The dew point is the temperature to which the air must cool for the water vapor it contains to reach a state of saturation and begin to condense into dew. Simply put, this is the temperature at which condensation occurs.
The dew point temperature is determined by only two parameters: temperature and relative humidity. The higher the relative humidity, the higher the dew point and the closer to the actual air temperature. The lower the relative humidity, the lower the dew point than the actual temperature.
Dew point table
Table with dew point temperature for different meanings temperatures (from -5°С to 35°С) and relative humidity (from 40% to 95%) of indoor air can be found in the reference Appendix R to SP 23-101-2004 “Design of thermal protection of buildings”. Unfortunately, there are several typos in this table. I have prepared it for you, the typos have been corrected.
Formula for calculating dew point
You can use the formula to approximately calculate the dew point Tp (°C) depending on the air temperature T (°C) and its relative humidity Rh (%):
The formula has an error of ±0.4 °C in the range of air temperature T from 0°C to 60°C, dew point temperature Tp from 0°C to 50°C, relative humidity Rh from 1% to 100%.
Devices with dew point determination
Psychrometer (psychrometric hygrometer) is a device for measuring air humidity and temperature. The psychrometer consists of two alcohol thermometers, one of them is a regular dry thermometer, and the second has a humidification device. Due to the evaporation of moisture, the moistened thermometer cools. The lower the humidity, the lower its temperature. At 100% humidity, the thermometer readings are the same. To determine relative humidity, a psychrometric table is used. Such devices are currently used only in laboratory conditions.
The most convenient in the practice of inspecting buildings are portable electronic thermohygrometers with an indication of temperature and relative air humidity on a digital display. Some models of thermohygrometers also have a dew point indication.
Calculation of dew point in a thermal imager
Some thermal imager models have a built-in function for calculating the dew point in real time and displaying an isotherm on the thermogram, which clearly shows surfaces where the temperature is below the dew point during thermal imaging. This function is available, for example, in the line of thermal imagers construction purposes(B Series from Building) FLIR Systems.
The dew point isotherm can be added to the thermogram later in a computer processing program. To calculate, you will need to set the air temperature and humidity. The isotherm will shade on the thermogram all surfaces whose temperature is below the dew point. Do not forget that this function shows areas dangerous for condensation only under thermal imaging conditions. If the outside temperature rises and the indoor humidity drops, hazardous areas will disappear from the thermogram (the structures will be warmer and the dew point lower). Below are screenshots of the FLIR and TESTO programs.
Dew point in construction
I will write about the importance of condensation and dew point during the operation of building structures, the position of the dew point or the plane of possible condensation in walls, and the assessment of structural defects using the dew point criterion using thermal imaging in one of the following publications.
This is a determination of the temperature at which condensation forms.
This value allows you to determine the localization condensation formation, which is located on the surface of the wall or inside it. The feasibility of its calculation is related to determining the thickness of the wall to retain heat.
The importance of determining the dew point is determined by the fact that this process affects whether the wall will be wet outside or inside. The temperature at which condensation forms depends on the following factors indoors:
- humidity levels;
- air temperature.
For example, at an air temperature of +20 o C and a humidity of 60% in the room, the temperature value of condensation on any surface is below +12 o C. If the temperature outside has dropped, but inside it is stably constant, then the dew point will move in the thickness of the wall closer to the room.
The more accurately the value of the indicator is determined, the higher the likelihood of creating comfortable microclimate in buildings and structures. Dew point calculation allows you to calculate the highest humidity segments.
It is advisable to prevent these processes in order to avoid the development of rotting processes and the appearance fungus and mold.
This is achieved dew point shift closer to the outer surface, that is, measures for insulation from the outside.
Competent calculation the thickness of the insulation will prevent freezing of the walls as a result of freezing and thawing of condensate. It is optimal if condensation falls inside the insulating layer.
The main indicators required for calculation are humidity and indoor temperature. To determine them it is used household psychrometer.
This device determines both indicators. His work based on a combination of a thermometer cooled by a humidifying device. The higher the percentage of humidity, the higher the thermometer readings.
For construction needs, electronic devices have been developed that instantly calculate temperature and humidity values and display the indicators on the display. Some models also have a dew point calculation function. thermal imagers.
There are several methods for calculating dew point:
- according to the formula;
- according to the table;
- using an online calculator.
Calculation by formula
Calculation of dew point T using the formula, it is carried out at known humidity and temperature indicators. The final value will be considered approximate due to the neglect of some factors.
t- room temperature o C, φ - humidity%, and 17.27 and 237.7 are constant values.
For example, for a room the normal indicators are humidity 60% and room temperature 21 o C, the calculation will look like in the following way:
Thus, the dew point calculation looks like this:
The condensation temperature is 12.92 o C. Thus, insulating the walls from the outside will prevent losses heat from the room and freezing of the wall.
Calculation according to the table
The dew point can be determined using a table created by specialists. In order to determine the dew point, for example for 21 o C at 60% humidity, we look for line intersection temperature with the humidity column and we get a value of 12.9 o C.
Table 1. Dew point definitions. 
Calculation using an online calculator
How to work with an online calculator to calculate the dew point in a wall, watch the video:
Regulations
The need to calculate the dew point is regulated building codes and rules. SP 23-101-2004 “Design of thermal protection of buildings”, as well as SNiP 23-02 “Thermal protection of buildings”. Insufficient insulation shifts the dew point closer to the room.
Since the temperature in the area of window units or doors is lower than the overall calculated dew point, condensation in these segments is inevitable during the cold season. Dew point determination important for implementation of the decision which side to carry out insulation work and what thickness is best to purchase insulation.
Important! The lower the thermal conductivity coefficient of the insulation, the smaller the thickness of the insulating layer required. For example, the thickness of insulation from mineral wool 0.12 m will be enough when you need more than 5 meters of reinforced concrete to retain heat in the room.
table 2. Dependence of wall material thickness on thermal conductivity
| Wall material | Coeff. heat-conductingI, W/(m*oC) | Required thickness in meters |
| 0,039 | 0,12 | |
| 0,041 | 0,13 | |
| Reinforced concrete | 1,7 | 5,33 |
| Solid silicate brick masonry | 0,76 | 2,38 |
| Perforated brickwork | 0,5 | 1,57 |
| Glued laminated timber | 0,16 | 0,5 |
| Expanded clay concrete | 0,47 | 1,48 |
| Gas silicate | 0,15 | 0,47 |
| Foam concrete | 0,3 | 0,94 |
| Cinder concrete | 0,6 | 1,88 |
Minimizing heat loss and maintaining a comfortable microclimate are priority tasks when designing and insulating buildings. Compliance with building rules and regulations, as well as sanitary and hygienic standards, will allow you to competently prepare engineering documentation and calculate the volume of required building materials.
Each of us has repeatedly witnessed the formation of water droplets on surrounding objects and structures. This is explained by the fact that the surrounding air cools over an object brought in from the cold. Saturation with water vapor occurs and dew condenses on the object.
Fogging of windows in an apartment has the same nature. The reason that “windows cry” is condensation processes, which are influenced by the humidity and temperature of the surrounding air.
The formation of condensation is closely related to the concept of dew point. For a better understanding of the described phenomena, it is simply necessary to consider this factor in more detail.
Dew point. What is this?
The dew point is the cooling temperature of the surrounding air at which the water vapor it contains begins to condense, forming dew, that is, it is the temperature at which condensation occurs.
This indicator depends on two factors: air temperature and relative humidity. The higher the relative humidity of a gas, the higher its dew point, that is, it approaches the actual ambient temperature. Conversely, the lower the humidity, the lower the dew point.
How to calculate dew point?
Calculating the dew point is important in many aspects of life, including in construction. The quality of life in new buildings and premises that have been commissioned for a long time depends on the correct determination of this indicator. So how do you determine the dew point?
To determine this indicator, use the formula for approximate calculation of the dew point temperature Tp (°C), which is determined by the dependence of relative humidity Rh (%) and air temperature T (°C):
What instruments are used to calculate it?
So how is dew point calculated in practice? This indicator is determined using a psychrometer - a device consisting of two that measures humidity and air temperature. These days it is mainly used in laboratories.
Portable thermohygrometers are used for - electronic devices, the digital display of which displays data on relative humidity and air temperature. Some models even display the dew point.
Some thermal imagers also have the function of calculating dew point. At the same time, a thermogram is displayed on the screen, which shows surfaces with temperatures below the dew point in real time.
Dew point calculation table
Using household psychrometers it is easy to measure the humidity and temperature of the surrounding air. Using the display readings, this condensation loss can be found using the table. The dew point is determined based on the calculated temperature and humidity indicators. Its calculation table looks like this:
How is dew temperature determined in construction?
Dew point measurement is very important stage construction of buildings, which must be carried out at the project development stage. The possibility of air condensation inside the room, and therefore the comfort of further living in it, as well as its durability, depends on its correctness.
Any wall has a certain humidity. That is why, depending on the material of the wall and the quality of the thermal insulation, condensation may form on it. The dew point temperature depends on:
- indoor air humidity;
- its temperature.
So, using the table given earlier, we can determine that in a room with a temperature of +25 degrees and a relative humidity of 65%, condensation will form on surfaces with a temperature of 17.5 degrees and below. A rule to remember is that the lower the humidity in the room, the greater the difference between the dew point and the room temperature.
The main factors that influence the location of the dew point include:
- climate;
- temperature inside and outside the room;
- humidity inside and outside;
- indoor living mode;
- quality of functioning of the heating and ventilation systems in room;
- wall thickness and material;
- ceiling, walls, etc.
Features of non-insulated walls
In many rooms, wall insulation is completely absent. Under such conditions, the following behavior options for the dew point are possible depending on its location:
- Between the outer surface and the center of the wall ( inner part the wall always remains dry).
- Between the inner surface and the center of the wall (at inner surface Condensation may appear if there is a sharp cooling in the air in the region).
- On the inner surface of the wall (the wall will remain wet throughout the winter period).
How to properly insulate a wall?
In an insulated wall, the dew point can be located in different places of the insulation, which depends on a number of factors:
- The thermal insulation properties of insulation decrease as its humidity level increases, since water is an excellent conductor of heat.
- The presence of insulation defects and gaps between the insulation and the wall surface creates good conditions for condensation to form.
- Dew droplets significantly reduce thermal insulation properties insulation, and also support the development of fungal colonies.
Thus, one should understand the risk of using moisture-permeable materials for wall insulation, since they are subject to loss of heat-protective qualities and gradual destruction.
In addition, be sure to pay attention to the ability of the materials chosen for wall insulation to resist fire. It is better to opt for materials with an organic content of less than 5%. They are considered non-flammable and are most suitable for insulating residential premises.
External wall insulation
The ideal option for protecting a room from dampness and cold is external wall insulation (provided that it is carried out in compliance with technology).
If the thickness of the insulation is chosen optimally, the dew point will be located in the insulation itself. The wall will remain absolutely dry throughout the entire cold period; even with a sharp cold snap, the dew point will not reach the inner surface of the wall.
If the thickness of the insulation has not been calculated correctly, some problems may arise. The dew point will move to the heat interface insulating material And outside walls. Condensation can occur in the cavities between the two materials and moisture can accumulate. IN winter period When the temperature drops below zero, the moisture will expand and turn into ice, causing the destruction of the insulation and part of the wall. In addition, constant moisture on surfaces will lead to the formation of mold.
In case of complete non-compliance with the technology and gross errors in calculations, it is possible that the dew point will shift to the inner surface of the wall, which will lead to the formation of condensation on it.
Internal wall insulation
Insulating a wall from the inside is initially not the best the best option. If the thermal insulation layer is thin, the dew point will be at the border of the insulating material and the inner surface of the wall. Warm air in the room thin layer thermal insulation will practically not be achieved inside walls, leading to the following consequences:
- high probability of the wall getting wet and freezing;
- moisture and, as a consequence, destruction of the insulation itself;
- excellent conditions for the development of mold colonies.
However, this method of insulating a room can be effective. To do this, some prerequisites must be met:
- must comply with standards and prevent excessive humidification of the surrounding air.
- thermal resistance of the fence structure, according to regulatory requirements, should not exceed 30%.
What are the consequences of ignoring condensation in construction?
In winter, when the temperature is almost constantly below zero degrees, warm air indoors, in contact with any cold surface, becomes supercooled and falls on its surface in the form of condensation. This occurs provided that the temperature of the corresponding surface is below the dew point calculated for the given air temperature and humidity.
If condensation occurs, the wall is in a damp state almost always at a low temperature. The result of this is the formation of mold and the development of a wide variety of harmful microorganisms in it. Subsequently, they move into the surrounding air, which leads to various diseases of residents who are often indoors, including asthmatic disorders.
In addition, houses affected by mold and fungal colonies are extremely short-lived. The destruction of the building is inevitable, and this process will begin with damp walls. That is why it is extremely important to make all calculations regarding the dew point correctly at the stage of design and construction of the building. This will allow you to do right choice relatively:
- wall thickness and material;
- thickness and material of insulation;
- method of wall insulation (internal or external insulation);
- choosing a ventilation and heating system that can provide an optimal microclimate in the room (the best ratio of relative humidity and temperature).
You can calculate the dew point in the wall yourself. In this case, one should take into account the characteristics of the climatic region of residence, as well as other previously mentioned nuances. But it’s still better to contact specialized construction organizations that deal with such calculations in practice. And the responsibility for the correctness of the calculations will lie not with the client, but with the representatives of the organization.
The concept of dew point (hereinafter referred to as TP) is used in the design of thermal protection of civil and industrial buildings, and 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 air temperature, condensed moisture is present on the substrate, which prevents the desired adhesion from being achieved. 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 the characteristics of air exchange within the premises.
The dew point temperature serves as a kind of indicator of the degree of air humidity from inside the living space. The dew point temperature determines the comfort level of living in the house. The higher the dew point in 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 heart patients and asthmatics is extremely suffocating and intolerable. Incorrect determination of the dew point in the wall of a residential building leads to the deposition of condensation 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 properties building structure.
The picture below shows damp wooden wall with fungal manifestations due to improper thermal insulation.
Physics of steam condensation
Water is present in the environment of our home 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's building structure: concrete or brick walls, ceilings, and the base of the floor. There are no ideally dry building materials in nature. In stable warm weather, the steam present in the air and the moisture in the walls of the home are in thermal equilibrium.
In this case, the partial pressure of steam in the air from 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 occurs through the wall. In frosty weather, the humidity of cold air is low, and the partial pressure of vapor in such air is low. In accordance with the laws of thermophysics, high-pressure steam (living space) begins to diffuse through the wall material to the cold street, where the pressure is lower.
All building materials from which the walls of houses are constructed have the property of vapor permeability. Even concrete or brick walls are capable of transmitting steam through their thickness, although concrete and brick have minimal vapor permeability.
When passing through the dew point in the wall, the steam turns into a liquid aggregate state, 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 will intensify, and the house will always be cold.
- During 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 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 is used) when TR gets inside the wall of the home.
TR calculation methods
The question of what dew point is is answered in the Code of Rules SP 50.13330.2012, which regulates the issues of thermal protection of buildings. In paragraph B.24, the concept of TP is interpreted as the temperature at which condensation moisture begins to form 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 air temperature parameter for which 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 TP, the temperature of condensation moisture depends on the values of two parameters:
- on 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. If the humidity of the same volume is within 80%, the temperature will already reach plus 6.7 °C. For 100% humidity, the values of TP and air t are the same = 10.0 °C.
When arranging thermal protection, it is very important to find a place where there may be a dew point in order to prevent the formation of condensation moisture in a place undesirable for providing effective thermal protection. It is almost impossible to visually determine the position of the TR as the place of initial condensation. For the dew point indicator, determination is carried out using several methods.
Calculation method
The following formula is very convenient for calculating TP in the positive temperature range up to 60°C:
T P = b*f(T,Rh)/(a-f(T,Rh), Where
- T R – the temperature at which condensation begins, that is, the dew point in the wall, insulation or ambient air;
- f(T,Rh) = a*T/(b+T) + ln(Rh);
- ln – 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 degrees. Ts, for example, T p ≈T – (1-RH)/0.05.
This formula can be used to calculate the relative humidity indicator using the already known temperature TR: RH≈1-0.05(T-T p).
Table method
Numerous special tables based on laboratory measurements indicate TP values depending on relative air humidity and temperature. The dew point parameter is determined in quite detail by the table in the reference appendix R of the Code of Rules SP 23-101-2004 “Design of thermal protection of buildings”. In Fig. Below is a similar dew point table that fully complies with the parameters from GOST and SP.
Table for determining dew point
| Tempera- tour air, (°C) | Dew point temperature (°C) at relative humidity (%) | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 30% | 35% | 40% | 45% | 50% | 55% | 60% | 65% | 70% | 75% | 80% | 85% | 90% | 95% | |
| 30 | 10,5 | 12,9 | 14,9 | 16,8 | 18,4 | 20 | 21,4 | 22,7 | 23,9 | 25,1 | 26,2 | 27,2 | 28,2 | 29,1 |
| 29 | 9,7 | 12 | 14 | 15,9 | 17,5 | 19 | 20,4 | 21,7 | 23 | 24,1 | 25,2 | 26,2 | 27,2 | 28,1 |
| 28 | 8,8 | 11,1 | 13,1 | 15 | 16,6 | 18,1 | 19,5 | 20,8 | 22 | 23,2 | 24,2 | 25,2 | 26,2 | 27,1 |
| 27 | 8 | 10,2 | 12,2 | 14,1 | 15,7 | 17,2 | 18,6 | 19,9 | 21,1 | 22,2 | 23,3 | 24,3 | 25,2 | 26,1 |
| 26 | 7,1 | 9,4 | 11,4 | 13,2 | 14,8 | 16,3 | 17,6 | 18,9 | 20,1 | 21,2 | 22,3 | 23,3 | 24,2 | 25,1 |
| 25 | 6,2 | 8,5 | 10,5 | 12,2 | 13,9 | 15,3 | 16,7 | 18 | 19,1 | 20,3 | 21,3 | 22,3 | 23,2 | 24,1 |
| 24 | 5,4 | 7,6 | 9,6 | 11,3 | 12,9 | 14,4 | 15,8 | 17 | 18,2 | 19,3 | 20,3 | 21,3 | 22,3 | 23,1 |
| 23 | 4,5 | 6,7 | 8,7 | 10,4 | 12 | 13,5 | 14,8 | 16,1 | 17,2 | 18,3 | 19,4 | 20,3 | 21,3 | 22,2 |
| 22 | 3,6 | 5,9 | 7,8 | 9,5 | 11,1 | 12,5 | 13,9 | 15,1 | 16,3 | 17,4 | 18,4 | 19,4 | 20,3 | 21,1 |
| 21 | 2,8 | 5 | 6,9 | 8,6 | 10,2 | 11,6 | 12,9 | 14,2 | 15,3 | 16,4 | 17,4 | 18,4 | 19,3 | 20,2 |
| 20 | 1,9 | 4,1 | 6 | 7,7 | 9,3 | 10,7 | 12 | 13,2 | 14,4 | 15,4 | 16,4 | 17,4 | 18,3 | 19,2 |
| 19 | 1 | 3,2 | 5,1 | 6,8 | 8,3 | 9,8 | 11,1 | 12,3 | 13,4 | 14,5 | 15,5 | 16,4 | 17,3 | 18,2 |
| 18 | 0,2 | 2,3 | 4,2 | 5,9 | 7,4 | 8,8 | 10,1 | 11,3 | 12,5 | 13,5 | 14,5 | 15,4 | 16,3 | 17,2 |
| 17 | -0,6 | 1,4 | 3,3 | 5 | 6,5 | 7,9 | 9,2 | 10,4 | 11,5 | 12,5 | 13,5 | 14,5 | 15,3 | 16,2 |
| 16 | -1,4 | 0,5 | 2,4 | 4,1 | 5,6 | 7 | 8,2 | 9,4 | 10,5 | 11,6 | 12,6 | 13,5 | 14,4 | 15,2 |
| 15 | -2,2 | -0,3 | 1,5 | 3,2 | 4,7 | 6,1 | 7,3 | 8,5 | 9,6 | 10,6 | 11,6 | 12,5 | 13,4 | 14,2 |
| 14 | -2,9 | -1 | 0,6 | 2,3 | 3,7 | 5,1 | 6,4 | 7,5 | 8,6 | 9,6 | 10,6 | 11,5 | 12,4 | 13,2 |
| 13 | -3,7 | -1,9 | -0,1 | 1,3 | 2,8 | 4,2 | 5,5 | 6,6 | 7,7 | 8,7 | 9,6 | 10,5 | 11,4 | 12,2 |
| 12 | -4,5 | -2,6 | -1 | 0,4 | 1,9 | 3,2 | 4,5 | 5,7 | 6,7 | 7,7 | 8,7 | 9,6 | 10,4 | 11,2 |
| 11 | -5,2 | -3,4 | -1,8 | -0,4 | 1 | 2,3 | 3,5 | 4,7 | 5,8 | 6,7 | 7,7 | 8,6 | 9,4 | 10,2 |
| 10 | -6 | -4,2 | -2,6 | -1,2 | 0,1 | 1,4 | 2,6 | 3,7 | 4,8 | 5,8 | 6,7 | 7,6 | 8,4 | 9,2 |
| * for intermediate indicators not indicated in the table, the average value is determined | ||||||||||||||
Using household psychrometers
Psychrometers, or more precisely, psychrometric hygrometers, are designed to measure air temperature and relative humidity. A modern hygrometer can be used as a device for determining dew point, since an image of a psychrometric table is printed on its body.
Using the readings of both thermometers of the device, the TP is determined from the table. The figure below shows models of modern household psychrometers equipped with psychrometric tables that help determine the dew point.
Portable electronic thermohygrometers
The dew point in construction during thermal inspection of premises is determined using portable thermohygrometers with displays equipped with an indication of the values of the ambient air temperature, its humidity and the TP parameter.
Thermal imager readings
There is no need to calculate TP if you use certain models of thermal imagers for construction purposes that have the function of calculating TP and display surfaces with temperatures below TP during thermal imaging. Given the given air parameters, it is possible to process thermal imaging data on a computer and show on thermograms all areas that risk falling into the condensation zone when insulating a wall or ceiling.
Housing options
The TP parameter is a kind of temperature boundary at which the meeting occurs internal heat and external cold. In wall enclosing structures warm air, diffusing during the cold winter months from a heated room onto a frosty street, becomes supercooled.
The vapor phase of water turns into a wet state, depositing on any surface that has a temperature below TP. The cause of condensation is not only the wall material ( wooden house, brick or aerated concrete), but also the method of arranging the thermal protection of the building, which determines in which direction the thermal protection is shifted.
The location of the TR depends on the following factors:
- indoor and outdoor humidity indicators;
- indoor and outdoor air temperature indicators;
- thickness of the wall and insulating layer;
- places where insulating material is placed.
Depending on these factors, TP 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 is within the thickness of the wall and can shift towards the street or room depending on changing temperature and humidity parameters.
In any case, is the dew point in aerated concrete or brick wall, condensation forms relatively far from the inner surface. Condensation moisture accumulates in the wall material and freezes in severe frosts. As temperatures warm, moisture thaws and evaporates out into the atmosphere.
There are three possible 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;
- TP falls between the geometric center of the wall and the inner surface of the room - the walls of the room may get wet during a sharp cold snap;
- The TR 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 aspect of the occurrence of TR in a wall is the gradual destruction of the wall structure.
Walls made of brick, aerated concrete, expanded clay blocks, etc., homogeneous in their design, have a TR in winter time inside the thickness of the material. Repeated freeze/thaw cycles worsen the strength properties of building materials and reduce the strength of the entire wall structure. Therefore the walls monolithic design homogeneous composition must be insulated with heat-insulating materials.
Insulation from the inside of the room
The following options are possible for the location of the TR:
- 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 moisture condensation inside the insulating layer (expanded polystyrene, etc.), then condensation will fall out at the boundary interior wall and insulating polystyrene board. The wall finish will begin to get wet, which will cause the formation of damp spots and mold;
- the wall material is in the zone subzero temperatures and exposed negative impacts temperature changes.
Insulation from the outside of the building
TP is brought into the outer heat-insulating layer. The possibility of condensation forming 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 a building from its outside. Then there is a greater chance that the TR will be in an area that does not allow moisture condensation inside the room.
The dew point is the temperature at which water vapor in the air becomes saturated. At the dew point temperature, the relative humidity becomes 100%. Consider such a phenomenon as the dew point in more detail
Is “breathable” wall material an advantage? Very controversial. Perhaps the walls should be strong and retain heat in the house, but they don’t have to allow steam to pass through; for this there is ventilation, natural and forced?
It’s clear why there are couples in the house. The air inside the home is always – generally – warmer than outside. Water flows in bathrooms and kitchens, indoor flowers are watered, and the house is often wet cleaned. The greater the temperature difference between the house and the street, the more water vapor tends to leave the room. This relationship is not linear, since there is another factor - humidity, and it is different, one in the house, and another outside. The lower the humidity at home and outside, the lower the risk of moisture appearing on the internal surfaces of walls in the form of condensation.
When water vapor comes through a wall, it makes the wall feel bad. The thermal conductivity of the wall material increases because water is present, which conducts heat very well, and in the form of steam too. Wall materials always have moisture capacity (if they are not made of metal), that is, they accumulate water. The steam coming through the breathing walls affects them bad influence, in fact, very slowly destroys, while simultaneously increasing heat loss from the premises. If in winter the accumulation of moisture in the wall is less than the standard value, then there will be no significant harm. But it is very desirable that the dew point in winter is outside the outer wall.
Dew point
Dew point is measured in degrees. This is the temperature at which the content of water vapor in the air is maximum. The dew point cannot be more important than the air temperature - condensation occurs. For example, in a kitchen where dishes are washed and cooked, the dew point will be the temperature of the window glass on which drops of water can be seen.
The dew point can be located both outside the wall and inside, it depends on the humidity and air temperature inside and outside the room, and on the thickness and vapor permeability of each layer of the wall “pie”.
Complex finishing and insulation of walls using technology " Wet facade" has undeniable advantages. But the first two options are somewhat different from the marketing presentations presented below. This is not entirely true.
Dew point in an uninsulated wall
- The dew point inside the wall is closer to its outer edge and does not reach the center of the wall - the inside of the wall is dry, everything is fine.
- The same, but the dew point is closer to the inner edge of the wall than the center of the wall - in this case, if the outside air temperature drops sharply, the inside of the wall will be wet for some time, about several days. Exactly how much depends on the water absorption and vapor permeability of the wall material. For example, ceramic bricks have excellent parameters; frosts will subside and moisture will escape. But for some time, as mentioned above, the wall will be wet.
- The worst option is the dew point on the inner surface of the wall. Most likely, the wall will be wet all winter, it all depends on how much steam there is in the room. You can’t keep the windows open all the time in winter.
Dew point in a wall with external insulation
- The dew point inside the insulation is normal, the thickness of the insulation is correct, the thermal calculation is correct, the wall inside is dry, and the insulation will release moisture outside when the temperature and air humidity change
- If the calculation is incorrect or the parameters have changed - the insulation is damaged, etc., then the dew point will be inside wall material, and not in the insulation layer. The consequences are the same as for an uninsulated wall according to points 2 and 3.
Dew point in a wall with internal insulation
The surface of the condensate moves inward, and again there are three options:
- The dew point between the insulation layer and the middle of the wall. If it gets colder sharply, the dew point will shift to their border. The wall will be dry.
- The dew point behind the insulation layer, inside the wall - the wall will be damp all winter.
- Dew point inside the insulation - all winter the insulation layer will absorb the condensation that forms.
Vapor permeability of building materials
We present in the table below the vapor permeability coefficients of building materials
To ensure that the microclimate in the house is normal, when constructing wall “pies”, both the thickness of each layer and its water absorption and vapor permeability properties are taken into account. The layers of the cake should be arranged in such a way and have such thicknesses that vapor permeability increases from the inside to the outside. This “rule of vapor permeability” is best observed. Otherwise, there are two options:
- Poor ventilation and high humidity in the house means you can get the dew point in the wrong place, and as a result, dampness and mold and fungus on the walls, and possible destruction of the walls.
- There is little humidity inside the house, and ventilation is organized - there will be no harmful consequences for the microclimate from violating the rule, except for the harmful effect of moisture on the wall material.
All this is true, the dew point must be taken into account, since it is a risk factor. But the degree of this risk depends on the real, actual amount of water condensed in the wall and on the properties of the wall material. The less water absorption a wall material has, that is, the less moisture it absorbs, the less it is at risk of destruction when this moisture freezes and expands in the pores. Brick Khrushchev buildings have been standing for more than 60 years, but they don’t even think about collapsing, although according to thermal engineering calculations they have condensation in the walls. Ceramic brick It has good characteristics in terms of frost resistance, the frosts end and the brick releases moisture into the air. But we must remember that the walls of the Khrushchev buildings are half a meter thick.
Dew point temperature calculation
It is possible and necessary to calculate the dew point; for this it is not necessary to study the science of heating engineering. It can be considered from calculators from the Internet, quite decent, working on the basis of thermal formulas and a database of material characteristics. It is better, of course, to entrust the final calculation to professionals.
Here is a table with the ability to calculate the dew point temperature.
Breathing walls
Regarding the issue of wall breathing. Perhaps this question relates not so much to construction physics as to ideology? There were once slit windows, they had wonderful vapor permeability, and the walls breathed with might and main. At the same time, you didn’t have to pay a good part of your salary for heating. Today the situation is different, and has been for a long time - the issue of energy saving for a private home is acute. Well-established phraseological units of the type - energy-saving house, energy-efficient building material - already speak volumes. Perhaps the walls of the house should keep the heat in, and breathing should be ensured by well-organized ventilation? After all, marketers know how to tell fairy tales, and about the breathing of houses that have grown gills thanks to innovative building materials... too.
