Air gap temperature distribution. Thermal protection of facades with a ventilated air gap. Heat and moisture transfer through external fences

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1.3 The building as a single energy system.
2. Heat and moisture transfer through external fences.
2.1 Fundamentals of heat transfer in a building .
2.1.1 Thermal conductivity.
2.1.2 Convection.
2.1.3 Radiation.
2.1.4 Thermal resistance of the air gap.
2.1.5 Heat transfer coefficients on the inner and outer surfaces.
2.1.6 Heat transfer through a multilayer wall.
2.1.7 Reduced resistance to heat transfer.
2.1.8 Temperature distribution over the section of the fence.
2.2 Moisture regime of enclosing structures.
2.2.1 Causes of moisture in fences.
2.2.2 Negative effects of dampening of external fences.
2.2.3 Communication of moisture with building materials.
2.2.4 Humid air.
2.2.5 Moisture content of the material.
2.2.6 Sorption and desorption.
2.2.7 Vapor permeability of fences.
2.3 Air permeability of external barriers.
2.3.1 Fundamentals.
2.3.2 Pressure difference on the outer and inner surfaces of the fences.
2.3.3 Air permeability of building materials.

2.1.4 Thermal resistance of the air gap.

For uniformity, heat transfer resistance closed air gaps located between the layers of the building envelope, called thermal resistance R vp, m². ºС/W.
The scheme of heat transfer through the air gap is shown in Fig.5.

Fig.5. Heat transfer in the air gap.

Heat flux passing through the air gap q v.p , W/m² , is made up of flows transmitted by thermal conductivity (2) q t , W/m² , convection (1) q c , W/m² , and radiation (3) q l , W/m² .

(2.12)

In this case, the share of the flux transmitted by radiation is the largest. Let us consider a closed vertical air gap, on the surfaces of which the temperature difference is 5ºС. With an increase in the interlayer thickness from 10 mm to 200 mm, the proportion of heat flux due to radiation increases from 60% to 80%. In this case, the share of heat transferred by thermal conductivity drops from 38% to 2%, and the share of convective heat flow increases from 2% to 20%.
The direct calculation of these components is rather cumbersome. Therefore, in normative documents data are given on the thermal resistance of closed air spaces, which in the 50s of the twentieth century was compiled by K.F. Fokin based on the results of experiments by M.A. Mikheev. If there is a heat-reflecting aluminum foil on one or both surfaces of the air gap, which hinders radiant heat transfer between the surfaces framing the air gap, the thermal resistance should be doubled. To increase the thermal resistance of closed air gaps, it is recommended to bear in mind the following conclusions from the studies:
1) thermally efficient are interlayers of small thickness;
2) it is more rational to make several layers of small thickness in the fence than one large one;
3) it is desirable to place air gaps closer to the outer surface of the fence, since in this case winter time the heat flux by radiation decreases;
4) vertical layers in the outer walls must be blocked by horizontal diaphragms at the level of interfloor ceilings;
5) to reduce the heat flux transmitted by radiation, it is possible to cover one of the surfaces of the interlayer aluminum foil, having an emissivity of about ε=0.05. Covering both surfaces of the air gap with foil does not significantly reduce heat transfer compared to covering one surface.
Questions for self-control
1. What is the heat transfer potential?
2. List the elementary types of heat transfer.
3. What is heat transfer?
4. What is thermal conductivity?
5. What is the thermal conductivity of the material?
6. Write the formula for the heat flux transferred by thermal conductivity in a multilayer wall at known temperatures of the inner tw and outer tn surfaces.
7. What is thermal resistance?
8. What is convection?
9. Write the formula for the heat flux transferred by convection from air to the surface.
10. physical meaning coefficient of convective heat transfer.
11. What is radiation?
12. Write the formula for the heat flux transmitted by radiation from one surface to another.
13. Physical meaning of the radiant heat transfer coefficient.
14. What is the name of the resistance to heat transfer of a closed air gap in the building envelope?
15. Of what nature does the total heat flow through the air gap consist of heat flows?
16. What nature of the heat flow prevails in the heat flow through the air gap?
17. How does the thickness of the air gap affect the distribution of flows in it.
18. How to reduce the heat flow through the air gap?

AIR GAP, one of the types of insulating layers that reduce the thermal conductivity of the medium. AT recent times the importance of the air layer has especially increased in connection with the use of hollow materials in the construction industry. In a medium separated by an air gap, heat is transferred: 1) by radiation from surfaces adjacent to the air gap, and by heat transfer between the surface and air, and 2) by heat transfer by air, if it is moving, or by heat transfer by some air particles to others due to heat conduction it, if it is motionless, and Nusselt's experiments prove that thinner layers, in which the air can be considered almost motionless, have a lower thermal conductivity coefficient k than thicker layers, but with convection currents arising in them. Nusselt gives the following expression for determining the amount of heat transferred per hour by the air gap:

where F is one of the surfaces limiting the air gap; λ 0 - conditional coefficient, the numerical values \u200b\u200bof which, depending on the width of the air gap (e), expressed in m, are given in the attached plate:

s 1 and s 2 - coefficients of radiation of both surfaces of the air gap; s is the radiation coefficient of a completely black body, equal to 4.61; θ 1 and θ 2 are the temperatures of the surfaces limiting the air gap. By substituting the appropriate values into the formula, it is possible to obtain the values \u200b\u200bfor calculations of k (thermal conductivity coefficient) and 1 / k (insulating ability) of air layers of various thicknesses. S. L. Prokhorov compiled, according to Nusselt's data, diagrams (see Fig.) showing the change in the values of k and 1/k of air layers depending on their thickness, and the most advantageous area is the area from 15 to 45 mm.

Smaller air gaps are practically difficult to implement, and large ones already give a significant thermal conductivity coefficient (about 0.07). The following table gives the values k and 1/k for various materials, and several values of these quantities are given for air depending on the layer thickness.

That. it can be seen that it is often more advantageous to make several thinner air layers than to use one or another insulating layer. An air gap up to 15 mm thick can be considered an insulator with a fixed air layer, with a thickness of 15-45 mm - with an almost fixed one, and, finally, air gaps over 45-50 mm thick should be recognized as layers with convection currents arising in them and therefore subject to calculation for general basis.

Description:

Enclosing structures with ventilated air gaps have long been used in the construction of buildings. The use of ventilated air spaces had one of the following goals

Thermal protection of facades with ventilated air gap

Part 1

The dependence of the maximum speed of air movement in the gap on the outside air temperature at different values thermal resistance of the wall with insulation

Dependence of the air velocity in the air gap on the outside air temperature at different values of the gap width d

Dependence of the thermal resistance of the air gap, R eff gap, on the outside temperature at various values of the thermal resistance of the wall, R pr therm. feature

Dependence of the effective thermal resistance of the air gap, R eff of the gap, on the width of the gap, d, at different values of the height of the facade, L

On fig. 7 shows the dependences of the maximum air velocity in the air gap on the outside air temperature for various values of the facade height, L, and the thermal resistance of the wall with insulation, R pr therm. feature , and in fig. 8 - at different values of the gap width d.

In all cases, the air velocity increases as the outside temperature decreases. Doubling the height of the façade results in a slight increase in air velocity. A decrease in the thermal resistance of the wall leads to an increase in air velocity, this is due to an increase in the heat flux, and hence the temperature difference in the gap. The gap width has a significant effect on the air speed, with a decrease in the values of d, the air speed decreases, which is explained by an increase in resistance.

On fig. 9 shows the dependences of the thermal resistance of the air gap, R eff gap, on the outside air temperature at various values of the height of the facade, L, and the thermal resistance of the wall with insulation, R pr therm. feature .

First of all, it should be noted the weak dependence of R eff of the gap on the outside air temperature. This is easily explained, since the difference between the air temperature in the gap and the temperature of the outside air and the difference between the temperature of the internal air and the air temperature in the gap change almost proportionally with a change in t n, therefore their ratio included in (3) almost does not change. So, with a decrease in t n from 0 to -40 ° C, the R eff of the gap decreases from 0.17 to 0.159 m 2 ° C / W. The gap R eff also depends insignificantly on the thermal resistance of the lining, with an increase in R pr therm. region from 0.06 to 0.14 m 2 °C / W, the value of R eff of the gap varies from 0.162 to 0.174 m 2 °C / W. This example shows the inefficiency of facade cladding insulation. Changes in the value of the effective thermal resistance of the air gap depending on the outdoor temperature and on the thermal resistance of the cladding are insignificant for their practical consideration.

On fig. 10 shows the dependences of the thermal resistance of the air gap, R eff of the gap, on the width of the gap, d, for various values of the height of the facade. The dependence of R eff of the gap on the width of the gap is most clearly expressed - with a decrease in the thickness of the gap, the value of R eff of the gap increases. This is due to a decrease in the height of temperature establishment in the gap x 0 and, accordingly, to an increase in the average air temperature in the gap (Fig. 8 and 6). If for other parameters the dependence is weak, since there is an overlap of various processes partially extinguishing each other, then in this case this is not the case - the thinner the gap, the faster it warms up, and the slower the air moves in the gap, the faster it heats up.

Generally nai greater value R eff gap can be achieved with minimum value d, the maximum value of L, the maximum value of R pr therm. feature . So, at d = 0.02 m, L = 20 m, R pr therm. feature \u003d 3.4 m 2 ° C / W, the calculated value of R eff of the gap is 0.24 m 2 ° C / W.

To calculate heat loss through the fence, the relative influence of the effective thermal resistance of the air gap is of greater importance, since it determines how much heat loss will decrease. Despite the fact that the largest absolute value of R eff gap is achieved at the maximum R pr therm. feature , greatest influence effective thermal resistance of the air gap to heat loss has a minimum value of R pr therm. feature . So, at R pr term. feature = = 1 m 2 °C/W and t n = 0 °C due to the air gap, heat loss is reduced by 14%.

With horizontally located guides to which facing elements are attached, when making calculations, it is advisable to take the width of the air gap equal to the smallest distance between the guides and the surface of the thermal insulation, since these sections determine the resistance to air movement (Fig. 11).

As shown by the calculations, the speed of air movement in the gap is small and is less than 1 m/s. The reasonableness of the adopted calculation model is indirectly confirmed by the literature data. So, in the work short review the results of experimental determinations of air velocity in the air gaps of various facades (see table). Unfortunately, the data contained in the article is incomplete and does not allow us to establish all the characteristics of the facades. However, they show that the air velocity in the gap is close to the values obtained by the calculations described above.

The presented method for calculating the temperature, air velocity and other parameters in the air gap makes it possible to evaluate the effectiveness of a particular design measure in terms of increasing operational properties facade. This method can be improved, first of all, it should relate to the effect of gaps between the facing plates. As follows from the results of calculations and the experimental data given in the literature, this improvement will not have a large impact on the reduced resistance of the structure, but it may affect other parameters.

Literature

1. Batinich R. Ventilated facades of buildings: Problems building thermal physics, microclimate and energy saving systems in buildings / Sat. report IV scientific-practical. conf. M.: NIISF, 1999.

2. Ezersky V. A., Monastyrev P. V. Mounting frame of a ventilated facade and temperature field outer wall // housing construction. 2003. № 10.

4. SNiP II-3-79*. Construction heat engineering. M.: GUP TsPP, 1998.

5. Bogoslovsky VN The thermal regime of the building. M., 1979.

6. Sedlbauer K., Kunzel H. M. Luftkonvektions einflusse auf den Warmedurchgang von belufteten Fassaden mit Mineralwolledammung // WKSB. 1999.Jg. 44.H.43.

To be continued.

List of symbols

c in \u003d 1 005 J / (kg ° С) - specific heat air

d - air gap width, m

L - facade height with ventilated gap, m

n to - the average number of brackets per m 2 of the wall, m–1

R about. feature , R pr o. region - reduced resistance to heat transfer of parts of the structure from the inner surface to the air gap and from the air gap to the outer surface of the structure, respectively, m 2 ° C / W

R about pr - reduced resistance to heat transfer of the entire structure, m 2 ° C / W

R cond. feature - resistance to heat transfer along the surface of the structure (excluding heat-conducting inclusions), m 2 ° C / W

R conditionally - resistance to heat transfer along the surface of the structure, is determined as the sum of the thermal resistances of the layers of the structure and the heat transfer resistances of the internal (equal to 1/av) and external (equal to 1/an) surfaces

R pr SNiP - reduced heat transfer resistance of the wall structure with insulation, determined in accordance with SNiP II-3-79 *, m 2 ° C / W

R pr therm. feature - thermal resistance of the wall with insulation (from internal air to the surface of the insulation in the air gap), m 2 ° C / W

R eff gap - effective thermal resistance of the air gap, m 2 ° C / W

Q n - calculated heat flux through an inhomogeneous structure, W

Q 0 - heat flow through a homogeneous structure of the same area, W

q - heat flux density through the structure, W / m 2

q 0 - heat flux density through a homogeneous structure, W / m 2

r - thermal uniformity coefficient

S - cross-sectional area of the bracket, m 2

t - temperature, °С

For uniformity, heat transfer resistance closed air gaps located between the layers of the building envelope, called thermal resistance Rv.p, m². ºС/W.
The scheme of heat transfer through the air gap is shown in Fig.5.

Fig.5. Heat transfer in the air gap.

The heat flux passing through the air gap qv.p, W/m², consists of flows transmitted by thermal conductivity (2) qt, W/m², convection (1) qc, W/m², and radiation (3) ql, W/m².

24. Conditional and reduced resistance to heat transfer. Coefficient of thermotechnical homogeneity of enclosing structures.

25. Rationing of resistance to heat transfer based on sanitary and hygienic conditions

, R0 = *

We normalize Δ t n, then R 0 tr = * , those. in order for Δ t≤ Δ t n Necessary

R 0 ≥ R 0 tr

SNiP extends this requirement to the reduced resistance. heat transfer.

R 0 pr ≥ R 0 tr

t in - design temperature of internal air, °С;

accept. according to design standards. building

t n - - calculated winter temperature of the outside air, ° С, equal to the average temperature of the coldest five-day period with a security of 0.92

A in (alpha) - heat transfer coefficient of the inner surface of enclosing structures, taken according to SNiP

Δt n - standard temperature difference between the temperature of the internal air and the temperature of the inner surface of the enclosing structure, taken according to SNiP

Required resistance to heat transfer R tr about doors and gates must be at least 0.6 R tr about walls of buildings and structures, determined by the formula (1) at the calculated winter temperature outdoor air, equal to the average temperature of the coldest five-day period with a security of 0.92.

When determining the required resistance to heat transfer of internal enclosing structures in formula (1), it should be taken instead of t n- the calculated air temperature of the colder room.

26. Thermotechnical calculation of the required thickness of the fence material based on the conditions for achieving the required resistance to heat transfer.

27. Humidity of the material. Reasons for wetting the structure

Humidity - physical quantity equal to the amount of water contained in the pores of the material.

It happens by weight and volume

1) Building moisture.(during the construction of the building). Depends on the design and construction method. Solid brickwork is worse than ceramic blocks. The most favorable wood (prefabricated walls). w / w not always. Should disappear in 2 = -3 years of operation. Measures: drying the walls

ground moisture. (capillary suction). It reaches the level of 2-2.5 m. waterproofing layers, with correct device does not affect.

2) Ground moisture, penetrates into the fence from the ground due to capillary suction

3)Atmospheric moisture. (slanting rain, snow). Especially important for roofs and cornices .. solid brick walls do not require protection if the jointing is done correctly. reinforced concrete, lightweight concrete panels attention to joints and window blocks, textured layer of waterproof materials. Protection = protective wall on the slope

4) Operating moisture. (in workshops industrial buildings, mainly in the floors and lower part of the walls) solution: waterproof floors, drainage device, cladding of the lower part ceramic tiles, waterproof plaster. Protection=protective cladding with ext. sides

5)Hygroscopic moisture. Due to the increased hygroscopicity of materials (property to absorb water vapor from humid air)

6) Condensation of moisture from the air: a) on the surface of the fence. b) in the thickness of the fence

28. Influence of humidity on the properties of structures

1) With an increase in humidity, the thermal conductivity of the structure increases.

2) Humidity deformations. Humidity is much worse than thermal expansion. Peeling of the plaster due to the accumulated moisture under it, then the moisture freezes, expands in volume and tears off the plaster. Non-moisture resistant materials deform when wet. For example, gypsum becomes creeping with increasing humidity, plywood swelling, delamination.

3) Decrease in durability - number of years of failure-free operation of the structure

4) Biological damage (fungus, mold) due to dew

5) Loss of aesthetic appearance

Therefore, when choosing materials, their moisture regime is taken into account and materials with the lowest moisture content are selected. Also, excessive humidity in the room can cause the spread of diseases and infections.

From a technical point of view, it leads to a loss of durability and structure and its frost-resistant properties. Some materials for high humidity lose mechanical strength, change shape. For example, gypsum becomes creeping with increasing humidity, plywood swelling, delamination. Corrosion of metal. deterioration in appearance.

29. Sorption of water vapor builds. mater. Sorption mechanisms. Hysteresis of sorption.

Sorption- the process of absorption of water vapor, which leads to an equilibrium moisture state of the material with air. 2 phenomena. 1. Absorption as a result of the collision of a vapor molecule with the surface of the pores and sticking to this surface (adsorption)2. Direct dissolution of moisture in the volume of the body (absorption). Humidity increases with increasing relative elasticity and decreasing temperature. "desorption" if a wet sample is placed in desiccators (solution of sulfuric acid), then it gives off moisture.

Sorption mechanisms:

1.Adsorption

2. Capillary condensation

3. Volumetric filling of micropores

4.Filling the interlayer space

1 stage. Adsorption is a phenomenon in which the surface of the pores is covered with one or more layers of water molecules (in mesopores and macropores).

2 stage. Polymolecular adsorption - a multilayer adsorbed layer is formed.

3 stage. capillary condensation.

CAUSE. The saturation vapor pressure over a concave surface is less than over a flat liquid surface. In small-radius capillaries, moisture forms concave minisks, so capillary condensation is possible. If D>2*10 -5 cm, then there will be no capillary condensation.

Desorption - natural drying process.

Hysteresis ("difference") of sorption consists in the difference between the sorption isotherm obtained when the material is moistened and the desorption isotherm obtained from the dried material. shows the % difference between weight moisture with sorption and weight with moisture desorption (desorption 4.3%, sorption 2.1%, hysteresis 2.2%) when moistening the sorption isotherm. When dried, desorption.

30. Mechanisms of moisture transfer in materials of building structures. Vapor permeability, capillary absorption of water.

1. In winter, due to the temperature difference and at different partial pressures, a stream of water vapor passes through the fence (from the inner surface to the outer) - diffusion of water vapor. In summer it's the other way around.

2. Convective transport of water vapor(with airflow)

3. Capillary water transfer(leakage) through porous materials.

4. Gravitational water leakage through cracks, holes, macropores.

Vapor permeability - the property of a material or structure made of them to pass water vapor through itself.

Permeability coefficient- Physical. the value is numerically equal to the number of steam that has passed through the plate at a unit area, at a unit pressure drop, at a unit thickness of the plate, at a unit time at a partial pressure drop on the sides of the plate e 1 Pa. Temperatures, mu decreases, with increasing humidity, mu increases.

Vapor resistance: R=thickness/mu

Mu - vapor permeability coefficient (determined according to SNIP 2379 heat engineering)

Capillary absorption of water by building materials - provides a constant transfer of liquid moisture through porous materials from a region of high concentration to a region of low concentration.

The thinner the capillaries, the greater the force of capillary suction, but in general the transfer rate decreases.

Capillary transport can be reduced or eliminated by providing an appropriate barrier (small air gap or capillary inactive layer (non-porous)).

31. Fick's law. Vapor permeability coefficient

P(amount of steam, g) \u003d (ev-en) F * z * (mu / thickness),

Mu- coefficient. vapor permeability (determined according to SNIP 2379 heat engineering)

Physical the value is numerically equal to the amount of steam that has passed through the plate at a unit area, at a unit pressure drop, at a unit plate thickness, at a unit time at a partial pressure drop on the sides of the plate e 1 Pa. [mg / (m 2 * Pa)]. The smallest mu has roofing material 0.00018, the largest min. cotton = 0.065g / m * h * mm Hg, window glass and metals are vapor-tight, air is the greatest vapor permeability. When decreasing Temperatures, mu decreases, with increasing humidity, mu increases. It depends on the physical properties of the material and reflects its ability to conduct water vapor diffusing through it. Anisotropic materials have different mu (for wood, along the fibers = 0.32, across = 0.6).

Equivalent resistance to vapor permeability of the fence with a sequential arrangement of layers. Fick's law.

Q \u003d (e 1 -e 2) / R n qR n1n =(e n1n-1 -e 2)

32 Calculation of the distribution of partial pressure of water vapor over the thickness of the structure.

One of the techniques that increase the thermal insulation qualities of fences is the installation of an air gap. It is used in the construction of external walls, ceilings, windows, stained-glass windows. In walls and ceilings, it is also used to prevent waterlogging of structures.

The air gap can be sealed or ventilated.

Consider heat transfer sealed air layer.

The thermal resistance of the air layer R al cannot be defined as the thermal conductivity resistance of the air layer, since heat transfer through the layer at a temperature difference on the surfaces occurs mainly by convection and radiation (Fig. 3.14). The amount of heat,

transmitted by thermal conductivity is small, since the coefficient of thermal conductivity of air is low (0.026 W / (m ºС)).

In the layers, in general, the air is in motion. In vertical - it moves up along the warm surface and down - along the cold. Convective heat transfer takes place, and its intensity increases with an increase in the thickness of the interlayer, since the friction of air jets against the walls decreases. When heat is transferred by convection, the resistance of the boundary layers of air at two surfaces is overcome, therefore, to calculate this amount of heat, the heat transfer coefficient α k should be halved.

To describe heat transfer jointly by convection and thermal conductivity, the convective heat transfer coefficient α "k is usually introduced, equal to

α" k \u003d 0.5 α k + λ a / δ al, (3.23)

where λ a and δ al are the thermal conductivity of air and the thickness of the air gap, respectively.

This ratio depends on geometric shape and sizes of air layers, direction of heat flow. By generalization a large number experimental data based on the theory of similarity, M.A. Mikheev established certain patterns for α "to. In Table 3.5, as an example, the values \u200b\u200bof the coefficients α" to, calculated by him at an average air temperature in a vertical layer t \u003d + 10º C.

Table 3.5

Coefficients of convective heat transfer in a vertical air gap

The coefficient of convective heat transfer in horizontal air layers depends on the direction of the heat flow. If a upper surface heated more than the lower one, there will be almost no air movement, since warm air concentrated at the top, and cold - at the bottom. Therefore, the equality

α" to \u003d λ a / δ al.

Consequently, the convective heat transfer decreases significantly, and the thermal resistance of the interlayer increases. Horizontal air gaps are effective, for example, when used in insulated basement ceilings above cold undergrounds, where the heat flow is directed from top to bottom.

If the heat flow is directed from the bottom up, then there are ascending and descending air flows. Heat transfer by convection plays a significant role, and the value of α" k increases.

To take into account the effect of thermal radiation, the coefficient of radiant heat transfer α l is introduced (Chapter 2, p. 2.5).

Using formulas (2.13), (2.17), (2.18) we determine the coefficient of heat transfer by radiation α l in the air gap between constructive layers brickwork. Surface temperatures: t 1 = + 15 ºС, t 2 = + 5 ºС; the degree of blackness of the brick: ε 1 = ε 2 = 0.9.

By formula (2.13) we find that ε = 0.82. Temperature coefficient θ = 0.91. Then α l \u003d 0.82 ∙ 5.7 ∙ 0.91 \u003d 4.25 W / (m 2 ºС).

The value of α l is much greater than α "to (see Table 3.5), therefore, the main amount of heat through the interlayer is transferred by radiation. In order to reduce this heat flux and increase the heat transfer resistance of the air layer, it is recommended to use reflective insulation, that is, a coating of one or both surfaces, for example, with aluminum foil (the so-called "reinforcement"). Such a coating is usually arranged on a warm surface to avoid moisture condensation, which worsens the reflective properties of the foil. "Reinforcement" of the surface reduces the radiant flux by about 10 times.

The thermal resistance of a sealed air gap at a constant temperature difference on its surfaces is determined by the formula

Table 3.6

Thermal resistance of closed air spaces

Air layer thickness, m	R al, m 2 °C / W
for horizontal layers with heat flow from bottom to top and for vertical layers	for horizontal layers with heat flow from top to bottom
summer	winter	summer	winter
0,01	0,13	0,15	0,14	0,15
0,02	0,14	0,15	0,15	0,19
0,03	0,14	0,16	0,16	0,21
0,05	0,14	0,17	0,17	0,22
0,1	0,15	0,18	0,18	0,23
0,15	0,15	0,18	0,19	0,24
0,2-0.3	0,15	0,19	0,19	0,24

R al values for closed flat air gaps are given in Table 3.6. These include, for example, interlayers between layers of dense concrete, which practically does not allow air to pass through. It has been experimentally shown that in brickwork with insufficient filling of the seams between the bricks with mortar, there is a violation of tightness, that is, the penetration of outside air into the layer and a sharp decrease in its resistance to heat transfer.

When covering one or both surfaces of the interlayer with aluminum foil, its thermal resistance should be doubled.

At present, walls with ventilated air layer (walls with a ventilated facade). A hinged ventilated facade is a structure consisting of cladding materials and a substructure, which is attached to the wall in such a way that an air gap remains between the protective and decorative cladding and the wall. For additional insulation external structures, a heat-insulating layer is installed between the wall and the cladding, so that ventilation gap left between the cladding and thermal insulation.

The design scheme of the ventilated facade is shown in Figure 3.15. According to SP 23-101, the thickness of the air gap should be in the range from 60 to 150 mm.

Structural layers located between the air gap and the outer surface are not taken into account in the heat engineering calculation. Therefore, thermal resistance outer cladding is not included in the heat transfer resistance of the wall, determined by formula (3.6). As noted in clause 2.5, the heat transfer coefficient of the outer surface of the building envelope with ventilated air spaces α ext for the cold period is 10.8 W / (m 2 ºС).

The design of a ventilated facade has a number of significant advantages. In paragraph 3.2, the temperature distributions in the cold period in two-layer walls with internal and external insulation were compared (Fig. 3.4). A wall with external insulation is more

“warm”, since the main temperature difference occurs in the heat-insulating layer. There is no condensation inside the wall, its heat-shielding properties do not deteriorate, additional vapor barrier is not required (chapter 5).

The air flow that occurs in the layer due to the pressure drop contributes to the evaporation of moisture from the surface of the insulation. It should be noted that a significant mistake is the use of vapor barrier on the outer surface of the heat-insulating layer, as it prevents the free removal of water vapor to the outside.