How to find out the dew point. What is dew point? How to calculate it? Consequences of improper insulation from the inside

When designing thermal insulation of residential buildings, specialists always calculate the dew point in order to determine its position in external wall. This allows you to understand in which place there is a high probability of a significant amount of condensate being released, and thus find out how well the selected fencing material corresponds to operating conditions.

We will not post here the calculation of the dew point using formulas, which is usually done in construction, since it is quite complicated and cumbersome. By the way, many unscrupulous sellers of building materials take advantage of this, telling us about the release of moisture inside certain insulation materials. The purpose of this article is to help the average homeowner determine the dew point in the wall themselves and use it in practice.

What is dew point

You must understand that air always contains water vapor, the amount of which depends on many conditions. Indoors, steam is emitted from a person and from various daily processes of his life - washing, cleaning, cooking, and so on.

Outside, the moisture content in the air depends on weather conditions, this is understandable. Moreover, the saturation of the air mixture with vapor has its own limit, upon reaching which moisture condensation begins and fog appears.

It is generally accepted that at this moment the air has absorbed the maximum possible amount of steam and its relative humidity (denoted by the letter ω) is 100%. Further saturation leads to the appearance of fog - small droplets of water in suspension. Nevertheless, everyone has had the opportunity to observe condensation on various surfaces and without any fog.

This happens when air that is not completely saturated with vapor (humidity less than 100%) comes into contact with a surface whose temperature is several degrees lower than its own. The trick is that the air mixture at different temperatures can accommodate different quantities pair. The higher the temperature, the more moisture it can absorb. Therefore, when a mixture with a relative humidity of 80% comes into contact with a colder object, it cools sharply, its saturation limit decreases, and the relative humidity reaches 100%.

At this moment, condensation begins to fall on the surface, the so-called Dew point. This is exactly the phenomenon that can be observed on the grass in summer. In the morning, the ground and grass are still cold, and the sun quickly warms the air, its humidity near the ground quickly reaches 100% and dew falls. It is noteworthy that the condensation process is accompanied by the release of thermal energy that was previously spent on vaporization. That's why the dew disappears quickly.

It turns out that the dew point temperature is a variable value and depends on relative humidity and air temperature at a certain moment. In practice, these values ​​are determined using various meters - thermometers and psychrometers. That is, by measuring the temperature and humidity of the air, we can assume at what surface temperature the dew point will occur according to the tables, which will be discussed further.

For reference. To determine the humidity of the outside air, now it is not at all necessary to take any measurements; just look at the weather forecast on the Internet. The relative humidity is also indicated there.

Dew point determination

On this moment there is no point in thinking about how to calculate the dew point, since this has already been done by specialists a long time ago, and the results are summarized in a table. It indicates the surface temperatures below which condensation begins to form from air with different humidity levels.

As you can see, purple The standard room temperature is highlighted here winter time year - 20 °C, and the green sector is indicated, which covers the range of normalized humidity - from 50 to 60%. At the same time, the dew point ranges from 9.3 to 12 °C. That is, if all standards are observed, moisture condensation inside the house is impossible, since there are no surfaces with such a temperature.

Another thing is the outer wall. From the inside it is washed by air heated to +20 °C, and from the outside - minus 20 °C, or even more. This means that in the thickness of the wall the temperature gradually increases from minus 20 °C to + 20 °C and in some place it will definitely be equal to 12 °C, which at a humidity of 60% will give the dew point. But for this it is still necessary for water vapor to reach this place through the fencing material. And here another factor arises that influences the determination of the dew point - the vapor permeability of the material, which is always taken into account during construction.

Now we can list all the factors influencing the formation of moisture inside external walls during operation:

• air temperature;
• relative humidity;
• temperature in the thickness of the wall;
• vapor permeability of the fencing material.

Note. There are no sensors or analyzers to measure these indicators in the thickness of the walls being used; they can only be obtained by calculation.

Vapor permeability is a characteristic that shows how much water vapor a particular material can pass through over a certain period of time. All are permeable construction materials with open pores - concrete, brick, wood and so on. There is a popular expression that houses built from them “breathe.” Examples of porous insulation are mineral wool and expanded clay.

From all of the above, we can conclude that in ordinary and insulated walls there are always conditions for the occurrence of a dew point. This is where many fables and horror stories appear, associated with a huge amount of water directly flowing out of the walls during condensation, and a mass of mold growing on them. In reality, everything is not so scary, because this point does not occupy a stationary position in the fence. Over time, conditions on both sides of the structure are constantly changing, causing the dew point in the wall to move. In construction, this is called the zone of possible condensation.

Since the fence is permeable, it is able to independently get rid of released moisture, and ventilation on both sides plays an important role. External wall insulation for a reason mineral wool it is made ventilated, because the dew point in this case is in the insulation. If everything is done correctly, the moisture released inside the cotton leaves it through the pores and is carried away by the ventilation air flow.

This is why it is so important to arrange good ventilation in residential areas; it not only removes harmful substances, but also excess moisture. The wall gets wet only in one case: when condensation occurs constantly and over a long period of time, and the moisture has nowhere to go. IN normal conditions the material simply does not have time to become saturated with water.

Modern polymer insulation materials practically do not allow steam to pass through, so when insulating walls it is better to place them outside. Then the temperature necessary for condensation will be inside the polystyrene foam or polystyrene foam, but the vapors will not reach this place, and therefore humidification will not occur. Conversely, it is not worth insulating with polymer from the inside, since the dew point will remain in the wall, and moisture will begin to be released at the junction of the two materials.

An example of such condensation is a window with one glass in winter; it does not allow vapor to pass through, which is why inner surface water is formed.

Internal insulation is feasible under the following conditions:

• the wall is quite dry and relatively warm;
• the insulation must be vapor-permeable so that the released moisture can leave the structure;
• There should be good ventilation in the house.

Conclusion

So the dew point is inside building structures is always present, but it is very difficult to calculate the amount of moisture formed using formulas; you can only determine the condensation zone. And this makes it possible to take measures to remove moisture, and sometimes even prevent its appearance with the help of vapor-tight insulation.

The dew point is the air cooled to certain temperature, in a state in which steam begins to condense and turn into dew. In general, this parameter depends on the air pressure indoors and outdoors. Determining the value is not always easy, but it must be done, since this is one of the most important factors during construction and for comfortable life, and human existence indoors.

If the dew point is too high, concrete, metal, wood and many other building materials will not give the desired effect when building or renovating a house and will not last long. When laying polymer floors, if condensation gets on the surface of the material, defects such as swelling of the floor, shagreen, peeling of the coating and much more may occur in the future. It is impossible to visually determine the parameter in the room; for this you need to use a non-contact thermometer and a table.

What factors influence

• the thickness of the wall in the room and what materials were used for insulation;
• temperature, in different parts around the world it is different and the temperature coefficient of the north from the south is very different;
• humidity, if the air space contains moisture, the dew point will be higher.

To understand more clearly what it is and how the value can be affected by certain factors, let’s look at an illustrative example:

1. Uninsulated wall in the room. The dew point will shift depending on the weather conditions outside. In case of stable weather without sharp fluctuations, the dew point will be located closer to the outer wall, towards the street. In this case, there are no harmful indicators for the premises itself. If a sharp cold snap occurs, the dew point will slowly move closer to the inside of the wall - this can lead to the room being saturated with condensation and the wall surfaces slowly getting wet.
2. Externally insulated wall. The dew point has a position inside the walls (insulation). When choosing a material for insulation, you should count on this factor and correctly calculate the thickness of the selected material.
3. Wall insulated from the inside. The dew point is between the center of the wall and the insulation. Is not the best option, if the weather conditions are too humid, since with a sharp cold snap, in this case the dew point will sharply move to the junction between the insulation and the wall, and this in turn can lead to disastrous consequences for the wall of the house itself. It is possible to insulate a wall from the inside in a humid climate if there is good system heating system, which is able to maintain an even temperature in each room.

If the home renovation is done without taking into account weather conditions, it will be almost impossible to eliminate the problems that have arisen; the only way out is to start the work again and clean up everything that has been done, which entails a lot of spending money.

How to correctly determine and calculate (table and formula)

Dew point can be affected by temperature and humidity

It is quite difficult for a person to live in comfort with high humidity. Condensation causes problems both for health (there is a chance of developing asthma) and for the house itself, especially for its walls. Ceiling and walls from high humidity can become covered with mold that is harmful to humans and difficult to remove; in rare cases, it is necessary to completely replace the walls and ceiling in order to kill all harmful microorganisms present.

In order to prevent this from happening, you should make a calculation and find out whether it is worth undertaking repairs in a particular building, insulating the walls, or even building housing in this place. It is important to know that for each building the dew point is individual, which means that its calculation will be carried out with slight differences.

Before starting the calculation, you should take into account such factors as: climatic conditions in a particular region, the thickness of the walls and the material from which they are made, and even the presence strong winds. Absolutely all materials contain low, permissible humidity; a person should make sure that this humidity does not increase and a dew point does not form. When you call a specialist to measure the value in case of high humidity, you will most likely be given the answer that the thermal insulation of the house was not done correctly, the thickness of the material is not suitable, or an error was made during installation. To some extent, this person will be right, since it is correct repair in the house has a greater influence on the change in dew point and the appearance of condensation on the walls.

Table: indicators for determining dew point

Schedule

Thanks to the graph, you can determine the optimal indicators

How to calculate: necessary tools and sequence of actions

• thermometer;
• hygrometer;
• non-contact thermometer (can be replaced with a regular one).

Formula for calculations in frame, brick, multi-layer walls with insulation

To calculate the dew point with insulation, the following formulas are used: 10.8 °C

Using the obtained indicators, draw up a graph with the temperature range T1 placed in the wall and the remaining °C for the insulation. IN in the right place mark the dew point.

What to do if the value is defined incorrectly?

Let's consider the places where the dew point can be located in an uninsulated wall:

• Closer to the outer surface of the wall. In this case, the appearance of dew point in the house is minimal; as a rule, the inner wall remains dry.
• Closer to the inner surface of the wall. In this case, condensation may appear when it gets cold outside.
• In the rarest cases, the dew point is at interior wall building. In this case, it is almost impossible to get rid of it, and most likely the walls in the house will be a little damp all winter.

In these cases, the problem can be solved by adding layers of vapor barrier to the walls. This will help retain water vapor so that it does not pass through the walls into the room, which will prevent dew points from appearing on the walls and ceiling. If the climate is too cold and most year the temperature remains more than minus 10 degrees, it is worth considering the option of forcing heated air into the room. This can be done using a heat exchanger or air heater.

Video: why condensation and mold appear on the walls

It is important to correctly determine the dew point during the construction phase. This will help to properly insulate the wall and further avoid the appearance of condensation and mold in the house.

Dew point is the temperature at which water vapor from the air begins to condense on surfaces. It happens that in heating season we can observe moisture condensation on windows and sometimes walls. In the latter case, condensation can even lead to the formation of mold.

In this article we will try to understand the concept of “dew point” and learn how to determine the temperature of condensation on surfaces.

What does the dew point depend on?

• Indoor air humidity
• Air temperatures

Let's consider a simple example for understanding: the indoor air has a temperature of +20°C and with an air humidity of 60%, condensation will form on a surface with a temperature below +12°C.

Thanks to the nomogram below, the dew point temperature can be determined more accurately.

Nomogram for determining dew point

• Regular hygrometer— shows relative air humidity as a percentage. It is enough to simply take his testimony.
• Psychometric hygrometer— has two alcohol thermometers with a division value of 0.1-0.5°C. One thermometer is dry, the second has a humidification device. For the convenience of determining the relative humidity in the room, a psychometric table is used.

Having measured these values, then on the nomogram using a ruler we draw a ray from the room temperature scale to the known air humidity, in the place where the ray intersects the “Dew point temperature” scale and will be the desired surface temperature value for your case.

To determine the level of humidity in a room, it will be useful to buy a hygrometer.

Contained in an isobarically cooled gas, it becomes saturated above the flat surface of water.

The chart below shows the maximum amount of water vapor in the air at sea level as a function of temperature. The higher the temperature, the higher the equilibrium partial pressure of vapor.

The dew point is determined by the relative humidity of the air. The higher the relative humidity, the higher the dew point and the closer to the actual air temperature. The lower the relative humidity, the lower the dew point than the actual temperature. If the relative humidity is 100%, then the dew point is the same as the actual temperature.

Formula for approximate calculation of dew point in degrees Celsius (only for positive temperatures):

The formula has an error of ±0.4 °C in the following range of values:

0 °C< T < 60 °C 0.01 < RH < 1.0 0 °C < T r < 50 °C

Dew point and corrosion

Air dew point - the most important parameter with anti-corrosion protection, indicates humidity and the possibility of condensation. If the dew point of the air is higher than the temperature of the substrate (substrate, usually a metal surface), then moisture condensation will occur on the substrate.

Paint applied to a substrate with condensation will not achieve proper adhesion unless specially formulated paints are used (Certificate available from Technological map product or paint specification).

Thus, the consequence of applying paint to a substrate with condensation will be poor adhesion and the formation of defects such as peeling, blistering, etc., leading to premature corrosion and/or fouling.

Dew point determination

Dew point values ​​in degrees °C for a number of situations are determined using a sling psychrometer and special tables. First, determine the air temperature, then the humidity, the temperature of the substrate, and using the Dew Point table, determine the temperature at which it is not recommended to apply coatings to the surface.

If you cannot find your exact reading on a sling psychrometer, then find one reading one division higher on both scales, both relative humidity and temperature, and another reading correspondingly one division lower, and interpolate the required value between them. The ISO 8502-4 standard is used to determine the relative humidity and dew point of a steel surface prepared for painting.

Temperature table

Dew point values ​​(°C) in different conditions are given in the table.

Comfort range

A person feels uncomfortable at high dew point values. In continental climates, conditions with a dew point between 15 and 20 °C cause some discomfort, and air with a dew point above 21 °C is perceived as stuffy. Low dew point, less than 10°C, correlates with lower temperature environment, and the body requires less cooling. Low dew point may go along with high temperature only at very low relative humidity.

see also

• Psychrometric chart (Molier chart)

Literature

• Burtsev S. I., Tsvetkov Yu. N. Wet air. Composition and properties (djvu, full text)
• Independent calculation of dew point inside building envelopes

Wikimedia Foundation. 2010.

Dew point- this is the temperature of the environment at which water in the air turns from gas into liquid (condenses).

Air humidity depends on its temperature. With the same amount of water vapor, cold air will be more humid than warm.

If we gradually cool the air, then a temperature will come at which the humidity will become one hundred percent. At this moment it falls out liquid condensate(dew). This temperature is called the dew point.

The temperature and humidity of the air on the inner edge of the wall or roof of the house is much higher than on the street. This leads to that same gradual decrease in temperature inside the structure. If the insulation is not selected correctly, there is a possibility that inside the wall there will be such humidity and temperature at which condensation forms.

Dew point position in the design of the wall depends on:

• How warm is it inside the room?
• how cold it is outside;
• width and thermal conductivity building materials, of which the wall consists;
• indoor humidity;
• air humidity outside.

Let's find out how the position of the dew point changes in different situations.

In a wall without insulation. If the wall of your house is not insulated at all, then the temperature inside its structure will gradually decrease from the inner edge to the outer one.

And then the dew point may be located: in the middle of the wall; close to the outer edge of the wall; close to the inner edge of the wall.

In a wall with insulation on the outside. In the place where the insulation of sufficient thickness is located, there will be sharp drop temperatures Then the dew point will be inside the insulation, and the entire wall will be warm.

If the insulation is too thin, the dew point may move to the middle of the wall or to its inner edge.

In a wall with insulation inside. In this case, there will also be a sharp rise in temperature in the place where the insulation is located. In this case, the wall will be in a cold zone and the dew point will shift to the inner edge of the wall.

To find dew point temperature you need to use the table.

For this find in the table appropriate room temperature and at the intersection with the current humidity readings the dew point temperature will be found.

Which ones exist? methods for determining dew point The article “Determining the dew point: secrets and nuances” will tell you.

Consequences of illiterate choice and calculation

The dew point that occurs inside the material causes it to become moist. Wet walls has the following consequences:

• Moistened concrete and brick have less heat-insulating properties.
• On wet wall Fungus and mold may develop.
• Indoors with damp walls there will be an unpleasant microclimate.
• If the moisture inside the wall freezes and crystallizes, its crystals will destroy the material of the structure. Several freeze and thaw cycles can cause the material to lose strength.
• For insulation, humidity is dangerous due to the deterioration of its heat-shielding properties.

Advice! Wet insulation can be easily dried if special ventilation is provided. This is the technology used to operate a ventilated façade.

Calculation of insulation comes down to selecting its thickness in each specific case. In this calculation it is necessary pay attention to the dew point position. An incorrect thickness of the layer of insulating material can lead to wetness and freezing of the entire wall structure.

In order to avoid getting wet load-bearing structure walls, it is best to place the insulation outside. In this case, it is necessary to provide good ventilation of the insulation layer, as well as its protection from bad weather.

Insulation can only be placed indoors at low humidity air or a slight temperature difference between the inside and outside of your room.

Dew point - what is it and how to determine it in a wall, watch the video:

Watch the video tutorial about dew point temperature: