---
title: "Window condensation in winter: causes and how to fix it"
description: "Why windows mist up in the heating season, what humidity to aim for, how to ventilate properly and how to wipe off condensation and clean frames and seals."
url: https://uklid123.cz/en/blog/kondenzace-na-oknech-vlhkost-v-byte
language: en
updated: 2026-10-31
publisher: Úklid 1-2-3 (info@uklid123.cz, +420 608 486 486)
---

# Window condensation: why windows mist up in winter and what to do

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# Window condensation: why windows mist up in winter and what to do

Misted windows are a sign that your home is too humid or not ventilated enough. We explain how to measure and reduce humidity, and how to treat condensation, frames and seals.

Úklid 1-2-3 editorial team · Published 31 October 2026 · Last reviewed 31 October 2026 · 8 min read

## In brief

- Windows mist up when warm, humid air in a room cools against cold glass.
- You can measure humidity with an inexpensive hygrometer; the US EPA recommends ideally keeping it between 30 and 50%.
- What helps most is airing briefly with the windows wide open several times a day and limiting sources of steam.
- Wipe off condensation every morning so that water does not linger in the frames and seals.
- Condensation inside a double-glazed unit is a glazing defect that neither cleaning nor ventilation will solve.

Condensation on windows forms when warm, humid air from your home meets cold glass and the vapour condenses into droplets. The main thing that helps is reducing humidity: air the rooms briefly with the windows wide open several times a day, do not dry laundry in living rooms, extract steam when cooking and showering, and keep the temperature steady. Wipe off condensation every morning so that it does not seep into the frames and seals.

## Why windows mist up in winter

Warm air can hold more water vapour than cold air. When it cools on a cold surface, some of the vapour condenses as water. The window is usually the coldest surface in a room, which is why it mists up first. According to a brochure from the roof window manufacturer [VELUX](https://contenthub.velux.com/api/public/content/f6f9646711d3434abc45a6e498c9397e?v=3e69adb0) , for example, at a relative humidity of 60% and a room temperature of 21 °C, condensation starts to form as soon as the inner pane cools below 13 °C.

According to the same source, the following contribute to condensation:

- high relative humidity indoors,
- the temperature inside the room and outside,
- air circulation along the window,
- the thermal insulation properties of the glazing.

Modern windows seal much better than old ones. Moisture that used to escape through gaps now stays in the home, so it has to be removed actively by ventilation.

### Where the humidity in your home comes from

Steam is added to the air by cooking, showering and bathing, washing and drying laundry, houseplants, and also by the breathing of people and pets. The VELUX brochure states that a household of three people releases approximately 12 litres of water in 24 hours. That is why windows most often mist up in the kitchen, bathroom, bedroom and in the room where laundry is dried.

## How to measure humidity in your home

You can measure relative humidity with a hygrometer. The US Environmental Protection Agency ( [EPA](https://www.epa.gov/mold/brief-guide-mold-moisture-and-your-home) ) describes it as a small, inexpensive device from an ordinary hardware store and recommends keeping indoor humidity below 60%, ideally between 30 and 50%. In its brochure, the window manufacturer VELUX gives a target of a room temperature of around 21 °C and a relative humidity of approximately 40–50%.

- Place the hygrometer in the room where the windows mist up the most, away from radiators, windows and direct sunlight.
- Let it settle for a few hours, then note down the readings in the morning, after cooking or showering, and in the evening.
- If the readings stay above 60% for a long time, focus on ventilation and sources of steam (see below).
- If, on the other hand, they drop well below 30% in winter, the air is dry and you can shorten the airing a little.

To give you an idea of the limits: [Decree No. 6/2003 Sb.](https://www.zakonyprolidi.cz/cs/2003-6) sets, for habitable rooms in certain buildings (for example schools, accommodation facilities or commercial buildings), a relative humidity of at least 30% in the cold season and at most 65% in the warm season. This decree does not apply to flats; it is only an indicative comparison.

## How to ventilate during the heating season

Brief, intensive airing is the most effective. The VELUX brochure recommends opening windows wide 2–3 times a day for 5–10 minutes, ideally so that a draught is created. On its [service and maintenance page](https://www.velux.cz/zakaznicka-podpora/servis-a-udrzba) , the same manufacturer advises airing for 5 minutes three to four times a day. In such a short time the walls and furniture do not have time to cool down, and the dry air that replaces the old air warms up quickly.

- In the morning after getting up, air the bedroom with the windows wide open, even if the windows are not misted up yet.
- When cooking, switch on the extractor hood and close the kitchen door; air the room briefly after cooking.
- After a shower, leave the fan running or air the bathroom, and keep the door to the rest of the flat closed.
- During the day, air all occupied rooms briefly and all at once, ideally with a draught.
- If your thermostat lowers the temperature at night, air the rooms just before it does, so that excess steam does not condense on the windows.

The roof window manufacturer expressly advises against airing in winter only through the ventilation flap, because it needlessly loses heat and increases the risk of condensation. Instead of a permanently tilted window, therefore, choose brief airing with the window wide open.

## How to reduce sources of moisture

- If possible, do not dry laundry in living areas; if there is no alternative, dry it in one room with the door closed and air that room more often.
- When cooking, use lids and, if you have one, an extractor hood that vents outside.
- Keep the bathroom and kitchen doors closed while steam is being produced, so that it does not spread through the flat.
- Keep the temperature steady; the EPA lists raising the air temperature as one way to reduce condensation.
- Do not cover the radiator under the window with heavy curtains or a wide windowsill with no gap; the VELUX brochure stresses that warm air from the radiator should flow freely past the glass.
- Where humidity is persistently high and ventilation is not enough, consider a dehumidifier, as the EPA also mentions.

## How to wipe off condensation and clean frames and seals

Water running down the glass collects in the lower corners of the sash, in the frame groove and on the windowsill. The EPA points out that condensation needs to be dealt with quickly: dry the wet surface and reduce the source of moisture; if a wet area dries within 24–48 hours, mould will in most cases not grow.

### What you will need

- a window squeegee or an absorbent microfibre cloth,
- a soft, lint-free cloth or a chamois leather,
- a bowl of lukewarm water with a few drops of mild detergent,
- cotton buds or a soft brush for the grooves,
- a dry cloth for the final drying.

### Routine every morning

- Squeegee the condensation from top to bottom, or soak it up with an absorbent cloth.
- Wipe the lower rail of the frame, the corners of the sash and the windowsill.
- Air the room briefly so that the remaining moisture escapes outside.

### Thorough cleaning from time to time

- Open the window and clean the grooves in the frame of dust and dirt, which hold moisture.
- Wipe the frame and seals with a cloth dipped in lukewarm water with detergent. Do not scrape the rubber seal and do not use abrasive products.
- Clean the glass with a soft cloth; according to VELUX, clean water is normally enough, and abrasive and aggressive products are unsuitable. A streak-free method is described in the article [how to clean windows without streaks](https://uklid123.cz/blog/jak-umyt-okna-bez-smouh) .
- Dry everything with a dry cloth and leave the window open for a while.
- While cleaning, check the seal: VELUX recommends checking that it works properly once a year and replacing a hardened or deformed seal after 10 years at the latest.

For wooden windows, the VELUX brochure points out that where condensation is frequent, the varnish needs to be maintained more often in the places the water reaches, especially in the lower corners of the sash. When painting the frame, cover the seal so that the varnish does not stick to it. Always check the type of varnish and the maintenance procedure in the manual from your window manufacturer.

If you can already see dark spots on the seal or in the corners of the frame, use any cleaning product only according to its label, wearing gloves and with the window open, and test it first on a hidden spot. Never mix chlorine-based products with acids or ammonia; you will find details in the overview of [cleaning products that must not be mixed](https://uklid123.cz/blog/nebezpecne-kombinace-cisticich-prostredku) . What to do when mould appears on the wall around the window is described in the article [mould on walls: how to remove it safely](https://uklid123.cz/blog/plisen-na-zdi-jak-odstranit) .

## When it is not ordinary condensation

Where the water appears What it usually means

On the inside of the glass, mainly at the bottom High humidity in the room; ventilation and reducing steam will help.

On the outer part of the frame in older windows Humid air is penetrating through gaps in the frame; according to VELUX, ventilation, additional sealing or mechanical ventilation will help.

Between the panes of the insulating glazing A glazing defect; according to VELUX, the solution is usually to replace the glazing.

If your windows mist up even at a humidity of around 40–50% and with regular airing, or if water seeps under the windowsill, it is advisable to have the window and its installation checked by a window specialist.

## Include windows in your regular cleaning

Wiping off condensation in the morning takes a few minutes and helps most when it becomes a habit. You can schedule a thorough cleaning of the frames, grooves and seals, say, once a month during the heating season; for inspiration, see the [household cleaning plan](https://uklid123.cz/blog/uklidovy-plan-domacnosti) . If you do not feel like washing the windows with their frames, it is also part of [deep cleaning](https://uklid123.cz/sluzby/generalni-uklid) by Úklid 1-2-3.

## Frequently asked questions

### Is window condensation in winter normal?

Occasional misting of the lower part of the glass in the morning or after cooking is a common physical phenomenon, because the window is the coldest surface in the room. However, if your windows mist up significantly every day and water runs down onto the windowsill, your home is too humid or not aired enough. Measure the humidity with a hygrometer, air the room briefly with the windows wide open several times a day and wipe off the condensation every morning so that it does not linger in the frames.

### What should the humidity in a home be in winter?

The US Environmental Protection Agency (EPA) recommends keeping indoor relative humidity below 60%, ideally between 30 and 50%. The window manufacturer VELUX gives a target of approximately 40–50% at a room temperature of around 21 °C. Check the values with a hygrometer in the room where the windows mist up the most, and monitor them at different times of day, for example in the morning and after cooking.

### Does it help to leave the window tilted open all day?

For removing moisture, brief airing with the windows wide open is more effective. The window manufacturer VELUX recommends opening windows fully for 5–10 minutes several times a day in winter, ideally creating a draught, because in such a short time the walls and furniture do not cool down and the fresh air is quickly warmed up. According to VELUX, permanent ventilation through a small gap or only through a ventilation flap needlessly loses heat and increases the risk of condensation.

### What should I use to clean window seals?

For routine cleaning, a soft cloth and lukewarm water with a few drops of mild detergent are enough; finally, dry the seals. Do not use abrasive products or sharp objects. VELUX recommends checking once a year whether the seal has hardened or become deformed, and replacing such a seal after 10 years at the latest. Check the exact procedure in the manual from your window manufacturer.

### What should I do if the window mists up between the panes?

Condensation in the cavity of the insulating glazing, that is, between the panes, has nothing to do with humidity in the home or with ventilation. According to a VELUX manufacturer brochure, it is a product defect that may have arisen during production, storage, transport or installation. The solution is usually to replace the glazing, or possibly the whole window. Contact the window supplier or manufacturer, especially if the window is still under warranty.

## Sources

- [A Brief Guide to Mold, Moisture and Your Home – US EPA](https://www.epa.gov/mold/brief-guide-mold-moisture-and-your-home) (checked 31 October 2026)
- [Jak se bránit kondenzaci na oknech: Příčiny a náprava kondenzace – VELUX Česká republika](https://contenthub.velux.com/api/public/content/f6f9646711d3434abc45a6e498c9397e?v=3e69adb0) (checked 31 October 2026)
- [Servis a údržba – VELUX](https://www.velux.cz/zakaznicka-podpora/servis-a-udrzba) (checked 31 October 2026)
- [Vyhláška č. 6/2003 Sb., kterou se stanoví hygienické limity chemických, fyzikálních a biologických ukazatelů pro vnitřní prostředí pobytových místností některých staveb – Zákony pro lidi](https://www.zakonyprolidi.cz/cs/2003-6) (checked 31 October 2026)

Prepared by the Úklid 1-2-3 editorial team with the help of AI and verified against the sources listed. Manufacturers' instructions and regulations change: always follow the current wording of the source and the product label. This article is general information, not expert or legal advice. This article was translated from Czech with the help of AI. The sources are in the original language.

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