Affordable, effective, low-cost, vapor-permeable wall material. Comparison of different types of insulation. Use of conductive qualities

There is a legend about a “breathing wall”, and tales about the “healthy breathing of a cinder block, which creates a unique atmosphere in the house.” In fact, the vapor permeability of the wall is not large, the amount of steam passing through it is insignificant, and much less than the amount of steam carried by air when it is exchanged in the room.

Vapor permeability is one of the most important parameters, used in calculating insulation. We can say that the vapor permeability of materials determines the entire insulation design.

What is vapor permeability

The movement of steam through the wall occurs when there is a difference in partial pressure on the sides of the wall ( different humidity). At the same time, the differences atmospheric pressure there may not be.

Vapor permeability is the ability of a material to pass steam through itself. According to the domestic classification, it is determined by the vapor permeability coefficient m, mg/(m*hour*Pa).

The resistance of a layer of material will depend on its thickness.
Determined by dividing the thickness by the vapor permeability coefficient. Measured in (m sq.*hour*Pa)/mg.

For example, the vapor permeability coefficient brickwork accepted as 0.11 mg/(m*hour*Pa). With a brick wall thickness of 0.36 m, its resistance to steam movement will be 0.36/0.11=3.3 (m sq.*hour*Pa)/mg.

What is the vapor permeability of building materials?

Below are the values ​​of the vapor permeability coefficient for several building materials(according to normative document), which are most widely used, mg/(m*hour*Pa).
Bitumen 0.008
Heavy concrete 0.03
Autoclaved aerated concrete 0,12
Expanded clay concrete 0.075 - 0.09
Slag concrete 0.075 - 0.14
Burnt clay (brick) 0.11 - 0.15 (in the form of masonry on cement mortar)
Mortar 0,12
Drywall, gypsum 0.075
Cement-sand plaster 0.09
Limestone (depending on density) 0.06 - 0.11
Metals 0
Chipboard 0.12 0.24
Linoleum 0.002
Foam plastic 0.05-0.23
Polyurethane solid, polyurethane foam
0,05
Mineral wool 0.3-0.6
Foam glass 0.02 -0.03
Vermiculite 0.23 - 0.3
Expanded clay 0.21-0.26
Wood across the grain 0.06
Wood along the grain 0.32
Brickwork made of sand-lime brick on cement mortar 0.11

Data on the vapor permeability of layers must be taken into account when designing any insulation.

How to design insulation - based on vapor barrier qualities

The basic rule of insulation is that the vapor transparency of layers should increase towards the outside. Then, during the cold season, it is more likely that water will not accumulate in the layers when condensation occurs at the dew point.

The basic principle helps to make a decision in any case. Even when everything is “turned upside down,” they insulate from the inside, despite persistent recommendations to do insulation only from the outside.

To avoid a catastrophe with the walls getting wet, it is enough to remember that the inner layer should most stubbornly resist steam, and based on this, for internal insulation apply extruded polystyrene foam in a thick layer - a material with very low vapor permeability.

Or don’t forget to use even more “airy” mineral wool on the outside for very “breathable” aerated concrete.

Separation of layers with a vapor barrier

Another option for applying the principle of vapor transparency of materials in a multilayer structure is to separate the most significant layers with a vapor barrier. Or the use of a significant layer, which is an absolute vapor barrier.

For example, insulating a brick wall with foam glass. It would seem that this contradicts the above principle, since it is possible for moisture to accumulate in the brick?

But this does not happen, due to the fact that the directional movement of steam is completely interrupted (at sub-zero temperatures from the room to the outside). After all, foam glass is a complete vapor barrier or close to it.

Therefore, in in this case the brick will enter an equilibrium state with internal atmosphere at home, and will serve as an accumulator of humidity during sudden fluctuations indoors, making the indoor climate more pleasant.

The principle of layer separation is also used when using mineral wool - an insulation material that is especially dangerous due to moisture accumulation. For example, in a three-layer structure, when mineral wool is located inside a wall without ventilation, it is recommended to place a vapor barrier under the wool and thus leave it in the outside atmosphere.

International classification of vapor barrier qualities of materials

The international classification of materials based on vapor barrier properties differs from the domestic one.

According to the international standard ISO/FDIS 10456:2007(E), materials are characterized by a coefficient of resistance to vapor movement. This coefficient indicates how many times more the material resists the movement of steam compared to air. Those. for air, the coefficient of resistance to steam movement is 1, and for extruded polystyrene foam it is already 150, i.e. Expanded polystyrene is 150 times less permeable to steam than air.

It is also customary in international standards to determine vapor permeability for dry and moistened materials. The internal humidity of the material is 70% as the boundary between the concepts of “dry” and “moistened”.
Below are the values ​​of the steam resistance coefficient for various materials according to international standards.

Steam resistance coefficient

Data are given first for dry material, and separated by commas for moistened material (more than 70% humidity).
Air 1, 1
Bitumen 50,000, 50,000
Plastics, rubber, silicone - >5,000, >5,000
Heavy concrete 130, 80
Concrete medium density 100, 60
Polystyrene concrete 120, 60
Autoclaved aerated concrete 10, 6
Lightweight concrete 15, 10
Fake diamond 150, 120
Expanded clay concrete 6-8, 4
Slag concrete 30, 20
Fired clay (brick) 16, 10
Lime mortar 20, 10
Drywall, gypsum 10, 4
Gypsum plaster 10, 6
Cement-sand plaster 10, 6
Clay, sand, gravel 50, 50
Sandstone 40, 30
Limestone (depending on density) 30-250, 20-200
Ceramic tile?, ?
Metals?, ?
OSB-2 (DIN 52612) 50, 30
OSB-3 (DIN 52612) 107, 64
OSB-4 (DIN 52612) 300, 135
Chipboard 50, 10-20
Linoleum 1000, 800
Underlay for plastic laminate 10,000, 10,000
Underlay for laminate cork 20, 10
Foam plastic 60, 60
EPPS 150, 150
Solid polyurethane, polyurethane foam 50, 50
Mineral wool 1, 1
Foam glass?, ?
Perlite panels 5, 5
Perlite 2, 2
Vermiculite 3, 2
Ecowool 2, 2
Expanded clay 2, 2
Wood across the grain 50-200, 20-50

It should be noted that the data on resistance to steam movement here and “there” are very different. For example, foam glass is standardized in our country, and the international standard says that it is an absolute vapor barrier.

Where did the legend of the breathing wall come from?

A lot of companies produce mineral wool. This is the most vapor permeable insulation. According to international standards, its vapor permeability resistance coefficient (not to be confused with the domestic vapor permeability coefficient) is 1.0. Those. in fact, mineral wool is no different in this respect from air.

Indeed, this is a “breathable” insulation. To sell as much mineral wool as possible, you need beautiful fairy tale. For example, if you insulate a brick wall from the outside mineral wool, then it will not lose anything in terms of vapor permeability. And this is the absolute truth!

The insidious lie is hidden in the fact that through brick walls 36 centimeters thick, with a humidity difference of 20% (on the street 50%, in the house - 70%) about a liter of water will leave the house per day. While with the exchange of air, about 10 times more should come out so that the humidity in the house does not increase.

And if the wall is insulated from the outside or inside, for example with a layer of paint, vinyl wallpaper, thick cement plaster, (which in general is “the most common thing”), then the vapor permeability of the wall will decrease by several times, and with complete insulation - by tens and hundreds of times.

Therefore always brick wall and it will be absolutely the same for household members whether the house is covered with mineral wool with “raging breath”, or with “sadly sniffling” polystyrene foam.

When making decisions on insulating houses and apartments, you should proceed from the basic principle - outer layer should be more vapor permeable, preferably several times more.

If for some reason it is not possible to withstand this, then you can separate the layers with a continuous vapor barrier (use a completely vapor-proof layer) and stop the movement of steam in the structure, which will lead to a state of dynamic equilibrium of the layers with the environment in which they will be located.

So I waited. I don’t know about you, but I’ve been wanting to experiment for a long time. Otherwise it’s all theory and theory. She didn't answer my questions. I mean thermal engineering calculation according to DBN. So I collected samples and decided to experiment with them. I'm interested in how the material will behave when exposed to steam.

Armed himself with whatever he could. Two steamers, pans with cold accumulators, a stopwatch and a pyrometer. Oh, yes... Another bucket of water for the fourth experiment with immersing samples. And off we went... :)

I summarized the results of the experiment on vapor permeability and inertia in a table.

In general, the experience went wrong. Despite different thermal conductivity materials, the surface temperature of the samples in the first experiment with a vapor barrier layer was practically the same. I suspect that the steam from the steamer, which escaped, also heated the surface of the samples. As soon as I blew air on the samples, the temperature dropped by 1-2 degrees. Although, in principle, the dynamics of temperature growth remained the same. But I was more interested in this, because the very conditions of the experiment are far from real.

Which surprised me. This is Bethol. Second experiment without vapor barrier. This behavior of the insulation should not be considered a disadvantage. In my experience, Betol itself was a representative of vapor-permeable insulation. Think mineral wool insulation would behave the same way, but with faster dynamics.

Experience is very revealing. A sharp increase temperatures (large heat losses) due to vapor permeability and subsequent cooling of the material when water begins to evaporate from the surface. The insulation warmed up so much that it allowed it to release water in a vapor state and thus cool itself.

Gas block 420 kg/m3. He disappointed me. No! Not in terms of quality! He just clearly showed that he is selfish! 🙂 It’s better not to design with it multilayer walls. Due to its higher vapor permeability, it retained warm steam worse than a dense foam block. This suggests that if this material is used, the entire temperature and humidity shock will be absorbed by the vapor-permeable insulation. In general, take a denser, thicker gas block, and interior walls glue materials with low vapor permeability ( vinyl wallpapers, plastic lining, oil painting etc)...

How do you like the foam block with high density(representative of inertial materials)? Well, isn't this lovely? After all, he clearly showed us how inertial material behaves when heat accumulates. I would like to note that when I removed it from the steamer it was hot. Its temperature was clearly higher than Betol and Gas-Block. During the same exposure time, it was able to accumulate more heat, which led to more high temperature material by 2-3 degrees.

Analyzing the table, I received many answers and became even more convinced that in our climate it is necessary to build inertial houses and you will definitely save on heating...

Sincerely, Alexander Terekhov.

Everyone knows that comfortable temperature regime, and correspondingly, favorable microclimate in the house is ensured largely due to high-quality thermal insulation. Lately there has been a lot of debate about what ideal thermal insulation should be and what characteristics it should have.

There are a number of thermal insulation properties, the importance of which is beyond doubt: thermal conductivity, strength and environmental friendliness. It is quite obvious that effective thermal insulation must have a low thermal conductivity coefficient, be strong and durable, not contain substances harmful to humans and environment.

However, there is one property of thermal insulation that raises a lot of questions - vapor permeability. Should insulation be permeable to water vapor? Low vapor permeability– is this an advantage or a disadvantage?

Points for and against"

Proponents of cotton insulation assure that high vapor permeability is a definite plus; vapor-permeable insulation will allow the walls of your home to “breathe”, which will create a favorable microclimate in the room even in the absence of any additional system ventilation.

Adherents of Penoplex and its analogues say: the insulation should work like a thermos, and not like a leaky “quilted jacket”. In their defense they give the following arguments:

1. Walls are not at all the “breathing organs” of the house. They perform a completely different function - they protect the house from environmental influences. Respiratory organs for the home are ventilation system, and also, partially, windows and doorways.

In many European countries supply and exhaust ventilation is installed without fail in any residential premises and is perceived as the same norm as centralized system heating in our country.

2. The penetration of water vapor through walls is a natural physical process. But at the same time, the amount of this penetrating steam in a living room with normal mode operation is so low that it can be ignored (from 0.2 to 3%* depending on the presence/absence of a ventilation system and its efficiency).

* Pogorzelski J.A., Kasperkiewicz K. Thermal protection multi-panel houses and energy saving, planning topic NF-34/00, (typescript), ITB library.

Thus, we see that high vapor permeability cannot act as a cultivated advantage when choosing thermal insulation material. Now let's try to find out whether this property be considered a disadvantage?

Why is high vapor permeability of insulation dangerous?

IN winter time years, with sub-zero temperature outside the home, the dew point (the conditions under which water vapor reaches saturation and condenses) should be in the insulation (extruded polystyrene foam is taken as an example).

Fig. 1 Dew point in EPS slabs in houses with insulation cladding

Fig. 2 Dew point in EPS slabs in frame-type houses

It turns out that if thermal insulation has high vapor permeability, then condensation can accumulate in it. Now let's find out why condensation in insulation is dangerous?

Firstly, When condensation forms in the insulation, it becomes damp. Accordingly, it decreases thermal insulation characteristics and, conversely, thermal conductivity increases. Thus, the insulation begins to perform the opposite function - remove heat from the room.

Well-known expert in the field of thermophysics, Doctor of Technical Sciences, Professor, K.F. Fokin concludes: “Hygienists view the breathability of enclosures as positive quality, providing natural ventilation premises. But from a thermal technical point of view, the air permeability of fences is more likely negative quality, since in winter, infiltration (air movement from inside to outside) causes additional heat loss from the fences and cooling of the premises, and exfiltration (air movement from outside to inside) can adversely affect the humidity regime of external fences, promoting moisture condensation.”

In addition, SP 23-02-2003 “Thermal protection of buildings” section No. 8 states that the air permeability of building envelopes for residential buildings should be no more than 0.5 kg/(m²∙h).

Secondly, due to wetting, the heat insulator becomes heavier. If we are dealing with cotton insulation, then it sags and cold bridges form. In addition, the load on bearing structures. After several cycles: frost - thaw, such insulation begins to deteriorate. To protect moisture-permeable insulation from getting wet, it is covered with special films. A paradox arises: the insulation breathes, but it requires protection with polyethylene or a special membrane, which negates all its “breathing”.

Neither polyethylene nor the membrane allow water molecules to pass into the insulation. From school course physicists know that air molecules (nitrogen, oxygen, carbon dioxide) are larger in size than a water molecule. Accordingly, air is also unable to pass through such protective films. As a result, we get a room with breathable insulation, but covered with an airtight film - a kind of polyethylene greenhouse.

In domestic standards, vapor permeability resistance ( vapor permeation resistance Rп, m2. h. Pa/mg) is standardized in Chapter 6 “Vapor Permeability Resistance of Enclosing Structures” SNiP II-3-79 (1998) “Building Heat Engineering”.

International standards for vapor permeability of building materials are given in ISO TC 163/SC 2 and ISO/FDIS 10456:2007(E) - 2007.

Indicators of the coefficient of resistance to vapor permeability are determined on the basis of the international standard ISO 12572 "Thermal properties of building materials and products - Determination of vapor permeability." Vapor permeability indicators for international ISO standards were determined in the laboratory on time-old (not just released) samples of building materials. Vapor permeability was determined for building materials in dry and wet states.
The domestic SNiP provides only calculated data on vapor permeability at a mass ratio of moisture in the material w, % equal to zero.
Therefore, to select building materials based on vapor permeability at dacha construction better focus on international standards ISO, which determine the vapor permeability of “dry” building materials with a humidity of less than 70% and “wet” building materials with a humidity of more than 70%. Remember that when leaving “pies” of vapor-permeable walls, the vapor permeability of the materials from the inside to the outside should not decrease, otherwise “soaking” will gradually occur. inner layers building materials and their thermal conductivity will increase significantly.

The vapor permeability of materials from the inside to the outside of a heated house should decrease: SP 23-101-2004 Design of thermal protection of buildings, clause 8.8: To provide the best performance characteristics in multi-layer building structures, layers should be placed on the warm side greater thermal conductivity and with greater resistance to vapor permeation than the outer layers. According to T. Rogers (Rogers T.S. Design of thermal protection of buildings. / Translated from English - Moscow: si, 1966) Individual layers in multi-layer fences should be placed in such a sequence that the vapor permeability of each layer increases from the inner surface to external With this arrangement of layers, water vapor entering the fence through inner surface with increasing ease, will pass through all the joints of the fence and be removed from the fence from the outer surface. The enclosing structure will function normally if, subject to the stated principle, the vapor permeability of the outer layer is at least 5 times higher than the vapor permeability of the inner layer.

The mechanism of vapor permeability of building materials:

At low relative humidity moisture from the atmosphere in the form of individual water vapor molecules. As the relative humidity increases, the pores of building materials begin to fill with liquid and the mechanisms of wetting and capillary suction begin to work. As the humidity of a building material increases, its vapor permeability increases (the vapor permeability resistance coefficient decreases).

The vapor permeability indicators for “dry” building materials according to ISO/FDIS 10456:2007(E) are applicable for internal structures heated buildings. Vapor permeability indicators for “wet” building materials are applicable to all external structures and internal structures of unheated buildings or country houses with variable (temporary) heating mode.

We supply building materials to the cities: Moscow, St. Petersburg, Novosibirsk, Nizhny Novgorod, Kazan, Samara, Omsk, Chelyabinsk, Rostov-on-Don, Ufa, Perm, Volgograd, Krasnoyarsk, Voronezh, Saratov, Krasnodar, Tolyatti, Izhevsk, Yaroslavl, Ulyanovsk, Barnaul, Irkutsk, Khabarovsk, Tyumen, Vladivostok, Novokuznetsk, Orenburg , Kemerovo, Naberezhnye Chelny, Ryazan, Tomsk, Penza, Astrakhan, Lipetsk, Tula, Kirov, Cheboksary, Kursk, Tver, Magnitogorsk, Bryansk, Ivanovo, Ulan-Ude, Nizhny Tagil, Stavropol, Surgut, Kamensk-Uralsky, Serov, Pervouralsk , Revda, Komsomolsk-on-Amur, Abakan, etc.

08-03-2013

30-10-2012

World wine production is expected to fall by 6.1 percent in 2012 due to poor harvests in several countries of the world,

What is vapor permeability

Vapor permeability, according to the set of rules for design and construction 23-101-2000, is the property of a material to transmit air moisture under the influence of a difference (difference) in the partial pressures of water vapor in the air on the inner and outer surfaces of the material layer. The air pressure on both sides of the material layer is the same. The density of a stationary flow of water vapor G n (mg/m 2 h), passing under isothermal conditions through a layer of material 5 (m) thick in the direction of decreasing absolute air humidity is equal to G n = cLr p / 5, where c (mg/m h Pa ) - coefficient of vapor permeability, Ar p (Pa) - difference in partial pressures of water vapor in the air at opposite surfaces of the material layer. The inverse value of c is called vapor permeation resistance R n = 5/c and refers not to the material, but to a layer of material with a thickness of 5.

Unlike air permeability, the term “vapor permeability” is an abstract property, and not a specific amount of water vapor flow, which is a terminological shortcoming of SP 23-101-2000. It would be more correct to call vapor permeability the value of the density of the stationary flow of water vapor G n through a layer of material.

If, in the presence of air pressure differences, the spatial transfer of water vapor is carried out by mass movements of the entire air together with water vapor (wind) and is assessed using the concept of air permeability, then in the absence of air pressure differences there is no mass movement of air, and the spatial transfer of water vapor occurs through chaotic movement water molecules in still air in through channels in a porous material, that is, not convective, but diffusion.

Air is a mixture of molecules of nitrogen, oxygen, carbon dioxide, argon, water and other components with approximately the same average speeds, equal to the speed of sound. Therefore, all air molecules diffuse (chaotically move from one zone of gas to another, continuously colliding with other molecules) with approximately at the same speeds. So the speed of movement of water molecules is comparable to the speed of movement of molecules of both nitrogen and oxygen. As a result, the European standard EN12086 uses, instead of the concept of vapor permeability coefficient μ, the more precise term diffusion coefficient (which is numerically equal to 1.39 μ) or diffusion resistance coefficient 0.72/μ.

Rice. 20. The principle of measuring the vapor permeability of building materials. 1 - glass cup with distilled water, 2 - glass cup with a drying composition (concentrated solution of magnesium nitrate), 3 - material to be studied, 4 - sealant (plasticine or paraffin mixture with rosin), 5 - sealed thermostated cabinet, 6 - thermometer, 7 - hygrometer.

The essence of the concept of vapor permeability is explained by the method for determining the numerical values ​​of the vapor permeability coefficient GOST 25898-83. A glass cup with distilled water is hermetically covered with the sheet material being tested, weighed and placed in a sealed cabinet located in a thermostated room (Fig. 20). An air dehumidifier (a concentrated solution of magnesium nitrate, providing a relative air humidity of 54%) and instruments for monitoring temperature and relative air humidity (a thermograph and a hygrograph that continuously records are desirable) are placed in the cabinet.

After a week of exposure, the cup of water is weighed, and the vapor permeability coefficient is calculated from the amount of water that has evaporated (passed through the test material). The calculations take into account that the vapor permeability of the air itself (between the surface of the water and the sample) is 1 mg/m hour Pa. The partial pressures of water vapor are taken to be equal to p p = spo, where po is the saturated vapor pressure at a given temperature, cp is the relative air humidity equal to one (100%) inside the cup above the water and 0.54 (54%) in the cabinet above the material.

Data on vapor permeability are given in tables 4 and 5. Recall that the partial pressure of water vapor is the ratio of the number of water molecules in the air to total number molecules (nitrogen, oxygen, carbon dioxide, water, etc.) in the air, i.e., the relative countable number of water molecules in the air. The given values ​​of the heat absorption coefficient (with a period of 24 hours) of the material in the structure are calculated using the formula s=0.27(A,poCo) 0 "5, where A, po and Co - table values thermal conductivity coefficient, density and specific heat capacity.

Table 5 Vapor permeation resistance sheet materials and thin layers of vapor barrier (Appendix 11 to SNiP P-3-79*)

Conversion of pressure from atmospheres (atm) to pascals (Pa) and kilopascals (1 kPa = 1000 Pa) is carried out taking into account the ratio 1 atm = 100,000 Pa. In bath practice, it is much more convenient to characterize the content of water vapor in the air by the concept of absolute air humidity (equal to the mass of moisture in 1 m 3 of air), since it clearly shows how much water needs to be added to the heater (or evaporated in a steam generator). Absolute air humidity is equal to the product of relative humidity and saturated vapor density:

Since the characteristic level of absolute air humidity in baths of 0.05 kg/m 3 corresponds to a partial pressure of water vapor of 7300 Pa, and the characteristic values ​​of partial pressure of water vapor in the atmosphere (outdoors) are at 50% relative air humidity 1200 Pa in the summer (20 °C) and 130 Pa in winter (-10 °C), then the characteristic differences in partial pressures of water vapor on the walls of the baths reach values ​​of 6000-7000 Pa. It follows that the typical levels of water vapor flows through the timber walls of bathhouses 10 cm thick are (3-4) g/m 2 hour in complete calm conditions, and based on 20 m 2 walls - (60-80) g/hour.

This is not so much, considering that a bath with a volume of 10 m 3 contains about 500 g of water vapor. In any case, if the walls are air permeable, during strong (10 m/sec) gusts of wind (1-10) kg/m 2 hour, the transfer of water vapor by the wind through timber walls can reach (50-500) g/m 2 hour. All this means that the vapor permeability of timber walls and ceilings of bathhouses does not significantly reduce the moisture content of wood wetted with hot dew during supply, so that the ceiling is steam bath and in fact, it can get wet and work as a steam generator, mainly humidifying only the air in the bathhouse, but only if the ceiling is carefully protected from gusts of wind.

If the bathhouse is cold, then the differences in water vapor pressure on the walls of the bathhouse cannot exceed 1000 Pa in the summer (at 100% humidity inside the wall and 60% air humidity outside at 20°C). Therefore, the characteristic drying rate of timber walls in summer due to vapor permeation is at the level of 0.5 g/m 2 hour, and due to air permeability in a light wind of 1 m/sec - (0.2-2) g/m 2 hour and with gusts of wind 10 m/sec - (20-200) g/m 2 hour (although inside the walls the movements of air masses occur at speeds less than 1 mm/sec). It is clear that vapor permeation processes become significant in the moisture balance only with good wind protection of the building walls.

Thus, for quick drying of building walls (for example, after emergency roof leaks), it is better to provide vents (ventilated façade channels) inside the walls. So, if in a closed bath you wet the inner surface of a timber wall with water in the amount of 1 kg/m2, then such a wall, allowing water vapor to pass through it to the outside, will dry out in the wind in a few days, but if timber wall plastered on the outside (that is, windproofed), it will dry out without heating in only a few months. Fortunately, wood is saturated with water very slowly, so drops of water on the wall do not have time to penetrate deep into the wood, and it is not typical for walls to dry out for such a long time.

But if the crown of the log house lies in a puddle on the base or on wet (and even damp) ground for weeks, then subsequent drying is only possible by the wind through the cracks.

In everyday life (and even in professional construction) it is in the area of ​​vapor barrier that there is greatest number misunderstandings, sometimes the most unexpected. For example, it is often believed that hot bath air supposedly “dries out” a cold floor, and cold dank air from the underground is “absorbed” and supposedly “moisturizes” the floor, although everything happens just the opposite.

Or, for example, they seriously believe that thermal insulation (glass wool, expanded clay, etc.) “sucks” moisture and thereby “dries out” the walls, without asking the question about future fate this supposedly endlessly “absorbed” moisture. It is useless to refute such everyday considerations and images in everyday life, if only because in the general public no one is seriously interested (and even more so during “bathroom chatter”) in the nature of the phenomenon of vapor permeability.

But if a summer resident, having the appropriate technical education, actually wants to figure out how and where water vapor penetrates the walls and how they exit from there, then he will have, first of all, to assess the real moisture content in the air in all areas of interest (inside and outside the bathhouse ), and objectively expressed in mass units or partial pressure, and then, using the given data on air permeability and vapor permeability, determine how and where water vapor flows move and whether they can condense in certain zones, taking into account real temperatures.

We will get acquainted with these questions in the following sections. We emphasize that for approximate estimates the following characteristic values ​​of pressure drops can be used:

Air pressure differences (to assess the transfer of water vapor along with air masses - by wind) range from (1-10) Pa (for one-story bathhouses or weak winds of 1 m/sec), (10-100) Pa (for multi-story buildings or moderate winds 10 m/sec), more than 700 Pa during hurricanes;

Changes in partial pressure of water vapor in the air from 1000 Pa (in residential premises) to 10,000 Pa (in baths).

In conclusion, we note that people often confuse the concepts of hygroscopicity and vapor permeability, although they have completely different physical meanings. Hygroscopic (“breathing”) walls absorb water vapor from the air, converting water vapor into compact water in very small capillaries (pores), even though the partial pressure of water vapor may be lower than the saturated vapor pressure.

Vapor-permeable walls simply allow water vapor to pass through without condensation, but if in some part of the wall there is a cold zone in which the partial pressure of water vapor becomes higher than the pressure of saturated vapor, then condensation, of course, is possible in the same way as on any surfaces. At the same time, vapor-permeable hygroscopic walls are moistened more than vapor-permeable non-hygroscopic walls.