Insulation of the house allows not only to live in comfort, but also to pay less for heating. The process of insulation begins with the choice of the method of thermal insulation and the selection of thermal insulation materials. At first glance, everything seems simple: add a layer of good thermal insulation material to the thickness of the wall and enjoy the warmth and comfort!
In reality, everything turns out to be much more complicated. There are a lot of videos on the Internet with stories about mold on the walls and the destruction of buildings, the cause of which was only improper insulation of the building, or rather, the position of the dew point inside the house or in the wall array, which led to the accumulation of moisture on the surface of the walls.
The correct determination of the dew point in the wall is the main condition for high-quality, reliable and efficient home insulation.
In physics, the dew point is the temperature at which the water vapor present in it, at constant pressure, changes from a gaseous state to a liquid state. In this case, condensation forms in the air, or, as is often said, dew falls.
The dew point is inextricably linked to the concentration of water vapor in the air: the higher it is, the higher the dew point temperature. A simple example, in a bathhouse, in a steam room, condensate forms even at a temperature close to 100 C. To form water droplets in a steam room, it is enough to come into contact with any surface, the heating of which is at least slightly below its temperature.
The concentration of water vapor in the air is called humidity. A hygrometer is used to measure humidity. In a residential area at an air temperature of 20-25 C, humidity of 40-60% is considered normal.
You can determine the dew point for a dwelling using heat engineering tables.
For an average residential area, its value is in the range from 6 to 12 C. This means that on any surface that has a temperature equal to the dew point temperature and below it (12 C and below), placed in a residential area, condensation is bound to form. It is this phenomenon that can be observed on the surface of bad windows in the cold season.
And what about the walls?
You ask, because their inner surface in a heated apartment or house is always warm and has an ambient temperature, and in places where radiators are installed, it exceeds it.
Indeed, condensation does not form on the inner surface of the walls ... until you decide to insulate them from the inside, using any heat-insulating material you like for this. It does not matter whether you take a vapor-permeable insulation based on stone wool or give preference to non-polystyrene - the effect will be approximately the same. Over time, moisture forms on the inner surface of the walls under the insulation layer, the accumulation of which can lead to mold. This is due to the dew point on the inner surface of the walls.
Where is she, the dew point?
The temperature of the inner surface of the wall of the house is equal to the temperature of the room, and the temperature of the outer surface of the wall of the house is equal to the ambient temperature. In the cold season, there may be a temperature difference between inside and outside of 30 degrees or more.
Heat loss through the surface of the wall can be represented graphically by connecting the temperature mark inside and outside the house with a straight line. The temperature drop in the thickness of the wall is gradual and the more intense, the smaller the thickness of the wall or the higher the thermal conductivity of the materials from which it is made, but in any case, with a homogeneous composition of the wall (for example, only from brick), the dew point temperature (12 C and below) will be inside the wall.
It is here, inside the wall, that condensation occurs, which leads to freezing of the walls and their destruction during repeated cycles of freezing and thawing. For this reason, it is recommended to heat the house constantly, maintaining the temperature of the walls at the same level, trying to exclude periods of thawing of the building and new freezing.
It should be noted that no matter what material the house is built from, its walls are always vapor-permeable to one degree or another. There is always some moisture inside the wall.
If the walls are insulated from the inside
With the location of the heat-insulating material from the inner surface of the walls (Fig. 1), the main temperature drop will fall on the thickness of the heat-insulation. As a result, the temperature of its surface inside the house will be equal to the temperature of the room, and the temperature of the outer surface, depending on the thickness of the heat-insulating material and its quality, will be below the dew point temperature. At the same time, the temperature of the wall behind the thermal insulation layer will be even lower by 1-3 C, which will invariably lead to condensation.
It turns out that the water vapor in the house, trying to go outside, passes through the heat-insulating material, cools down and condenses on the inner walls without getting into their thickness, even if building material with good vapor permeability is used for the walls.
There can be only one conclusion: it is impossible to insulate the house from the inside!
How to bring the dew point outside?
When the thermal insulation material is located outside the walls, the ambient temperature will not be the wall, but the outer layer of thermal insulation. The temperature drop graph in this case will be more gentle, and the dew point temperature on it, depending on the temperature difference outside and inside the house, will be outside the wall in the thickness of the heat-insulating material or in the wall, but in close proximity to its outer surface.
It turns out that the thicker the thermal insulation layer, the more likely it is that the dew point is outside the wall, which means that the walls of the house that are well insulated from the outside will always be dry, which will increase the life of the building.
How to calculate dew point?
To calculate the dew point in the wall, the method for designing the thermal protection of buildings is used, which is detailed in the Code of Rules for the Design and Construction of SP 23-101-2004. An approximate primitive calculation is unlikely to help in this.
You can get reliable results using the services of appropriate online calculators, which are easy to find on the Internet.
What thermal insulation material to prefer
The concept of the dew point in the wall allows you to better understand and imagine the physical processes associated with the loss of heat through the plane of the wall and to choose the right heat-insulating material, while determining the methods of its installation.
As a rule, you have to choose between mineral wool and polystyrene foam.
Thermal insulation materials based on mineral wool are characterized by vapor permeability and, when the dew point is in their array, do not prevent the movement of steam and its release to the outside, into the atmosphere. Of course, we are talking about only part of the water vapor. The rest will turn into water and flow down the insulation layer. By the way, all heat-insulating materials made of basalt and fiberglass are resistant to moisture, are not susceptible to mold and perfectly tolerate repeated cycles of thawing and freezing. So the position of the dew point in the thermal insulation layer will not harm it.
Expanded polystyrene is not vapor permeable. Therefore, moisture will accumulate on its inner surface. To remove it between the wall and the thermal insulation layer, you need to leave a groove, making guides in them. Only in this case can we talk about the safety of the walls and the high quality of their insulation.