In the summer, people often assume that window condensation is only a winter problem. In fact, condensation can also form on windows during the warmer months. If windows become foggy in the morning, or if moisture or even mold appears around the window frames, this usually indicates excessive humidity or inadequate ventilation.
In this article, we’ll look at the 7 main reasons why windows fog up in the summer and how to solve the problem.
1. High humidity in the room
The most common cause is excessively high relative humidity. In the summer, the following sources contribute to humidity:
- taking a shower
- drying laundry indoors
- food preparation
- a large number of houseplants
- people and pets
- Rainy weather
When humidity rises above 60–65%, it begins to condense on cooler surfaces.
Solution
Measure the humidity with a hygrometer and use a dehumidifier if necessary.
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2. Inadequate ventilation
In many apartments and houses, the ventilation doesn’t work well enough. Moist air remains in the rooms and condenses on the windows.
Typical characteristics:
- stale air
- condensation on the windows in the morning
- laundry that dries slowly
- mold around the windows
Solution
Check that the ventilation system is working properly and, if necessary, install a ventilation system with heat recovery.
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3. Incorrect setup of the heat recovery unit
Many modern heat recovery units have a summer bypass function. If it does not work properly or is set incorrectly, moisture can accumulate indoors.
Solution
Check the condition of the filters and the device’s summer operating mode settings.
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4. A cold glass surface and warm outside air
In the summer, an air conditioner can cool a room to the point where the window glass becomes colder than the outside air. As a result, condensation forms on the outside surface of the window.
This is not usually a problem and does not indicate an error.
Solution
This is a physical phenomenon and generally does not require intervention.
5. Drying Laundry Indoors
A single load of laundry can release several liters of water into the air. If ventilation is insufficient, the humidity level will rise rapidly.
Solution
Use ventilation or a clothes dryer when drying your laundry.
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6. Excess moisture after a rainy period
Long periods of rain increase indoor humidity as well. This particularly affects:
- basements
- summer cottages
- older buildings
- poorly ventilated rooms
Solution
Use a dehumidifier with automatic humidity control.
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7. Mold Growth Around Windows
If windows are constantly fogging up, mold may form around the window frames. This affects both the condition of the building and the indoor air quality.
Solution
The root cause of the problem must be eliminated:
- improve ventilation
- reduce humidity
- Use a dehumidifier if necessary
- check for the presence of cold bridges
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What is a normal level of humidity?
The recommended relative humidity at home is:
- In winter: 30–45%
- In the summer: 40–60%
If the humidity level remains above 60–65% for an extended period, the risk of mold and condensation increases.
Summary
In the summer, windows usually fog up due to excess moisture or inadequate ventilation. Ignoring this problem can lead to mold, a decline in indoor air quality, and damage to the building’s structure.
If you regularly notice damp windows, it’s a good idea to check the humidity level and, if necessary, use a dehumidifier or improve ventilation.
Pro Air OÜ helps you find the right solution for apartments, single-family homes, and commercial spaces alike.
Frequently Asked Questions (FAQ)
Why do windows get condensation on them in the summer?
In the summer, windows usually fog up due to excessive humidity. Moisture is caused, for example, by drying laundry indoors, taking a shower, cooking, and inadequate ventilation. When moist air comes into contact with a cooler window surface, condensation forms.
Is condensation on windows dangerous?
Occasional condensation is not usually a problem. However, if windows constantly fog up, it can lead to mold growth, damage to window frames, and a decline in indoor air quality.
What should the humidity level be at home?
The recommended relative humidity is:
- 30–45% in winter
- 40–60% in the summer
If the humidity exceeds 60–65% over an extended period, the risk of mold and condensation increases.
Does a dehumidifier help prevent condensation on windows?
Yes. A dehumidifier reduces the humidity in a room and helps prevent condensation from forming on windows. A dehumidifier is especially useful in bathrooms, basements, laundry rooms, and poorly ventilated apartments.
Does ventilation help prevent condensation on windows?
Yes. Properly functioning ventilation removes humid air and replaces it with fresh outdoor air. This helps keep humidity levels under control and reduces condensation on windows.
Why do new triple-pane windows fog up?
The problem is usually not the windows, but the building’s airtightness. New windows reduce natural air exchange, and if ventilation is insufficient, moisture can accumulate indoors.
Does a heat recovery ventilator help reduce condensation on windows?
Yes. Heat-recovery ventilation, or a heat exchanger, removes moist indoor air and helps maintain stable humidity levels. A properly configured heat exchanger significantly reduces the risk of condensation.
Why do windows get more foggy in the morning?
Rooms are usually cooler at night, and people release moisture while they sleep. If ventilation is inadequate, moisture can build up, and condensation may appear on the windows in the morning.
Can drying laundry indoors cause windows to fog up?
Yes. A single load of laundry can release several liters of water into the air. If there is no ventilation or dehumidifier, the humidity will rise quickly and the windows may start to fog up.
When should you be concerned about condensation on windows?
The situation is cause for concern when:
- The windows fog up every day;
- Mold is forming around the window frames;
- moisture stains appear on the walls;
- The room constantly has stale air or a musty smell.
In that case, you should check that the ventilation is working properly and measure the humidity.
