Calculate the dew point in the insulation. The concept of dew point and methods for calculating it
Construction technologies involve taking into account many nuances that affect the durability of the structure and the ability to resist from negative influence external factors. One of the main enemies of most buildings and structures is constant high humidity. A variety of techniques are used to combat it.
It is necessary to take into account all factors at the initial design stage, when it is possible to influence the use of materials and the formation of the exterior of buildings. Important place In such a situation, it is necessary to make competent calculations when insulating buildings. A mandatory attribute in them is the determination of dew point temperature.
Basic knowledge
Large construction projects use complex and cumbersome calculation programs. Many coefficients and mathematical formulas are used. In domestic conditions, the technique is significantly simplified. Many roundings and approximations are used in the calculation, while the error is minimal.
Homeowners or builders will be able to independently calculate the dew point in the wall without involving third-party specialists.
To understand how to find a point, you need knowledge about the surrounding air and the presence of water vapor in it. It is formed as a result of many events, for example, particles of water are separated from residents, any sources of liquid, containers with water, appear after wet cleaning premises, etc.
Air capacity has a certain maximum. When this parameter is obtained, water particles begin to interact with each other, forming larger water droplets. This is how condensation occurs. In nature, it is noticeable in the form of fog or droplets on plants.
When the air is saturated with liquid as much as possible and can no longer receive replenishment from it without turning into condensation, it is said that in this case the relative humidity has reached the level of 100%. Subsequent saturations turn the air into fog, which is a large number of suspended water droplets in the air.
The peculiarity of this event is that different temperatures air can provide varying degrees saturation with moisture until it turns into condensation. There is a direct dependence on high temperature and the amount of dissolved liquid in the air. Moreover, when air with a moisture content of 70-80% comes into contact with a cooled object, the saturation limit occurs, and the degree of humidity in the contact plane instantly reaches 100%.
What causes condensation to form
Events lead to condensation. This interaction largely explains what a dew point is. Considering this example, it is obvious that this parameter in construction or in another field is a variable value. It is expressed in degrees. The main parameters that affect it:
- relative humidity in this moment;
- current air temperature;
- air speed;
- thickness of materials.
To obtain calculated values, measuring instruments are used: psychrometers and thermometers. A special table helps calculate the location of the desired value in the wall. Values for development can not only be measured, but also learned from the current weather forecast. Many websites provide information not only about temperature, but also about humidity.
VIDEO: Why condensation appears on the walls
The role of the concept in the construction process
We recommend using a special table prepared by specialists to determine the location of the dew point in the wall. It is preferable to use your own definition of the parameter, without resorting to the help of a set online calculators. Often, the built-in algorithms in them do not take into account important factors.
The table below uses the step principle. For values intermediate between two adjacent ones, the arithmetic mean can be used.
The table is easy to use. We draw a horizontal line from the measured room temperature. We draw a vertical line from the measured humidity value. At the intersection we obtain the required temperature number. Visually it will look like this.
Let's look at an example. Let's imagine a house whose walls are made of brick. Inside the room, for example, the temperature will be +20°C, and outside it will be cooler, for example -10°C. The air humidity in the room is 60%. By connecting the horizontal and vertical lines in the table (20 and 60), we get 12°C at the intersection.
Each brick will have a non-uniform temperature. Its inner surface will have the maximum high value(+20°C), and the outer part will be with the lowest possible parameter (-10°C). In the middle of the brick there will be a plane with a temperature of +12°C. Moisture will begin to condense in this area. The process will also occur throughout the entire volume with lower values.
The use of various insulation materials helps to turn the situation in a positive direction. They help shift the position of the dew point in the wall. Depending on which side the home owners installed the insulation, the condensation plane will move. If everything is done correctly, then this point will not be in the wall of the house, but in the insulating fence. This way there will be no structural damage.
It must be taken into account that without insulation, the plane with the dew point in our climate will be located directly in the depths of the wall. This is shown in the first picture, so moisture will cause damage to the structure, ensuring the spread of fungus and mold in the room. The dew point in the wall will be located at a depth that depends on the vapor permeability of a particular building material.
It is necessary that water vapor penetrates to a place with a design temperature. This factor is taken into account when choosing a material.
Requirements for insulation and thermal insulation
Vapor permeability is usually called a value that demonstrates how much water vapor a building material can pass through in a given time. Almost all popular materials are permeable by this criterion:
- tree;
- concrete;
- brick, etc.
From some builders you can hear such a concept as “walls breathe.” Porous materials may also be included in the list (expanded clay, mineral wool etc.).
There is no need to be afraid that there is some kind of stationary part with a dew point in the wall, since this happens in a certain area. Builders call the area a possible condensation zone. Considering that most fences are “breathable,” a lot of moisture escapes outside.
The correct construction of a building is such an arrangement of materials in which the determination of the dew point in the wall falls into the outer insulation layer. It is also important to provide the room with high-quality ventilation, in which excess moisture leaves the apartment or house. Under such conditions, the material does not have time to become saturated with liquid.
Offered by manufacturers various insulation materials Due to their design, polymers practically do not allow steam to pass through. Due to this property, they are recommended to be placed outside the walls. In this case, the dew point at which condensation occurs will move inside the foam or polystyrene. However, water vapor will not be able to get to this zone. No moisture will form.
It is not recommended to use extruded foam sterol for façade insulation. It is used only for foundations or closed building systems. As a result of constant temperature changes and exposure to direct sun rays after only a year and a half it begins to crumble.
The same will happen in the reverse process. You should not insulate internal walls with polymers, because the dew point will be located in the wall. In this case, unwanted moisture will seep into the joint of materials.
It is reasonable to use internal insulation in the following cases:
- the wall is almost always warm and dry;
- the residential building has high-quality ventilation;
- It is necessary to use high-quality permeable insulation that ensures the removal of excess moisture.
Conclusion
It is quite difficult to identify a specific place with a dew point, since this zone is floating and depends on external factors. It is advisable to use external insulation to move the point to insulation material. Use good ventilation in the room to remove water vapor.
VIDEO: Proper insulation or How to remove dew point from a wall
Dew point determines the ratio of air temperature, air humidity and surface temperature at which water begins to condense on the surface.
Production and sale of materials, performance of work: Polymer floors Self-leveling floors
Dew point definition
Dew point determination is extremely important factor when installing any polymer floors, coatings and self-leveling floors on any base: concrete, metal, wood, etc. The appearance of a dew point and, accordingly, water condensation on the surface of the base at the time of laying polymer floors, self-leveling floors and coatings can cause the appearance of a variety of defects: shagreen, swelling and cavities; complete detachment of the coating from the base. Visually determining the dew point - the appearance of moisture on the surface - is almost impossible, so the technology given below is used to calculate the dew point.
Dew point table
The dew point table is very easy to use - hover your mouse over it... Dew point table - download
For example: air temperature +16°C, relative humidity 65%.
Find a cell at the intersection of air temperature +16°C and air humidity 65%. It turned out to be +9°C - this is the dew point.
This means that if the surface temperature is equal to or below +9°C, moisture will condense on the surface.
To apply polymer coatings, the surface temperature must be at least 4°C above the dew point!
| Tempe- ratio air | Dew point temperature at relative air humidity (%) | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 30% | 35% | 40% | 45% | 50% | 55% | 60% | 65% | 70% | 75% | 80% | 85% | 90% | 95% | |
| -10°С | -23,2 | -21,8 | -20,4 | -19 | -17,8 | -16,7 | -15,8 | -14,9 | -14,1 | -13,3 | -12,6 | -11,9 | -10,6 | -10 |
| -5°C | -18,9 | -17,2 | -15,8 | -14,5 | -13,3 | -11,9 | -10,9 | -10,2 | -9,3 | -8,8 | -8,1 | -7,7 | -6,5 | -5,8 |
| 0°С | -14,5 | -12,8 | -11,3 | -9,9 | -8,7 | -7,5 | -6,2 | -5,3 | -4,4 | -3,5 | -2,8 | -2 | -1,3 | -0,7 |
| +2°С | -12,8 | -11 | -9,5 | -8,1 | -6,8 | -5,8 | -4,7 | -3,6 | -2,6 | -1,7 | -1 | -0,2 | -0,6 | 1,3 |
| +4°С | -11,3 | -9,5 | -7,9 | -6,5 | -4,9 | -4 | -3 | -1,9 | -1 | 0 | 0,8 | 1,6 | 2,4 | 3,2 |
| +5°С | -10,5 | -8,7 | -7,3 | -5,7 | -4,3 | -3,3 | -2,2 | -1,1 | -0,1 | 0,7 | 1,6 | 2,5 | 3,3 | 4,1 |
| +6°С | -9,5 | -7,7 | -6 | -4,5 | -3,3 | -2,3 | -1,1 | -0,1 | 0,8 | 1,8 | 2,7 | 3,6 | 4,5 | 5,3 |
| +7°С | -9 | -7,2 | -5,5 | -4 | -2,8 | -1,5 | -0,5 | 0,7 | 1,6 | 2,5 | 3,4 | 4,3 | 5,2 | 6,1 |
| +8°С | -8,2 | -6,3 | -4,7 | -3,3 | -2,1 | -0,9 | 0,3 | 1,3 | 2,3 | 3,4 | 4,5 | 5,4 | 6,2 | 7,1 |
| +9°С | -7,5 | -5,5 | -3,9 | -2,5 | -1,2 | 0 | 1,2 | 2,4 | 3,4 | 4,5 | 5,5 | 6,4 | 7,3 | 8,2 |
| +10°С | -6,7 | -5,2 | -3,2 | -1,7 | -0,3 | 0,8 | 2,2 | 3,2 | 4,4 | 5,5 | 6,4 | 7,3 | 8,2 | 9,1 |
| +11°С | -6 | -4 | -2,4 | -0,9 | 0,5 | 1,8 | 3 | 4,2 | 5,3 | 6,3 | 7,4 | 8,3 | 9,2 | 10,1 |
| +12°С | -4,9 | -3,3 | -1,6 | -0,1 | 1,6 | 2,8 | 4,1 | 5,2 | 6,3 | 7,5 | 8,6 | 9,5 | 10,4 | 11,7 |
| +13°С | -4,3 | -2,5 | -0,7 | 0,7 | 2,2 | 3,6 | 5,2 | 6,4 | 7,5 | 8,4 | 9,5 | 10,5 | 11,5 | 12,3 |
| +14°С | -3,7 | -1,7 | 0 | 1,5 | 3 | 4,5 | 5,8 | 7 | 8,2 | 9,3 | 10,3 | 11,2 | 12,1 | 13,1 |
| +15°С | -2,9 | -1 | 0,8 | 2,4 | 4 | 5,5 | 6,7 | 8 | 9,2 | 10,2 | 11,2 | 12,2 | 13,1 | 14,1 |
| +16°С | -2,1 | -0,1 | 1,5 | 3,2 | 5 | 6,3 | 7,6 | 9 | 10,2 | 11,3 | 12,2 | 13,2 | 14,2 | 15,1 |
| +17°С | -1,3 | 0,6 | 2,5 | 4,3 | 5,9 | 7,2 | 8,8 | 10 | 11,2 | 12,2 | 13,5 | 14,3 | 15,2 | 16,6 |
| +18°С | -0,5 | 1,5 | 3,2 | 5,3 | 6,8 | 8,2 | 9,6 | 11 | 12,2 | 13,2 | 14,2 | 15,3 | 16,2 | 17,1 |
| +19°С | 0,3 | 2,2 | 4,2 | 6 | 7,7 | 9,2 | 10,5 | 11,7 | 13 | 14,2 | 15,2 | 16,3 | 17,2 | 18,1 |
| +20°С | 1 | 3,1 | 5,2 | 7 | 8,7 | 10,2 | 11,5 | 12,8 | 14 | 15,2 | 16,2 | 17,2 | 18,1 | 19,1 |
| +21°С | 1,8 | 4 | 6 | 7,9 | 9,5 | 11,1 | 12,4 | 13,5 | 15 | 16,2 | 17,2 | 18,1 | 19,1 | 20 |
| +22°С | 2,5 | 5 | 6,9 | 8,8 | 10,5 | 11,9 | 13,5 | 14,8 | 16 | 17 | 18 | 19 | 20 | 21 |
| +23°С | 3,5 | 5,7 | 7,8 | 9,8 | 11,5 | 12,9 | 14,3 | 15,7 | 16,9 | 18,1 | 19,1 | 20 | 21 | 22 |
| +24°С | 4,3 | 6,7 | 8,8 | 10,8 | 12,3 | 13,8 | 15,3 | 16,5 | 17,8 | 19 | 20,1 | 21,1 | 22 | 23 |
| +25°С | 5,2 | 7,5 | 9,7 | 11,5 | 13,1 | 14,7 | 16,2 | 17,5 | 18,8 | 20 | 21,1 | 22,1 | 23 | 24 |
| +26°С | 6 | 8,5 | 10,6 | 12,4 | 14,2 | 15,8 | 17,2 | 18,5 | 19,8 | 21 | 22,2 | 23,1 | 24,1 | 25,1 |
| +27°С | 6,9 | 9,5 | 11,4 | 13,3 | 15,2 | 16,5 | 18,1 | 19,5 | 20,7 | 21,9 | 23,1 | 24,1 | 25 | 26,1 |
| +28°С | 7,7 | 10,2 | 12,2 | 14,2 | 16 | 17,5 | 19 | 20,5 | 21,7 | 22,8 | 24 | 25,1 | 26,1 | 27 |
| +29°С | 8,7 | 11,1 | 13,1 | 15,1 | 16,8 | 18,5 | 19,9 | 21,3 | 22,5 | 22,8 | 25 | 26 | 27 | 28 |
| +30°С | 9,5 | 11,8 | 13,9 | 16 | 17,7 | 19,7 | 21,3 | 22,5 | 23,8 | 25 | 26,1 | 27,1 | 28,1 | 29 |
| +32°С | 11,2 | 13,8 | 16 | 17,9 | 19,7 | 21,4 | 22,8 | 24,3 | 25,6 | 26,7 | 28 | 29,2 | 30,2 | 31,1 |
| +34°С | 12,5 | 15,2 | 17,2 | 19,2 | 21,4 | 22,8 | 24,2 | 25,7 | 27 | 28,3 | 29,4 | 31,1 | 31,9 | 33 |
| +36°С | 14,6 | 17,1 | 19,4 | 21,5 | 23,2 | 25 | 26,3 | 28 | 29,3 | 30,7 | 31,8 | 32,8 | 34 | 35,1 |
| +38°С | 16,3 | 18,8 | 21,3 | 23,4 | 25,1 | 26,7 | 28,3 | 29,9 | 31,2 | 32,3 | 33,5 | 34,6 | 35,7 | 36,9 |
| +40°С | 17,9 | 20,6 | 22,6 | 25 | 26,9 | 28,7 | 30,3 | 31,7 | 33 | 34,3 | 35,6 | 36,8 | 38 | 39 |
Dew point calculation
To calculate the dew point, you need instruments: a thermometer, a hygrometer.
- Measure the temperature at a height of 50-60cm from the floor (or from the surface) and the relative humidity.
- Use the table to determine the “dew point” temperature.
- Measure the surface temperature. If you do not have a special non-contact thermometer, place a regular thermometer on the surface and cover it to insulate it from the air. After 10-15 minutes, take readings.
- The surface temperature must be at least four (4) degrees above the dew point.
Otherwise, it is IMPOSSIBLE to carry out work on applying polymer floors and polymer coatings!
There are devices that immediately calculate the dew point in degrees C.
In this case, a thermometer, hygrometer and dew point table are not required - they are all combined in this device.
Different polymer coatings they “treat” moisture on the surface differently during application. Most “sensitive” to the occurrence of dew point polyurethane materials: paint coatings, polyurethane self-leveling floors, varnishes, etc. This is due to the fact that water is a hardener for polyurethane, and when there is excess moisture, the polymerization reaction occurs very quickly. As a result, a variety of coating defects appear. A particularly unpleasant defect is a decrease in adhesion, which cannot be immediately determined, and over time this leads to partial or complete peeling of the coating or polymer floor.
It is important to consider that the dew point is dangerous not only at the time of coating application, but also during its curing. This is especially dangerous for self-leveling floors, since their initial curing time is quite long (up to a day).
Epoxy self-leveling floors and coatings are “less sensitive” to moisture, but, nevertheless, determining the dew point is a guarantee of quality when installing any polymer floors and paint coatings.
The concept of dew point
The dew point is the temperature at which precipitation or condensation of moisture occurs from the air, which was previously in a vapor state. In other words, the dew point in construction is the boundary of transition from low air temperature outside the building envelope to warm temperature 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 a regulatory document SP 23-101-2004 “Design of thermal protection of buildings” (Moscow, 2004) and SNiP 23-02 “Thermal protection of buildings” conditions are regulated regarding the accounting and value of the dew point :
“6.2 SNiP 23-02 establishes three mandatory mutually related standardized indicators for the thermal protection of a building, based on:
“a” – standardized values of heat transfer resistance for individual building envelopes for thermal protection of the building;
“b” – standardized 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 standardized specific indicator of thermal energy consumption for heating, which allows one to vary the values of the heat-protective properties of enclosing structures, taking into account the choice of systems for maintaining standardized microclimate parameters.
The requirements of SNiP 23-02 will be met if, when designing residential and public buildings the requirements of the indicators of groups “a” and “b” or “b” and “c” will be met.
Condensation of water vapor most easily occurs on some surface, but moisture can also appear inside the thickness of structures. In relation to wall construction: in the case where the dew point is located close to or directly on the inner surface, under certain temperature conditions During the cold season, condensation will inevitably form on surfaces. If the enclosing structures are not sufficiently insulated or are constructed without installing 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 proliferation of microorganisms, such as fungus and mold, the spores of which 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 enclosing structures, including calculating the dew point.
According to the instructions of the regulatory document SP 23-101-2004 “Design of thermal protection of buildings” (Moscow, 2004):
“5.2.3 Temperature of the internal surfaces of the external fences of the building, where there are heat-conducting inclusions (diaphragms, through inclusions of cement-sand mortar or concrete, interpanel 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 indoor wall or window units is lower than calculated value dew point, then condensation is likely to appear in the cold season, when the outside air temperature drops to negative values.
Solving the problem - how to find the dew point, its physical quantity, 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.
Calculation of dew point value
- using the table of the regulatory document;
- according to the formula;
- using an online calculator.
Calculation using a table
Calculation of the dew point when insulating a house can be done using the table of the regulatory document SP 23-101-2004 “Design of thermal protection of buildings” (Moscow, 2004)
To determine the temperature of condensation, it is enough to look at the intersection of the temperature and humidity values established by the standards for each category of premises.
Calculation by formula
Another way to determine the dew point in a wall is using a simplified formula:
$$\quicklatex(size=25)\boxed(T_(p)= \frac(b\times \lambda (T,RH))(a — \lambda(T,RH)))$$
Values:
Тр – 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 fractions of volume ranging from 0.01 to 1;
ln – natural logarithm.
For example, let’s calculate the required value in a room where it should be maintained optimal temperature 20 °C with a relative humidity of 55%, which is established by standards for residential buildings. In this case, we first calculate the coefficient λ(T,RH):
λ(T,RH) = (17.27 x 20) / (237.7 + 20) + Ln 0.55 = 0.742
Then the temperature of condensation from the air will be equal to:
Tr = (237.7 x 0.742)/(17.27 – 0.742) = 176.37/ 16.528 = 10.67 °C
If we compare the temperature value obtained from the formula and the value obtained from the table (10.69°C), we will 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 using an online calculator
The examples show that such a task as determining the dew point is not particularly difficult. Online calculators are developed based on tables and formulas, 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 website. To make the calculation, it is enough to fill in two fields - enter the indicators of the established standard indoor temperature and relative humidity.
Determining the position of the dew point in the wall
In order to ensure the normal thermal protection qualities of the enclosing structures, it is necessary not only to know the value of the condensation temperature, 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 condensation boundary can be different:
- the structure was built without a device 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 there may come a time when this boundary will end up indoors, and then condensation will appear on the internal surfaces.
It should be noted that the dew point in a wooden house with correctly selected wall thickness - made of logs or timber - will be located closer to the outer surfaces, since wood is natural material With unique properties, having very low thermal conductivity with high vapor permeability. Wooden walls in most cases do not require additional insulation;
- the structure was built with an additional layer of insulation with outside. With the correct calculation of the thickness of all materials, the dew point when insulating with foam plastic or other types effective insulation materials will be located inside the insulating layer, and condensation will not appear indoors;
- the structure is insulated with inside. In this case, the boundary for the appearance of condensation will be located close to the inside and, during severe cold weather, can shift to the inner surface, to the junction with the insulation. In this case, it is also likely that moisture will appear indoors, leading to unpleasant consequences. Therefore, this insulation option is not recommended and is carried out only in cases where there are no other solutions. At the same time, it is necessary to provide additional measures to prevent negative consequences– provide between the insulation and the cladding air gap, holes for ventilation, arrange additional ventilation of rooms to remove water vapor, air conditioning to reduce humidity.
- wall thickness, including base material (h1, in meters) and insulation (h2, m);
- thermal conductivity coefficients for load-bearing structure(λ1, W/(m*°C) and insulation (λ1, W/(m*°C);
- standard room temperature (t1, °C);
- outdoor air temperature, taken for the coldest time of year in a given region (t2, °C);
- standard relative humidity in the room (%);
- standard dew point value at given temperature and humidity (°C)
We will accept the following conditions for calculation:
- wall brick thick h1 = 0.51 m, insulation – polystyrene foam thickness h2 = 0.1 m;
- thermal conductivity coefficient established according to regulatory document For sand-lime brick, laid on cement-sand mortar, according to the table in Appendix “D” SP 23-101-2004λ1 = 0.7 W/(m*°C);
- thermal conductivity coefficient for EPS insulation - expanded polystyrene, having a density of 100 kg/m² according to the table in Appendix “D” SP 23-101-2004λ2 = 0.041 W/(m*°C);
- indoor temperature +22 °C, as established by standards within 20-22 °C according to table 1 SP 23-101-2004 for residential premises;
- outside air temperature –15 °C for the coldest time of year in a conventional area;
- indoor humidity – 50%, also within the standard range (no more than 55% according to Table 1 SP 23-101-2004) for residential premises;
- the dew point value for the given values of temperature and humidity, which we take from the table above, is 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 load-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 the thickness to the thermal conductivity coefficient: 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;
- temperature difference in the layer brickwork will be: T = t1 – t2xN= 22 - (-15) x 0.292 = 37 x 0.292 = 10.8 °C;
- the temperature at the junction of the masonry and insulation will be: 24 – 10.8 = 13.2 °C.
Based on the calculation results, we will plot the temperature change in the wall mass and determine the exact position of the dew point.
According to the graph, we see that the dew point, the value of 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. When the outside air temperature decreases, the condensation boundary may shift to this joint and further inside the wall. In principle, this will not cause any special consequences and condensation cannot form on the surface indoors.
The calculation conditions were accepted for middle zone Russia. In the climatic conditions of regions located in more northern latitudes, a greater thickness of the wall and, accordingly, the insulation is accepted, which will ensure that the boundary of condensation formation is located within the insulating layer.
In the case of insulation from the inside under all the same conditions: thickness of the supporting structure and insulation, external and internal temperature, humidity, accepted in the calculation example given, 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 as the outside temperature drops will increase significantly.
Dew point and vapor permeability of structures
When designing enclosing structures, ensuring regulatory thermal protection of premises great importance takes into account the vapor permeability of materials. The amount of vapor permeability depends on the volume of water vapor that can pass through this material per unit of 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 this case, three principles must be observed:
- there should be no obstacles to removing moisture if it condenses on one of the surfaces or inside the material;
- the vapor permeability of enclosing structures should increase from the interior to the outside;
- the thermal resistance of the materials from which external walls are constructed should also increase towards the outside.
In the diagram we see correct composition designs of external walls, providing regulatory thermal protection of interior spaces and removing moisture from materials when it condenses on surfaces or inside the thickness of the wall.
The above principles are violated with internal insulation, therefore this method of thermal protection is recommended only as a last resort.
All modern designs external walls are based on these principles. However, some insulation materials that are included in wall construction have almost zero vapor permeability. For example, polystyrene foam, 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 condensation formation is within the insulation.
Opinion of portal experts
According to experts on the website portal, calculating the dew point value and its position in the enclosing structures is one of the defining moments in ensuring the protection of buildings from heat loss. Most best option- this is when the boundary of condensation 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 so as to prevent the dew point from shifting into the thickness of the wall and towards the surfaces inside the premises.
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 during application anti-corrosion coatings on 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 premises) begins to diffuse through wall material on a 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 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 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 more simple formulas, operating 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%. Negative point occurrence of TR in the wall is a gradual destruction 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 with 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.
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 occurring 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). The 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 along the wall thickness to indoors) 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 humidity in the room, the lower the dew point is than the actual indoor air temperature.
2. If the indoor temperature is +20 degrees, and the indoor humidity is 40%, then condensation will form on any surface with a temperature below +6 degrees.
The higher the humidity in the room, the higher the dew point and the closer to the actual indoor air temperature.
3. If the indoor temperature is +20 degrees, and the indoor humidity is 80%, then condensation will form 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. If the indoor temperature is +20 degrees, and the indoor humidity is 100%, then condensation will form on any surface with a temperature below +20 degrees.
Dew point location
A 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 the room.
Let's look at what happens to the position of the dew point:
- in a wall that is 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 a location of the dew point.
Location of the dew point in an uninsulated wall
By dew point location there may be such 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 is 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 is between the middle of the wall and the inner surface, the wall is not insulated
In this case, the wall is dry and can become damp when the outside temperature drops sharply (lower than the calculated temperature according to DBN/SNiP in the region for several days). During these few days, the dew point position may shift to the inner surface of the wall.
3. Location of the dew point on the inner surface.
Location of the dew point on the inner surface of the wall, the wall is not insulated
Almost the entire wall is wet inside winter period.
As has already been discussed, the position of the dew point depends on 5 factors described in the part above.
Location of the dew point in an externally insulated wall
By dew point location in the wall, insulated outside, there may be the following options:
1. If the insulation is taken according to the required thermal engineering calculation thickness, then the position of the dew point is inside the insulation.
Location of the dew point in the insulation, the wall is insulated from the outside
This is the correct dew point position. The wall in this version is dry.
2. If the insulation is taken with a thickness smaller than required according to the thermal engineering calculation, then all three options described above for an uninsulated wall are possible. The consequences are described there.
Location of the dew point in a wall insulated from the outside (if the insulation is taken less than the calculated thickness)
Location of the dew point in an internally insulated wall
According to the location of the dew point in the wall, insulated from the inside. When we insulate a wall from the inside, we, as it were, “fence it off” from 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 the following options:
1. Location of the dew point in the thickness of the wall.
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 and can become damp when the outside temperature drops sharply (lower than the calculated temperature according to DBN\SNiP in the region for several days). During these few days, the dew point position may shift to the inner surface of the wall.
2. Location of the dew point on the inner surface of the wall, under the insulation.
Location of the dew point on the inner surface of the wall, under the insulation, the wall is insulated from the inside
In this case, the wall is sealed under the insulation throughout the winter period.
3. Location of the dew point inside the insulation.
Location of the dew point in the insulation, the wall is insulated from the inside
In this case, the wall is soaked throughout the entire winter period, except for the wall, the insulation is also wet.
When is it possible or not to insulate walls from the inside?
Now let's look at when it is possible to insulate a wall from the inside, when it is not, what it depends on and how it depends. What is this “no”, what are the consequences?
The main “is it possible or not” is what will happen to the wall after insulating it from the inside. If the wall is dry, it’s possible. If the wall is dry, and only during a sharp, unexpected (which happens once every ten years) cold snap can it become wet, you can try to insulate it from the inside (at the discretion of the customer). If the wall is consistently wet throughout the winter calculation period (with the usual winter temperature according to 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 this 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 it. This part of the article was prompted by questions from readers of the following nature: “Why can the reader in the next thread be insulated from the inside, but I can’t, because he and I (further options) have the same apartment layout, or the houses are built from the same material, or the same city of residence, or same wall thickness, etc.
Let's figure it out. As we have already found out above, the consequences internal insulation depends on:
- dew point (condensation temperature);
- the position of the dew point in the wall before and after insulation.
In turn, the dew point (temperature) depends on: room humidity and room temperature. And the humidity in the room depends on:
- Mode of residence (permanently or temporarily);
- Ventilation (both supply and exhaust, are they sufficient according to calculations).
And the room temperature depends on:
- Quality of heating operation;
- The degree of insulation of the remaining structures of the house/apartment, except for the walls (ceiling/roof, windows, floor).
The position of the dew point depends on:
- thickness and material of all layers of the wall;
- indoor temperature. What it depends on was clarified above;
- temperature outside the room. It depends on whether it is outside or another room, as well as on the climate zone;
- indoor humidity. What it depends on was found out above;
- humidity outside the room. It depends on whether it is outside or another room (and on the mode of operation of this room), as well as on the climate zone.
Now, if we collect ALL factors influencing dew point And dew point position, we will receive a list of influencing factors that must be taken into account when deciding the question “whether or not in a particular situation it is possible to insulate a specific wall from the inside.” Here is a list of these factors:
- mode of residence in the premises (permanently or temporarily);
- ventilation (both supply and exhaust, are they sufficient according to calculation);
- quality of indoor heating;
- the degree of insulation of the remaining structures of the house/apartment, except for the walls (ceiling/roof, windows, floor);
- thickness and material of all layers of the wall;
- indoor temperature;
- indoor humidity;
- outside temperature;
- humidity outside the room;
- climate zone;
- what is behind the wall, a street or another room (its mode of operation).
It becomes clear that there may not be two identical situations regarding insulation from the inside. Let's see what (approximately, without specifics) the situation looks like when insulation from the inside is possible:
- permanent residence premises,
- ventilation is carried out according to the norm (for this room),
- The heating works well and is done according to standard,
- the remaining structures are insulated according to the standard,
- the wall that is planned to be insulated is thick and quite warm. According to the calculation of additional insulation for it, it should not be more than 50mm (foam plastic, cotton wool, EPS). In terms of heat transfer resistance, the wall “falls short” of the norm by 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 “input 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. This is true. 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, it is necessary to insulate from the outside.
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 polystyrene foam or EPS does not. But that doesn't change things. The result is mold and mildew on the walls. The time for the effects to appear is from one to three years.