Dew point in the wall - what is it? How to calculate the dew point What is the dew point for

This article will address the following questions:

• What happens in a wall insulated from the inside;
• How to determine when you can insulate from the inside, and when you can’t. Factors on which it depends.

Definition of "dew point"

In order to understand the processes taking place in the wall, I will first dwell on such a concept as the dew point in construction.

Dew point determination- This is the temperature at which condensation occurs (moisture from the air turns into water). A point with this temperature is located in a certain place (on the wall outside, somewhere in the thickness of the wall or on the wall inside). Depending on the location of the dew point (further or closer in wall thickness to the interior), the wall is either dry or wet inside. The dew point (condensation temperature) depends on:

• indoor humidity;
• indoor air temperature.

1. If the indoor temperature is +20 degrees, and the indoor humidity is 60%, then condensation will form on any surface with a temperature below +12 degrees.

The lower the indoor humidity, the lower the dew point is the actual indoor air temperature.

2. At an indoor temperature of +20 degrees, and indoor humidity of 40%, then condensation will fall on any surface with a temperature below +6 degrees.

The higher the indoor humidity, the higher the dew point and closer to the actual indoor air temperature.

3. At an indoor temperature of +20 degrees, and indoor humidity of 80%, then condensation will fall on any surface with a temperature below +16, 44 degrees.

If the relative humidity is 100%, then the dew point is the same as the actual indoor temperature.

4. When the indoor temperature is +20 degrees, and indoor humidity is 100%, then condensation will fall on any surface with a temperature below +20 degrees.

Dew point location

BUT dew point position in the wall depends on:

• thickness and material of all layers of the wall,
• indoor temperature,
• outside temperature,
• indoor humidity,
• humidity outside.

Let's analyze what happens to the position of the dew point:

• in a wall not insulated at all;
• in a wall insulated from the outside;
• in a wall insulated from the inside.

Immediately, for each option, we will consider the consequences of such an arrangement of the dew point.

Location of the dew point in an uninsulated wall

By dew point location there may be options not insulated walls:

1. The location of the dew point between the middle of the wall and the outer surface of the wall.

The location of the dew point in the wall between the middle of the wall and the outer surface, the wall is not insulated

In this case, the wall is dry.

2. The location of the dew point between the middle of the wall and the inner surface.

The location of the dew point between the middle of the wall and the inner surface, the wall is not insulated

In this case, the wall is dry, it can freeze with a sharp drop in outside temperature (lower than the design temperature according to DBN / SNiP in the region for several days). The position of the dew point in these few days may shift to the inner surface of the wall.

3. The location of the dew point on the inner surface.

The location of the dew point on the inner surface of the wall, the wall is not insulated

The wall is wet inside almost the entire winter period.

As already discussed, the position of the dew point depends on the 5 factors described in the part above.

The location of the dew point in the wall insulated from the outside

By dew point location in the wall, insulated outside, there may be such options:

1. If the insulation is taken with the thickness required according to the heat engineering calculation, then the position of the dew point is inside the insulation.

The location of the dew point in the insulation, the wall is insulated from the outside

This is the correct position of the dew point. This wall is dry.

2. If the insulation is taken with a smaller thickness than it should be according to the heat engineering calculation, then all three options described above for an uninsulated wall are possible. The consequences are described there.

The location of the dew point in the wall, insulated from the outside (if the insulation is taken less than the calculated thickness)

The location of the dew point in the wall insulated from the inside

According to the location of the dew point in the wall, insulated from within. When we insulate a wall from the inside, we kind of “fence off” it from the room heat. Thus, we shift the position of the dew point inside the room and lower the temperature of the wall itself under the insulation. That is, both the dew point (temperature) and its position become such that condensation is more likely to form. There may be such options:

1. The location of the dew point in the thickness of the wall.

The location of the dew point in the thickness of the wall, the wall is insulated from the inside

In this case, the wall is dry, it can freeze with a sharp drop in outside temperature (lower than the design temperature according to DBN \ SNiP in the region for several days). The position of the dew point in these few days may shift to the inner surface of the wall.

2. The location of the dew point on the inner surface of the wall, under the insulation.

The location of the dew point on the inner surface of the wall, under the insulation, the wall is insulated from the inside

The wall in this case freezes under the insulation for the entire winter period.

3. The location of the dew point inside the insulation.

The location of the dew point in the insulation, the wall is insulated from the inside

The wall in this case freezes the entire winter period, except for the wall, the insulation is also wet.

When it is possible or impossible to insulate the walls from the inside

Now we will analyze when it is possible to insulate the wall from the inside, when it is impossible, what it depends on and how it depends. What is this “no”, what are the consequences.

The main "possible or impossible" is what will happen to the wall after it is insulated from the inside. If the wall is dry, you can. If the wall is dry, and only with a sharp, unexpected (which happens once every ten years) cold snap can get wet, you can try to insulate from the inside (at the discretion of the customer). If the wall is consistently wet throughout the winter settlement period (with the usual winter temperature in the region), it is impossible to insulate from the inside. As we have already found out above, these consequences depend on the position of the dew point. And the position of the dew point in the wall can be calculated, and then it will be clear (BEFORE insulation) whether it is possible or not to insulate a particular wall from the inside.

Note: We do such a calculation, ask questions in the section and we will calculate your specific situation.

Now a little discussion on the topic of what affects the possibility of insulation from the inside, and how it affects. This part of the article is caused by questions from readers of the following nature: “Why can a reader in the next branch be insulated from the inside, but I can’t, because we have the same apartment layout with him (further options), or houses are built from the same material, or one city of residence, or the same thickness of the wall and so on.

Let's figure it out. As we have already found out above, the consequences of internal insulation depend on:

• dew points (condensation temperature);
• position of the dew point in the wall before and after insulation.

In turn, the dew point (temperature) depends on: the humidity in the room and the temperature in the room. And the humidity in the room depends on:

• Mode of residence (permanently or temporarily);
• Ventilation (both inflow and exhaust, are they enough according to the calculation).

And the temperature in the room depends on:

• The quality of the heating;
• The degree of insulation of other structures of the house / apartment, except for walls (ceiling / roof, windows, floor).

The position of the dew point depends on:

• thickness and material of all layers of the wall;
• indoor temperature. On what it depends - found out above;
• outside temperature. It depends on whether the street is outside or another room, as well as on the climatic zone;
• indoor humidity. On what it depends, found out above;
• humidity outside. It depends on whether the street is outside or another room (and on the mode of operation of this room), as well as on the climatic zone.

Now, if we collect ALL factors of influence on dew point and dew point position, we will get a list of factors of influence that must be taken into account when deciding whether it is possible or not in a particular situation to insulate a particular wall from the inside. Here is a list of these factors:

• mode of residence in the premises (permanently or temporarily);
• ventilation (both inflow and exhaust, are they enough according to the calculation);
• the quality of the heating in the room;
• the degree of insulation of other structures of the house / apartment, except for walls (ceiling / roof, windows, floor);
• thickness and material of all layers of the wall;
• indoor temperature;
• indoor humidity;
• outside temperature;
• humidity outside the room;
• climatic zone;
• what is behind the wall, street or other room (its mode of operation).

It becomes clear that there may not be two identical situations for warming from the inside. Let's see how (approximately, without specifics) the situation looks like when insulation from the inside is possible:

• permanent residence,
• ventilation is carried out according to the norm (for this room),
• heating works well and is carried out according to the norm,
• other structures are insulated according to the norm,
• the wall to be insulated is thick and warm enough. According to the calculation for additional insulation for it, it should not be more than 50 mm (polystyrene, cotton wool, EPS). In terms of heat transfer resistance, the wall “does not reach” the norm of 30% or less.

To simplify it completely, it turns out like this: the warmer the region, the better your heating and ventilation, the thicker and warmer the wall, the more likely it is that you can insulate from the inside. I think it is clear that in each specific case you need to consider your "incoming data" and then make a decision.

Everything that is written above gives the impression that there are very few cases when internal insulation is possible and not harmful. It really is. In our experience, out of 100 who came up with the idea of ​​internal insulation, only 10 can do it without consequences. In other cases, you need to insulate the outside.

The consequences of improper insulation from the inside

What are the consequences of insulation, when they insulated from the inside, but it was “impossible”. As a rule, these are wet walls at first. Then, depending on the type of insulation, wet insulation. Cotton wool gets wet, but Styrofoam or XPS does not. But that doesn't change things. The end result is mold and fungus on the walls. The time of occurrence of consequences is from one to three years.

The concept of dew point (hereinafter referred to as TP) is used in the design of thermal protection of civil and industrial buildings, it is a convenient parameter in the calculations of air drying systems and pneumatic installations. The dew point of the ambient air is taken into account when applying anti-corrosion coatings to metal substrates.

When the substrate temperature is lower than the FR of air, condensed moisture is present on the substrate, which does not allow to achieve the desired adhesion. On the painted surface, defects such as peeling or bubbling of the paint layer are formed, which contribute to the occurrence of premature corrosion. A correctly performed calculation of the dew point determines what the thermal insulation of a residential building should be, taking into account heat consumption, air humidity and air exchange characteristics within the premises.

The dew point temperature serves as a kind of indicator of the degree of humidity inside the living space. The dew point temperature value determines the comfort level of living in a house. The higher the dew point in a frame house, the higher the humidity in the room. If the dew point temperature exceeds 20 ° C, then for most people being in the room will be very uncomfortable.

The atmosphere in such a room for hearts and asthmatics is extremely suffocating and unbearable. An incorrect determination of the dew point in the wall of a residential building leads to the deposition of condensate on the surface of the walls and ceiling of the room. Wet walls provoke the formation of mold and the development of microorganisms that enter the human body along with the inhaled air. Condensed moisture in the materials of wet walls and ceilings freezes in winter, sharply increasing in volume and weakening the strength qualities of the building structure.

The picture below shows a damp wooden wall with fungal manifestations due to improper thermal insulation.

Physics of steam condensation

Water is present in the environment of our dwelling in two states of aggregation:

• liquid - this is water for cooking and sanitary needs;
• gaseous - steam over boiling water or as one of the fractions of exhaled air.

In addition to such obvious places, traces of moisture are necessarily present in the materials of the elements of the building structure of the building: concrete or brick walls, ceilings, floor base. Ideally dry building materials do not exist in nature. In stable warm weather, the vapor present in the air and the moisture in the walls of the dwelling are in thermal equilibrium.

At the same time, the partial pressure of vapor in the air from the side of the street (outer side of the wall) and inside the house (inner side of the wall) is the same. This means that no movement of water vapor through the wall occurs. In frosty weather, the humidity of cold air is low, the partial vapor pressure in such air is reduced. In accordance with the laws of thermophysics, high-pressure steam (living space) begins to diffuse through the wall material into a cold street, where the pressure is lower.

All building materials from which the walls of houses are built have the property of vapor permeability. Even concrete or brick walls are able to pass steam through their thickness, although concrete and brick have minimal vapor permeability.

When passing through the dew point in the wall, the vapor passes into a liquid state of aggregation, forming condensate moisture.

The appearance of moisture in the wall structure is accompanied by a number of negative factors:

• The thermal conductivity of a damp wall increases several times. This will mean that the heat exchange between the heated room and the street is intensified, the house will always be cold.
• In the cold season, periodic freezing of condensate moisture in the wall occurs, followed by thawing. The cyclical nature of freezing has a destructive effect on the structure of the building material, reducing the period of trouble-free operation of the building.

The figure below schematically shows the transformation of vaporous moisture into a liquid state (blue color used) when TR enters the interior of the dwelling wall.

Methods for calculating TP

The question of what is the dew point is answered in the Code of Rules SP 50.13330.2012, which regulates the thermal protection of buildings. In paragraph B.24, the concept of TP is interpreted as the temperature of the beginning of the formation of condensate moisture in the air with specific parameters of temperature and relative humidity.

The value of TP is indicated in degrees C! It should be taken into account that the TP value can never exceed the actual temperature parameter of the air for which the TP is determined. Only in the case of 100% relative humidity will the TR coincide with the air temperature.

In accordance with the definition of TR, the temperature of precipitation of condensate moisture depends on the values ​​of two parameters:

• from air temperature;
• on the relative humidity of the surrounding air.

For example, for air masses with a humidity of 40% and a temperature of 10 ° C, the TP indicator will be minus 2.9 ° C. With a humidity of the same volume within 80%, the TR will already reach plus 6.7 °C. For 100% humidity, the values ​​of TP and t of air are the same = 10.0 °C.

When arranging thermal protection, it is very important to find a place where the dew point can be in order to prevent the formation of condensate moisture in a place that is not desirable for effective thermal protection. It is almost impossible to visually determine the position of the TR as the place of initial condensate precipitation. For the dew point indicator, the determination is carried out by several methods.

Calculation method

The following formula is very convenient for calculating TE in the plus temperature range up to 60°C:

T P \u003d b * f (T, Rh) / (a-f (T, Rh), where

• T P - temperature of the beginning of condensation, that is, the dew point in the wall, insulation or ambient air;
• f(T,Rh) = a*T/(b+T) + ln(Rh);
• ln is the natural logarithm;
• a=17.27;
• b=237.7;
• Т – air temperature in °C;
• Rh - relative humidity, indicated in volume fractions (from 0.01 to 1.00).

This formula works with an error of ±0.4 degrees Celsius.

There are simpler formulas that work with an error within ±1.0 deg. C, for example, T p ≈T - (1-RH) / 0.05.

This formula can be used to calculate the relative humidity index through the already known temperature of the TR: RH≈1-0.05(T-T p).

Tabular method

In special numerous tables, based on laboratory measurements, the values ​​\u200b\u200bof TP are indicated depending on the indicators of the relative humidity of the air and its temperature. The table of reference appendix R of the Code of Rules SP 23-101-2004 "Design of thermal protection of buildings" defines the dew point parameter in some detail. On fig. below is a similar dew point table that fully complies with the parameters from GOST and SP.

Table for determining the dew point

Use of household psychrometers

Psychrometers, more precisely, psychrometric hygrometers, are designed to measure air temperature and its relative humidity. A modern hygrometer can be used as a device for determining the dew point, since an image of a psychrometric table is applied to its body.

Using the readings of both thermometers of the device, TP is determined from the table. The figure below shows models of modern household psychrometers equipped with psychrometric tables to help you determine the dew point.

Portable electronic thermohygrometers

The dew point in construction during a thermal engineering survey of premises is determined using portable thermohygrometers with displays equipped with an indication of the ambient temperature, its humidity and the TP parameter.

Indications of thermal imagers

It is not necessary to calculate TP if you use separate models of thermal imagers for construction purposes that have the function of TP calculation and display surfaces with a temperature below TP during thermal imaging. With the given air parameters on the computer, it is possible to process thermal imaging data and show on thermograms all areas that are at risk of falling into the condensation zone when the wall or ceiling is insulated.

Housing options

The TP parameter is a kind of temperature boundary in which the meeting of internal heat and external cold occurs. In wall enclosing structures, warm air diffusing during the cold winter months from a heated room to a frosty street is supercooled.

The vapor phase of water goes into a wet state, settling on any surface that has a temperature below TP. The reason for the occurrence of condensate is not only the material of the wall (wooden house, brick or aerated concrete), but also the method of arranging the thermal protection of the building, which determines in which direction the TR is shifted.

The location of the TR depends on the following factors:

• humidity indicators indoors and outdoors;
• indicators of air temperature indoors and outdoors;
• wall thickness and insulation layer;
• places where the insulating material is placed.

Depending on these factors, TR can be located not only on the surface of the wall, but also in the thickness of the wall or insulating material. Options for the location of the TR in the “wall plus insulation” system provide for the placement of the insulation inside the room or on the outside of the enclosing wall (see figure below).

Wall without insulation

The location of the TR falls on the thickness of the wall and is able to shift towards the street or premises, depending on the changing parameters of temperature and humidity.

In any case, if the dew point is in aerated concrete or in a brick wall, the condensate forms relatively far from the inner surface. Condensate moisture accumulates in the wall material, in severe frosts it freezes. When it warms, the moisture thaws and evaporates out into the atmosphere.

There are three options for placing the TR in the wall:

• the TP indicator found by calculation or tabular method fell between the geometric center of the wall thickness and the outer surface - the inner wall remained dry;
• TR falls between the geometric center of the wall and the inner surface of the room - the walls of the room can get wet during a sharp cold snap;
• TP exactly hit the coordinate of the inner surface - the wall will be damp all winter.

Heat loss with an uninsulated wall reaches 80%. The negative moment of the occurrence of TR in the wall is the gradual destruction of the wall structure.

Walls made of bricks, aerated concrete, expanded clay blocks, etc., homogeneous in their design, have TR in winter inside the thickness of the material. Multiple freeze/thaw cycles degrade the strength properties of building materials and reduce the strength of the entire wall structure. Therefore, the walls of a monolithic structure of a homogeneous composition must be insulated with heat-insulating materials.

Insulation from the inside of the room

The following options are available for the TR location:

• if the dew point is in the insulation, then the insulation will be wet throughout the frosty period;
• if the structure of the insulation material does not allow condensation of moisture inside the insulation layer (expanded polystyrene, etc.), then the condensate will fall on the border of the inner wall and the insulation polystyrene plate. The wall decoration will begin to get wet, which will provoke the formation of damp spots and mold;
• the material of the wall is in the zone of sub-zero temperatures and is exposed to the negative effects of temperature changes.

Insulation on the outside of the building

TR is brought into the outer heat-insulating layer. The possibility of condensation in the room is excluded, the walls will be dry.

Video: dew point in the wall

Theory and practice show that it is preferable to equip the thermal protection of the building from its outside. Then there are more chances that the TR will be in a zone that does not allow moisture condensation inside the room.

The concept of dew point

The dew point is the temperature at which moisture falls out or condenses from the air, which was previously in the vapor state. In other words, the dew point in construction is the boundary of the transition from the low air temperature outside the building envelope to the warm temperature of the internal heated rooms, where moisture may appear, its location depends on the materials used, their thickness and characteristics, the location of the insulating layer and its properties.

In the normative document SP 23-101-2004 "Design of thermal protection of buildings" (Moscow, 2004) and SNiP 23-02 "Thermal protection of buildings" regulates the conditions regarding the accounting and dew point value :

“6.2 SNiP 23-02 establishes three mandatory mutually related standardized indicators for the thermal protection of the building, based on:

"a" - standardized values ​​of heat transfer resistance for individual enclosing structures of the thermal protection of the building;

"b" - normalized values ​​of the temperature difference between the temperatures of the internal air and on the surface of the enclosing structure and the temperature on the inner surface of the enclosing structure above the dew point temperature;

"c" - a normalized specific indicator of thermal energy consumption for heating, which makes it possible to vary the values ​​​​of the heat-shielding properties of enclosing structures, taking into account the choice of systems for maintaining normalized microclimate parameters.

The requirements of SNiP 23-02 will be met if the requirements of indicators of groups "a" and "b" or "b" and "c" are met when designing residential and public buildings.

Condensation of water vapor occurs most easily on some surface, but moisture can also appear inside the thickness of structures. In relation to the construction of walls: in the case when the dew point is located close to or directly on the inner surface, under certain temperature conditions during the cold season, condensation will inevitably fall on the surfaces. If the enclosing structures are not sufficiently insulated or constructed without an additional insulating layer at all, then the dew point will always be located closer to the internal surfaces of the premises.

The appearance of moisture on the surfaces of structures is fraught with unpleasant consequences - this creates a favorable environment for the reproduction of microorganisms, such as fungus and mold, whose spores are always present in the air. In order to avoid these negative phenomena, it is necessary to correctly calculate the thickness of all elements that make up the building envelope, including calculating the dew point.

According to the instructions of the normative document SP 23-101-2004 "Design of thermal protection of buildings" (Moscow, 2004):

"5.2.3 The temperature of the internal surfaces of the building's external enclosures, where there are heat-conducting inclusions (diaphragms, through inclusions of cement-sand mortar or concrete, inter-panel joints, rigid connections and flexible connections in multilayer panels, window frames, etc.), in the corners and on window slopes should not be lower than the dew point temperature of the air inside the building ... ".

If the surface temperature of the wall inside the premises or window blocks is lower than the calculated dew point value, then condensate is likely to appear during the cold season, when the outdoor air temperature drops to negative values.

The solution of the problem - how to find the dew point, its physical value, is one of the criteria for ensuring the required protection of buildings from heat loss and maintaining normal microclimate parameters in the premises, in accordance with the conditions of SNiP and sanitary and hygienic standards.

Dew Point Calculation

• using the table of the normative document;
• according to the formula;
• using an online calculator.

Calculation using a table

The calculation of the dew point when insulating a house can be made using the table of the regulatory document SP 23-101-2004 "Design of thermal protection of buildings" (Moscow, 2004)

To determine the value of the condensation temperature, it is enough to look at the intersection of the temperature and humidity values ​​established by the standards for each category of premises.

Formula calculation

Another way to determine the dew point in a wall is with a simplified formula:
$$\quicklatex(size=25)\boxed(T_(p)= \frac(b\times \lambda (T,RH))(a - \lambda(T,RH)))$$

Values:

Tr is the desired dew point;

a – constant = 17.27;

b – constant = 237.7 °C;

λ(Т,RH) – coefficient calculated by the formula:
$$\quicklatex(size=25)\boxed(\lambda(T,RH) = \frac(((a\times T)))((b + T) + (\ln RH)))$$
Where:
Т – indoor air temperature in °C;

RH - humidity in volume fractions ranging from 0.01 to 1;

ln is the natural logarithm.

As an example, let's calculate the desired value in a room where the optimum temperature of 20 ° C with a relative humidity of 55%, which is set by the standards for residential buildings, should be maintained. In this case, we first calculate the coefficient λ(Т,RH):

λ(T, RH) = (17.27 x 20) / (237.7 + 20) + Ln 0.55 = 0.742

Then the value of the temperature of condensation from the air will be equal to:

Tr \u003d (237.7 x 0.742) / (17.27 - 0.742) \u003d 176.37 / 16.528 \u003d 10.67 ° C

If we compare the temperature value obtained from the formula and the value obtained from the table (10.69°C), we see that the difference is only 0.02°C. This means that both methods allow you to find the desired value with high accuracy.

Calculation with an online calculator

The examples show that such a task as determining the dew point is not particularly difficult. Based on tables and formulas, online calculators are being developed, so if you are faced with the problem of how to calculate the dew point in a wall, a calculator for this is available on the site. For the calculation, it is enough to fill in two fields - to enter the indicators of the established standard indoor temperature and relative humidity.

Determination of the position of the dew point in the wall

In order to ensure the normal qualities of enclosing structures in terms of thermal protection, it is necessary not only to know the value of the temperature of condensation, but also its position within the enclosing structure. The construction of external walls is now carried out in three main options, and in each case, the location of the condensate boundary may be different:

• the structure was built without additional insulation - from masonry, concrete, wood, etc. In this case, in the warm season, the dew point is located closer to the outer edge, but if the air temperature drops, it will gradually shift towards the inner surface, and may a moment will come when this boundary will be inside the room, and then condensate will appear on the internal surfaces.

It should be noted that the dew point in a wooden house with a properly selected wall thickness - from a log or timber - will be located closer to the outer surfaces, since wood is a natural material with unique properties that has very low thermal conductivity with high vapor permeability. Wooden walls in most cases do not require additional insulation;

• the structure was erected with an additional layer of insulation from the outside. With the correct calculation of the thickness of all materials, the dew point during insulation with foam plastic or other types of effective heaters will be located inside the insulation layer, and condensation will not appear inside the premises;
• the structure is insulated from the inside. In this case, the boundary of the appearance of condensate will be located close to the inner side and, in case of severe cooling, can shift to the inner surface, to the junction with the insulation. In this case, it is also likely that moisture will appear inside the premises, leading to unpleasant consequences. Therefore, this option of insulation is not recommended and is performed only in cases where there are no other solutions. At the same time, it is necessary to provide additional measures to prevent negative consequences - to provide an air gap between the insulation and the cladding, ventilation holes, arrange additional ventilation of the premises to remove water vapor, air conditioning with a decrease in humidity.

• wall thickness, including base material (h1, in meters) and insulation (h2, m);
• thermal conductivity coefficients for the supporting structure (λ1, W/(m*°C) and insulation (λ1, W/(m*°C);
• normative room temperature (t1, °C);
• air temperature outside the premises, taken for the coldest season in the region (t2, °C);
• standard relative humidity in the room (%);
• standard dew point value at given temperature and humidity (°C)

We accept the following conditions for the calculation:

• brick wall thickness h1 = 0.51 m, insulation - polystyrene foam thickness h2 = 0.1 m;
• thermal conductivity coefficient established according to the normative document for silicate brick laid on a cement-sand mortar, according to the table of Appendix "D" SP 23-101-2004λ1 = 0.7 W/(m*°C);
• thermal conductivity coefficient for PPS insulation - polystyrene foam having a density of 100 kg / m² according to the table of Appendix "D" SP 23-101-2004λ2 = 0.041 W/(m*°C);
• indoor temperature +22 °C, as set by the regulations within the range of 20-22 °C according to table 1 SP 23-101-2004 for residential premises;
• outside air temperature -15 °C for the coldest season in the conditional area;
• humidity in the premises - 50%, also within the limits of the standard (no more than 55% according to table 1 SP 23-101-2004) for residential premises;
• the value of the dew point for the given values ​​​​of temperature and humidity, which we take from the above table - 12.94 ° C.

First, we determine the thermal resistances of each layer that makes up the wall, and the ratio of these values ​​to each other. Next, we calculate the temperature difference in the bearing layer of the masonry and at the boundary between the masonry and the insulation:

• the thermal resistance of the masonry is calculated as the ratio of thickness to the coefficient of thermal conductivity: h1 / λ1 = 0.51 / 0.7 = 0.729 W / (m² * ° C);
• the thermal resistance of the insulation will be equal to: h2 / λ2 = 0.1 / 0.041 = 2.5 W / (m² * ° C);
• thermal resistance ratio: N = 0.729/2.5 = 0.292;
• the temperature difference in the layer of brickwork will be: T \u003d t1 - t2xN \u003d 22 - (-15) x 0.292 \u003d 37 x 0.292 \u003d 10.8 ° C;
• the temperature at the junction of the masonry and insulation will be: 24 - 10.8 \u003d 13.2 ° C.

Based on the results of the calculation, we will plot the temperature change in the wall massif and determine the exact position of the dew point.

From the graph, we can see that the dew point, which is 12.94 ° C, is within the thickness of the insulation, which is the best option, but very close to the junction between the wall surface and the insulation. With a decrease in the outside air temperature, the condensate boundary may shift to this joint and further into the wall. In principle, this will not cause any special consequences and condensation cannot form on the surface inside the premises.

The calculation conditions were adopted for central Russia. In the climatic conditions of regions located in more northern latitudes, a large thickness of the wall and, accordingly, the insulation is accepted, which will ensure that the boundary of condensate formation is located within the insulation layer.

In the case of insulation from the inside, under all the same conditions: thickness of the supporting structure and insulation, external and internal temperatures, humidity, taken in the above calculation example, the graph of temperature changes in the thickness of the wall and at the boundaries will look like this:

We see that the boundary of condensation from the air in this case will shift almost to the inner surface and the likelihood of moisture appearing in the room when the outside temperature drops will increase significantly.

Dew point and vapor permeability of structures

When designing enclosing structures, ensuring the normative thermal protection of premises, it is of great importance to take into account the vapor permeability of materials. The value of vapor permeability depends on the volume of water vapor that a given material can pass per unit time. Almost all materials used in modern construction - concrete, brick, wood and many others - have small pores through which air carrying water vapor can circulate. Therefore, designers, when developing enclosing structures and selecting materials for their construction, must take into account vapor permeability. In doing so, three principles must be observed:

• there should be no obstacles to remove moisture in case of condensation on one of the surfaces or inside the material;
• the vapor permeability of enclosing structures should increase from the inside to the outside;
• the thermal resistance of the materials from which the outer walls are constructed must also increase towards the outside.

In the diagram, we see the correct composition of the structure of the outer walls, which provides the normative thermal protection of the interior and the removal of moisture from the materials when it condenses on the surfaces or inside the thickness of the wall.

The above principles are violated with internal insulation, so this method of thermal protection is recommended only as a last resort.

All modern exterior wall designs are based on these principles. However, some heaters, which are included in the structure of the walls, have almost zero vapor permeability. For example, expanded polystyrene, which has a closed cellular structure, does not allow air and, accordingly, water vapor to pass through. In this case, it is especially important to accurately calculate the thickness of the structure and insulation so that the boundary of condensate formation is within the insulation.

Opinion of portal experts

According to the experts of the site portal, the calculation of the dew point and its position in the building envelope is one of the defining moments in ensuring the protection of buildings from heat loss. The most optimal option is when the condensate boundary is within the thickness of the insulation in a structure with external insulation. It is necessary to calculate the thickness of the layers of enclosing structures for certain materials in such a way as to exclude the dew point shift into the thickness of the wall and towards the surfaces inside the premises.

Colorer! Remember that in order to prevent moisture condensation during painting work, the temperature of the surface to be painted must be 3°C higher than the dew point temperature!

When carrying out painting work, the presence of ice and frost on the painted surface is unacceptable. It is unacceptable to carry out work during precipitation and on a wet surface

What happens if the difference between the surface temperature and the dew point temperature is less than 3°C?

It's simple, the likelihood of moisture condensation on the painted surface increases. In the future, this may adversely affect the quality and performance properties of the paintwork.

What is important to know when determining the dew point?

The temperature at which the air becomes saturated and the water vapor present in the air begins to condense into a liquid state is called dew point. Water that condenses from the air can settle on surfaces, including painted or prepared for painting.

When does water condensation occur on the surface?

Water condensation on the surface usually occurs when the air temperature drops. The greater the initial relative humidity of the air, the smaller the temperature difference required for water to condense on the surface. Outdoors, condensation is most likely on calm, clear evenings when temperatures drop. High probability of condensation in changeable weather.

Often, moisture condensation occurs on cold surfaces surrounded by warm, moist air, such as on the outer surfaces of containers or pipes if they are filled with cold liquid.

What are the consequences of moisture condensation on the painted surface?

• occurrence of under-film corrosion processes;
• violation of the wettability of the surface of the paintwork material;
• reducing the adhesion of the paintwork to the painted surface;
• the formation of defects in the paint film (pores, craters, wrinkles, etc.).

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 affects the change in the dew point and the appearance of condensation on the walls to a greater extent.

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.