Vapor permeability of thermal insulation. Should insulation “breathe”? Polyurethane foam is an effective insulation material

Last time we determined . Today we will compare insulation materials. Table with general characteristics you can find in the article summary. We have selected the most popular materials, including mineral wool, polyurethane foam, penoizol, polystyrene foam and ecowool. As you can see, this is universal insulation with a wide range of applications.

Comparison of thermal conductivity of insulation

The higher the thermal conductivity, the worse the material works as insulation.

It is not without reason that we start comparing insulation materials based on thermal conductivity, since this is undoubtedly the most important characteristic. It shows how much heat a material transmits, not over a certain period of time, but constantly. Thermal conductivity is expressed by a coefficient and is calculated in watts per square meter. For example, a coefficient of 0.05 W/m*K indicates that square meter constant heat loss is 0.05 Watt. The higher the coefficient, the better material conducts heat, and accordingly, it works worse as insulation.

Below is a table comparing popular insulation materials by thermal conductivity:

Having studied the above types of insulation and their characteristics, we can conclude that, with equal thickness, the most effective thermal insulation among all is liquid two-component polyurethane foam (PPU).

The thickness of the thermal insulation is of utmost importance; it must be calculated for each case individually. The result is influenced by the region, the material and thickness of the walls, and the presence of air buffer zones.

Comparative characteristics of insulation materials show that thermal conductivity is affected by the density of the material, especially for mineral wool. The higher the density, the less air there is in the insulation structure. As is known, air has a low thermal conductivity coefficient, which is less than 0.022 W/m*K. Based on this, as density increases, the coefficient of thermal conductivity also increases, which negatively affects the ability of the material to retain heat.

Comparison of vapor permeability of insulation

High vapor permeability = no condensation.

Vapor permeability is the ability of a material to allow air to pass through, and with it steam. That is, the thermal insulation can breathe. On this characteristic of home insulation Lately Manufacturers pay a lot of attention. In fact, high vapor permeability is only needed when . In all other cases, this criterion is not categorically important.

Characteristics of insulation in terms of vapor permeability, table:

A comparison of wall insulation materials showed that the highest degree of vapor permeability is achieved by natural materials, while polymer insulation has an extremely low coefficient. This indicates that materials such as polyurethane foam and polystyrene foam have the ability to retain steam, that is, they perform . Penoizol is also a kind of polymer that is made from resins. Its difference from polyurethane foam and polystyrene foam lies in the structure of the cells that open. In other words, it is a material with an open-cell structure. The ability of thermal insulation to transmit steam is closely related to the following characteristic– moisture absorption.

Review of hygroscopicity of thermal insulation

High hygroscopicity is a disadvantage that needs to be eliminated.

Hygroscopicity is the ability of a material to absorb moisture, measured as a percentage of its own weight of insulation. Hygroscopicity can be called weak side thermal insulation and the higher this value, the more serious measures will be required to neutralize it. The fact is that water, getting into the structure of the material, reduces the effectiveness of the insulation. Comparison of hygroscopicity of the most common thermal insulation materials in civil engineering:

A comparison of the hygroscopicity of home insulation showed the high moisture absorption of foam insulation, while this thermal insulation has the ability to distribute and remove moisture. Thanks to this, even when wet by 30%, the thermal conductivity coefficient does not decrease. Despite the fact that mineral wool has a low percentage of moisture absorption, it especially needs protection. Having absorbed the water, it holds it, preventing it from leaving. At the same time, the ability to prevent heat loss is catastrophically reduced.

To prevent moisture from entering the mineral wool, vapor barrier films and diffusion membranes are used. Basically, polymers are resistant to prolonged exposure to moisture, with the exception of ordinary polystyrene foam, which quickly deteriorates. In any case, water does not benefit any thermal insulation material, so it is extremely important to exclude or minimize their contact.

Installation and operational efficiency

Installation of polyurethane foam is quick and easy.

Comparison of the characteristics of insulation materials should be carried out taking into account installation, because this is also important. Easiest to work with liquid thermal insulation, such as polyurethane foam and penoizol, but this requires special equipment. It is also easy to lay ecowool (cellulose) on horizontal surfaces, for example, when or attic floor. To spray ecowool on walls using the wet method, special devices are also needed.

Polystyrene foam is laid both over the sheathing and directly onto the work surface. In principle, this also applies to stone wool slabs. Moreover, slab insulation can be laid on both vertical and horizontal surfaces (including under screed). Soft glass wool in rolls is laid only on the sheathing.

During operation, the thermal insulation layer may undergo some undesirable changes:

• absorb moisture;
• shrink;
• become a home for mice;
• collapse from exposure to IR rays, water, solvents, etc.

In addition to all of the above, the fire safety of thermal insulation is important. Comparison of insulation materials, flammability group table:

Results

Today we reviewed the most commonly used home insulation materials. Based on comparison results different characteristics we obtained data regarding thermal conductivity, vapor permeability, hygroscopicity and the degree of flammability of each of the insulation materials. All this data can be combined into one common table:

In addition to these characteristics, we have determined that it is easiest to work with liquid insulation and ecowool. PPU, penoizol and ecowool (installation using the wet method) are simply sprayed onto the work surface. Dry ecowool is poured manually.

Almost any advertising and information brochure or article describing the advantages of cotton insulation certainly mentions such a property as high vapor permeability - i.e. the ability to pass water vapor through. This property is closely related to the concept of “breathing walls”, around which heated debates and discussions on many pages regularly flare up on various construction forums and portals.

If we go to the official Russian (Ukrainian, Belarusian) website of any manufacturer of cotton insulation (ISOVER, ROCKWOOL, etc.), we will definitely find information about the high vapor permeability of the material, which ensures the “breathing” of the walls and a favorable microclimate in the room.

An interesting fact is that such information is completely absent from the English-language websites of the above-mentioned companies. Moreover, most of the information materials on these portals promote the idea of ​​​​creating completely airtight, airtight house structures. For example, consider the official website of the Isover company in the *com domain zone.

We bring to your attention the “golden rules of insulation” from the point of view of ISOVER.

1. Insulation performance
2. Good air tightness
3. Controlled ventilation
4. Quality fitting

Below we provide some translated quotes from this article:

“On average, a family of 4 people produces steam equal to 12 liters of water. Under no circumstances should this steam escape through the walls and roof! Only a ventilation system suitable for a particular home and the mode of living in it can prevent the occurrence of dark spots inside the room, streams of water flowing down the walls, damage to coatings and, ultimately, to the entire building.”

“Ventilation cannot be carried out by breaking the tightness of walls, windows, frames, shutters. All this only leads to the penetration of polluted air into the room, which disrupts the quality of air exchange inside the house, harms the building structures, the operation of the chimney and ventilation shafts. Under no circumstances should so-called 'breathing walls' be used as a design solution for home ventilation."

Having familiarized ourselves with the English-language websites of most manufacturers of cotton insulation, we can find out that the high vapor permeability of the produced material is not mentioned as an advantage on any of them. Moreover, these sites completely lack information about vapor permeability as a property of insulation.

Thus, we can come to the conclusion that cultivating the myth of vapor permeability is a successful marketing ploy representative offices of company data in Russia and the CIS countries, used to discredit manufacturers vapor-tight insulation– extruded polystyrene foam and foam glass.

However, despite the dissemination of such misleading information, manufacturers of cotton insulation on Russian websites post Constructive decisions on insulation of roofs and walls using vapor barriers, which makes their discussions about “breathable” structures devoid of common sense.

"WITH inside The roof must be provided with a vapor barrier layer. ISOVER recommends using ISOVER VS 80 or ISOVER VARIO membranes.

When installing a vapor barrier, it is necessary to maintain the integrity of the membrane, install it overlapping, and seal the joints with vapor-proof mounting tape. This will ensure the safety of the roof for many years.”

1. External skin
2. Waterproofing membrane
3. Metal or wooden frame
4. Thermal and sound insulation ISOVER
5. Vapor barrier ISOVER VARIO KM Duplex UV or ISOVER VS 80
6. Drywall (eg GYPROC)

"For guard thermal insulation material from humidification with internal air vapors is established vapor barrier film from the inner “warm” side of the insulation. To protect the wall from blowing with outside It is advisable to provide a windproof layer for insulation.”

Similar information can be heard directly from company representatives:

Ekaterina Kolotushkina, head of the " Frame house construction", Saint-Gobain ISOVER company:

“I would like to note that the durability of the entire roof structure depends not only on the similar indicator of the load-bearing elements, but is also determined by the service life of all materials used. To maintain this parameter when insulating the roof, it is necessary to use steam, hydro, and wind insulating membranes to protect the structure from steam from inside the room and moisture from outside.”

NATALIA CHUPYRA, head of the “Retail Products” department of the company “SAINT-GOBAIN IZOVER”, states approximately the same thing, “My Home” magazine.

“ISOVER recommends a roofing “pie” of the following design (layer-by-layer): roof covering, hydro-windproof membrane, counter-lattice, rafters with thermal insulation between them, vapor barrier membrane, interior finishing.”

Natalia also recognizes the importance of the ventilation system in the house:

“When insulating a house from the inside, many people neglect supply and exhaust ventilation. This is fundamentally wrong, because it provides the correct microclimate in the house. There is a certain air exchange rate that needs to be maintained in the room.”

As we see, the manufacturers of cotton insulation and their representatives themselves admit that the vapor barrier layer is a necessary component of almost any structure in which such thermal insulation is used. And this is not surprising, because the penetration of water molecules into a hygroscopic thermal insulation material leads to its wetting and, as a result, an increase in the thermal conductivity coefficient.

Thus, the high vapor permeability of insulation is more of a disadvantage than an advantage. Many manufacturers of vapor-tight thermal insulation have repeatedly tried to draw the attention of consumers to this fact, citing as arguments the opinions of scientists and qualified specialists in the field of construction.

For example, a well-known expert in the field of thermophysics, Doctor of Technical Sciences, Professor, K.F. Fokin states: “From a thermotechnical point of view, the air permeability of fences is more likely negative quality, since in winter time 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.”

Wet insulation requires additional protection as waterproofing and vapor barrier membranes. Otherwise, the thermal insulation material ceases to fulfill its main task - to retain heat indoors. In addition, wet insulation becomes a favorable environment for the development of fungi, mold and other harmful microorganisms, which negatively affects the health of household members and also leads to the destruction of structures in which it is a part.

Thus, a high-quality thermal insulation material must have such undeniable advantages, such as low thermal conductivity, high strength, water resistance, environmental friendliness and safety for humans and the environment, as well as low vapor permeability. The use of such thermal insulation material will not make the walls of your house “breathable”, but will allow them to fulfill their direct function - to maintain a favorable microclimate in the house and provide reliable protection from negative factors environment.

We supply Construction Materials to cities: Moscow, St. Petersburg, Novosibirsk, Nizhny Novgorod, Kazan, Samara, Omsk, Chelyabinsk, Rostov-on-Don, Ufa, Perm, Volgograd, Krasnoyarsk, Voronezh, Saratov, Krasnodar, Togliatti, 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.

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) at approximately 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 are the tabulated values ​​of the 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 using the concept of absolute air humidity ( equal mass 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 in a steam bath can actually get wet and work as a steam generator, mainly humidifying only the air in the bathhouse, but only when carefully protecting the ceiling 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 field of vapor barrier that there is the greatest number of 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 up” moisture and thereby “dries out” the walls, without asking the question about the further fate of 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.

Recently, various external insulation systems have been increasingly used in construction: “wet” type; ventilated facades; modified well masonry etc. What they all have in common is that they are multilayer enclosing structures. And for multilayer structures questions vapor permeability layers, moisture transfer, quantification of condensate that falls are issues of paramount importance.

As practice shows, unfortunately, both designers and architects do not pay due attention to these issues.

We have already noted that the Russian construction market is oversaturated with imported materials. Yes, of course, the laws of construction physics are the same and operate in the same way, for example, both in Russia and in Germany, but the approach methods and regulatory framework are very often very different.

Let us explain this using the example of vapor permeability. DIN 52615 introduces the concept of vapor permeability through the vapor permeability coefficient μ and air equivalent gap s d .

If we compare the vapor permeability of a layer of air 1 m thick with the vapor permeability of a layer of material of the same thickness, we obtain the vapor permeability coefficient

μ DIN (dimensionless) = air vapor permeability/material vapor permeability

Compare the concept of vapor permeability coefficient μ SNiP in Russia is introduced through SNiP II-3-79* “Construction Heat Engineering”, has the dimension mg/(m*h*Pa) and characterizes the amount of water vapor in mg that passes through one meter of thickness of a particular material in one hour at a pressure difference of 1 Pa.

Each layer of material in the structure has its own final thickness d, m. Obviously, the amount of water vapor passing through this layer will be less, the greater its thickness. If you multiply μ DIN And d, then we get the so-called air equivalent gap or diffuse equivalent thickness of the air layer s d

s d = μ DIN * d[m]

Thus, according to DIN 52615, s d characterizes the thickness of the air layer [m], which has equal vapor permeability with a layer of a specific material thickness d[m] and vapor permeability coefficient μ DIN. Resistance to vapor permeation 1/Δ defined as

1/Δ= μ DIN * d / δ in[(m² * h * Pa) / mg],

Where δ in- coefficient of air vapor permeability.

SNiP II-3-79* "Construction Heat Engineering" determines vapor permeation resistance R P How

R P = δ / μ SNiP[(m² * h * Pa) / mg],

Where δ - layer thickness, m.

Compare, according to DIN and SNiP, vapor permeability resistance, respectively, 1/Δ And R P have the same dimension.

We have no doubt that our reader already understands that the issue of linking quantitative indicators The coefficient of vapor permeability according to DIN and SNiP lies in determining the vapor permeability of air δ in.

According to DIN 52615, air vapor permeability is defined as

δ in =0.083 / (R 0 * T) * (p 0 / P) * (T / 273) 1.81,

Where R0- gas constant of water vapor equal to 462 N*m/(kg*K);

T- indoor temperature, K;

p 0- average indoor air pressure, hPa;

P- atmospheric pressure in normal condition, equal to 1013.25 hPa.

Without going deeply into the theory, we note that the quantity δ in depends to a small extent on temperature and can be considered with sufficient accuracy in practical calculations as a constant equal to 0.625 mg/(m*h*Pa).

Then, if the vapor permeability is known μ DIN easy to go to μ SNiP, i.e. μ SNiP = 0,625/ μ DIN

Above we have already noted the importance of the issue of vapor permeability for multilayer structures. No less important, from the point of view of building physics, is the issue of the sequence of layers, in particular, the position of the insulation.

If we consider the probability of temperature distribution t, saturated steam pressure Rn and unsaturated (real) vapor pressure Pp through the thickness of the enclosing structure, then from the point of view of the process of diffusion of water vapor, the most preferable sequence of layers is in which the resistance to heat transfer decreases, and the resistance to vapor permeation increases from the outside to the inside.

Violation of this condition, even without calculation, indicates the possibility of condensation in the section of the enclosing structure (Fig. A1).

Rice. P1

Note that the arrangement of layers from various materials does not affect the value of the overall thermal resistance, however, the diffusion of water vapor, the possibility and location of condensation predetermine the location of the insulation on the outer surface of the load-bearing wall.

Calculation of vapor permeability resistance and checking the possibility of condensation loss must be carried out according to SNiP II-3-79* “Construction Heat Engineering”.

Recently we have had to deal with the fact that our designers are provided with calculations performed using foreign computer methods. Let's express our point of view.

· Such calculations obviously have no legal force.

· The methods are designed for higher winter temperatures. Thus, the German “Bautherm” method no longer works at temperatures below -20 °C.

· Many important characteristics as initial conditions are not linked to ours regulatory framework. Thus, the thermal conductivity coefficient for insulation materials is given in a dry state, and according to SNiP II-3-79* “Building Heat Engineering” it should be taken under conditions of sorption humidity for operating zones A and B.

· The balance of moisture gain and loss is calculated for completely different climatic conditions.

Obviously, the number of winter months with negative temperatures for Germany and, say, Siberia are completely different.

The concept of “breathing walls” is considered a positive characteristic of the materials from which they are made. But few people think about the reasons that allow this breathing. Materials that can pass both air and steam are vapor permeable.

A clear example of building materials with high vapor permeability:

• wood;
• expanded clay slabs;
• foam concrete.

Concrete or brick walls are less permeable to steam than wood or expanded clay.

Indoor steam sources

Human breathing, cooking, water vapor from the bathroom and many other sources of steam in the absence of an exhaust device create high level indoor humidity. You can often observe the formation of perspiration on window glass in winter, or in cold weather water pipes. These are examples of water vapor forming inside a home.

What is vapor permeability

The design and construction rules give the following definition of the term: vapor permeability of materials is the ability to pass through droplets of moisture contained in the air due to different values ​​of partial vapor pressures on opposite sides at identical values air pressure. It is also defined as density steam flow passing through a certain thickness of material.

The table containing the coefficient of vapor permeability, compiled for building materials, is of a conditional nature, since the specified calculated values ​​of humidity and atmospheric conditions do not always correspond to real conditions. The dew point can be calculated based on approximate data.

Wall design taking into account vapor permeability

Even if the walls are built from a material that has high vapor permeability, this cannot be a guarantee that it will not turn into water within the thickness of the wall. To prevent this from happening, you need to protect the material from the difference in partial vapor pressure from inside and outside. Protection against the formation of steam condensate is carried out using OSB boards, insulating materials such as penoplex and vapor-proof films or membranes that prevent steam from penetrating into the insulation.

The walls are insulated so that closer to the outer edge there is a layer of insulation that is unable to form moisture condensation and pushes back the dew point (water formation). In parallel with the protective layers in roofing pie Proper ventilation gap must be ensured.

Destructive effects of steam

If the wall cake has a weak ability to absorb steam, it is not in danger of destruction due to the expansion of moisture from frost. The main condition is to prevent moisture from accumulating in the thickness of the wall, but to ensure its free passage and weathering. It is equally important to arrange forced exhaust excess moisture and steam from the room, connect a powerful ventilation system. By observing the above conditions, you can protect the walls from cracking and increase the service life of the entire house. The constant passage of moisture through building materials accelerates their destruction.

Use of conductive qualities

Taking into account the peculiarities of building operation, the following insulation principle is applied: the most vapor-conducting insulating materials are located outside. Thanks to this arrangement of layers, the likelihood of water accumulating when the outside temperature drops is reduced. To prevent the walls from getting wet from the inside, the inner layer is insulated with a material that has low vapor permeability, for example, a thick layer of extruded polystyrene foam.

The opposite method of using the vapor-conducting effects of building materials has been successfully used. It consists in the fact that brick wall covered with a vapor barrier layer of foam glass, which interrupts the moving flow of steam from the house to the street during low temperatures. The brick begins to accumulate moisture in the rooms, creating a pleasant indoor climate thanks to a reliable vapor barrier.

Compliance with the basic principle when constructing walls

Walls must have a minimum ability to conduct steam and heat, but at the same time be heat-intensive and heat-resistant. When using one type of material, the required effects cannot be achieved. The outer wall part must retain cold masses and prevent their impact on internal heat-intensive materials that maintain a comfortable thermal regime inside the room.

Ideal for inner layer reinforced concrete, its heat capacity, density and strength have maximum performance. Concrete successfully smoothes out the difference between night and day temperature changes.

When conducting construction work wall pies are made taking into account the basic principle: the vapor permeability of each layer should increase in the direction from the inner layers to the outer ones.

Rules for the location of vapor barrier layers

If this rule is followed, it will not be difficult for water vapor trapped in the warm layer of the wall to quickly escape through more porous materials.

If this condition is not met, the inner layers of building materials harden and become more thermally conductive.

Introduction to the table of vapor permeability of materials

When designing a house, the characteristics are taken into account construction raw materials. The Code of Rules contains a table with information about what coefficient of vapor permeability building materials have under normal conditions. atmospheric pressure and average air temperature.

The importance of the vapor permeability table of materials

The vapor permeability coefficient is an important parameter that is used to calculate the thickness of the layer of insulating materials. The quality of insulation of the entire structure depends on the correctness of the results obtained.

Sergey Novozhilov - expert on roofing materials with 9 years experience practical work in the field of engineering solutions in construction.