It was already mentioned in the previous articles of the section that even a perfectly executed waterproofing of the roof with liquid rubber will not save from leaks over time if the vapor barrier layer was initially poorly or incorrectly laid.

The task of vapor barrier is to reduce (ideally to 0) the diffusion of gases through the vapor barrier layer in order to prevent warm air from entering the lower heated rooms into the cold space of the roofing pie above the reinforced concrete floor slab. Why is it necessary?

Because warm air from below, ending up in a cold space, acts as a source of moisture formation. This phenomenon is directly related to such a term as Dew point. And the dew point, in turn, is directly related to absolute and relative humidity. Let's explain this phenomenon.


As noted on the page, the same amount of moisture in the air corresponds to different relative humidity values. In particular, relative humidity is inversely proportional to air temperature:

  • when the air temperature rises, the relative humidity decreases, because warm air can "contain" more moisture in the form of steam;
  • when the air temperature drops, the relative humidity increases (and at some point it will reach 100% and then absolute humidity will take place), because cold air can "take in" a smaller amount of moisture in a gaseous state.

For example, there is a closed cubic meter of air, in which it is known that there is 9.41 grams of moisture. When the air temperature in this cube changes, the relative humidity will also change, namely:

  • 9.41 grams of moisture at +30 degrees C correspond to 31% relative humidity;
  • 9.41 grams of moisture at +25 degrees C correspond to 41% relative humidity;
  • 9.41 grams of moisture at +20 degrees C corresponds to 54% relative humidity;
  • 9.41 grams of moisture at +15 degrees C correspond to 73% relative humidity;
  • 9.41 grams of moisture at +10 degrees C correspond to 100% relative humidity;
  • 9.41 grams of moisture at +5 degrees C correspond to 138% relative humidity;
  • 9.41 grams of moisture at 0 degrees C corresponds to 194% relative humidity.

We recall (see the article at http://www..html) that air can "accept", "accommodate", "contain" only a limited amount of moisture in the gaseous state, the maximum of which corresponds to 100% relative humidity (it in this case, absolute humidity). Therefore, all moisture that does not fit into 100% relative humidity condenses in the form of water. Specifically, for the example above:

  • At a temperature of +5 degrees C, 2.61 grams of water falls out of 9.41 grams of steam;
  • At a temperature of 0 degrees C, 4.56 grams of water falls out of 9.41 grams of steam.

Chart can be enlarged, click on it.

It can be seen from the example that a relative humidity of 100% at a temperature of +10°C corresponds to the same amount of moisture in the gaseous state as a relative humidity of 31%, but at a temperature of +30°C. And this despite the fact that 100% > 31%. Thus, it is obvious that relative humidity data by themselves are not a sufficient informative base if the air temperature is not indicated.

Thus, the air containing steam, when cooled, reaching a certain temperature, is no longer able to "retain" part of the moisture in the gaseous state and gives it out in the form of water droplets, and the amount of water "given away" in this way depends on the cooling temperature.

Air dew point in soft roof layers

Everything that is described above is directly related to the issue of roof waterproofing with liquid rubber (or waterproofing or repairing a flat soft roof with a traditional roll). The fact is that in the cold season, especially during the heating season, warm air from the building diffuses through the floor slab into the under-roof space, i.e. where the insulation is laid, the screed under the slope and the waterproofing coating.

Theoretically, such penetration is impossible if a vapor barrier is made on the floor slab. But, as practice shows, the vapor barrier device is not always performed properly. By the way, that is why it is convenient to use liquid rubber for vapor barrier. In this case, the entire base of the reinforced concrete floor slab is covered with a monolithic, seamless, excellent adhesion vapor barrier material with a giant diffusion resistance coefficient of 150,000. Moreover, it is enough to apply a layer of only 1 ... 1.2 mm of liquid rubber.

But few people use liquid rubber for vapor barrier. For many, even waterproofing with liquid rubber raises doubts and mental anguish (mainly because of the price), and vapor barrier is even more so. It’s good if at least a cheap vapor barrier film is spread over concrete, because there are those who don’t understand why a vapor barrier is needed at all.

So warm air which contains! water vapor, diffuses through the floor slab and ends up in the under-roof space. Obviously, the air temperature under the stove is significantly higher than the air temperature above the stove. In other words, the temperature of the diffusing air containing water vapor decreases.

Depending on the insulation used on the roof (thermal conductivity coefficient), the thickness of the insulation, the air temperature outside, the air temperature in the under-roof space also changes. This temperature decreases in the direction from the floor slab to the top layer of the roof (for non-exploited - this is waterproofing).

On the surface of the roof, the temperature is already the same as on the street (if there is no snow cover on the roof). Obviously, at some point in the roofing pie, the temperature drops to such a value (this is the temperature that is called dew point), when the relative humidity reaches 100%, after which there is an "excess" of steam, which can no longer be held in a gaseous state. They fall out in the form of droplets of moisture. Those. water enters the roof space. Obviously, this is not good.

The figure illustrates the above. From top to bottom we see the following layers (there is no vapor barrier):

  • waterproofing
  • insulation
  • floor slab;
  • plaster.

And, if, for example, in a room under a floor slab + 18 degrees C and a relative humidity of 45%, then Dew point 5.9 0 C (see below dew point table depending on temperature and relative humidity).

It can be seen from the figure that the temperature directly on the surface of the concrete slab is + 7 degrees C, and above - in the heater, the temperature decreases, passes through 0 0 C and on the surface of the insulation is already close to the outside temperature of -15 0 C. The figure shows, that the dew point value of +5.9 0 С for this example is above the floor slab, in the heat-insulating layer, and since there is no vapor barrier layer, then water will condense and accumulate in the insulation.

Dew point definition

dew point the temperature of that point [of the under-roof space] is called, below which the water vapor contained in the gas cooled isobarically (at constant pressure and mass of gas) becomes saturated ("excess" falls out in the form of water droplets).

The device and repair of flat roofs must be carried out in such a way as to prevent the ingress of condensed moisture into the under-roof space, because. this leads to damage to the functional layers of the soft roof and negatively affects the bearing capacity of the roof base. Knowing the coefficients of thermal conductivity and the thickness of the materials of all layers of soft roofing, it is possible to initially calculate the dynamics of temperature decrease in the direction from the floor slab to the street.

In order to design a roof correctly, it is necessary to know at what temperature, and therefore, at what point in the roof space, a part of the moisture will pass from the air to the liquid state. In other words, it is necessary to understand where the dew point will be located in the roofing pie.

Dew point temperature table

Below is dew point table depending on the initial temperature and humidity, focusing on which you can find out the dew point for various climatic indicators. Click on the picture to enlarge.

How to use the dew point table?

On the left, find the value of the air temperature in the room, and on the top, the value of the relative humidity of the air in this room. At the intersection - the dew point temperature for this room, i.e. to what temperature at a given humidity should it "get colder" so that the "excess" steam from the air falls out in the form of water droplets.

Dew point in construction

Already at the design stage, consideration should be given to dew point in construction. To avoid damage to building structures (both roofs and walls) due to the transition of part of the vapor into a liquid state when the air is cooled, two basic conditions must be met:

  1. Moisture due to air condensation must not form on the covering of walls and ceilings of interior spaces bordering on the street.
  2. In the structure of roofing layers or walls, moisture accumulation of more than 1 kg/m2 is unacceptable.

The fulfillment of the first condition is achieved by the device outside the heat-insulating layer of sufficient thickness. In this case, the dew point temperature is after the vapor barrier layer. And, therefore, the temperature on the surface of walls and ceilings in the interior, bordering the street, will always be above the dew point.

As for the fulfillment of the second condition, it should be understood that it is impossible to 100% avoid the formation of condensation moisture in the layers of a soft roof. But, it is quite possible to minimize the formation of condensate if the vapor barrier device is correctly performed. And, moreover, condensation moisture does not pose a threat to building structures, incl. and roofs, if:

  • Condensate water, which is formed, as a rule, in winter, during the heating season, can be discharged outside without damaging the roof in summer. This is discussed in detail in the article on.
  • Building materials in contact with condensate water are not subject to rotting, corrosion, fungal infection or other damage.
  • The amount of water-condensate does not exceed 1.0 kg per 1 m2, which corresponds to a liquid film with a thickness of 1 mm per 1 m2.

To sum up, with regard to dew point in construction, it should be noted that when constructing roofs and walls, the vapor barrier layer should never be neglected.

For the safety and durability of building structures, it is required to minimize (ideally - to 0) the diffusion of warm air from the interior to the outside. And for this you need a vapor barrier. But a competent vapor barrier device is possible only in combination with the correct thermal insulation, the thickness of which determines both the physical location and the dew point temperature in the wall or roof.

Therefore, a continuous, seamless vapor barrier (e.g. liquid rubber) under a sufficiently thick layer of thermal insulation (in this case ensuring that the dew point is above the vapor barrier material) prevents water vapor from inside the building from penetrating into the thermal insulation, cooling and condensing there, causing damage. and destruction of soft flat roof layers.

The following article from the section Vapor barrier on www.website tells how to properly implement a vapor barrier device, taking into account the dew point.

Recently, we are increasingly convinced of how dependent a person is on the nature around him. The disturbed ecology by him as a response in the form of global warming, climate change and other violations of normal (habitual) conditions of existence has a negative impact on the health of not individual individuals suffering from this or that disease, but almost all people inhabiting planet Earth.

The relevance of this problem is confirmed by the reports of the World Health Organization, the growth of somatic and infectious diseases, the decrease in life expectancy, as well as the manifestation of other negative trends in a wide range of medical statistics. And this is all at the current level of development of medical technologies and vast experience in the treatment and prevention of almost all human diseases.

You can solve at least part of the problem by creating a favorable microclimate in your homes and workplaces. We have long learned to regulate the temperature at the level of established medical and sanitary standards or to make it simply comfortable for a person. But besides temperature indicators, there are many other evaluation criteria. Among them, relative humidity plays an important role. The amount of moisture in the air determines the well-being of people, their performance, the ability to get a full-fledged high-quality rest within a particular room.

Low humidity adversely affects the skin and mucous membranes. Their drying can lead to the appearance of microcracks, and these are open paths for the penetration of various infections. High humidity causes a feeling of heaviness, adversely affects the functioning of the cardiovascular system and other systems of the human body.

Optimal has a clear framework. To classify this indicator, you can use the following standards:

From 20% and below - corresponds to extremely low humidity;

30 to 40% is just low;

From 40 to 55% - the most optimal humidity;

From 60 to 65% - is already an indicator of high humidity;

From 70 to 75% - high humidity;

From 75% and above - the humidity is very (extremely) high.

To recall what relative humidity is, let's turn to a high school level physics course. From the first reading, the definition of this quantity looks confusing and a little incomprehensible. But with a little understanding, it becomes clear that relative humidity is a value equal to the ratio the amount of moisture (steam) in a certain volume of air to the amount of the maximum possible amount of water that can be contained in the air at this temperature. This value is measured in percent.

If the first part of the expression does not raise any special questions, then the second part needs to be dealt with. The fact is that water vapor cannot dissolve in the air indefinitely, there comes a moment when the vapor becomes saturated, and the excess moisture entering the air falls in the form of condensate, rain, fog or dew. This indicator depends on temperature and pressure.

The determination of the relative humidity of the air is carried out using a variety of instruments. Modern industry has launched the production of various thermohygrometers, wet bulb temperature and humidity meters, moisture meters, anemometers and other instruments. But most importantly, technical devices have appeared on the market that can regulate the level of humidity.

The relative humidity of the air increases with the help of special humidifiers, and air conditioners equipped with this function are used to dry the air.

"Adaptation of organisms" - The fitness of organisms is relative and is the result of natural selection. Hawk hawk. Physiological adaptations. Mimicry. Dissecting coloration. The opposite of disguise. Insectivorousness in plants Chemical interaction. Caring for offspring. It does not allow to perceive the animal's body as a whole.

"Humidity for humans" - Recommendations for the regulation of humidity and air temperature. Changes in air humidity during the day in room 9. Change in air humidity during the air in the 7th room. Problems with deviations from optimal humidity. Humidity,%. Scientific work in physics on the topic "The influence of air humidity on human well-being."

"Air humidity class 10" - Weather forecast for Mon 25 January. Psychrometric table. Ultrasonic humidifiers are the most efficient humidifiers available. Humidity: Dehumidifiers. Humidifiers. Any technical device requires a verification of the physical environment. Air is the object of study. Hygrometers psychrometers condensation hair.

"Air humidity lesson" - 3. Relative humidity in the evening at 16? C is 55%. Humidity value. Type of lesson: consolidation and improvement of knowledge. The dew point is measured using a thermometer, and then the relative humidity of the air is determined. Table "Humidity". Will there be dew if the temperature drops to 8°C at night? Hygrometer Psychrometer Psychrometric Table Calculator< Casio>.

“Air temperature and humidity” - People say: Salt gets wet - to rain. Psychrometer. Normal air humidity is 60%. The value of air humidity. Hair hygrometer. Saturated steam … Condensation hygrometer. Psychrometric table. Hospitals, clinics, pharmacies. Tobacco becomes damp - to damp weather. Signs are witnesses of folk wisdom.

"Air humidity" - A. laundry dries out due to condensation of water vapor. Humidity is the amount of water vapor in the air. Consolidation. B. Cooling occurs due to the condensation of water vapor. C. When water vapor condenses, heat is absorbed. Ficus. The concept of air humidity. 1. An open pot of water was placed on the stove.

We open a series of three articles on the topic of air humidity. The topic is large and multifaceted. Some have such dry air in the apartment that all the furniture is shocked. Others have such dampness in the house that mold appears on the walls, floor and ceiling. In three articles of our series, we will tell you what the humidity in the apartment should be in the norm, how to deal with both low and high humidity in the apartment.

In the first part, we will talk about what humidity is in general, how it depends on temperature, and how you can determine the humidity in an apartment at home.

A bit of physics: absolute and relative humidity

The total amount of water in a cubic meter of air is its absolute humidity. If one cubic meter of air contains 0 grams of water, then the air is completely dry and its absolute humidity is 0 g/m3. "Pour" into this cubic meter 5 grams of water - we get an absolute humidity of 5 g / m3.

Absolute humidity cannot increase indefinitely, it has a limit. The maximum absolute humidity is called moisture capacity, that is, how much water can hold a cubic meter of air under given conditions. If the moisture capacity is 10 g / m3, then we will not “pour” more than 10 grams of water into a cubic meter of such air. Any excess will "pour" out of the air as condensate.

With absolute humidity sorted out. Now about the relative humidity of the air. It shows how much the air is "filled" with water.

The maximum possible absolute humidity is taken as 100%. If it is equal to 10 g/m3, and in reality there is 1 gram of water in a cubic meter of air, then the relative humidity is 10%. Normal air humidity in an apartment is considered to be 40-60%.

Air humidity and temperature

The relative humidity of the air depends on the temperature. Let's take two cubic meters of air: one is in a room at a temperature of 25°C, and the other is outside the window, at 0°C. The relative humidity of both air samples is 100%. But if we "squeeze out" each of them, we find that there was much more water in the warm air than in the cold one. Why did it happen, because the volume of air is the same (one cubic meter), and both cubic meters were "filled" equally to the eyeballs?

But because the moisture content of air depends on temperature: the warmer the air, the more water it can hold. At a room temperature of 25 ° C, a cubic meter of air can contain up to 20 grams, and at zero temperature - less than 5 grams. This means that 100% humidity in the room and on the street is not the same thing. In the heat it is 20 g/m3, and in the cold it is at most 5 g/m3.

What happens to the air when it enters the house from the street?

Let's say outside the window the temperature is zero, the relative humidity is 100%. As we already know, there is less than 5 grams of water in a cubic meter of such air. When it enters a warm room (25°C), the air heats up, its moisture capacity rises to 20 g/m3. And there is no more water in the air. Therefore, the same 5 grams of water is no longer 100% relative humidity, as on the street, but only 25%. Thus, the humidity in the apartment decreases when cold outside air enters the house. Therefore, any supply ventilation dries the air in winter (we are talking about an ordinary open window, and about special intake devices).

The reverse situation: when there is no air flow from the street at all at home. Then there may be increased humidity in the apartment. We wrote about this earlier.

How to measure the humidity in the apartment without special devices

To accurately measure the humidity in the apartment, you need a special device, a hygrometer. But there is a simpler, albeit less accurate, way.You will need: a glass, water and a refrigerator. The sequence of actions is elementary:

  1. We fill the glass with water.
  2. We put it in the refrigerator for several hours. Our goal is to cool the water down to 3-5°C.
  3. We take the glass out of the refrigerator and put it on the table in the room where we want to measure the humidity of the air. Important condition: put the glass away from batteries, heaters and other heat sources.


  1. The walls of the glass were first covered with small droplets of water, and then rapidly dried up in 5-10 minutes. This indicates that the air is very dry.
  2. The walls were covered with large drops, which are in no hurry to dry and flow down. This means that the air is very humid.
  3. The walls were covered with drops again, but after 5-10 minutes the drops did not dry up and did not flow. In this case, the humidity in the apartment can be considered normal.

Try to measure the humidity in your apartment in the described way. If the result is dry air in the apartment, then you should consider a humidifier. If, on the contrary, you find increased humidity in the apartment, then you should think about fresh ventilation.

We will talk about how low or high humidity in the apartment affects health in the second part of this cycle. And in the third article we will introduce you to the humidifiers on the market.