During the construction of a building or its individual parts, the concept of dew point often arises before the developer.
This term has been heard by everyone who has ever changed windows, insulated walls or changed the heating system in their homes.
So, let's consider what the dew point is, why you need to know its location in the wall and how it can be determined using the available tools at hand.
At high temperature and humidity, cold walls are covered with dew
In simple terms, the dew point is the moment when the internal temperature of the room and humidity significantly exceed the temperature of the floor surface. At the same time, moisture from the air inevitably condenses on the surface of the wall. This moment is influenced by:
- indoor air humidity;
- temperature of walls or ceilings;
- temperature inside the building.
If the room is humid and hot, then dew drops immediately form on a cold glass.
Why is this term used in construction? Any fences: a wall or a window is the border with the outside world, which means that their surface temperature differs from the average in the room.
This means that in the place where the dew point is located on the wall, moisture will regularly accumulate. The determination of the dew point is influenced by:
- characteristics of materials used in construction and their thickness;
- the place of installation, the number of layers and the quality of the insulation.
It is important that the dew point is on the outside of the building wall. Otherwise, we get a constantly wet surface and, as a result, the formation of mold, fungus, the destruction of the decorative layer and the bearing characteristics of the structure.
Dew point calculation
Many owners of square meters are interested in the question of how to independently calculate the dew point in the wall. Purely theoretically, there is nothing complicated in this, especially if you are a mathematician, physicist, or just remember the school curriculum well. 
To do this, you need to use the formula:
TP = (b * λ(T,RH)) / (a * λ(T,RH)), where:
- TP is the desired point;
- a is a constant equal to 17.27;
- b is a constant equal to the value 237.7;
- λ(Т,RH) – coefficient, which is calculated as follows:
λ(Т,RH) = (а*Т) / (b*T+ lnRH), where:
- T is the internal temperature of the room;
- RH - indoor humidity, the value is taken in fractions, not in percentages: from 0.01 to 1;
- ln is the natural logarithm.
If in school you were more into playing basketball or reading Dostoevsky than logarithms, don't be discouraged. Everything has already been calculated in the thermal protection data table numbered SP 23-101-2004, compiled on the basis of measurements and calculations by scientific and design organizations.
The most likely values in average Russian conditions are shown in the table below:
Practical use
Knowing the value of the dew point value is important when planning the insulation of a building. In practice, the meaning of the dew point term is important when insulating the walls of a building. To ensure optimal thermal insulation characteristics of the enclosing parts of the building, it is necessary to know not only the value of the dew point, but also its position on the surface or in the body of the wall.
Modern construction methods allow 3 options for carrying out work, and in each case, the point of condensation may be different:
An exception in the case of the same type of wall will be, perhaps, wooden log cabins. Wood is a natural material with excellent quality characteristics of low thermal conductivity and high vapor permeability. In such buildings, the dew point will always be located closer to the outer surface. Wooden log cabins almost never require additional thermal insulation work.
The latter option is highly undesirable and is performed only when there is no other way out. For information on how to properly insulate the walls of the house, see this video:
If, nevertheless, the insulation is laid inside the building, then additional measures should be taken:
- leave an air pocket between the thermal insulation layer and the cladding;
- provide for the arrangement of ventilation openings and heating of the room with an additional decrease in the level of humidity.
What to do to bring the dew point out of the house?
What is the right thing to do when the house has already been built and is in operation, and the walls have begun to dampen? All of the above tells us that it is necessary to change the factors that affect the dew point. This means that you can either increase the heating in order to reduce the level of humidity, or reduce the difference in the temperature of the coatings, namely, lay a layer of external thermal insulation.
Wall insulation options Why do we insulate the walls from the outside? First, it's convenient. Secondly, in this case, the temperature of the external environment will not have a wall of the house, but a layer of thermal insulation. The temperature drop curve will become flatter and the dew point will actually move to the edge of the thermal insulation layer. For important tips on this issue, see this video:
The thicker the coating, the more likely it is that the dew point will shift into the thermal insulation body outside the wall of the house. As a result, houses that are well insulated from the outside last longer and do not require large heating costs.
Thermal insulation material
Penoplex is recommended for external wall insulation As we have already figured out, it is better to use a heat-insulating material that can be mounted on the outside of the building. As a rule, we are talking about foam or mineral wool.
Material based on mineral wool has good vapor permeability. At the same time, moisture is partially retained in the insulation and flows down under the action of gravity. This circumstance does not threaten the insulation, since basalt or glass fiber is resistant to moisture.
It would not be out of place to arrange a layer of waterproofing at the bottom of the building to prevent the destruction of the foundation.
Penoplex materials are vapor-tight, so when installing them, an air pocket should be left to remove moisture from the inner surface of the material.
Subject to these conditions, we can talk about the safety of the walls and the effectiveness of insulation.