What cause condensation: Condensation The What, Why & How It Happens






Condensation is the process where water vapor becomes liquid


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Condensation is the process where water vapor becomes liquid. It is the reverse action of evaporation, where liquid water becomes a vapor.

Condensation happens one of two ways: Either the air is cooled to its dew point or it becomes so saturated with water vapor that it cannot hold any more water.

Dew Point
Dew point is the temperature at which condensation happens. (Dew is simply condensed water in the atmosphere.) Air temperatures can reach or fall below the dew point naturally, as they often do at night. That’s why the ground, structures, and objects left outside are often coated with water droplets in the morning.

Condensation can also produce water droplets on the outside of soda cans or glasses of cold water. When warm air hits the cold surface, it reaches its dew point and condenses. This leaves droplets of water on the glass or can.

When a pocket of air becomes full of water vapor, clouds form. The point at which condensation starts can be easily viewed in cumulus clouds, which have flat bottoms. Those flat bottoms are where vapor begins to condense into water droplets.

Clouds are simply masses of water droplets in the atmosphere. Molecules in water vapor are far apart from one another. As more water vapor collects in clouds, they can become saturated with water vapor. Saturated clouds cannot hold any more water vapor. When clouds are saturated with water vapor, the density, or closeness, of the molecules increases. The vapor condenses and becomes rain.

Cold air holds less water vapor than warm air. This is why warm climates are often more humid than cold ones: Water vapor remains in the air instead of condensing into rain. Cold climates are more likely to have rain, because water vapor condenses more easily there.

Fast Fact

Making Rain
Cloud condensation nuclei (CCN) are microscopic bits of clay, salt, or solid pollutants such as ash from smoke. Water in clouds condenses around these condensation nuclei to form raindrops.


National Weather Service: Clouds and ContrailsNational Geographic Science: Clouds, explainedNOAA: Cloud Classification


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Last Updated

June 15, 2023

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Condensation The What, Why & How It Happens


Learn about condensation, why and when it occurs and which of its causes you can control to reduce its likelihood of happening in your home.

What is condensation?

Condensation in a household setting is when airborne water vapour condenses into a liquid and is deposited on interior (or exterior) surfaces.

If the temperature of an object (e.g. grass, metal, the glass of a windowpane) falls below what is known as the dew point temperature for a given relative humidity of the surrounding air, water vapour from the atmosphere condenses into water droplets on its surface.

This dew point varies according to the amount of water in the atmosphere and air temperature (known as relative humidity). In humid conditions, condensation occurs at higher temperatures. In cold conditions, condensation occurs despite relatively low humidity.

With regard to windows and doors, it is the difference in temperature between the environment, be it internal or external, and the glass, that causes condensation to form.

Why does it occur in homes?

The air surrounding us in our homes always contains water vapour, which is invisible. A typical example is the steam cloud from a kettle, which rapidly becomes invisible – it has in fact been absorbed into the atmosphere.

The warmer the air, the more water vapour it can hold – but there is a limit to the amount it can hold for a given temperature. When that limit is reached, the air is said to be ‘saturated’.

When saturated air comes into contact with a surface that is at a lower temperature than itself, the air is chilled at the point of contact and sheds its surplus water vapour on that surface – initially in the form of a mist and, if excessive, eventually in the form of droplets of moisture.

An example of this is when a person breathes onto a mirror: condensation occurs because the exhaled air is saturated and its temperature is higher than that of the mirror (which is at room temperature).

The factors governing condensation

1. The water vapour content of the air


This is produced by normal living activities such as washing, cooking, bathing, etc. , and can be controlled by the use of extractor fans, cowlings, and ventilation at appropriate places.

2. Inside room temperature

This can be controlled to some extent by replacing single glazing with energy-efficient double or triple glazing, thereby maintaining a higher surface temperature of the glass on the room side.

This will help to retain the room’s air temperature, which, along with adequate ventilation, will enable the room to hold more water vapour without condensing.

3. Outside temperature   

This cannot be controlled, but its effect on the inside room temperature can be reduced by the installation of energy efficient double or triple glazing.

4. Internal and external temperature variation

This cannot be controlled as the main variant is the outside temperature. However, this variation may also be affected by building orientation, localised atmospheric conditions, shelter from nearby trees or buildings, air currents, wind speeds and nearby vegetation.

Some things to note:

– It is often the case that external condensation will appear on some windows but not on others due to variable microclimates in differing locations.

– Condensation may occur on the exterior of a window or door when the surface temperature of the outer pane is below the dew point. This can be the result of the reduction in the transfer of heat from inside to out and is visible evidence of the energy efficiency of the window or door.

– Following the installation of replacement windows and doors, it is important that adequate ventilation is included to remove the airborne vapour. Failure to do this may result in this vapour condensing on the coldest surface which would no longer be the window but could be an outside wall.

For more advice and information about reducing condensation in your home, get in touch with a local GGF Member company.

Related topics

– Window condensation – where the water vapour comes from
– How double or triple glazing helps reduce window condensation
– Where condensation can form on a window and how to reduce it
– Top tips on reducing condensation room by room
– A guide to home ventilation

How and why does condensation form on the inner glass? – Blog

How and why does condensation form on the inner glass? – Blog | Eurologi. lv

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  • How and why does condensation form on the inner glass?
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This phenomenon occurs if, along with high humidity in the room, the temperature of the inner surface of the double-glazed window is significantly lower than the air temperature in the room. Here are some typical examples of condensation on the surface of the inner glass:

  • When doing laundry or taking a shower in the room, the temperature and humidity of the air increase significantly.
  • Condensation forms on cooled walls and interior windows during the cold season as the temperature drops below the dew point.
  • Cooking always causes high humidity. Because of this, in the cold season, condensation forms on the surface of the internal glasses.
  • Every person releases a significant amount of moisture into the air during sleep. Therefore, in the morning in the bedroom is usually high humidity.
  • If the windows are not fitted with roller shutters or folding shutters, condensation will form on the interior glass in winter. Condensation can also appear on roller shutter or swing shutter windows when the shutters are opened in the morning and the glass surface cools rapidly.

In addition, there are several other factors that contribute to the formation of condensation:

  • Too many plants Blinds on the side of the room that interfere with air circulation
  • Thermal bridges (places of heat leakage)

The frequency of condensation on the glass surface from the room side is primarily influenced by the K factor of the double-glazed window. Consider the swimming pool example: At a room temperature of 28°C and a relative humidity of 50%, the dew point in this case is 16.6°C. If conventional double-glazed windows are installed in the pool, then at an outside temperature of -10 ° C, the temperature of the inner glasses will be 13.8 ° C, which will lead to the formation of condensate. If heat-insulating double-glazed windows with a coefficient of K = 1.3 W / m2K are installed in the pool, the temperature of the internal glasses does not reach the dew point and condensate does not form: Tvs = 28 + 1.3 x (-10-28) / 8 = 21.8 ° C (> 16.6 °C) So, heat-insulating double-glazed windows reduce the likelihood of condensation on the glass surface from the side of the room. Conclusion: the lower the K coefficient of an insulating glass unit consisting of several glasses, the higher the temperature of the glass from the side of the room and the lower the likelihood of condensation on the glass surface.

Other articles

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  • What has changed as a result of technical innovations?
  • Low surface temperature of building parts
  • How are the isotherms?

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Condensation problem. Causes, consequences and remedies.

The occurrence of condensation on windows (and at negative temperatures – ice and hoarfrost), which we may encounter during the cold season, is a fairly common problem that has several interrelated causes, among which are high humidity and poor ventilation. Modern windows and double-glazed windows that hermetically protect our housing from the penetration of cold air into the room do not allow ventilation of the room with a natural ventilation system in the house, since, in order to ensure multiple air exchange, it is necessary to provide air flow. As a result, the exhausted humid air is not removed from the room, but when it comes into contact with the window, it turns into water, i. e. into the condensate. And the more moisture is contained in the air, the higher the intensity of condensate. What is the source of moisture? People, animals, plants and even furniture, interior items and (furniture, linen, carpets, etc.), which, due to their porous structure, accumulate part of the moisture emitted by the residents and then give it away in their absence.

In accordance with the regulations, the air exchange rate in living rooms should be 0.5-1.0. In kitchens, bathrooms, bathrooms – at least 3.0. With a multiplicity of less than 0.5, a person may experience a feeling of stuffiness. Multiplicity means that in 1 hour the air in the room will change twice.

The probability of condensation on a window (glazed loggia, stained-glass window, etc.) can be predicted in advance and even accurately calculated. To do this, you need to get acquainted with such a parameter as the Dew Point.

Dew point

The dew point is the minimum temperature at which dew (condensation) begins to form. At negative temperatures, dew will turn into ice or frost. In everyday life, we encounter the dew point every day. For example, when taking a shower in the bathroom, when the mirror fogs up, or when we go into a warm room from a frost and a cold object, such as glasses, immediately fogs up. Fog on the roads is another manifestation of the dew point. This phenomenon occurs when the temperature decreases with constant humidity and the effect of condensation appears directly in the air. There are many such examples. In an apartment, condensation can occur primarily in the kitchen, during cooking, if you do not use the hood.

For window constructions The dew point defines the temperature limit at which moisture begins to form on the inner glass of an insulating glass unit or on window profiles. The dew point directly depends on two other characteristics – air temperature and relative humidity in the room.

Calculate the Dew Point in degrees Celsius using the following formula:

Tp = (b*f (T, Rh))/(a-f (T, Rh))
f (T, Rh) = a*T/(b+T)+ln(Rh/100)
Тр – dew point temperature, °С;
a = 17. 27;
b = 237.7;
Т – room temperature, °С;
Rh—relative humidity, %;
Ln is the natural logarithm.

(The formula has an error of ±0.4 °C in the range of air temperature T from 0°C to 60°C, dew point temperature Tp from 0°C to 50°C, relative humidity Rh from 1% to 100%)

Other measurement options available Dewpoints

  • You can use a table with already calculated dewpoint temperature for different temperatures (from -5°C to 35°C) and relative humidity (from 40% to 95%) of indoor air can be found in the reference Appendix R to SP 23-101-2004 “Design of thermal protection of buildings”
  • Use a special device – a psychometer (psychrometric hygrometer) to measure air humidity and temperature, as well as calculate the dew point.

Let’s calculate the Dew Point for specific temperatures and humidity. To do this, consider a few examples. We will change the value of the relative humidity in the room, and leave the air temperature at a constant value of + 21 ° C

Example #1

Room relative humidity 60%
Room air temperature + 21 °С
Let’s determine the dew point. Substituting these values ​​into the formula, we obtain the value of the Dew Point +12.4 °C.

This means that on any surface whose temperature is less than +12.4 °C, condensation will begin to form. Those. if the surface temperature of the window is less than +12.4 ° C, then it will begin to “cry”. Everything is very simple. What do you think – under what condition can the temperature of the window surface be lower than this value, if the temperature in the room is +21°C? For example, if there is a severe frost outside, and you have a single-chamber double-glazed window installed.

What happens if the humidity is reduced to 40%? (Remind that the room temperature remains at the same level)

Example #2

Room relative humidity 40%
Air temperature +21 °С,
Let’s determine the dew point. Substituting these values ​​into the formula, we get the value of the Dew Point +6.8 °C.

i.e. Condensation will appear if the temperature on the window is equal to or lower than +6. 8 °C. Thus, by reducing the humidity in the room by 20%, we changed the dew point temperature by 2 times! The probability of condensation at such a humidity, even with a single-chamber double-glazed window with I-glass in the climatic zone of St. Petersburg and Leningrad Region, is extremely small.

Now, on the contrary, increase the humidity to 80% (Remember that the temperature remains the same)

Example #3

Room relative humidity 40%
Air temperature +21 °С,

Let’s determine the dew point. Substituting these values ​​into the formula, we get the value of the Dew Point +17.2 °C.

With such a high humidity of almost 100%, it can be said that at sub-zero temperatures outside, the window will condense heavily, even with the warmest double-glazed windows and I-glass.

IMPORTANT! The higher the relative humidity value in the room, the higher the Dew Point value is also, which negatively affects the risk of window frosting. By lowering the humidity to acceptable values ​​in the range of 30-40% (NOT HIGHER!!), we significantly reduce the risk of condensation even in severe frosts outside the window!

What can cause high humidity? First of all – the lack of normal ventilation of the room.
According to the norms, in a residential area per 1 m2 of an apartment or house, air exchange should be at least 3 m3 / h, and for kitchens 6-9m3/hour. High humidity is caused by people living in the apartment, if there are a lot of them, and the rooms are small in size, pets and plants in large numbers.
High humidity adversely affects the well-being and comfort level of living people.

In example no. 2, we calculated the Dew Point at 40% indoor humidity and +21°C air temperature. The Dew Point value was +6°C. And what will happen to the Dew Point if the indoor temperature is +16°C? For example, when insulating a loggia, you laid not a heating cable, but a mat, and now, when it is cold outside, the underfloor heating system is not able to warm the air in the room than 16 degrees Celsius. Or your builders poorly insulated the surface of the walls, ceiling, floor and heat loss is such that the temperature does not rise Above this mark.

Example #4

Room relative humidity 40%
Air temperature +16 °С
Substituting these values ​​into the formula, we get + 2. 4 ° C! (at +21°C the dew point value was +6.8°C)

I.e. by reducing the air temperature by 5 ° C, the dew point value decreased by 3.4 ° C! Recall that the value of +3.2 ° C means that if the window temperature is less than this mark, then it will condense.

Reduced heat transfer resistance

Knowing how the humidity in the room affects the dew point, you can calculate at what temperature outside the window the windows will begin to “sweat”. To calculate such a temperature value, you need to know another important parameter of any light-transparent structure, which is called – Reduced heat transfer resistance.
It is denoted by the letter R and is measured in m2 * C / W. This is the reciprocal of the thermal conductivity coefficient K. (Thermal conductivity coefficient is the amount of heat in watts that passes through 1 m2 of the structure with a temperature difference on both sides of the window). The lower the value of K, the lower the heat transfer through the window, i. e. higher thermal insulation properties. The value of R, on the contrary, the higher it is, the better the light-transparent design retains heat inside the room and prevents the penetration of cold from the outside. Therefore, one should always strive to ensure that the reduced heat transfer resistance of the window is maximum.
There are calculated standard values ​​of R for each of the climatic zones. For the zone of St. Petersburg and Leningrad Region, this value for window structures installed in residential premises should be at least 0.51 m2 * C / W.

It is important to pay attention to the obvious fact that any window, stained-glass window, double-glazed window does not heat up by itself, i.e. do not form or generate heat. The positive temperature on the loggia, in the room, in the house is provided by the heating system, and the window structure only effectively saves this heat. (there are double-glazed windows with heating, but in this case we will not consider them)

If we neglect the direction and speed of the wind, the cardinal direction on which the window is facing (south, north) and the influence of solar energy, then it is logical to assume that the surface temperature of the window inside the room depends on two parameters

  • outdoor temperature
  • indoor temperature

On the one hand, a window, stained-glass window, glazed loggia is cooled by cold air from the street, and on the other hand, it is heated by warm air inside the room. The higher the R value (reduced heat transfer resistance) of the window structure, the less the influence of cold from the outside, and the better the light-transparent structure is heated. Accordingly, the risk of condensation is reduced.

Possible ways to increase the value of R (reduced heat transfer resistance) for any window construction, such as the use of double-glazed windows, low-e glass, plastic thermal frames, etc. we have covered in another article on our website. Here is a link for familiarization http://www.eurofasad.ru/faq/sposoby-uvelichit-teploizoljaciju.html)

Knowing parameters such as outdoor air temperature, relative humidity and indoor air temperature and R value, you can accurately predict the presence or absence of moisture condensation on your window.

If the temperature (hereinafter referred to as T) of the inner glass/window profile is above the Dew Point, condensation will not form on the glass/profile.
If the T of the inner glass/window profile is below the dew point, the glass/profile will sweat.

The calculation is carried out according to the formula:
T of internal glass/profile = T inside – ( T inside – T outside) / ( R def * ?int )

Example No. 5

indoor temperature +21°С
outside air temperature -21°С
a window made of a 3-chamber Gealan 3000 system with a low-emission double-glazed window 32 mm (4-10Ar-4-10Ar-4Low-E).
The average value of the reduced heat transfer resistance R=0.64 (with a relative glazing area = 0.7)

According to the formula, we obtain that the temperature of the inner surface of the window is +12.8 °C.

What does this value give us? Earlier in example 1, we calculated that at a humidity of 60%, at a given temperature, the dew point would be + 12.4 ° C.
Therefore, even at 60% humidity, or any value below, there will be no dew. But if the humidity is, for example, 65%, then you will see a slight condensate. If the humidity is higher, the condensation will be abundant.

Now let’s see how the temperature of the inner surface of the window will change if it is made of a warmer profile.

Example #6

room temperature +21°C
outside air temperature -21°C
a window made of five-chamber PVC system Gealan 8000 with a low-emission double-glazed window 44 mm (4-16Ar-4-16Ar-4low-E).
Average value of reduced heat transfer resistance R=0.78 (with relative glazing area = 0.7)

According to the formula, we obtain that the temperature of the inner surface of the window is 14.27 °C.
This temperature guarantees that even with a relative indoor humidity of 65%, condensation will not occur!

We can also consider the reverse situation. Let’s try to determine at what external temperature the internal temperature of the window will be equal to the Dew Point, i.e. we find such a value of the outdoor air temperature at which condensate begins to form.

To do this, use the formula
T outside = T inside + ?int * Ropr * ( Dew point – T inside )

Example #7

indoor temperature +21°C
indoor humidity 40%
a window made of a 3-chamber Gealan 3000 system with a low-emission double-glazed window 32 mm (4-10Ar-4-10Ar-4Low-E).
The average value of the reduced heat transfer resistance R=0.64 (with a relative glazing area = 0.7)

Let’s substitute the values ​​into the formula, we get that the outdoor air temperature at which condensation starts to form should be – 51.31°C!

Example #8

indoor temperature 21°C
indoor humidity 40%
a window made of five-chamber PVC system Gealan 8000 with a low-emission double-glazed window 44 mm (4-16Ar-4-16Ar-4low-E).
The average value of the reduced heat transfer resistance R=0.78 (with a relative glazing area = 0.7)

Let’s substitute the values ​​into the formula, we get that the outdoor air temperature at which condensation starts to form should be – 67.13°C!

And now we will do the same calculations, for humidity 60%

Example No. 9

indoor temperature 21
indoor humidity 60%
a window made of a 3-chamber Gealan 3000 system with a low-emission double-glazed window 32 mm (4-10Ar-4-10Ar-4Low-E).
The average value of the reduced heat transfer resistance R=0.64 (with a relative glazing area = 0.7)

Let’s substitute the values ​​into the formula, we get that the outdoor air temperature at which condensation begins to form should be – 20.31°C!

i.e. the difference between 60% and 40% humidity is 31 degrees!!!

Example #10

room temperature 21
indoor humidity 60%
a window made of five-chamber PVC system Gealan 8000 with a low-emission double-glazed window 44 mm (4-16Ar-4-16Ar-4low-E).
Average value of reduced heat transfer resistance R=0.78 (with relative glazing area = 0.7)


i.e. the difference between 60% and 40% humidity is 37 degrees!!!


  • when glazing loggias and balconies – provide for sufficient thermal insulation of walls, ceiling, floor and place the necessary heat sources inside the room, which will allow reaching the recommended values ​​of 20-22 degrees Celsius, even under frosty conditions. It is recommended to place the heat source as close to the glazing as possible.
  • provide for forced ventilation of the premises in order to avoid an increase in humidity inside due to impaired air exchange. The recommended humidity values ​​during the cold season, according to SNIP, are not higher than 40%.
  • it is better to choose window structures that have the highest values ​​of heat transfer resistance in order to reduce the risk of condensation and retain heat as much as possible

As a rule, balconies and loggias have the largest area of ​​glazing. Therefore, when glazing or replacing cold glazing with warm postlendim, special attention must be paid.
If you have already completed the repair and regularly encounter condensation on the loggia windows, we will give you some effective tips.

Simple tips to reduce humidity

The most important advice. Ventilate the room! Better several times a day and longer. The more fresh air will come from the street, the more exhausted moist air will be removed from the apartment. Fresh air, saturated with oxygen, kills fungi and bacteria. You will sleep better and your body will thank you.

If available, consider a forced ventilation system. More details can be found on our website in this section:

  • Remove damp laundry from the house. Just don’t dry it in rooms. You can use special dryers or dry it on an open balcony
  • close the bathroom door when using it
  • Purchase dehumidifiers. Most modern air conditioners have this feature.
  • when preparing food in the kitchen, be sure to turn on the cooker hood
  • make sure that the interior doors do not interfere with the flow of air to the hood in the bathroom, and the ventilation grill itself is not dirty or blocked. If a fan is installed there, then try to use it in active mode during the cold season.

Simple tips on how to raise the temperature

  • without a positive temperature on the loggia or balcony, we should not expect that we will safely avoid condensation (recommended temperatures are 20-22°C) . If, in severe frosts, the specified temperature is still not reached on the balcony (say thanks to your builders and finishers), then place any additional heater in the glazing area. It is desirable that the heating system is located under the window or glazing line. Rising heat flows will remove the convection of cold air and eliminate the effect of a draft from the window that occurs during the cold season.
  • , consider installing a warm plinus heating system (our specialists are ready to install this system). In our opinion, this is the most effective way of heating loggias and balconies, both in terms of design and efficiency.
  • do not close the balcony door to the loggia or balcony so that the heat from the room is distributed to the glazed balcony or loggia, this will give you an extra 2-5 degrees of heat.

During the period of renovation, when general construction work is underway, the humidity is always above the norm, so do not be surprised when after plastering the walls or sticking wallpaper you will encounter abundant condensate.