  4. Repaint with a paint suited to the room, in a dry spell with the fan running.

People with asthma, a weakened immune system or large areas of mould (more than a small patch) should read their council’s advice and speak to their GP or landlord before cleaning it themselves. If you rent, report persistent damp to your landlord in writing.

Our perspective: fix the moisture, then the finish

We get calls that start as “can you repaint this ceiling?” and turn into something else once we look at the room. A painter who covers mould without asking why it is there has done the customer no favours, because it will be back by February.

Here is how we approach it as a local building firm in Hereford. We work on one project at a time with a dedicated team, so the person who sees the problem on the site visit is the person who does the work. That matters for condensation jobs, because the fix is rarely just one trade. A single room can need a de-misted or replaced glass unit, replaced sealant, a trickle vent that works, a skim of fresh plaster and a repaint. We would rather quote for all of it clearly in writing than patch one symptom.

For homeowners, we record what is already there before recommending anything: fans, vents, how the room is used and what damage is visible. For landlords and small businesses, we treat ventilation as part of the maintenance job. GOV.UK states that building work must not make ventilation worse, and the 2025 Wolverhampton booklet notes that rented properties should have extract ventilation in the kitchen and bathroom. Any job we do should leave a property no worse ventilated than we found it.

Some honest rules we follow:

  • We will not tell you to replace windows when only a glass unit has misted.
  • We will not tell you new windows will end condensation. They reduce cold-glass problems, but you still have to ventilate.
  • We will not paint over active mould and call it done.
  • We keep the site clean, because dust and sticky dust sheets are the last thing you need in a room you are already worried about.

— Trey

When it is time to call a builder

Most condensation you can manage yourself. Call someone when:

  • mist or haze appears between the panes,
  • frames are soft, swollen or the sealant is split and black,
  • plaster is blown, flaking or stained after repeated wetting,
  • there is mould on a large area or it returns within weeks of cleaning,
  • you are planning new windows and want ventilation to be designed in,
  • you manage a rental or commercial property and need the whole job assessed and recorded.

How we can help

We offer painting and decorating, carpentry, drywall and plastering, UPVC windows and doors, fascia, soffits and guttering, and general maintenance for homes and commercial properties in Hereford and the surrounding area. We give free, no-obligation written quotations after a site visit, we are fully insured, and flexible payment options are available. Tell us what you are seeing and we will look at it properly before we recommend a thing.

A winter checklist you can do this weekend

  1. Open every trickle vent and make sure none is painted shut or blocked.
  2. Test the bathroom and kitchen fans with a tissue held to the grille. If it does not hold, the fan is weak or clogged.
  3. Clean the extractor filters if the maker’s instructions say so.
  4. Wipe wet glass and sills in the morning and wring the cloth out.
  5. Keep furniture a hand’s width from external walls so air can circulate.
  6. Keep the main rooms at a steady 18 to 21°C when you are in.
  7. Photograph any mist between panes, black sealant or stained plaster so you have a record for a quote or your landlord.

FAQ

Does putting the heating on reduce condensation?

It can, because warmer glass is less likely to reach the dew point. But Warwick District Council warns that heating alone may only reduce it temporarily. Pair it with extraction and ventilation so the moisture leaves the house.

Will a hair dryer get rid of condensation between glass panes?

No, not in any lasting way. Mist between the panes means the sealed unit’s seal has failed and moisture is trapped inside. The unit needs replacing, and a glazier can tell you whether the frame is sound enough to keep.

What causes condensation on the outside of windows?

Usually efficient glazing on a cool, still night. The outer pane stays colder than the humid outdoor air, so dew forms, mostly around sunrise. It is generally not a defect, though water entering the frame or wall is worth having checked.

How much does it cost to fix misted double glazing?

Published UK trade guides for 2026 give roughly £150 to £350 per pane for a replacement sealed unit, and less for de-misting. Size, glass type and access change the figure a lot, so get a written, site-specific quotation.

Sources

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