What is "dew point". Dew point in construction - what is it and how to calculate Why do we need a dew point

The dew point is called the cooled air to a certain temperature, in which the vapor begins to condense and turn into dew. In general, this parameter depends on the air pressure in the room and on the street. It is not always easy to determine the value, but it is necessary to do this, since this is one of the most important factors in construction and for a comfortable life and human existence in the room.

With an overestimated dew point, concrete, metal, wood and many other building materials will not give the desired effect when building or repairing a house and will not last long. During the laying of polymer floors, if condensate gets on the surface of the material, in the future, such defects as swelling of the floor, shagreen, peeling of the coating and much more may occur. It is impossible to visually determine the parameter in the room, for this it is necessary to use a non-contact thermometer and a table.

What factors influence

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

To better understand what it is, and how certain factors can affect the value, consider an illustrative example:

1. Insulated wall in the room. The dew point will shift depending on the outdoor weather conditions. In the 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 room itself. If a sharp cold snap occurs, the dew point will slowly move closer to the inside of the wall - this can lead to saturation of the room with condensate and slow wetting of the wall surface.
2. Externally insulated wall. The dew point has a position inside the walls (insulation). When choosing a material for insulation, you should rely on this factor and correctly calculate the thickness of the selected material.
3. Insulated wall from the inside. The dew point is between the center of the wall and the insulation. This is not the best option if the weather conditions are too humid, because with a sharp cold snap, in this case, the dew point will sharply shift 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 the house has a good heating system that is able to maintain a uniform temperature in each room.

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

How to correctly identify 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 possibility of getting sick with asthma) and for the house itself, especially for its walls. The 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 change 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 starting repairs in a particular building, insulating 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 proceeding with the calculation, one 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 of 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 an answer that the thermal insulation of the house is not done correctly, the thickness of the material is not suitable, or a mistake was made during installation. To some extent, this person will be right, since it is the correct repair in the house that to a greater extent affects the change in the dew point and the appearance of condensation on the walls.

Table: indicators for determining the dew point

Schedule

Thanks to the graph, you can determine the optimal performance

How to calculate: necessary tools and sequence of actions

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

Formula for calculation in frame, brick, multilayer 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. Mark the dew point at the desired location.

What if the value is defined incorrectly?

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 a 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 occur during a sharp cold snap outside.
• In the rarest of cases, the dew point is on the inside wall of a 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 vapor barrier layers to the walls. This will help keep water vapor from escaping through the walls into the room, preventing dew points from appearing on the walls and ceiling. If the climate is too cold and most of the year the temperature is more than minus 10 degrees, it is worth considering the option of forced entry of heated air into the room. This can be done using a heat exchanger or an 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 correctly insulate the wall and in the future to avoid the appearance of condensation and mold in the house.

In climatic zones, in which conditions change depending on the time of year or time of day, builders face the difficult task of choosing and calculating the right amount of building materials for arranging a home and creating a comfortable microclimate in it. There is a question of protection from sub-zero temperatures, winds and humidity. The answer is a simple word - warming. But its effectiveness directly depends on the dew point, which shows how much water vapor is in the air.

Dew point: a bit of history

For a long time, people who did not even think about determining some kind of dew point built their own dwellings in order to relax in conditions of complete silence, when neither insects nor animals disturb you, not to mention weather phenomena. If a simple house made of natural materials is enough on a tropical island, since this area maintains a comfortable air temperature all year round, then the situation may be completely different in another climatic zone, where the indoor temperature differs from the outdoor temperature by several tens of degrees. How to maintain comfortable conditions in the home under such weather conditions? That's right, build walls from the appropriate material that can withstand all these misfortunes, and insulate them. But this is not enough - you still need to know what the dew point is, and learn how to calculate it correctly.

There are building codes in which different values ​​are calculated for any region, including the thickness of the building walls for a certain material, the thickness of the insulation for a certain wall thickness, etc. Unfortunately, some customers, in order to save materials and reduce construction costs, take these indicators at the lower limits. Here they can stumble upon "underwater reefs". In an ever-changing climate, there is no guarantee that the frost will not be too severe this winter. As a result, if during construction the determination of the dew point was not correct or was not carried out at all, due to changes in temperature and humidity outside, the walls inside the room begin to get wet, and mold and fungus appear over time.

Our ancestors tried many building materials that provided reliable protection from frost and snow, water and rain in spring and autumn and heat in summer in regions with severe winters. It was enough for them to build a hut with thick breathing walls (with a margin, so to speak), put a good stove inside, ensuring the correct circulation of warm air, and the job was done. In such a dwelling, any person felt comfortable, without even wondering about the calculation of the dew point. But time passed...

With the advent of large cities and their development, people began to build multi-storey buildings. They began to use new materials in construction. Construction firms and private traders began to save on material, being guided during construction by the lower limits of the values ​​of building regulations. In addition, in the cold and winter season in apartment buildings (and not only) they began to use not heat from the stove, but central heating or individual heating systems that work on the same principle.

Why do you need the correct calculation of the dew point when insulating a house?

Have you ever seen an uninsulated house in which (especially when it is minus outside) the walls near the ceiling or floor are damp? What is this moisture? Turns out it's dew. Indoors? Yes! And not only indoors, but also in the walls, in the floor.

Humidity, temperature and atmospheric pressure. When these three values ​​change, precipitation occurs. Precipitation comes in the form of rain, snow and dew. Let's talk about the latter in more detail.

As a result of the contact of a cold surface and humid warm air, its humidity drops, and condensation begins to form on this surface. This process can be observed on the walls of a glass with a cold drink.

The temperature at which this condensation occurs is called the dew point temperature (DTP). At a certain value of temperature and atmospheric pressure, with an increase in air humidity, the dew point value, which is expressed in degrees, also increases. The walls of a glass of ice are at dew point temperature. Thus, this concept is used to somehow indicate the content of water vapor in the air. Having calculated everything correctly, you will find out the temperature value at which the humidity of the air reaches 100 percent. If this temperature is equal to the air temperature (it cannot exceed it), then fog or rain is formed (depending on pressure). If it is much less, there will be no precipitation.

So, if there is moist air and an object whose temperature has a TTR, moisture will accumulate on this object. That is why it is necessary to determine the dew point during various construction works, including the erection of walls, their insulation, pouring self-leveling floors, thermal insulation of roofs of buildings, etc. For example, at , if the temperature outside is such that the dew point is in the area from the center of the wall, closer to its inner edge, you will see a wet spot in your room until the temperature outside rises. If this continues for some time, a fungus will form on these surfaces, loving the combination of moisture, heat and carbon dioxide (which we exhale from the lungs). Now we have come to the main point.

Dew point determination

Example #1

Let's take, for example, a very common case in construction: the installation of self-leveling floors. The humidity of the air in the room and the temperature of the substrate to be coated play an important role. After all, if the floor has a TTR, the moisture released in it can adversely affect the strength of the future coating - all kinds of deformations appear, which soon turn into delamination of the coating. To avoid this, it is necessary to measure the humidity in the room (with a hygrometer) and the air temperature. We calculate the dew point using the formula:

Or based on the finished table:

Click to enlarge

For example, if the dew point turned out to be 11 degrees Celsius, and the base temperature is not higher than the dew point by 5 degrees, it is not recommended to install a self-leveling floor.

Example #2

Arrangement of external insulation of the house with foam plastic or. In this case, the situation is much more complicated. After all, you need to measure the temperature and humidity outside and inside the room in all sorts of combinations that happen in your climate zone. To help builders, the norms SP 23-101-2004 "Design of thermal protection of buildings" and SNiP 23-02-2003 "Thermal protection of buildings" have been developed. Also, manufacturers of insulation systems provide on their websites special calculators for calculating the thickness of the insulator, depending on the parameters of the walls and climatic conditions, so that the dew point does not end up in an unnecessary place.

Properly executed thermal insulation provides favorable living conditions and reduces the cost of maintaining a comfortable temperature. With the seeming simplicity of the insulation process and the presence of a large selection of thermal insulation materials, it is important to choose the right location for the insulation. This will prevent the formation of mold caused by the accumulation of moisture. That is why the dew point in construction is an important concept that characterizes the temperature of condensation. It is important to understand where it is in a particular case and how it is calculated.

What is dew point in construction

Many have heard, but not everyone can correctly answer what semantic load the widely used concept carries - the dew point. Its definition in construction is unambiguous. This is the temperature threshold at which moisture in the air condenses and turns into water droplets. The condensate formation area can be located both inside the main wall, and from the outside or inside of the building. The location of the condensate fallout zone is determined by a set of the following indicators:

• moisture concentration in the room;
• temperature conditions of the room.

At a constant temperature and an increase in relative humidity, the temperature threshold for the formation of condensate increases accordingly. For a correct understanding of the processes, let's consider how the threshold for condensation increases at a room temperature of 20 ° C:

• at a humidity of 40%, moisture turns into drops of water at a surface temperature of plus 6 ° C and below;
• an increase in relative humidity up to 60% causes condensation at 12 °C;
• when the moisture concentration reaches 80%, moisture condenses at 16.5 °C;
• at 100% humidity, the condensation temperature corresponds to the internal one and is 20 °C.

The difference between dew point and temperature can indirectly estimate relative humidity:

• with a small difference - high humidity;
• with a significant discrepancy - the vapor concentration is negligible.

Depending on how far the dew point is from the room in the wall, the state of the surface changes - it can be wet or completely dry. This is due to the condensation of moisture that occurs when a cold surface comes into contact with warm air. Professional builders attach great importance to this parameter, since it is inextricably linked with the issues of thermal insulation of buildings and the creation of a comfortable microclimate.

Wall dew point - location options

The location of the dew point in the capital structures of the building is determined by the following factors:

• material used for manufacturing;
• the distance from the street surface of the wall to its plane, located in the room;
• outdoor and indoor air temperature;
• relative humidity outside the room;
• moisture concentration in the house.

Consider how likely the formation of condensate inside the room for different types of walls:

• not thermally insulated;
• thermally insulated from the outside;
• insulated from the side of the room.

For the non-insulated version, the following location options are possible:

• closer to the outer surface. At the same time, moisture condensation is impossible and the wall of the room is absolutely dry;
• with an offset from the middle of the wall into the room. There is no condensate, but it can occur when the outdoor air cools sharply;
• on the inside of the wall. With a sharp cold snap, moisture actively condenses.

With an external location of thermal insulation, the following options for locating the problem area are possible:

• in an array of thermal insulation material. This is the optimum position to guarantee a dry surface;
• in any of the three zones, similarly to the non-insulated version. The displacement is associated with incorrect calculations and the use of insulation of insufficient thickness.

Internal insulation significantly shifts the position of the condensation area towards the room and contributes to the cooling of the walls located under the heat insulator. This significantly increases the likelihood of moisture accumulation in any of these areas:

• inside the wall. The surface is dry, but can be moistened with significant temperature fluctuations with a shift towards the room;
• between wall and insulation. Condensation is inevitable during winter cooling;
• deep inside the heater. Droplets of moisture in winter are constantly collected, moistening the insulation. The result is dampness and mold formation.

The correct location of the insulation avoids the formation of dampness caused by an increased concentration of condensed moisture.

To determine the temperature threshold for the formation of condensate, various methods are used:

• settlement. Calculations are made according to a cumbersome formula that takes into account a number of coefficients, as well as the actual values ​​of climatic conditions. The calculation method involves determining the natural logarithm of relative humidity and performing a number of calculations. This makes it difficult to use it for the rapid determination of the threshold level of condensate formation;
• tabular. This method is very convenient for practical conditions, when it is important to quickly determine the threshold of condensate formation. A ready-made table is used, in which, in small increments, the values ​​\u200b\u200bof room temperature and relative humidity are indicated. Knowing the value of these indicators, it is easy to determine the value of the required parameter from the table;
• using an online calculator. Using a free program hosted on specialized sites, it is easy to determine the required value. It is necessary to select a building material in a simple and accessible calculator shell, as well as indicate its thickness. It remains to press the "calculate" button and the calculated value will appear on the screen.

Unfortunately, qualifications do not always allow you to independently perform calculations using special formulas. On the practical side, in order to quickly obtain reliable values, it is advisable to use a standard table. When using online calculators, only trusted sites should be used. The choice of calculation method for each specific case is determined individually.

Calculation of the dew point in the wall - example of the definition

Consider how to determine the dew point in the wall. To perform calculations, it is necessary to first determine the actual values ​​of the parameters using special devices:

• pyrometer, which is a non-contact type thermometer;
• hygrometer required to determine humidity:
• conventional household thermometer.

The sequence of operations for calculating the dew point for a particular room:

1. Use a tape measure to measure the level at a distance of 0.5-0.6 m from the floor.
2. Determine the air temperature and humidity at this mark using instruments.
3. Find in the table the required indicator corresponding to the measurement results.
4. Measure the degree of cooling on any surface with a pyrometer at the same level.
5. Compare the temperature readings and determine the difference in values.

With a difference exceeding 4 degrees Celsius, there is a high probability of condensation on the surface. This must be taken into account when performing construction work on insulation.

For example, according to the results of measurements, the following data were obtained:

• air temperature - 22 degrees Celsius;
• relative humidity at a given level - 70%.

Then we perform the following steps:

• we determine, using the table, the temperature of condensate formation, equal to 16.3 degrees Celsius;
• we measure the temperature of the wall with a non-contact device, the value of which, for example, is 18 degrees Celsius;
• we calculate the temperature difference - 18-16.3 \u003d 2.3 degrees Celsius.

The indicated value is less than 4, which confirms the absence of condensation during measurements and indicates normal humidity. In this case, the dew point is located in the wall array not far from the inner surface. When cooling a non-insulated wall as a result of a sharp drop in temperature to a value of 16.3 degrees Celsius, the condensation zone will shift to the inner surface.

Dew point for insulation from the inside - when internal insulation is allowed

In order to make a decision on the possibility of performing internal thermal insulation, it is necessary to analyze the following factors:

• nature of residence in the premises (permanent or episodic);
• functioning of the supply and exhaust air exchange system;
• efficiency of the heating circuit;
• the degree of thermal insulation of all building structures (floor, roof, ceiling);
• the material used in the construction of the walls and their thickness;
• temperature conditions and humidity from the outside and inside the building;
• features of the climatic zone;
• presence from the outside of the street or neighboring premises.

As a result of a carefully performed analysis, it can be concluded that internal thermal insulation is possible under the following conditions:

• permanent residence;
• normal operation of ventilation;
• absence of internal temperature differences;
• stable operation of heating;
• insulation of building structures;
• increased wall thickness;
• living in a region with a relatively warm climate.

In each specific situation, the decision is made individually. At the same time, there remains the likelihood of problem situations with poor-quality internal insulation. Instruct professionals to perform calculations, making internal wall insulation. The dew point of the walls, with an unskilled approach, can reach their inner surface and manifest itself negatively. Decision-making and execution of work should be entrusted to specialists. This will prevent annoying mistakes.

Dew point in the building - what is fraught with improper thermal insulation from the inside

The price of an error is quite high if thermal calculations are incorrectly performed, as well as violation of the requirements for the choice of thermal insulation materials. Especially if they are installed from the inside of the room. Regardless of the intensity of the heating system, warmer air inevitably cools when it comes into contact with a cold surface. In this case, moisture concentration occurs and a number of serious problems arise:

• moistening the surface of the walls;
• destruction of heat-insulating material by moisture;
• the appearance of unpleasant odors;
• the presence of constant dampness;
• development of fungal colonies;
• abundant mold formation;
• peeling of facing materials;
• rotting wood;
• development of microorganisms;
• an increase in the incidence rate.

The formation of condensation on the cooled surface of window panes is a clear example of the manifestation of the dew point and indicates the presence of deviations in the internal microclimate. To minimize the chance of condensation:

• maintaining a comfortable humidity level of 40-50% and a temperature of 19-22 degrees Celsius;
• ensuring normal air circulation. In residential premises, the volume of air exchange should be more than 3 cubic meters per hour per square meter, and in kitchens - up to 9 cubic meters.

You should responsibly approach the choice of thermal insulation materials and correctly determine the place for their installation.

Summing up

It is not difficult to independently calculate the temperature threshold of condensate formation. It is important to understand the seriousness of the consequences of improper placement of thermal insulation materials and the use of heaters of insufficient thickness. When performing calculations, take into account the climate and the whole complex of determining factors. Performance of heat engineering calculations must be carried out at the stage of building construction.

The dew point is the temperature of water vapor below which the moisture it contains cools isobarically.

Everyone wants to make the house dry and warm. Therefore, many use a variety of heaters. But the work of thermal insulation of walls is not as simple as it seems. Quite often it happens that insulated, it suddenly starts to get wet, traces of condensation are noticeable on it. They do not appear immediately, usually only a year or three after all the insulation work has been completed.

Therefore, not everyone realizes that the occurrence of condensate on the surface is associated with improperly performed thermal insulation. What is the cause of such an unpleasant phenomenon? It's very simple: this is the dew point.

What is dew point?

Scheme of formation of a dew point in a wall.

When you insulate the surface from the inside of the room, you fence it off from the heat of the room. Thus, the position of the dew point moves inward, closer to the room, the temperature of the wall itself decreases. And what conclusion can be drawn from this? The occurrence of condensate.

By definition, the dew point is the temperature at which condensation begins to fall, that is, the moisture in the air turns into water and settles on the surface. This point can be in different places (outside, inside, in the middle, closer to any of its surface).

Depending on this indicator, the wall remains dry all year round or gets wet when the temperature drops outside.

The location of the dew point depends on the level of humidity inside the house, the temperature.

For example, if the temperature in the room is +20°C, and the humidity level is 60%, then condensation will form on any surface even when the temperature drops to +12°C. If the humidity level is higher and is 80%, then dew can already be seen at +16.5°C. At 100% humidity, the surface gets wet at 20°C.

Consider the situations that arise when insulating with foam plastic from the outside or from the inside:

1. Point position for an uninsulated surface. It can be located in the thickness of the wall closer to the street, approximately between the outer surface and the middle. The wall does not get wet at any temperature drop, it remains dry. It often happens that the point is closer to the inner surface, then the wall is dry in most cases, but gets wet when the temperature drops sharply. When the indicator is on the inner surface, the wall remains wet all winter.
2. When insulating with foam plastic outside the house, several situations may occur. If the choice of insulation, or rather its thickness, was carried out correctly, then the dew point will be in the insulation. This is the most correct location, in which case the wall will remain dry under any circumstances. If the heat insulator layer was taken less, then three options for the location of the dew point are possible:

• in the middle between the central part of the wall and the outer one - the wall remains dry almost all the time;
• closer to the inner surface - when it gets cold, dew falls;
• on the inner surface - in winter the wall is constantly wet.

To determine the condensation rate, you can use the following formula:

Tp=(b*y(T,RH))/(a-y(N,RH))

Tp is the dew point

constant values: a=17.27 and b=237.7 degrees (Celsius).

y(T,RH) = (aT/(b+T))+ln(RH)

T - temperature,

RH - level relative to humidity (greater than zero, but less than one),

Ln is the logarithm.

When using the formula, it is necessary to take into account what material the walls are made of, what is their thickness, and much more. It is better to perform such calculations using special computer programs.

When is internal insulation possible?

Table for determining the dew point depending on air temperature and humidity.

It is not always possible to carry out insulation from the inside, since if the actions are not performed correctly, the dew will constantly fall out from the inside, rendering all building materials completely unusable, creating an uncomfortable microclimate inside. Consider when it is not recommended to make insulation from the inside, what it depends on.

Is it possible or not to insulate from the inside? The solution to this issue largely depends on what will happen to the structure after the work is completed. If the wall remains dry all year round, then work on its thermal insulation from the inside of the room can be carried out, and in many cases even necessary. But if it constantly gets wet every winter, then it is impossible to carry out thermal insulation categorically. Warming is allowed only if the structure is dry, and it gets wet extremely rarely, for example, once every ten years. But even in this case, the work must be carried out very carefully, since otherwise such a phenomenon as the dew point will be observed constantly.

Consider what determines the occurrence of the dew point, how to find out whether or not the walls of the house can be insulated from the inside.

As already mentioned, the dew point occurs due to factors such as:

• humidity;
• indoor temperature.

Humidity in the room depends on the presence of ventilation (exhaust, forced ventilation, air conditioners, etc.) and on the mode of residence, temporary or permanent. The temperature inside is affected by how well the insulation was laid, what is the level of thermal insulation of all other structures of the house, including windows, doors, roofs.

From this we can conclude that the consequences for internal insulation depend on:

• temperature of precipitation of condensate moisture, that is, from the dew point;
• from the position of this point to the thermal insulation and after it.

How to determine where the dew point is? This value depends on many parameters, among which it is necessary to highlight:

• thickness, wall material;
• average indoor temperature;
• average temperature outside (influenced by the climatic zone, average weather conditions throughout the year);
• indoor humidity;
• the level of humidity in the street, which depends not only on the climate, but also on the operating conditions of the house.

Putting all the factors together

Thermal resistance graph and dew point shift when using insulation.

Now we can collect all the factors that affect where the dew point will be located:

• mode of residence and operation of the house;
• the presence of ventilation and its type;
• the quality of the heating system;
• the quality of work when insulating with foam plastic or other material of all house structures, including the roof, doors, windows;
• thickness of individual layers of the wall;
• temperature inside the room, outside;
• humidity indoors, outside;
• climatic zone;
• operating mode, i.e. what is outside: street, garden, other premises, attached garage, greenhouse.

Warming from the inside is possible, based on all the above factors, in such cases:

• with permanent residence in the house;
• when installing ventilation in accordance with all standards for a particular room;
• during normal operation of the heating system;
• with a heater that is laid for all house structures that need thermal insulation;
• if the wall is dry, it has the required thickness. According to the standards, when insulating with foam, mineral wool and other materials, the thickness of such a layer should not be more than 50 mm.

In other cases, it is impossible to perform insulation from the inside. As practice shows, in 90% of cases the walls of the house can only be thermally insulated from the outside, since it is quite difficult to provide all the conditions, and often not entirely feasible.

Consequences of improper insulation

Cases of improper insulation of the house are rare. Most often this happens when it is impossible to lay thermal insulation from the inside, but you did it. In this case, even with the best insulation, various problems will quickly begin to arise, but at first these are wet walls. As a result, the decorative finish loses its attractive appearance. After that, the heater gradually gets wet.

It all depends on what kind of material was used during the work: the foam does not get wet, while many other materials simply will not have time to dry, after which traces of mold and fungus begin to appear on the surface, which it is already impossible to get rid of. Therefore, it is much easier to immediately foresee how and under what conditions it is possible to carry out work on insulating a house with foam plastic or other material than to spend money and time on eliminating the consequences of improper work.

The dew point is the temperature level at which condensation occurs. The appearance of moisture during foam insulation depends on many factors, including the internal temperature in the room, and the values ​​​​of the humidity level. Quite often, constant high humidity, and hence traces of mold on the surface, occurs due to improperly carried out home insulation work, so you should not only understand what a dew point is, but also under what circumstances it occurs, how to avoid this negative phenomenon .

The dew point is the temperature at which the vapor contained in the air turns into condensate in the form of dew. This parameter is important to consider when building and insulating walls. Therefore, it is important to find out in advance what the dew point (TP) is and how to determine it correctly in order to find out where a lot of condensate is likely to collect and take appropriate measures.

The air in the environment always includes water vapor, the concentration of which depends on many factors. Inside buildings, steam is emitted by people and other living organisms. It also enters the internal space from various everyday processes - washing, ironing, cleaning, cooking, and so on.

Outside, the percentage of moisture in the atmosphere is dependent on weather conditions. Moreover, filling the air with vapors has its own limit, upon reaching which the process of moisture condensation and fog generation follows.

At this point, the air mixture absorbs the maximum amount of steam and its relative humidity is 100%. Subsequent saturation leads to the appearance of fog - small droplets of water in the atmosphere.

When an incompletely vaporized air mass (humidity less than 100%) comes into contact with a surface whose temperature is a few degrees lower than its own, condensation forms even without fog.

The fact is that air at different temperatures can accommodate a different amount of steam. The higher the temperature, the more moisture it can absorb. Therefore, when an air mixture with a relative humidity of 80% comes into contact with a cooler object, it cools rapidly, its saturation limit decreases, and the relative humidity reaches 100%.

Then condensation occurs, that is, a dew point appears. It is this phenomenon that can be observed in the early summer morning on the grass. At dawn, the soil and grass are still cold, and the sun quickly heats the air, its humidity near the ground quickly reaches 100% and dew falls. The condensation process is associated with the release of thermal energy, which was previously spent on vaporization. Therefore, the dew quickly disappears.

Thus, the dew point temperature is a variable that depends on the relative humidity and temperature of the air at a particular moment. To determine the dew point and its temperature, various meters are used - thermohygrometers, psychrometers and thermal imagers.

The dew point depends on the relative humidity of the air. The higher it is, the closer the TP is to the actual air temperature. If the relative humidity is 100%, then the dew point is the same as the actual temperature.

The dew point in construction is necessary in order to understand whether the degree of wall insulation corresponds to the fact that condensation does not form.

At dew point values ​​​​more than 20 ° C, physical discomfort is felt, the air seems stuffy; over 25 °C, people with heart or respiratory disease are at risk. But such values ​​are achieved very rarely even in tropical countries.

How to determine the dew point?

In fact, to determine the dew point, you do not need to make complex technical calculations using formulas, measure the relative humidity of the air, etc. It makes no sense to think about how to calculate the dew point, as this has long been done by experts. And the results of their calculations are listed in the table, which shows the values ​​​​of surface temperatures, below which condensate begins to form from air with different humidity.

The purple color indicates the temperature according to the snip in the room in winter - 20 ° C, and the green sector is highlighted, which indicates the range of normalized humidity - from 50 to 60%. At the same time, TP ranges from 9.3 to 12 °C. That is, subject to all standards, condensation will not form inside the house, since there are no surfaces with such a temperature in the room.

The situation is different with the outer wall. From the inside, it is enveloped by air heated to +20 ° C, and from the outside it is exposed to -20 ° C or more. Accordingly, in the thickness of the wall, the temperature slowly rises from -20 °С to + 20 °С and in a certain zone it will necessarily be equal to 12 °С, which will give condensation at a humidity of 60%.

But for this it is still necessary that water vapor reaches this zone through the material of the supporting structure. Here another factor appears that affects the determination of the dew point - the vapor permeability of the material. This parameter must always be taken into account when building walls. .

So, the following factors influence the process of condensation formation inside the outer walls:

• ambient air temperature;
• relative humidity;
• temperature in the thickness of the wall;
• vapor permeability of the material of the erected walls.

To measure these indicators in the thickness of the wall, there are no analyzing devices. They can only be calculated by calculation.

Dew Point Formula

If you still want to calculate the dew point yourself, you can use the following formulas:

Tp = (b f (T, RH)) / (a ​​- f (T, RH)), where:

f (T, RH) = a T / (b + T) + ln (RH / 100), where:

Тр – dew point temperature, °С; a = 17.27; b = 237.7; Т – room temperature, °С; RH – relative humidity, %; Ln is the natural logarithm.

We will carry out the calculation for such values ​​of temperature and humidity:

• T = 21 °C;
• RH = 60%.

First we calculate the function f (T, RH)

f (T, RH) = a T / (b + T) + ln (RH / 100),

f (T, RH) = 17.27 * 21 / (237.7+21) + ln (60 / 100) = 1.401894 + (-0.51083) = 0.891068

Then we calculate the dew point temperature

Tp = (b f (T, RH)) / (a ​​- f (T, RH)),

Tp \u003d (237.7 * 0.891068) / (17.27 - 0.891068) \u003d 211.807 / 16.37893 \u003d 12.93167 ° С

So, the result of our calculations is Tr = 12.93167 °C.

Calculating the dew point using formulas is very complicated. It is better to use ready-made tables.

External or internal insulation?

Vapor permeability is a parameter that shows how much water vapor can pass through a certain type of material in a set period of time. Permeable materials include all building materials with open pores - concrete, mineral wool, brick, wood, expanded clay. They say that the houses built from them “breathe”.

In ordinary and insulated walls, there are always conditions for the formation of a dew point. However, this phenomenon does not occur in a specific place on the wall. Over time, the conditions on both sides of the structure change, so the dew point in the wall also shifts. In construction, this phenomenon is called the “zone of possible condensation”.

Since the supporting structures are permeable, they can independently get rid of the released moisture, while the arrangement of ventilation on both sides is important. It is not for nothing that the wall insulation with mineral wool from the outside becomes ventilated, because the dew point then moves into the insulation. If everything is done correctly, then the moisture that is released inside the mineral wool leaves it through the pores and is carried away by the ventilation air flow.

Therefore, it is important to equip good ventilation in residential premises, since it removes not only harmful substances, but also excess moisture. The wall gets wet only in one case: when condensation occurs constantly and for a long time, and there is nowhere for moisture to go. Under normal conditions, the material simply does not have time to be saturated with water.

Modern polymer insulation almost does not let steam through, so when insulating walls, it is better to place them outside. Then the temperature necessary for condensation will be inside the foam or polystyrene foam, but the vapors will not reach this place, and therefore there will be no moisture. And vice versa, it is not worth insulating with a polymer from the inside, since the dew point will remain in the wall, and moisture will begin to come out 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, so water forms on the inner surface.

It is rational to carry out internal insulation under such conditions:

• the wall is quite dry and relatively warm;
• the insulation must be vapor-permeable so that the released moisture can exit the structure;
• The building must have a good ventilation system.

Practice shows that it is preferable to equip the thermal protection of the structure from its outside. Then there is more chance that the TR will be in an area that will not allow moisture condensation inside the room.

Thus, the dew point in the construction of walls is always present, however, if you correctly calculate the amount of moisture generated and use the right insulation when insulating the walls from the outside, then the condensation zone can be shifted. As a result, moisture will not appear inside the room.