Determining the dew point in a wall is very simple. Below is an example of how to do the calculation. This can be done by anyone who is interested in the issue of proper insulation.
The dew point is the temperature at which water vapor begins to condense.
What is dew point
The dew point in a wall can move along its thickness as the temperatures inside and outside the room change. For example, if the temperature inside the room is stable, but it gets colder outside, then the dew point will move along the thickness of the wall closer to the room.
The temperature of the object on which the steam will begin to condense, i.e. dew point depends mainly on two parameters:
- air temperature;
- air humidity.
For example, when a refrigerator is opened, dew from the incoming warm air falls out inside it. It looks like "fog coming out of the fridge".
If it is cold outside, then somewhere in the wall there will be a temperature at which steam will begin to condense, and at this point there will be moisture. If the wall is thin, "cold", and its inner surface cools to 12.9 degrees or less (at the specified values of temperature and air humidity), then dew will fall on it, it will become wet, and very quickly acquire mold.
When insulating walls, house structures, it is useful to calculate the dew point for the highest and lowest values of humidity and temperature, in order to know in what boundaries of space the dew point will move when these parameters change.
How the calculation is done
In calculations of the dew point and the thickness of the insulation, some parameters are not taken into account - pressure, air velocity, material density ... Therefore, we can only talk about approximate values. But, this is not critical when it comes to determining the thickness of the insulation.To determine the dew point in the wall, the easiest way is to use the tables of ready-made approximate values, and do not try to do the calculations yourself. Moreover, you should not trust self-made programs from the Internet, they often do not take into account the parameters and give false values, and sometimes on the principle of random numbers.
Below is a table of calculated dew point values depending on air temperature and humidity. These are approximate values, since the influence of other factors is not taken into account.
For example, you can determine that for a room with an internal temperature of +22 degrees, and a humidity of 60%, the temperature at which water vapor will condense (dew point) will be 13.9 degrees.
Wall with insulation - how to determine the place of condensation
Solving the problem of finding the dew point in a wall is very simple.Need to know:
- thermal resistance coefficient of the wall, ?1, W/(m K);
- thermal resistance coefficient of the insulation, ?2, W/(m K);
- wall thickness, h1, m;
- insulation thickness, h2, m;
- indoor temperature, t1, deg. FROM;
- air humidity, which will reach the dew point,%;
- dew point for temperature and humidity data, deg. FROM;
- outside temperature, t2, deg. FROM.
The desired value is the temperature at the boundary of the layers of the wall and insulation. When it is found, you can build a graph of temperature changes in the "wall-insulation" layer and use it to find the position of the dew point.
For this, the ratio of the thermal resistance of the wall to the thermal resistance of the insulation is found, on the basis of which the temperature change in one of the layers is determined, which will make it possible to find out the temperature at the boundary.
Let's look at an example.
Calculation example
An example of conditions is the following.Reinforced concrete wall h1 = 36 cm, insulated with foam h2 = 10 cm. The coefficient of thermal resistance of reinforced concrete? 1 = 1.7 W / cmK, foam -? 2 = 0.04 W / cmK. Temperature inside t1=+20 degrees, outside t2=-10 degrees. Humidity indoors and outdoors is assumed to be the same - 50%. According to the table, the dew point will be 9.3 degrees.
The thermal resistance of the wall and insulation are defined as h/?, W/m2K.
In this example, the thermal resistance of the wall will be 0.36 / 1.7 \u003d 0.21 W / m2K., Insulation 0.1 / 0.04 \u003d 2.5 W / m2K.
The ratio of the thermal resistances of the first layer to the second (wall to foam) will be: n=0.21/2.5=0.084.
Then the temperature difference in the first layer (wall) will be, T \u003d t1- t2xn \u003d 20-(-10) x0.084 \u003d 2.52 degrees.
Accordingly, the temperature at the boundary of the layer will be equal to t1-T=20-2.52=17.48 degrees.
Now we can build an approximate graph of temperature drops in the wall-insulation layer on a scale and mark the dew point on it.
From approximate calculations and an approximate graph, you can find out the main thing - the dew point is in the insulation, far from the wall, i.e. even worsening conditions, taking into account the calculation errors, will not lead to detrimental wetting of the wall.
Example of determining the location of the condensation temperature inside the wall
The temperature inside is +22 degrees, outside - 15 degrees (region to the north), humidity - 50%, dew point - 11.1 degrees. A 38 cm thick brick wall (1.5 bricks + seam + plaster is taken as "brickwork").The coefficient of thermal resistance for brickwork is 0.7 W / cmK, for mineral wool - 0.05 W / cmK (taking into account its moisture in real operating conditions). 
Thermal resistance of the wall: 0.38/0.7=0.54 W/m2K, insulation 0.1/0.05= 2.0 W/m2K.
The ratio of the thermal resistances of the first layer to the second will be: n=0.54/2.0=0.27, and the temperature difference within the first layer will be T=22 - (-15)x0.27=9.99 deg. Temperature at the boundary of the layers: 22- 9.99 \u003d 12 degrees.
As you can see, the situation is "end-to-end". With an increase in humidity, which is a common occurrence, with a drop in indoor temperature, or in a cold winter, the dew point will “walk” inside the wall.
Such insulation for a relatively “warm” brick wall will already be considered insufficient, both by the position of the dew point and by the standard values of heat loss through the building envelope.
The dew point can also be shifted by heating the room with internal heating and dehumidifying it. Naturally, this is an extreme measure, which is used only when it's time to "dry the walls."
Dew point in the wall - calculation and finding
What values should be taken to calculate
Usually the temperature inside the room is 22 degrees, more often it is lower near the floor, and reaches 27 degrees under the ceiling. For the central regions, the minimum outside temperature is considered to be -15 degrees, (short-term temperature drops to -20 - -25 degrees are allowed).For the southern regions - -7 degrees, with a short-term decrease of -15 - -20 degrees.
(You can choose the minimum temperature yourself - what temperature does it keep in winter all the time? To what values does it drop for a short time?)
The humidity in the room is usually taken as medium (but not small) - 50%. There is usually some reserve here, since often in winter the air in the room is drier, due to actively working heating, - 30 - 40%. But many homes are struggling with dry air by installing humidifiers and planting plants. The optimum humidity is 50%, it is also calculated.
In autumn and spring, for passing heaters, steam will go in the opposite direction - from the street. To calculate the "demi-season" for vapor-permeable heaters, the humidity must be taken as about 90%.
Where should the dew point be?
The insulation of the fence is considered “normal” only when the dew point in cold weather is mainly (!) In the insulation and does not shift into the wall.What does "mostly" mean?
At maximum negative temperatures, which usually last several days, a week, and occur periodically, the dew point can also shift into the wall.
For a wall made of dense heavy materials, there is nothing dangerous in this. But for a wall made of porous materials, which as usual very well pass steam and absorb moisture, the appearance of the dew point should be short, especially when they are combined with vapor barrier insulation.
Such walls require the most insulation, especially considering that they are warm in themselves. To shift the dew point, you need 2 times more insulation. With vapor-transparent heaters, they combine much better, since moisture can be removed here, but only if the insulation is well ventilated.
Illustrative temperature graphs for various insulation schemes are given. The dew point is approximately indicated as 16 degrees, it is reached when the interior of the house is especially comfortable +25 degrees, 55 - 60% humidity. 
- 1 - wall without insulation;
- 2 - insufficient layer of insulation - the dew point is inside the wall. Its constant presence will cause wetting of a loose wall, an unhealthy atmosphere, the danger of material destruction if the wall has a layer of insulation that has a greater resistance to vapor movement than the wall itself (improper insulation);
- 3 - sufficient insulation, dew point in the insulation (main time), normal preservation of wall materials and heat in the house, if the thermal resistance of the structure is not less than the standard, because for very cold walls it is possible to shift the dew point from them with a small layer of insulation;
- 4 - internal insulation - the worst solution. The dew point on the surface of the wall or close to it leads to wetting of the wall, and damage to the health of residents, wet freezing and destruction of structures. It is used in desperate situations, provided that the wall is completely closed with a vapor barrier, which prevents the penetration of steam to the dew point. Those. condensation is not possible due to humidity close to 0.
More often, the standard requires a smaller thickness of the insulation than what is needed to shift the dew point into the insulation. Therefore, it is in principle desirable to select a heater for all surfaces, and according to the condition of shifting the dew point into the heater.
These values are compared with the regulatory requirement, and, as a rule, an even larger value is taken, a multiple of the thickness of the heaters that is on sale.