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How to Reduce Condensation in Winter

Without sounding too Game of Thrones, winter isn’t coming – Winter is here. In the last couple of weeks, the wintery weather has begun to hit the UK. Condensation is one of the biggest tell-tale signs in our homes that it’s cold outside.

In the cold winter weather, condensation will form on windows and walls around your home. It can be tempting to ignore condensation and assume it’s a necessary evil of ensuring your house is warm and comfortable, but ignoring condensation can have serious negative effects on the comfort and health of your home.

In this blog, Anna Marie Byrne, MVHR Technical Support Specialist from our MVHR team, talks you through some of the key things to know about condensation in your home and what you can do to help reduce it.

Condensation- What steps can you take to reduce it in your home?

Did you know a long relaxing hot bath will help improve relaxation and have a lower impact on indoor moisture levels than a ten-minute hot shower?

With winter upon us, we spend much longer in our homes than we typically would throughout the year. With the biggest contributor to indoor moisture problems in our homes actually being us as humans, it is important to consider what steps we might take to manage moisture in our homes to reduce the risk of mould buildup.

To reduce condensation, the 4 main steps are:
  1. Reduce moisture going into the air by drying clothes outside or in a vented drier
  2. Increase moist air leaving the house by ventilating.
  3. Reduce the (relative) humidity by heating the house adequately
  4. Ensure surfaces are warm enough by getting them insulated professionally and keeping walls clear of clutter and furniture

Continue reading below to learn how to implement the above four steps and to look at the biggest sources of moisture in your own home

How does condensation form in our homes?

Condensation happens when moist air touches a cold surface. Too much condensation causes mould. And too much mould makes you ill.

In winter, the inside surfaces of your house’s external walls/windows/floors will be colder than the air in the house and the internal wall surfaces.

(If you live in a Passivhaus, lucky you! All the external surfaces will be over 17 degrees C, and no water will condense on them when the indoor humidity is good (40-60%). In your Passivhaus, the indoor humidity will be good because you’ll have an MVHR system.)

However, if you are one of the 99% who doesn’t live in a Passivhaus in the UK, there will be many cold surfaces in your home, and the indoor humidity will likely be high (over 60%).  This is especially true in high-occupancy small houses (humans and animals will all contribute to the relative humidity).

Condensation will form where the surfaces are too cold for the damp air. The colder the surface and the wetter the air, the worse the condensation. If condensation is present for long enough, mould will form. 27% of UK households report a mould problem[i].

Mould is the cause of significant health problems and will form if the average relative humidity is above the levels shown for the period of time shown below (taken from the ventilation building regulations, part F1). In winter, the growth of mould is far more likely.

If you live in a rented home, then the landlord has a responsibility to the tenant to ensure the house is mould-free; see here for government guidance “Awaab’s law”[i]. Named after the two-year-old boy who tragically died as a result of mould in his home where his family were renting.

Is it a good idea to use a dehumidifier?

Many people imagine running a dehumidifier is the best way to reduce humidity. The problem with this solution is that you need ventilation for more reasons than just reducing humidity.  Reducing CO2 and VOCs are two other major reasons to ventilate your home.

The need to ventilate your house will help bring the humidity down towards the theoretical minimum level it can be, given the outside conditions (see above). But If you still have a condensation problem, it is more effectively solved by following these four steps, than using a dehumidifier.

If, however, these four steps are all followed, and you still have an issue, then using a dehumidifier willhelp, although for a price. A 10l/day dehumidifier, sized for a small house, will cost about £1-2 per day in electricity to run, and the fan makes about 50 – 60 dB(A) of noise. For comparison, an MVHR system costs about 20p per day in electricity to run and makes below 30dB(A) of noise in the rooms.

The Four Steps of Condensation Reduction

There are many sources of moisture in the home. To get an idea of the relative amounts from different sources, here’s a chart for a typical two person household. This scenario imagines that the couple has put a wash on, hung the clothes to dry inside, had a shower and a bath, cooked a meal and stayed inside all day breathing.

The most significant source of moisture (on a wash day) is the indoor clothes drying. So, any possible way of doing this differently would make a big difference.

Drying outside is the cheapest and simplest, but when that’s not possible, then here’s a good overview of tumble drier options[i]

The second largest moisture source in a home on a day like this is the people breathing. If you have an MVHR system or another programable ventilation system, setting the system to low for holidays and boost for parties will extract this moisture.

Regarding condensation, which is better, a shower or a bath?

A ten-minute hot shower makes the same amount of moisture in the air as an hour-long hot bath. And uses about the same amount of water, (assuming a low flow 5 litre/minute shower and a 50 litre bath). So, if you have long showers, consider having a bath instead.

Other moisture-reducing actions you can take around the home include:

  • Choosing electric over gas cookers,
  • Covering saucepans,
  • Squeegee down wet surfaces in bathrooms after use

You can get a cheap humidity meter called a Hygrometer, like this one to indicate the relative humidity in your house. You can position these around the home in key spaces like the bathroom and the kitchen to understand the humidity in that space.

Ideally, you want the humidity to be between 40-60% for most of the time. When producing lots of steam e.g. when showering, the humidity will rise, and that’s ok as long as it falls within the next half hour or so.

Increase moist air leaving the house

Ventilation is needed for many things in your home, but one of the main reasons is to help reduce moisture in the house. Whatever your ventilation strategy, ensure it works properly and is turned on. Different strategies have different effectiveness.

Here is a list of ventilation strategies from the least to most effective:

  1. No trickle vents or fans, and windows open in the kitchen and bathroom when used.
  2. Trickle vents and intermittent extract fans in the kitchen and bathroom.
  3. Positive input ventilation (continuous ventilation into the house)
  4. Mechanical extract ventilation (continuous ventilation out of the house)
  5. MVHR: Mechanical ventilation with heat recovery (continuous ventilation into and out of the house)

If there is no MVHR, an externally vented cooker hood extractor will help.

Something to be aware of if you’re watching the relative humidity of the house and wondering why it’s not dropping even though you’re ventilating it thoroughly is what is the outdoor air’s absolute humidity.

Humidity is measured in two ways:

  1. Absolute humidity (AH) is measured in grams of water vapour held in a kg of dry air. (g/kg)
  2. Relative humidity (RH) is measured as a percentage of the maximum amount of water vapour air at that temperature can hold. (%)

Outdoor relative humidity is usually between about 50 -100% (mist/fog is 100%)

In autumn/spring, when the temperature of the air outside is not too cold, the absolute humidity is high. When that air comes into the house and is heated, the relative humidity will drop, but not by much. It might not be possible to get the house below about 60% RH in autumn. If it’s still not too cold outside, this doesn’t matter because the external walls/windows won’t be very cold either.

In cold periods, ventilation will be really effective at reducing the humidity in the house because cold outside air can hold very little water. So, when it’s brought into the house and warmed up, it’ll be at a low relative humidity.  In large, low occupancy homes, this can even be a problem with too dry air but for most UK houses, this isn’t a problem though.

Reduce the (relative) humidity by heating the house adequately

The warmer the house is, the less the condensation will be on cold surfaces. Relative humidity drops as temperature rises: 60% relative humidity at 20 degrees C becomes 80% relative humidity at only 15 degrees C.

A common effect in homes is that bedrooms are kept cooler than the rest of the house. If moisture from kitchens and bathrooms isn’t vented at the source, then that warm damp air will rise to the bedrooms upstairs and condense on the colder surfaces, causing dampness. Ensuring bedrooms are warm enough and extracting moisture from wet rooms will prevent this from happening.

Ensure surfaces are warm enough (above the dew point)

Inadequately insulated buildings and gaps in insulation (thermal bridges) leave the external surfaces cold in winter. This is the hardest problem to solve and why it’d be good if new buildings were built to better standards like Passivhaus from the start when it’s easier and cheaper.

In a poorly insulated building, to avoid surfaces that are warm enough, here’s what you should and shouldn’t do:


Ensure air circulation to all cold external surfaces keeps them as warm as possible.

If you keep bathrooms and kitchens cooler than the rest of the house, then heating them an hour before using them will help reduce condensation by increasing the temperature of the surfaces when moisture is created in the room. It’s also nicer to use them warm!

If you can afford it, get the house properly insulated with higher spec windows/ doors and internal/ external or cavity insulation done by building professionals who understand the movement of water vapour in a way that protects the house from condensation.


Try to make surfaces warmer by putting insulating materials against them like fabric, cushions, books, vapour permeable insulation, etc. This will insulate the surface from the rest of the room so the surface becomes colder.  Moist air will still get to the surface through whatever is against the wall and condense on the surface, causing more of a problem. So avoid mattresses against external walls, bookcases without an air gap behind them, or clothes stored against external walls/floors, etc.

What is the science of moisture in the air?

If you like to get scientific and a little bit more technical to understand condensation even better then understanding more about how water is carried in air. The psychometric chart below, shows the properties of water in air:
The blue horizontal lines show the absolute humidity: how many grams of water there are in a gram of dry air.

The vertical green lines show the temperature in degrees C.

The red curved lines show the relative humidity: how much water can be held in the air as a percentage of the maximum amount of water held in the air at a certain temperature. The top left red line is 100% relative humidity; over that point, water will condense out of the air.

The red lines slope upwards because warm air can hold more water as vapour than cold air. They curve because it’s not a linear relationship.

To take a scenario, air outside in winter might be at 5 degrees C and 80% relative humidity, point A on the graph. The absolute humidity of that air is found by following the horizontal line across to the axis, where it says about 0.004g/g of dry air.

If we take that air into the house and it is warmed to 20 degrees C, then the absolute humidity remains the same, so we move horizontally on the graph to point B. Because warm air can hold more moisture, the relative humidity of the air falls to about 30% (curved red line).

If there were no one home and no other sources of moisture being added to the air, the indoor heated air would remain at 30% relative humidity.

Whatever way we put moisture into the air in the house, we are adding to the absolute humidity of the inside air. The amount of moisture we’re adding in grams of moisture per gram of air, depends on how quickly the air is flowing through the building.

Imagine filling a glass of water from the tap, but you keep the tap running, and the glass is constantly overflowing. If you then add Ribena to the glass, the concentration of Ribena will depend on how fast you are adding the Ribena and how fast the tap is flowing. It’s the same principle with ventilation (the running tap) and humidity or other indoor air pollutants (the Ribena).

For a typical small household with good ventilation, moisture is added to the air at a rate that will bring the moisture concentration in the air up to about 50% relative humidity, point C on the graph.

Suppose moisture is introduced into the house quickly (having a hot shower, for example). In that case, the room’s relative humidity will rise to 100%  (point D), and any more moisture will condense out of the air (misty shower air) and on the room surfaces, e.g. the walls, mirrors and tiles.

When introducing moisture at normal rates, and the relative humidity indoors is about 50%, moisture will only condense out if it is in contact with a surface below about 9 degrees C (point E on the graph).

You can see from the graph that the higher the relative humidity is in the house (moving up from point C on the graph to point D), the warmer the surfaces that will cause condensation. For example, at 70% relative humidity, condensation will form on any surface below 14 degrees C.

The psychometric chart helps understand different winter, spring/autumn scenarios, and cold houses.

Returning to the four steps we can take to reduce condensation, hopefully, you now understand better why they help:

  1. Reduce moisture going into the air = pour the Ribena more slowly
  2. Increase moist air leaving the house = run the tap more quickly
  3. Reduce the (relative) humidity by heating the house adequately = moving to the right on the psychometric chart
  4. Ensure surfaces are warm enough (above the dew point) = keeping surfaces higher than point E on the graph.

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