Arrange a Survey

What Causes Condensation Overnight?

Select a category

What Causes Condensation Overnight

By Ruth MacEachern

Product Manager

Sep 11, 2023

As the weather cools in the autumn, it becomes increasingly common to see condensation forming on your windows in the morning, but have you ever wondered what causes it?  Where does the moisture come from that you see pooling on your windowsills and forming drops on your windowpanes in the morning?  Would it surprise you to know that some of it comes from your breathing during the night?

What is condensation and where does it come from?

As air flows around our homes, it picks up moisture that is released by activities like cooking and bathing.  Steam from pans cooking on the stove, water vapour from showers, and the cloud of steam that emerges when you open the dishwasher or oven are some of the most visible sources of water vapour, but steam can also be released more slowly if you dry your towels or clothes indoors on radiators.

Air can carry a surprising amount of water.  At room temperature, every cubic metre of air in your home can hold approximately 17g of water vapour.  This creates humidity.

Some humidity is healthy, however if the air becomes saturated, and can no longer carry as much water vapour, it is released.  Usually onto cold surfaces like walls, tiles, and most visibly, windows.

The amount of water vapour that the air can hold is dependent on temperature.  As the temperature cools, the water condenses out of the air forming patches of moisture around your home.

The most obvious sources of humidity in your home are bathrooms and kitchens, but every time you breathe out, you also release a small amount of moisture – as you would have seen if you ever exhaled onto a mirror and wrote your name.  In fact, each day, an average person breathes around 14,000 litres of air, and exhales around 400g of water vapour into the atmosphere.

During the night, with few other activities going on in your home, respiration becomes one of the major sources of water vapour, and if you sleep with your bedroom door closed, there is limited ventilation, which means that there is nowhere for the moisture in the air to go.

Condensation on your bedroom windows in the morning

Overnight, the average person exhales around 130g of water vapour into the air.  During the summer, when the nighttime temperatures are higher, the air can easily accommodate this, but with cooling temperatures during the autumn and winter, the capacity of the air to hold moisture falls, which means that the moisture will be released.

Your bedroom windows are likely to be the coolest place in your room – after all, they are cooled by the outside air.  As a result, they are also the place where the air cools most rapidly.  As the air circulates in your room, it will meet your windows where condensation will form.

Is condensation dangerous?

Condensation is just water, and is harmless, however if it is allowed to build up in your home, it can cause damp to form, and those damp patches are the perfect environment for mould to grow. 

Mould is an all-too-common sight in poorly ventilated bathrooms, and should be cleaned away, however in a bedroom with poor airflow, mould can be particularly bad for your health.  Mould spores released by the mould colony can be inhaled and irritate your airways.  Some people have serious allergic reactions to mould and suffer from respiratory problems as a result.  The presence of mould is also associated with other health problems including skin irritation and even depression.  Toxic species of mould such as Stachybotrys Chartarum release mycotoxins which are extremely dangerous for children, older people, and those with weaker immune systems.

Preventing condensation

Stopping condensation in your home is important to prevent damp and mould from forming.  The most effective way of doing this is to improve ventilation.  Ensuring that the extractor fans in your bathroom and kitchen are working properly to remove moisture from the air and are used when you cook or shower will help to control condensation, but they will not reduce condensation on your bedroom windows overnight.

Whole house ventilation systems such as the EnviroVent Atmos improve airflow throughout your home.  They run continually, drawing fresh air in from outdoors to displace the humid air in your home.  This controls humidity levels throughout the night and stops condensation on your bedroom windows for good, which prevents mould from growing in your bedroom.

Find out more

If you are worried that condensation on your bedroom windows could lead to mould and cause health problems for you and your family, it is important to act quickly.  Our local ventilation specialists can visit your home to perform a free home survey.  During the survey, they will identify the causes of condensation in your property and advise you about the best way of dealing with it.  Enter your postcode below to find a specialist near you.

Need help with condensation, mould or damp problems?

One of our local experts will contact you to learn more about your problems, offer free expert advice and make recommendations for a permanent solution.

During the free survey we will

  • check Assess any condensation, damp or mould problems in your property
  • check Take readings of the relative humidity levels
  • check Identify any underlying problems and make recommendations for a permanent solution

Arrange a FREE Home Survey now