How closed air gaps work. Air layers

The article discusses the design of a thermal insulation system with a closed air gap between the thermal insulation and the wall of the building. It is proposed to use vapor-permeable inserts in thermal insulation in order to prevent moisture condensation in the air layer. A method for calculating the area of ​​inserts depending on the conditions of use of thermal insulation is given.

This paper describes the thermal insulating system having dead air space between the thermal insulation and the outer wall of the building. Water vapor-permeable inserts are proposed for use in the thermal insulation in order to prevent moisture condensation in the air space. The method for calculating the offered area of ​​the inserts has been depending on the conditions of the thermal insulation usage.

INTRODUCTION

The air gap is an element of many building envelopes. In this paper, the properties of enclosing structures with closed and ventilated air gaps are investigated. At the same time, the features of its application in many cases require solving the problems of building heat engineering in specific conditions of use.

Known and widely used in construction is the design of a heat-insulating system with a ventilated air gap. The main advantage of this system over light plaster systems is the ability to perform work on the insulation of buildings all year round. The insulation fastening system is first attached to the enclosing structure. The heater is attached to this system. The outer protection of the insulation is installed from it at some distance, so that an air gap is formed between the insulation and the outer fence. The design of the insulation system allows ventilation of the air gap in order to remove excess moisture, which reduces the amount of moisture in the insulation. The disadvantages of this system include the complexity and necessity, along with the use of insulation materials, to use siding systems that provide the necessary clearance for moving air.

Known ventilation system in which the air gap is adjacent directly to the wall of the building. Thermal insulation is made in the form of three-layer panels: the inner layer is thermal insulation material, the outer layers are aluminum and aluminum foil. This design protects the insulation from the penetration of both atmospheric moisture and moisture from the premises. Therefore, its properties do not deteriorate under any operating conditions, which saves up to 20% of insulation compared to conventional systems. The disadvantage of these systems is the need to ventilate the layer to remove moisture migrating from the premises of the building. This leads to a decrease in the thermal insulation properties of the system. In addition, the heat losses of the lower floors of buildings increase, since the cold air entering the interlayer through the holes at the bottom of the system takes some time to heat up to a steady temperature.

INSULATION SYSTEM WITH CLOSED AIR GAP

A thermal insulation system similar to that with a closed air gap is possible. Attention should be paid to the fact that the movement of air in the interlayer is necessary only to remove moisture. If we solve the problem of removing moisture in a different way, without ventilation, we get a thermal insulation system with a closed air gap without the above disadvantages.

To solve the problem, the thermal insulation system should have the form shown in Fig. 1. Thermal insulation of the building should be performed with vapor-permeable inserts made of thermal insulation material, such as mineral wool. The thermal insulation system must be arranged in such a way that steam is removed from the interlayer, and inside it the humidity is below the dew point in the interlayer.

1 - building wall; 2 - fasteners; 3 - heat-insulating panels; 4 - steam and heat-insulating inserts

Rice. one. Thermal insulation with vapor-permeable inserts

For the saturated vapor pressure in the interlayer, the following expression can be written:

Neglecting the thermal resistance of air in the interlayer, we determine the average temperature inside the interlayer by the formula

(2)

where T in, Tout- air temperature inside the building and outside air, respectively, about С;

R 1 , R 2 - resistance to heat transfer of the wall and thermal insulation, respectively, m 2 × o C / W.

For steam migrating from the room through the wall of the building, you can write the equation:

(3)

where Pin, P– partial vapor pressure in the room and interlayer, Pa;

S 1 - the area of ​​​​the outer wall of the building, m 2;

k pp1 - coefficient of vapor permeability of the wall, equal to:

here R pp1 = m 1 / l 1 ;

m 1 - vapor permeability coefficient of the wall material, mg / (m × h × Pa);

l 1 - wall thickness, m.

For steam migrating from the air gap through vapor-permeable inserts in the thermal insulation of a building, the following equation can be written:

(5)

where P out– partial vapor pressure in the outside air, Pa;

S 2 - the area of ​​vapor-permeable heat-insulating inserts in the thermal insulation of the building, m 2;

k pp2 - coefficient of vapor permeability of inserts, equal to:

here R pp2 \u003d m 2 / l 2 ;

m 2 - coefficient of vapor permeability of the material of the vapor-permeable insert, mg / (m × h × Pa);

l 2 – insert thickness, m.

Equating the right parts of equations (3) and (5) and solving the resulting equation for the vapor balance in the interlayer with respect to P, we obtain the value of vapor pressure in the interlayer in the form:

(7)

where e = S 2 /S 1 .

Having written the condition for the absence of moisture condensation in the air gap in the form of an inequality:

and solving it, we obtain the required value of the ratio of the total area of ​​vapor-permeable inserts to the area of ​​the wall:

Table 1 shows the data obtained for some options for enclosing structures. It was assumed in the calculations that the coefficient of thermal conductivity of the vapor-permeable insert is equal to the coefficient of thermal conductivity of the main thermal insulation in the system.

Table 1. Value of ε for various wall options

The examples given in Table 1 show that it is possible to design thermal insulation with a closed air gap between the thermal insulation and the wall of the building. For some wall structures, as in the first example from Table 1, vapor-permeable inserts can be dispensed with. In other cases, the area of ​​vapor-permeable inserts may be insignificant compared to the area of ​​the insulated wall.

THERMAL INSULATION SYSTEM WITH CONTROLLED THERMAL TECHNICAL CHARACTERISTICS

The design of thermal insulation systems has undergone significant development over the past fifty years, and today designers have a wide choice of materials and designs at their disposal, from the use of straw to vacuum thermal insulation. It is also possible to use active thermal insulation systems, the features of which allow them to be included in the energy supply system of buildings. In this case, the properties of the thermal insulation system can also change depending on the environmental conditions, ensuring a constant level of heat loss from the building, regardless of the outside temperature.

If you set a fixed level of heat loss Q through the building envelope, the required value of the reduced resistance to heat transfer will be determined by the formula

(10)

Such properties can be possessed by a heat-insulating system with a transparent outer layer or with a ventilated air gap. In the first case, solar energy is used, and in the second, the heat energy of the ground can be additionally used together with the ground heat exchanger.

In a system with transparent thermal insulation at a low position of the sun, its rays pass to the wall almost without loss, heat it, thereby reducing heat loss from the room. In summer, when the sun is high above the horizon, the sun's rays are almost completely reflected from the wall of the building, thereby preventing the building from overheating. In order to reduce the reverse heat flow, the heat-insulating layer is made in the form of a honeycomb structure, which plays the role of a trap for sunlight. The disadvantage of such a system is the impossibility of redistributing energy along the facades of the building and the absence of an accumulative effect. In addition, the efficiency of this system directly depends on the level of solar activity.

According to the authors, an ideal thermal insulation system should, to some extent, resemble a living organism and change its properties over a wide range depending on environmental conditions. When the outside temperature drops, the thermal insulation system should reduce heat loss from the building, and when the outside temperature rises, its thermal resistance may decrease. During the summer, solar energy input into the building should also depend on outdoor conditions.

The thermal insulation system proposed in many respects has the properties formulated above. On fig. 2a shows a diagram of the wall with the proposed thermal insulation system, in fig. 2b - temperature graph in the heat-insulating layer without and with the presence of an air gap.

The heat-insulating layer is made with a ventilated air gap. When air moves in it with a temperature higher than at the corresponding point on the graph, the value of the temperature gradient in the thermal insulation layer from the wall to the interlayer decreases compared to thermal insulation without an interlayer, which reduces heat loss from the building through the wall. At the same time, it should be borne in mind that the decrease in heat loss from the building will be compensated by the heat given off by the air flow in the interlayer. That is, the air temperature at the outlet of the interlayer will be less than at the inlet.

Rice. 2. Scheme of the thermal insulation system (a) and temperature graph (b)

The physical model of the problem of calculating heat losses through a wall with an air gap is shown in fig. 3. The heat balance equation for this model has the following form:

Rice. 3. Calculation scheme of heat loss through the building envelope

When calculating heat flows, the conductive, convective and radiative mechanisms of heat transfer are taken into account:

where Q 1 - heat flow from the room to the inner surface of the building envelope, W / m 2;

Q 2 - heat flow through the main wall, W / m 2;

Q 3 - heat flow through the air gap, W/m2;

Q 4 – heat flux through the thermal insulation layer behind the interlayer, W/m 2 ;

Q 5 - heat flow from the outer surface of the enclosing structure into the atmosphere, W / m 2;

T 1 , T 2, - temperature on the wall surface, o C;

T 3 , T 4 – temperature on the interlayer surface, о С;

Tk, T a- temperature in the room and outside air, respectively, about С;

s is the Stefan-Boltzmann constant;

l 1, l 2 - thermal conductivity of the main wall and thermal insulation, respectively, W / (m × o C);

e 1 , e 2 , e 12 - the emissivity of the inner surface of the wall, the outer surface of the thermal insulation layer and the reduced emissivity of the surfaces of the air gap, respectively;

a in, a n, a 0 - heat transfer coefficient on the inner surface of the wall, on the outer surface of the thermal insulation and on the surfaces limiting the air gap, respectively, W / (m 2 × o C).

Formula (14) is written for the case when the air in the interlayer is stationary. In the case when air with a temperature T u instead of Q 3, two flows are considered: from the blown air to the wall:

and from the blown air to the screen:

Then the system of equations splits into two systems:

The heat transfer coefficient is expressed in terms of the Nusselt number:

where L- characteristic size.

Formulas for calculating the Nusselt number were taken depending on the situation. When calculating the heat transfer coefficient on the inner and outer surfaces of the enclosing structures, the following formulas were used:

where Ra= Pr×Gr – Rayleigh criterion;

Gr= g×b ×D T× L 3 /n 2 is the Grashof number.

When determining the Grashof number, the difference between the wall temperature and the ambient air temperature was chosen as a characteristic temperature drop. For the characteristic dimensions were taken: the height of the wall and the thickness of the layer.

When calculating the heat transfer coefficient a 0 inside a closed air gap, the following formula was used to calculate the Nusselt number:

(22)

If the air inside the interlayer was moving, a simpler formula was used to calculate the Nusselt number from:

(23)

where Re = v×d /n is the Reynolds number;

d is the thickness of the air gap.

The values ​​of the Prandtl number Pr, kinematic viscosity n and the coefficient of thermal conductivity of air l in depending on the temperature were calculated by linear interpolation of tabular values ​​from . Systems of equations (11) or (19) were solved numerically by iterative refinement with respect to temperatures T 1 , T 2 , T 3 , T 4 . For numerical simulation, a thermal insulation system based on thermal insulation similar to expanded polystyrene with a thermal conductivity coefficient of 0.04 W/(m 2 × o C) was chosen. The air temperature at the inlet of the interlayer was assumed to be 8 ° C, the total thickness of the heat-insulating layer was 20 cm, the thickness of the interlayer d- 1 cm.

On fig. 4 shows graphs of specific heat loss through the insulating layer of a conventional heat insulator in the presence of a closed heat-insulating layer and with a ventilated air layer. A closed air gap almost does not improve the properties of thermal insulation. For the considered case, the presence of a heat-insulating layer with a moving air flow more than doubles the heat loss through the wall at an outdoor temperature of minus 20 ° C. The equivalent value of the heat transfer resistance of such heat insulation for this temperature is 10.5 m 2 × ° C / W, which corresponds to the layer expanded polystyrene with a thickness of more than 40.0 cm.

D d= 4 cm with still air; row 3 - air speed 0.5 m/s

Rice. 4. Graphs of dependence of specific heat losses

The effectiveness of the thermal insulation system increases as the outdoor temperature decreases. At an outside air temperature of 4 ° C, the efficiency of both systems is the same. A further increase in temperature makes the use of the system inappropriate, as it leads to an increase in the level of heat loss from the building.

On fig. 5 shows the dependence of the temperature of the outer surface of the wall on the temperature of the outside air. According to fig. 5, the presence of an air gap increases the temperature of the outer surface of the wall at a negative outdoor temperature compared to conventional thermal insulation. This is because the moving air gives off its heat to both the inner and outer layers of thermal insulation. At high outside air temperatures, such a thermal insulation system plays the role of a cooling layer (see Fig. 5).

Row 1 - ordinary thermal insulation, D= 20 cm; row 2 - in the thermal insulation there is an air gap 1 cm wide, d= 4 cm, air speed 0.5 m/s

Rice. 5. The dependence of the temperature of the outer surface of the wallfrom the outside air temperature

On fig. 6 shows the dependence of the temperature at the outlet of the interlayer on the temperature of the outside air. The air in the interlayer, cooling down, gives up its energy to the enclosing surfaces.

Rice. 6. Dependence of the temperature at the exit of the interlayerfrom the outside air temperature

On fig. 7 shows the dependence of heat loss on the thickness of the outer layer of thermal insulation at a minimum outdoor temperature. According to fig. 7, the minimum heat loss is observed at d= 4 cm.

Rice. 7. The dependence of heat loss on the thickness of the outer layer of thermal insulation at minimum outside temperature

On fig. 8 shows the dependence of heat loss for an outside temperature of minus 20 ° C on the air velocity in an interlayer with different thicknesses. The rise in air velocity above 0.5 m/s does not significantly affect the properties of thermal insulation.

Row 1 - d= 16 cm; row 2 - d= 18 cm; row 3 - d= 20 cm

Rice. eight. Dependence of heat loss on air speedwith different thickness of the air layer

Attention should be paid to the fact that a ventilated air layer allows you to effectively control the level of heat loss through the wall surface by changing the air velocity in the range from 0 to 0.5 m/s, which is impossible for conventional thermal insulation. On fig. Figure 9 shows the dependence of the air velocity on the outside temperature for a fixed level of heat loss through the wall. This approach to the thermal protection of buildings makes it possible to reduce the energy intensity of the ventilation system as the outdoor temperature rises.

Rice. nine. Dependence of air velocity on outdoor temperature for a fixed level of heat loss

When creating the thermal insulation system considered in the article, the main issue is the source of energy to increase the temperature of the pumped air. As such a source, it is supposed to take the heat of the soil under the building by using a soil heat exchanger. For more efficient use of soil energy, it is assumed that the ventilation system in the air gap should be closed, without atmospheric air suction. Since the temperature of the air entering the system in winter is lower than the ground temperature, the problem of moisture condensation does not exist here.

The authors see the most effective use of such a system in the combination of the use of two energy sources: solar and ground heat. If we turn to the previously mentioned systems with a transparent heat-insulating layer, it becomes obvious that the authors of these systems strive to implement the idea of ​​a thermal diode in one way or another, that is, to solve the problem of directional transfer of solar energy to the building wall, while taking measures to prevent the movement of heat energy flow in the opposite direction. direction.

A dark-colored metal plate can act as an outer absorbing layer. And the second absorbing layer can be an air gap in the thermal insulation of the building. The air moving in the layer, closing through the ground heat exchanger, in sunny weather heats the ground, accumulating solar energy and redistributing it over the facades of the building. Heat from the outer layer to the inner layer can be transferred using thermal diodes made on heat pipes with phase transitions.

Thus, the proposed thermal insulation system with controlled thermophysical characteristics is based on a structure with a thermal insulation layer having three features:

- a ventilated air layer parallel to the building envelope;

is the energy source for the air inside the interlayer;

– a system for controlling the parameters of the air flow in the interlayer depending on the external weather conditions and the air temperature in the room.

One of the possible design options is the use of a transparent thermal insulation system. In this case, the thermal insulation system must be supplemented with another air gap adjacent to the wall of the building and communicating with all the walls of the building, as shown in Fig. ten.

The thermal insulation system shown in fig. 10 has two air spaces. One of them is located between the thermal insulation and the transparent fence and serves to prevent the building from overheating. For this purpose, there are air valves connecting the interlayer to the outside air at the top and bottom of the thermal insulation panel. In the summer and at times of high solar activity, when there is a danger of overheating of the building, the dampers open, providing ventilation with outside air.

Rice. ten. Transparent thermal insulation system with ventilated air gap

The second air gap is adjacent to the wall of the building and serves to transport solar energy in the building envelope. Such a design will allow the use of solar energy by the entire surface of the building during daylight hours, providing, moreover, an effective accumulation of solar energy, since the entire volume of the walls of the building acts as an accumulator.

It is also possible to use traditional thermal insulation in the system. In this case, a ground heat exchanger can serve as a source of thermal energy, as shown in Fig. eleven.

Rice. eleven. Thermal insulation system with ground heat exchanger

As another option, building ventilation emissions can be proposed for this purpose. In this case, to prevent moisture condensation in the interlayer, it is necessary to pass the removed air through the heat exchanger, and let the outside air heated in the heat exchanger into the interlayer. From the interlayer, air can enter the room for ventilation. The air is heated, passing through the ground heat exchanger, and gives up its energy to the building envelope.

A necessary element of the thermal insulation system should be an automatic control system for its properties. On fig. 12 is a block diagram of the control system. The control is based on the analysis of information from temperature and humidity sensors by changing the operating mode or turning off the fan and opening and closing the air dampers.

Rice. 12. Block diagram of the control system

The block diagram of the operation algorithm of the ventilation system with controlled properties is shown in fig. thirteen.

At the initial stage of operation of the control system (see Fig. 12), the temperature in the air gap for the still air condition is calculated from the measured values ​​of the outdoor and indoor temperatures in the control unit. This value is compared with the air temperature in the layer of the southern facade during the design of the thermal insulation system, as in Fig. 10, or in a ground heat exchanger - when designing a thermal insulation system, as in fig. 11. If the calculated temperature is greater than or equal to the measured temperature, the fan remains off and the air dampers in the interlayer are closed.

Rice. thirteen. Block diagram of the ventilation system operation algorithm with managed properties

If the calculated temperature is less than the measured one, turn on the circulation fan and open the dampers. In this case, the energy of the heated air is given to the wall structures of the building, reducing the need for thermal energy for heating. At the same time, the value of air humidity in the interlayer is measured. If the humidity approaches the dew point, a damper opens, connecting the air gap with the outside air, which ensures that moisture does not condense on the surface of the walls of the gap.

Thus, the proposed system of thermal insulation allows you to really control the thermal properties.

TESTING THE LAYOUT OF THE THERMAL INSULATION SYSTEM WITH CONTROLLED THERMAL INSULATION BY USING THE BUILDING VENTILATION EMISSIONS

The scheme of the experiment is shown in fig. 14. The layout of the thermal insulation system is mounted on the brick wall of the room in the upper part of the elevator shaft. The layout consists of thermal insulation representing vapor-tight heat-insulating plates (one surface is aluminum 1.5 mm thick; the second is aluminum foil) filled with polyurethane foam 3.0 cm thick with a thermal conductivity coefficient of 0.03 W / (m 2 × o C). Heat transfer resistance of the plate - 1.0 m 2 × o C / W, brick wall - 0.6 m 2 × o C / W. Between the heat-insulating plates and the surface of the building envelope there is an air gap 5 cm thick. In order to determine the temperature regimes and the movement of heat flow through the building envelope, temperature and heat flow sensors were installed in it.

Rice. fourteen. Scheme of an experimental system with controlled thermal insulation

A photograph of the installed thermal insulation system with energy supply from the ventilation exhaust heat recovery system is shown in fig. fifteen.

Additional energy inside the layer is supplied with air taken at the outlet of the heat recovery system of the ventilation emissions of the building. Ventilation emissions were taken from the exit of the ventilation shaft of the building of the State Enterprise “Institute NIPTIS named after A.I. Ataeva S.S., were fed to the first input of the recuperator (see Fig. 15a). Air was supplied from the ventilation layer to the second inlet of the recuperator, and again to the ventilation layer from the second outlet of the recuperator. Ventilation exhaust air cannot be supplied directly into the air gap due to the danger of moisture condensation inside it. Therefore, the ventilation emissions of the building first passed through the heat exchanger-recuperator, the second inlet of which received air from the interlayer. In the heat exchanger, it was heated up and, with the help of a fan, was supplied to the air gap of the ventilation system through a flange mounted at the bottom of the heat-insulating panel. Through the second flange in the upper part of the thermal insulation, the air was removed from the panel and closed the cycle of its movement at the second inlet of the heat exchanger. In the process of work, the information received from the temperature and heat flow sensors installed according to the scheme of Fig. 1 was recorded. fourteen.

A special control and data processing unit was used to control the operation modes of the fans and to record and record the parameters of the experiment.

On fig. 16 shows graphs of temperature changes: outdoor air, indoor air and air in different parts of the layer. From 7.00 to 13.00 hours the system enters the stationary mode of operation. The difference between the temperature at the air inlet to the interlayer (sensor 6) and the temperature at its outlet (sensor 5) turned out to be about 3°C, which indicates the consumption of energy from the passing air.

Rice. sixteen. Temperature charts: a - outdoor air and indoor air;b - air in various parts of the interlayer

On fig. 17 shows graphs of the time dependence of the temperature of the wall surfaces and thermal insulation, as well as the temperature and heat flow through the enclosing surface of the building. On fig. 17b, a decrease in the heat flux from the room is clearly recorded after the supply of heated air to the ventilation layer.

Rice. 17. Graphs versus time: a - temperature of the surfaces of the wall and thermal insulation;b - temperature and heat flow through the enclosing surface of the building

The experimental results obtained by the authors confirm the possibility of controlling the properties of thermal insulation with a ventilated layer.

CONCLUSION

1 An important element of energy efficient buildings is its shell. The main directions for the development of reducing the heat loss of buildings through building envelopes are associated with active thermal insulation, when the building envelope plays an important role in shaping the parameters of the internal environment of the premises. The most obvious example is a building envelope with an air gap.

2 The authors proposed a thermal insulation design with a closed air gap between the thermal insulation and the wall of the building. In order to prevent moisture condensation in the air layer without reducing the heat-insulating properties, the possibility of using vapor-permeable inserts in thermal insulation is considered. A method has been developed for calculating the area of ​​inserts depending on the conditions of use of thermal insulation. For some wall structures, as in the first example from Table 1, vapor-permeable inserts can be dispensed with. In other cases, the area of ​​vapor-permeable inserts may be insignificant relative to the area of ​​the insulated wall.

3 A method for calculating thermal characteristics and design of a thermal insulation system with controlled thermal properties have been developed. The design is made in the form of a system with a ventilated air gap between two layers of thermal insulation. When moving in an air layer with a temperature higher than at the corresponding point of the wall with a conventional thermal insulation system, the magnitude of the temperature gradient in the thermal insulation layer from the wall to the layer decreases compared to thermal insulation without a layer, which reduces heat loss from the building through the wall. As energy for increasing the temperature of the pumped air, it is possible to use the heat of the soil under the building, using a soil heat exchanger, or solar energy. Methods for calculating the characteristics of such a system have been developed. Experimental confirmation of the reality of using a thermal insulation system with controlled thermal characteristics for buildings has been obtained.

BIBLIOGRAPHY

1. Bogoslovsky, V. N. Construction thermal physics / V. N. Bogoslovsky. - St. Petersburg: AVOK-NORTH-WEST, 2006. - 400 p.

2. Thermal insulation systems for buildings: TKP.

4. Design and installation of an insulation system with a ventilated air gap based on three-layer facade panels: R 1.04.032.07. - Minsk, 2007. - 117 p.

5. Danilevsky, LN On the issue of reducing the level of heat loss in a building. Experience of Belarusian-German cooperation in construction / LN Danilevsky. - Minsk: Strinko, 2000. - S. 76, 77.

6. Alfred Kerschberger "Solares Bauen mit transparenter Warmedammung". Systeme, Wirtschaftlichkeit, Perspektiven, BAUVERLAG GMBH, WEISBADEN UND BERLIN.

7. Die ESA-Solardassade – Dammen mit Licht / ESA-Energiesysteme, 3. Passivhaustagung 19 bis 21 Februar 1999. Bregenz. -R. 177–182.

8. Peter O. Braun, Innovative Gebaudehullen, Warmetechnik, 9, 1997, pp. 510–514.

9. Passive house as an adaptive life support system: abstracts of the Intern. scientific and technical conf. “From the thermal rehabilitation of buildings to the passive house. Problems and solutions” / L. N. Danilevsky. - Minsk, 1996. - S. 32-34.

10. Thermal insulation with controlled properties for buildings with low heat loss: Sat. tr. / SE "NIPTIS Institute named after. Ataeva S. S. "; L. N. Danilevsky. - Minsk, 1998. - S. 13-27.

11. Danilevsky, L. Thermal insulation system with controlled properties for a passive house / L. Danilevsky // Architecture and construction. - 1998. - No. 3. - S. 30, 31.

12. O. G. Martynenko, Free Convective Heat Transfer. Reference book / O. G. Martynenko, Yu. A. Sokovishin. - Minsk: Science and technology, 1982. - 400 p.

13. Mikheev, M. A. Fundamentals of heat transfer / M. A. Mikheev, I. M. Mikheeva. – M.: Energy, 1977. – 321 p.

14. External ventilated enclosure of the building: Pat. 010822 Evraz. Patent Office, IPC (2006.01) Е04В 2/28, Е04В 1/70 / L. N. Danilevsky; applicant State Enterprise "NIPTIS Institute named after Ataeva S.S. - No. 20060978; dec. 05.10.2006; publ. December 30, 2008 // Bull. Eurasian Patent Office. - 2008. - No. 6.

15. External ventilated enclosure of the building: Pat. 11343 Rep. Belarus, IPC (2006) E04B1 / 70, E04B2 / 28 / L. N. Danilevsky; applicant State Enterprise "NIPTIS Institute named after Ataeva S.S. - No. 20060978; dec. 05.10.2006; publ. 12/30/2008 // Afitsyyny bul. / National center intellectual. Ulasnastsi. – 2008.

Initial data for the layers of enclosing structures;
- wooden floor(grooved board); δ 1 = 0.04 m; λ 1 \u003d 0.18 W / m ° C;
- vapor barrier; insignificant.
- air gap: Rpr = 0.16 m2 °C/W; δ 2 \u003d 0.04 m λ 2 \u003d 0.18 W / m ° С; ( Thermal resistance of a closed air gap >>>.)
- insulation(styrofoam); δ ut = ? m; λ ut = 0.05 W/m °С;
- draft floor(board); δ 3 = 0.025 m; λ 3 \u003d 0.18 W / m ° С;

As we have already noted, to simplify the heat engineering calculation, a multiplying factor ( k), which approximates the value of the calculated thermal resistance to the recommended thermal resistances of enclosing structures; for basement and basement floors, this coefficient is 2.0. The required heat resistance is calculated based on the fact that the outdoor air temperature (in the subfield) is equal to; - 10°C. (however, everyone can set the temperature that he considers necessary for his particular case).

We believe:

Where Rtr- required thermal resistance,
tv- design temperature of internal air, °С. It is accepted according to SNiP and equals 18 ° С, but since we all love warmth, we suggest raising the temperature of the internal air to 21 ° С.
tn- design temperature of the outside air, °C, equal to the average temperature of the coldest five-day period in a given construction area. We offer the temperature in the subfield tn accept "-10°C", this is of course a large margin for the Moscow region, but here, in our opinion, it is better to re-mortgage than not to count. Well, if you follow the rules, then the outdoor temperature tn is taken in accordance with SNiP "Construction climatology". Also, the required standard value can be found in local construction organizations, or regional departments of architecture.
δt n α c- the product in the denominator of the fraction is: 34.8 W / m2 - for external walls, 26.1 W / m2 - for coatings and attic floors, 17.4 W / m2 ( in our case) - for basement ceilings.

Now we calculate the thickness of the insulation from extruded polystyrene foam (styrofoam).

Whereδ ut - insulation layer thickness, m;
δ 1 …… δ 3 - thickness of individual layers of enclosing structures, m;
λ 1 …… λ 3 - thermal conductivity coefficients of individual layers, W / m ° С (see the Builder's Handbook);
Rpr - thermal resistance of the air gap, m2 °С/W. If air is not provided in the enclosing structure, then this value is excluded from the formula;
α in, α n - heat transfer coefficients of the inner and outer surfaces of the floor, equal to 8.7 and 23 W/m2 °C, respectively;
λ ut - coefficient of thermal conductivity of the insulating layer(in our case, styrofoam is extruded polystyrene foam), W / m ° С.

Conclusion; In order to meet the requirements for the temperature regime of the operation of the house, the thickness of the insulation layer of polystyrene foam boards located in the basement floor over wooden beams (beam thickness 200 mm) must be at least 11 cm. Since we initially set too high parameters, the options may be as follows; it is either a cake of two layers of 50 mm Styrofoam boards (minimum), or a cake of four layers of 30 mm Styrofoam boards (maximum).

Construction of houses in the Moscow region:
- Building a house from a foam block in the Moscow region. The thickness of the walls of the house from foam blocks >>>
- Calculation of the thickness of brick walls during the construction of a house in the Moscow region. >>>
- Construction of a wooden log house in the Moscow region. The thickness of the wall of a timber house. >>>

Note. When pasting one or both surfaces of the air gap with aluminum foil, the thermal resistance should be increased by 2 times.

Application 5*

Schemes of heat-conducting inclusions in enclosing structures

Application 6*

(Informative)

Reduced heat transfer resistance of windows, balcony doors and skylights

* in steel bindings

Notes:

1. Soft selective glass coatings include coatings with thermal emission less than 0.15, and hard ones - more than 0.15.

2. The values ​​of the reduced resistance to heat transfer of the fillings of the light openings are given for cases where the ratio of the glazing area to the filling area of ​​the light opening is 0.75.

The values ​​of the reduced heat transfer resistances indicated in the table may be used as design values ​​in the absence of such values ​​in the standards or technical specifications for structures or not confirmed by test results.

3. The temperature of the inner surface of the structural elements of the windows of buildings (except for industrial ones) must be at least 3 ° C at the design temperature of the outside air.

.
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.

The gaps available to air flows are vents that worsen the thermal insulation characteristics of the walls. Closed gaps (as well as closed pores of foamed material) are heat-insulating elements. Windproof voids are widely used in construction to reduce heat loss through building envelopes (slots in bricks and blocks, channels in concrete panels, gaps in double-glazed windows, etc.). Voids in the form of windproof air layers are also used in the walls of baths, including frame ones. These voids are often the main elements of thermal protection. In particular, it is the presence of voids on the hot side of the wall that makes it possible to use low-melting foam plastics (expanded polystyrene and polyethylene foam) in the deep zones of the walls of high-temperature baths.

At the same time, the voids in the walls are the most insidious elements. It is worth disturbing the wind insulation in the slightest degree, and the entire system of voids can become a single blown cooling air, turning off all external heat-insulating layers from the wall thermal insulation system. Therefore, they try to make voids small in size and are guaranteed to be isolated from each other.

It is impossible to use the concept of thermal conductivity of air (and even more so to use the ultra-low value of the thermal conductivity of still air 0.024 W/m deg) to assess the processes of heat transfer through real air, since air in large voids is an extremely mobile substance. Therefore, in practice, for thermotechnical calculations of heat transfer processes, even through conditionally "stationary" air, empirical (experimental, experimental) ratios are used. Most often (in the simplest cases) in the theory of heat transfer, it is considered that the heat flux from air to the surface of a body in air is equal to Q = α∆T, where α - empirical heat transfer coefficient of "still" air, ∆T- the temperature difference between the surface of the body and air. Under normal conditions of residential premises, the heat transfer coefficient is approximately equal to α = 10 W/m² deg. It is this figure that we will adhere to when estimating the heating of the walls and the human body in the bath. With the help of air flows with a speed V (m / s), the heat flow increases by the value of the convective component Q=βV∆T, where β approximately equal to 6 W sec/m³ deg. All quantities depend on the spatial orientation and surface roughness. So, according to the current norms of SNiP 23-02-2003, the heat transfer coefficient from air to the internal surfaces of enclosing structures is assumed to be 8.7 W / m² deg for walls and smooth ceilings with slightly protruding ribs (with the ratio of the height of the ribs "h" to the distance "a » between faces of adjacent edges h/a< 0,3); 7,6 Вт/м² град для потолков с сильно выступающими рёбрами (при отношении h/a >0.3); 8.0 W/m² deg for windows and 9.9 W/m² deg for skylights. Finnish experts take the heat transfer coefficient in the “still” air of dry saunas to be 8 W/m² deg (which, within measurement errors, coincides with our value) and 23 W/m² deg in the presence of air flows with an average speed of 2 m/sec.

Such a low value of the heat transfer coefficient in conditionally "still" air α = 10 W/m² hail corresponds to the concept of air as a heat insulator and explains the need to use high temperatures in saunas to quickly warm the human body. With regard to walls, this means that with characteristic heat losses through the walls of the bath (50-200) W / m², the difference in air temperatures in the bath and the temperatures of the internal surfaces of the walls of the bath can reach (5-20) ° С. This is a very large value, often not taken into account by anyone. The presence of strong air convection in the bath makes it possible to reduce the temperature drop by half. Note that such high temperature differences, characteristic of baths, are unacceptable in residential premises. Thus, the temperature difference between air and walls, normalized in SNiP 23-02-2003, should not exceed 4 ° C in residential premises, 4.5 ° C in public and 12 ° C in industrial premises. Higher temperature differences in residential premises inevitably lead to cold sensations from the walls and dew on the walls.

Using the introduced concept of the heat transfer coefficient from the surface to the air, the voids inside the wall can be considered as a sequential arrangement of heat transfer surfaces (see Fig. 35). The near-wall air zones, where the above temperature differences ∆T are observed, are called boundary layers. If there are two void gaps in the wall (or double-glazed window) (for example, three glasses), then in fact there are 6 boundary layers. If a heat flux of 100 W / m² passes through such a wall (or a double-glazed window), then on each boundary layer the temperature changes by ∆T = 10°С, and on all six layers the temperature difference is 60°C. Given that the heat fluxes through each individual boundary layer and through the entire wall as a whole are equal to each other and still amount to 100 W / m², the resulting heat transfer coefficient for a wall without voids (“insulating glass unit” with one glass) will be 5 W / m² hail, for a wall with one hollow layer (double-glazed window with two glasses) 2.5 W / m² hail, and with two hollow layers (double-glazed window with three glasses) 1.67 W / m² hail. That is, the more voids (or the more glass), the warmer the wall. At the same time, the thermal conductivity of the wall material itself (glasses) in this calculation was assumed to be infinitely large. In other words, even from a very “cold” material (for example, steel), it is possible in principle to make a very warm wall, providing only for the presence of many air layers in the wall. Actually, all glass windows work on this principle.

To simplify the estimated calculations, it is more convenient to use not the heat transfer coefficient α, but its reciprocal value - the resistance to heat transfer (thermal resistance of the boundary layer) R = 1/α. The thermal resistance of two boundary layers corresponding to one layer of wall material (one glass) or one air gap (interlayer) is equal to R = 0.2 m² deg/W, and three layers of wall material (as in Figure 35) - the sum of the resistances of six boundary layers, that is, 0.6 m² deg / W. From the definition of the concept of resistance to heat transfer Q=∆T/R it follows that with the same heat flux of 100 W/m² and thermal resistance of 0.6 m² deg/W, the temperature drop on the wall with two air layers will be the same 60°C. If the number of air layers is increased to nine, then the temperature drop on the wall with the same heat flux of 100 W/m² will be 200°C, that is, the calculated temperature of the inner surface of the wall in the bath with a heat flux of 100 W/m² will increase from 60 °C to 200°С (if it is 0°С outside).

The heat transfer coefficient is the resulting indicator that comprehensively sums up the consequences of all physical processes occurring in the air near the surface of a heat-releasing or heat-receiving body. At small temperature differences (and low heat fluxes), convective air flows are small, heat transfer mainly occurs conductively due to the thermal conductivity of still air. The thickness of the boundary layer would be small, only a=λR=0.0024 m, where λ=0.024 W/m deg- coefficient of thermal conductivity of still air, R=0.1 m²grad/W-thermal resistance of the boundary layer. Within the boundaries of the boundary layer, the air has different temperatures, as a result of which, due to gravitational forces, the air at the hot vertical surface begins to rise (and at the cold one it sinks), picking up speed, and turbulizes (swirls). Due to the vortices, the heat transfer of air increases. If the contribution of this convective component is formally introduced into the value of the thermal conductivity coefficient λ, then an increase in this thermal conductivity coefficient will correspond to a formal increase in the thickness of the boundary layer a=λR(as we will see below, about 5-10 times from 0.24 cm to 1-3 cm). It is clear that this formally increased thickness of the boundary layer corresponds to the dimensions of air flows and eddies. Without delving into the subtleties of the structure of the boundary layer, we note that it is much more important to understand that the heat transferred to the air can “fly away” upwards with a convective flow without reaching the next plate of a multilayer wall or the next glass of an insulating glass unit. This corresponds to the case of calorific air heating, which will be considered below in the analysis of shielded metal furnaces. Here we consider the case when the air flows in the interlayer have a limited height, for example, 5–20 times greater than the thickness of the interlayer δ. In this case, circulation flows arise in the air layers, which actually participate in the heat transfer together with conductive heat flows.

At small thicknesses of the air gaps, the oncoming air flows at the opposite walls of the gap begin to influence each other (they mix). In other words, the thickness of the air gap becomes less than two undisturbed boundary layers, as a result of which the heat transfer coefficient increases, and the heat transfer resistance decreases accordingly. In addition, at elevated temperatures of the walls of the air spaces, the processes of heat transfer by radiation begin to play a role. Updated data in accordance with the official recommendations of SNiP P-3-79 * are given in Table 7, which shows that the thickness of the undisturbed boundary layers is 1-3 cm, but a significant change in heat transfer occurs only when the thickness of the air gaps is less than 1 cm. This means that in particular, that the air gaps between panes in an insulating glass unit should not be less than 1 cm thick.

Table 7 Thermal resistance of a closed air layer, m² deg/W

Their table 7 also shows that warmer air layers have lower thermal resistances (better pass heat through themselves). This is explained by the influence of the radiative mechanism on heat transfer, which we will consider in the next section. Note that the viscosity of air increases with temperature, so that warm air becomes less turbulent.

Rice. 36. . The designations are the same as in Figure 35. Due to the low thermal conductivity of the wall material, temperature drops occur ∆Тc = QRc, where Rc is the thermal resistance of the wall Rc = δc / λc(δc - wall thickness, λc - coefficient of thermal conductivity of the wall material). As c increases, the temperature drops ∆Tc decrease, but the temperature drops on the boundary layers ∆T remain unchanged. This is illustrated by the distribution of Tint, referring to the case of a higher thermal conductivity of the wall material. Heat flow through the entire wall Q = ∆T/R = ∆Tc/Rc = (Tin - Text) /(3Rc+6R). The thermal resistance of the boundary layers R and their thickness a do not depend on the thermal conductivity of the wall material λc and their thermal resistance Rc.

Rice. 37.: a - three layers of metal (or glass) separated from each other with gaps of 1.5 cm, equivalent to wood (wooden board) 3.6 cm thick; b - five layers of metal with gaps of 1.5 cm, equivalent to wood 7.2 cm thick; c - three layers of plywood 4 mm thick with gaps of 1.5 cm, equivalent to wood 4.8 cm thick; d - three layers of polyethylene foam 4 mm thick with gaps of 1.5 cm, equivalent to wood 7.8 cm thick; e - three layers of metal with gaps of 1.5 cm filled with effective insulation (polystyrene foam, polyethylene foam or mineral wool), equivalent to wood 10.5 cm thick. gap sizes within (1-30) cm.

If the structural material of the wall has low thermal conductivity, then in the calculations it is necessary to take into account its contribution to the thermal resistance of the wall (Fig. 36). Although the contribution of voids, as a rule, is significant, filling all voids with effective insulation allows (due to the complete stop of air movement) to significantly (by 3-10 times) increase the thermal resistance of the wall (Fig. 37).

In itself, the possibility of obtaining warm walls quite suitable for baths (at least in summer) from several layers of “cold” metal is, of course, interesting and is used, for example, by the Finns for fire protection of walls in saunas near the stove. In practice, however, such a solution turns out to be very complicated due to the need for mechanical fixation of parallel metal layers with numerous jumpers, which play the role of undesirable cold “bridges”. One way or another, even one layer of metal or fabric "warms" if it is not blown by the wind. Tents, yurts, chums are based on this phenomenon, which, as you know, are still used (and have been used for centuries) as baths in nomadic conditions. So, one layer of fabric (it doesn’t matter what, as long as it is windproof) is only twice as “cold” as a brick wall 6 cm thick, and it warms up hundreds of times faster. However, the fabric of the tent remains much colder than the air in the tent, which does not allow for any long-term steam regimes. In addition, any (even small) tissue ruptures immediately lead to powerful convective heat losses.

The most important in the bath (as well as in residential buildings) are the air gaps in the windows. At the same time, the reduced heat transfer resistance of windows is measured and calculated for the entire area of ​​the window opening, that is, not only for the glass part, but also for the binding (wooden, steel, aluminum, plastic), which, as a rule, has better heat-insulating characteristics than glass. For orientation, we present the normative values ​​of the thermal resistance of windows of various types according to SNiP P-3-79 * and honeycomb materials, taking into account the thermal resistance of the outer boundary layers inside and outside the premises (see table 8).

Table 8 Reduced heat transfer resistance of windows and window materials