What is the dew point: its relationship with construction + calculation methodology

Alexey Dedyulin
Checked by a specialist: Alexey Dedyulin
Author: Maria Khodun
Last update: May 2019

The physical condition of the water contained in heaters, hygroscopic building materials, and air is affected by the ambient temperature. According to the laws of heat engineering, the dew point is a certain value of the temperature at which the vaporous water becomes condensate, that is, dew.

You will learn all about how to determine the dew point in order to take it into account when developing a construction project from our article. We will tell you how the point at which the steam turns into condensate is calculated and how it affects the operation of the house. We will give advice on options for localizing this phenomenon.

Connection of dew point and construction

The numerical value of the dew point is directly dependent on such indicators: relative humidity and temperature on the street, and in the room itself. For example, if outside the window t = 8 ˚С, and in the house t = 22 ˚С and relative humidity is 45%, then condensation will form on the outer wall.

There are additional factors that form the dew point, namely: features of the regional climate, the degree of insulation of all enclosing surfaces, the quality and type of heating system, the period of residence - can be permanent (house, apartment) or temporary, for example, a summer residence or garage, ventilation .

It is very important for builders to know the dew point number in order to calculate the exact localization of condensate on the walls, as well as to determine the required thickness of the insulation. Indeed, thanks to this knowledge, it is possible to minimize the loss of heat during the cold season.

The position of the dew point may wander along the thickness of the wall. It depends on the thickness and type of materials of the wall itself and the insulation, on the indicators of temperature and humidity in the room and on the street.

Each material used for construction and wall decoration, except for metal, has its own degree of vapor permeability. This indicator, from the point of view of physics, shows the amount of steam that any material can pass in a certain time.

Vapor permeability table
Vapor permeability is one of the decisive factors that affect the choice of materials for insulation, also this parameter is important for the analysis of the condition of external walls

During periods of low temperatures, steam from the room under pressure will tend to go outside through all layers of the external walls. The lower the coefficient of vapor permeability of the insulation, the smaller the layer should be laid. Its coefficient should increase from the inside to the outside, as well as thermal conductivity.

If all calculations are carried out without errors, then the location of the dew point will be in wall insulating layercloser to the outer surface. It is there that the steam will turn into condensate and only moisturize the wall. Thus, steam will accumulate in the winter, and in summer it is necessary to create conditions for the evaporation of accumulated moisture.

Quality insulation
The main condition for high-quality insulation is the creation of conditions for the evaporation of accumulated moisture. For this, special calculations are carried out and finishing materials are selected

The position of the dew point in the supporting wall of the house will be less suitable. This happens if the type and thickness of the insulation is incorrectly selected.

The worst case scenario is the location of condensation on the inside of the wall. This situation is possible if the wall is not insulated at all or the insulation is indoors. In the latter case, mold may form under the insulation layer, and wet insulation will not retain heat at all.

Options for calculating the dew point

The methodology and rules for calculating the dew point are regulated at the legislative level by such documents as SNiP 23-02 Thermal protection of buildings and SP 23-101-2004 Design of thermal protection of buildings.

In SNiP in paragraph 6.2, three standardized values ​​for thermal protection are prescribed, namely:

  1. Resistance to heat transfer of walls and insulation.
  2. Temperature values ​​indoors and on the surface of an external wall.
  3. Indicator of approximate heat consumption for heating, including ventilation.

Norms are considered fulfilled if requirements 1 and 2 or 2 and 3 are met.

In order to determine the dew point as accurately as possible, some experts turn to the regional meteorological service for information on the exact temperature and wind rose in a certain area.

But everyone can carry out such calculations. There are several ways to determine the dew point.

Method number 1 - the use of formulas

For these calculations, several formulas were created. For example, the formula for deducing the dew point at t is from 0 ˚С to +60 ˚С. Its error is ± 0.4 ˚С. To carry out the calculations, you will need the temperature in the room at a height of 50-60 cm from the floor and humidity. Then just fill in the data and get the result.

Formula for dew point
This is one of the most popular formulas in which T is temperature in degrees Celsius, Rh is relative humidity in%, Ln is the natural logarithm

Method number 2 - the use of the finished table

Specialists have developed a table for instant computing. Please note that the table contains approximate data. It shows the temperature and humidity, and at their intersection you will find the dew point.

Dew point table
You can find out the dew point number thanks to the data from the table presented in SP 23-101-2004. You need to select a value at the intersection of temperature and humidity

Method number 3 - measuring instruments

Now there are several types of special apparatus for such measurements. For example, some thermal imager modelsIn addition to key characteristics, they can display both the localization of the dew point and the room thermogram. They are used by professional builders and heat engineers.

Thermogram thermal imager
The thermal imager is a professional device with which you can create a thermal image of the room. Some models have a dew point calculation function.

A portable thermo-hygrometer will help to find out not only the temperature and humidity in the room, but also calculate the dew point.

A psychrometer will help measure two key indicators in a room: humidity and air temperature. The device consists of wet and dry thermometers in one unit.

Mobile heat hygrometer
Using a mobile heat hygrometer, it is easy to find out the humidity and temperature in all parts of the wall, roof in any room

Method number 4 - calculations on an online calculator

There are a lot of services providing such calculators. Moreover, this method is considered one of the most unreliable, because as a result you can get numbers from the ceiling or with a big error.

If you are unsure of the results, then trust the professionals and contact a specialized company. They will analyze the walls and offer the best option.

Localization of the dew point

The location of the dew point depends on which side the insulation is located. So, in a wall without insulation, it will be shifted along the wall thickness depending on changes in air temperature and humidity. With a minimum temperature difference, it will be located in the thickness of the wall between the center and the outer surface.

Subsequently, the inside of the wall will remain dry. When its position is between the inner surface and the center of the wall, the latter gets wet inside during a cold snap or during frost.

Dew Point Position Diagram
A wall can be insulated from the outside or the outside, or it may not be insulated at all. The location of the dew point will depend on this

In the wall with insulation on the outside the dew point location will be optimal. Indeed, in this case, it will be located inside the insulation, and thus the inner surface of the wall will be dry. This is the best option.

But, if the thickness of the insulation was chosen incorrectly, a dew point may shift, which is fraught with the appearance of fungus, mold, rapid destruction of the walls.

In the wall with insulated inside condensate forms in the wall closer to the living room, the temperature of the wall under the heat-insulating layer decreases, creating optimal conditions for the growth of mold.

Localization can be like this:

  • between the center of the wall and the insulation, and during frost or a sharp drop in temperature at their border;
  • on the inner surface of the wall, which will be wet under the insulation during the whole winter period;
  • inside the insulation, which, like the wall under it, will be wet during the entire cold period.

As you can see, the dew point location has a significant impact on comfort and human health.

The consequences of incorrect calculations

When choosing materials for insulation, remember that one of the effective ways to protect external walls from moisture is to correctly position the insulation layers.

Heat loss in insulated and insulated walls
High-quality heat insulation will help significantly reduce heat loss and maintain comfort in the house, as well as extend the life of the walls

A dense layer that will not allow steam to pass through should be located on the inside of the load-bearing wall, and a porous, moisture-permeable layer should be outside.

It is also necessary to create conditions for ventilation at the condensation point. In this case, the condensate will evaporate without obstruction.

Correct dew point location
A properly insulated outer wall will help to reduce heat loss during the heating period from 45 to 95% and create comfort in the house

If the insulation was chosen incorrectly, then the moisture in it will accumulate gradually and the number of thermal resistance of the wall will decrease. Therefore, in the second, maximum for the fifth heating season, heating costs will increase, if it is a private house, the apartment will simply be much colder in the winter.

Professional insulation is a long and expensive process. Today there are many materials for insulation.Do not try to save on them, since cheap materials will become unusable and begin to collapse after a few heating seasons.

There are several consequences of incorrect calculations, but some of them can negatively affect the quality of life. The main consequence will be constantly wet walls, as a result of fungus, mold, microbes on the walls, which entails the appearance of many chronic diseases.

Mold and fungus on the walls
Constantly wet walls become a hotbed for the growth of fungus and mold, because their spores fly in the air and can cause illness

Since a humid room is difficult to heat, the level of comfort drops. And high humidity inside such walls can provoke respiratory diseases.

Another unpleasant consequence of incorrect calculations is the destruction of finishing materials - the tiles crumble, the brick crumbles on the external wall, and the indoor surface on the walls begins to swell.

Destruction of the wall
Dry condensate, this is the key cause of bloating and delamination of the finishing materials on the external wall.

To correct the situation, you should contact specialists to analyze the condition of the walls and insulation. With the right calculations, you can correct all the mistakes and create comfortable and warm conditions in your home.

With the rules and formulas of the heat engineering calculation for the competent design of the house will introduce next article, which we highly recommend reading.

Conclusions and useful video on the topic

You can learn about how to determine the dew point and what it represents from the following video:

The ways of wall insulation and the right choice of materials will be discussed in the following video:

You can find out the dew point both independently and by contacting professionals. The number of dew points allows the specialist to correctly select the material and properly insulate the walls of a residential building or any other room.

Not only warmth and comfort in the house, but also the health of its residents depends on the accuracy of measurements. Professionals recommend insulating the wall from the inside only as a last resort and after a professional consultation.

Please write comments and ask questions about controversial issues, publish photos and posts with your opinion in the block below. Share useful information and methods for determining the dew point that are not described in the article. Tell us about personal experience in resolving this issue.

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