What is dew point and how to calculate the dew point in a wall.

It is essential to comprehend dew point in the context of construction in order to preserve a long-lasting and healthy building environment. The term "dew point" describes the temperature at which moisture in the air becomes saturated and causes surfaces to condense. This phenomenon is frequently seen in construction, where moisture buildup is caused by temperature differences between indoor and outdoor environments.

You must take into account both the temperature and the humidity content when determining the dew point inside a wall. Warm, humid air cools when it comes into contact with a wall or other cooler surface. Condensation occurs when the air’s dew point temperature is lowered below the surface. This may eventually cause structural damage, the growth of mold, and the degradation of insulation materials.

The relationship between temperature and humidity must be understood in order to calculate the dew point. Using an internet calculator or a psychrometric chart is an easy method to determine the dew point. These instruments calculate the point at which condensation is most likely to occur by considering variables like the surface temperature of the wall, relative humidity, and indoor temperature.

Preserving the integrity of buildings requires preventing problems with dew points in walls. The three most important tactics are sufficient ventilation, efficient vapor barriers, and proper insulation. Insulation lowers the chance of condensation by assisting in the maintenance of constant temperatures inside walls. While proper ventilation aids in regulating indoor humidity levels, vapor barriers keep moisture from permeating walls.

Term	Description
Dew Point	The temperature at which air becomes saturated with moisture and water droplets begin to form.
Why It Matters	Understanding the dew point helps prevent condensation in walls, which can lead to mold and damage.
Factors Affecting Dew Point	Indoor humidity levels and temperatures, as well as the wall’s insulation and construction materials.
How to Calculate	Measure indoor temperature and relative humidity. Use a dew point calculator or formula to find the dew point temperature.
Formula	Dew Point = Temperature – ((100 – Relative Humidity) / 5)
Example	For a temperature of 20°C and relative humidity of 60%, the dew point is approximately 12°C.

The concept of dew point

The temperature at which moisture begins to precipitate or condense from previously vaporized air is known as the dew point. Stated differently, the dew point in construction refers to the boundary where moisture may appear as the temperature inside heated rooms reaches a warm point, depending on the location of the insulating layer and its properties, as well as the thickness and characteristics of the materials used.

The following regulations govern the conditions pertaining to the accounting and value of the dew point: SNiP 23-02, "Thermal protection of buildings," and SP 23-101-2004, "Design of thermal protection of buildings."

"6.2 SNiP 23-02 creates three required, interrelated, standardized indicators for a building’s thermal protection, based on:

"a": the uniform heat transfer resistance values for each building envelope to ensure the structure is thermally protected;

"b" represents the standardised values of the temperature differential between the interior air and exterior surfaces of the enclosing structure, as well as the temperature on the interior surface above the dew point;

"c" is a standardized specific indicator of thermal energy consumption for heating that lets you adjust the values of enclosing structures’ heat-protective qualities while accounting for the systems you choose to maintain standardized microclimate parameters.

When designing residential and public buildings, SNiP 23-02 requirements will be satisfied if the requirements of indicators of groups "a" and "b" or "b" and "c" are met.

Water vapor condenses most readily on certain surfaces, but moisture can also form inside the walls of structures. Regarding wall construction, under specific temperature conditions during the cold season, condensation will unavoidably form on the surfaces if the dew point is situated near or directly on the inner surface. The dew point will always be closer to the interior surfaces of the building if the enclosing structures are not built with extra insulation installed or are constructed with insufficient insulation.

The presence of moisture on building surfaces can lead to unfavorable outcomes because it fosters the growth of microorganisms like mold and fungus, whose spores are constantly in the atmosphere. Accurately determining the thickness of each component that makes up the enclosing structures—including the dew point—is essential to preventing these undesirable phenomena.

The guidelines in the regulatory document SP 23-101-2004, "Design of thermal protection of buildings," state as follows:

"5.2.3 The temperature of the internal surfaces of the building’s external fences in corners and on window slopes should not be lower than the dew point temperature of the air inside 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. d.)."

Condensation is likely to occur during the cold season, when the outside air temperature drops to negative values, if the surface temperature of the walls inside the building or window units is lower than the estimated dew point value.

One of the requirements for guaranteeing the necessary 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, is solving the problem of how to find the dew point, its physical value.

Calculation of dew point value

Here, we’ll examine several methods for calculating dew point:

• using the table of the regulatory document;
• according to the formula;
• using an online calculator.

Calculation using a table

Utilizing the table found in the regulatory document SP 23-101-2004, "Design of thermal protection of buildings," one can determine the dew point when insulating a home.

The temperature of the room, in degrees Celsius

Temperature of the dew point at humidity,%

It is sufficient to look at the point where the temperature and humidity values specified by the standards for each category of premises intersect to determine the temperature of condensation.

Calculation by formula

Another method for figuring out a wall’s dew point is to apply this straightforward formula:

Тр, the intended dew point

A – constant equals 17.27.

A constant of b is 237.7 °C.

The coefficient λ(΢,RH) is computed using the following formula:

Where: T, the degree Celsius of the interior air;

RH is humidity expressed as a fraction of volume between 0.01 and 1;

Natural logarithm, or ln.

For illustration, let’s figure out the necessary value in a room where standards for residential buildings dictate that the ideal temperature must be kept at 20 °C with a relative humidity of 55%. In this instance, we begin by determining the coefficient λ(T,RH):

Λ(T,RH) = Ln 0.55 + (17.27 x 20) / (237.7 + 20) = 0.742

The air’s condensation temperature will then be equal to:

Tr is equivalent to (237.7 x 0.742)/(17.27 – 0.742) = 176.37 / 16.528 = 10.67 °C.

The temperature difference between the value (10.69°C) found in the table and the value obtained from the formula is only 0.02°C. This indicates that you can find the desired value with high accuracy using both methods.

Knowing the dew point is essential for avoiding moisture-related problems in building and remodeling, such as wall decay and mold. The temperature at which moisture in the air becomes saturated and condensation occurs is known as the dew point. When determining a wall’s dew point, variables like humidity and surface temperature must be taken into account. Builders can ascertain whether vapor barriers or insulation are required to maintain dry and sturdy walls by knowing the dew point. The definition of the dew point, its effects on walls, and useful computation techniques for preserving ideal building conditions are all covered in this article.

Determining the position of the dew point in the wall

The position of the condensation temperature within the building envelope as well as its value must be known in order to guarantee the building envelope’s normal thermal protection qualities. There are currently three primary methods for building external walls, and the condensation boundary may be located differently in each:

• the structure was built without additional insulation – from masonry, concrete, wood, etc. P. 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 is inside the room, and then condensation appears on the inner 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 a 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 on the outside. If the thickness of all materials is correctly calculated, the dew point when insulating with foam plastic or other types of effective insulation will be located inside the insulating layer, and condensation will not appear indoors;
• the structure is insulated from the inside. In this case, the boundary for the appearance of condensation will be located close to the inner side and, with severe cold weather, can shift to the inner surface, at 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 take additional measures to prevent negative consequences – provide an air gap between the insulation and the cladding, ventilation holes, arrange additional ventilation of rooms to remove water vapor, air conditioning to reduce humidity.

Let’s examine an example of how to determine the condensation boundary’s location in a structure that has exterior insulation. The computation will need the following information:

• wall thickness, including base material (h1, in meters) and insulation (h2, m);
• thermal conductivity coefficients for the supporting 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)

The following criteria will be accepted for computation:

• brick wall with thickness h1 = 0.51 m, insulation – expanded polystyrene with thickness h2 = 0.1 m;
• thermal conductivity coefficient established according to the regulatory document for sand-lime brick laid on cement-sand mortar, according to the table in Appendix “D” SP 23-101-2004 λ1 = 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 23-101-2004 λ2 = 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 limits (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 above table – 12.94 °C.

Initially, we ascertain the thermal resistances of every stratum comprising the wall, along with the proportionality of these figures. Next, we compute the temperature differential at the masonry’s load-bearing layer and the masonry-insulation boundary:

• 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;
• the temperature difference in the brickwork layer 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.

We will plot the temperature change in the wall mass and pinpoint the precise location of the dew point based on the computation results.

The graph shows that the dew point, which has a value of 12.94 °C, is within the insulation’s thickness, making it the best choice, but it is also quite close to the point where the insulation and wall surface meet. The condensation boundary may move to this joint and farther inside the wall when the outside air temperature drops. Generally speaking, there won’t be any particular effects from this, and condensation cannot form inside on the surface.

Central Russia was chosen as the calculation region. Greater wall thickness and insulation are acceptable in areas with more northern latitudes due to the climate, as this will guarantee that the boundary of condensation formation is inside the insulating layer.

Under identical conditions (thickness of the supporting structure and insulation, internal and external temperature, humidity), as stipulated in the provided calculation example, in the case of insulation from the inside, the temperature change graph at the boundaries and in the thickness of the wall will resemble this one:

As the outside temperature drops, we can see that the boundary of condensation from the air will in this case move almost to the inner surface, increasing the likelihood that moisture will appear in the room.

The portal has a calculator that can be used to quickly determine the dew point value if necessary.

Knowing the dew point is essential to preserving your home’s comfort and structural integrity. The temperature at which air becomes saturated with moisture and can no longer hold it in vapor form—a condition that results in condensation—is known as the dew point. If left unchecked, this can lead to structural damage, mold growth, and moisture buildup in walls.

Temperature, humidity, and the materials used in construction are some of the variables that must be taken into account when calculating the dew point in a wall. You can take precautionary steps to guarantee that the inside of your walls stay dry and free from moisture-related issues by being aware of the dew point.

Adding moisture barriers, insulation, and adequate ventilation are good ways to manage the dew point in your walls. You can address condensation problems before they become serious ones by keeping an eye on them and performing routine maintenance and monitoring to help spot early signs of the problems.

In addition to shielding your house from potential harm, proactive dew point management also improves the quality of life in your house. Monitoring these elements will help guarantee that your house remains secure and comfortable for many years to come.

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