As winter rolls in, many homeowners and renters alike notice condensation forming on the inside of their windows. This phenomenon, often appearing as a thin layer of water or fog on the glass, is not just an aesthetic nuisance; it’s a natural result of the interplay between temperature, humidity, and the properties of glass. Understanding why condensation forms and how to manage it is key to maintaining a comfortable and energy-efficient home during the colder months.

What is Condensation?

Condensation occurs when water vapor in the air cools and transforms into liquid water. It’s the same process that happens when you take a cold drink outside on a warm day, and the glass sweats with moisture. In the context of windows, the water vapor in your indoor air comes into contact with the cold surface of the window. When the surface temperature of the window drops below the “dew point” — the temperature at which air can no longer hold all the moisture it contains — the water vapor condenses into tiny droplets.

Why Does Condensation Happen More in Winter?

Condensation on windows is more common in the winter due to several factors:

1. Cold Exterior Temperatures: During winter, the outside air temperature drops significantly, cooling the window’s surface from the outside. The greater the difference between the temperature inside and outside the home, the colder the window surface becomes, making condensation more likely.
2. Indoor Humidity: Winter air is typically drier outdoors, but when we heat our homes to stay warm, the relative humidity inside can rise. Activities like cooking, showering, drying clothes indoors, or even breathing contribute to higher moisture levels in the air. The warmer indoor air can hold more moisture, but as it comes into contact with the cold window, it cools, and the moisture condenses.
3. Reduced Ventilation: During colder months, windows and doors are often kept shut, limiting airflow and ventilation. Without adequate air circulation, moist air remains trapped inside, leading to higher humidity levels and an increased chance of condensation.

The Impact of Condensation on Your Home

While condensation may seem like a minor issue, it can have serious implications for both your home and health:

1. Moisture Damage: Over time, repeated condensation can lead to water damage. The moisture that collects on windows can drip down onto the sill, frames, or nearby surfaces, potentially causing rot, peeling paint, and warping wood. This can be particularly problematic in older homes where the seals around windows may not be as tight.
2. Mold Growth: If condensation persists, it can create a damp environment, which is a perfect breeding ground for mold. Mold thrives in moist, poorly ventilated spaces, and the buildup of condensation on windows can lead to mold growth on walls, window frames, and around the edges of the glass.
3. Energy Efficiency: Excess moisture on windows can affect the thermal performance of your home. When windows become foggy due to condensation, it can indicate that heat is escaping through the glass. This reduces your home’s energy efficiency and can lead to higher heating costs.

How to Prevent and Manage Condensation

While condensation is a common issue in winter, there are several ways to manage and prevent it:

1. Improve Ventilation: One of the best ways to reduce humidity indoors is by improving ventilation. Use exhaust fans in kitchens and bathrooms to remove excess moisture from the air. If possible, open windows slightly for a few minutes each day to allow fresh air to circulate, especially after cooking or showering.
2. Use a Dehumidifier: A dehumidifier can help remove excess moisture from the air, lowering indoor humidity levels. This is particularly effective in areas with heavy condensation.
3. Maintain a Consistent Temperature: Keep the temperature in your home relatively stable and avoid drastic fluctuations. Consistent warmth can prevent windows from cooling too much and reaching the dew point.
4. Install Double-Glazed Windows: Double-glazed windows, or insulated windows, are designed to keep the inner and outer panes of glass separated, helping to maintain a more stable indoor temperature on the window surface. This reduces the likelihood of condensation forming. If you already have double-glazed windows, ensure that the seals are intact to avoid drafts.
5. Use Window Insulation Films: Window insulation films are an affordable way to reduce heat loss and prevent condensation. These thin plastic sheets adhere to the window and provide an additional layer of insulation.
6. Increase Airflow Around Windows: Ensure that furniture and curtains are not blocking airflow around your windows. By allowing air to circulate freely around the glass, you can help maintain a more uniform temperature on the window surface.
7. Use Absorbent Materials: If condensation is a persistent problem, you can place absorbent materials like silica gel or moisture-absorbing crystals near the windows to capture excess moisture. These products can help keep your indoor environment drier.

Conclusion

Condensation on windows during winter is a natural process that occurs when moisture-laden warm air meets cold glass. While it may seem harmless at first, prolonged condensation can lead to moisture damage, mold growth, and reduced energy efficiency. By understanding the causes of condensation and implementing simple solutions such as improving ventilation, using dehumidifiers, and investing in energy-efficient windows, you can reduce its impact and keep your home comfortable and healthy throughout the winter season.

Additional information about condensation on glass
We see water vapour either on the glass or inside the double or triple glazed glass units. This is a common issue in the morning during the winter in the UK. Condensation (water droplets) on the glass window can be wiped off with a towel; if not, then the double or triple glazed glass unit has failed and so the view through the glass looks misty as if clouds inside the glass unit. See an example below between glass with mist and newly replaced glass without mist.

How does mist form…humidity?
Condensation occurs on windows, because glass and window frames are usually the coolest surface in the room, so water vapour in the air condenses on them first. There are many sources of moisture in the home; for example showering, cooking, dishwasher, washing machine and even breathing out. Drying clothes or towels on a radiator also produces a lot of condensation (i.e. water from those wet clothes evaporates into the room air during the drying process.)

Does condensation mean the window is faulty?
It depends. If condensation can be wiped off with a towel (i.e. it’s not inside the window glass panes,) then the window glass is not faulty, meaning humidity is too high. You need to take action to reduce humidity. If condensation is inside the glass panes, then the glass unit is faulty and that glass unit needs replacement. So water droplets running down on the glass surface, doesn’t mean the window is faulty.

How to measure or check humidity?
We can measure humidity in the air with a hygrometer. Ideal humidity level is between 40% – 60%. If humidity is too low, then we feel uncomfortable as the room air becomes too dry. Common symptoms are dry throat, sore eyes, stuffy nose, itchy or dry skin. Also young children may experience nose bleeding, as dry air can damage or crack the delicate nasal membrane lining inside their nose. Higher humidity above 60% or more causes condensation on glass or windows and mould in our room walls.

How to reduce humidity and prevent mould?
Room air can be treated like a sponge; hotter air has a higher capacity to hold water before water vapour starts to sit on another surface. Increasing heating during the winter will increase the capacity of air to hold more water, alternatively we will need fresh air supply in the room to prevent condensation. However, fresh air supply can’t be maintained easily during the winter as UK weather is very cold. So a balance needs to be struck between fresh air supply and more healing (i.e. more heating equals higher bills.) It’s a good idea to leave the window slightly ajar but secure, when outside and increase heating while we are in the room.

Is misted glass a serious problem?
Condensation can ruin the appearance of windows and the rest of the house, it will reduce the amount of light that enters the home and obscure our view through the window. Excessive condensation can damage wooden frames and cause mould on room walls. Windows can remain misted, if water gathers between the panes of glass; this can slowly rot window frames, dampen walls and allow mould to grow.

Can new double glazed glass be misted?
Double or triple glazed window glass usually lasts 10 years or more, before showing signs of mist inside the glass units. The two panes of glass are joined with a sealant. These sealants lose integrity after many years, which creates weak points around the edge of the glass unit and allows moisture to enter between the two panes of glass. This occurs naturally and cannot be prevented as high summer temperatures cause the air between the panes to expand and contract during the winter, causing sealant to become weaker over time.

How can misted windows be fixed?
Misted glass needs to be replaced. You would need to contact a professional to replace misted window glass. New glass will usually last over 10 years, before showing any sign of mist. Keeping humidity in check will reduce the chance of glass becoming misty. Also keeping window sill, frame and seals clean helps glass units to last longer.