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11FACTORS THAT REDUCE THE HEAT-SHIELDING PROPERTIES OF
ENCLOSING STRUCTURES
Bakhromjon Adhamovich Dilfuza Tillavoldiyevna Madinakhon Tolib qizi Otakulov Sobirova Yokubova
Fergana Polytechnic Institute
ABSTRACT
This article is devoted to the issues of improving the energy efficiency of buildings and structures, energy resources, thermal insulation, energy efficiency in the design of buildings, increasing the thermal insulation of wall structures.
Keywords: fuel - energy resources, thermal insulation, energy efficiency
As building envelopes, external walls are exposed to a number of factors closely related to the processes taking place both outside the building and inside it. These factors include: precipitation, snow depth, depth of soil freezing, outdoor humidity, water vapor contained in the indoor air of a building, soil moisture, wind, solar radiation, the number of sunny and cloudy days per year, outdoor temperature, etc. temperature drops, chemically aggressive substances contained in the air and other factors.
Precipitation. Oblique rain with wind has the greatest negative impact on the outer walls of buildings. Rainwater can enter a wall through porous surface structures, holes, cracks, crevices and loose joints. Tops of walls and corners are exposed to the heaviest rainfall.
Defective gutters and pipes can also cause walls to get wet. Vertical seams of downpipes should be arranged on the opposite side of the wall to prevent water from entering the wall. The distance between the wall and the gutters must be at least 30 mm.
Incorrectly executed window slopes can also allow rainwater to enter the interior of the wall structure. The outer edges of window slopes should be at a distance of 30 mm from the wall, moreover, they should have a sufficient slope, not less than 30 ^Incorrectly executed vertical and horizontal joints of large-panel buildings contribute to the ingress of moisture into the three-layer panels during heavy rains, significantly reducing their operational properties.
Fire escapes, flagpoles, lamps, advertising posters, balcony railings, etc. must be installed in such a way that they do not direct rainwater down the wall.
Surface water on the ground, snow drifts and splashes of rainwater affect the plinth and the lower part of the façade. In order to neutralize the negative effects of this type of load, it is necessary to provide for the device of a slope of the land adjacent to the building.
Outside air humidity. The humidity of the outside air has a huge impact on the thermal properties of the enclosing structures. If the poured structures of building materials are filled with humid air or moisture penetrates them, then the thermal insulation qualities of these materials deteriorate significantly. The air always contains some moisture in the form of water vapor. Its amount contained in 1 m3 of air is measured in grams and is called absolute humidity (g / m3). However, absolute humidity does not characterize the degree of air saturation with moisture, since at different temperatures the maximum moisture content in the air is not the same. The higher the temperature, the more moisture can be in the air. Therefore, the concept of relative humidity is introduced, which is expressed as a percentage (%), as the ratio of the actual elasticity of water vapor e in air to its maximum elasticity at this temperature.
The amount of moisture evaporating from the surface of the fence depends on the relative humidity of the air. The higher the relative humidity of the air, the slower the evaporation takes place.
Extremely fast drying of the outer layers of enclosing structures and products, for example concrete, in the initial period of concrete setting can cause the formation of cracks and a significant decrease in the strength of the products. The appearance of cracks in the walls contributes to a decrease in their thermophysical characteristics during the operation of the building. At low relative air humidity, the drying of the outer layers of concrete occurs faster than the reaction of cement hydration proceeds, which will lead to a deterioration in the structural and mechanical properties of the outer layers of the product or structure.
When the air temperature of a given humidity rises, its relative humidity decreases. This is due to the fact that the water vapor pressure remains unchanged, and the maximum elasticity increases. Quite the opposite is observed when air is cooled: its relative humidity increases due to a decrease in maximum elasticity. At a certain temperature, the value will reach a value, and the air will acquire a relative humidity of 100%, that is, it will reach full saturation. The temperature at which air with a given water vapor pressure reaches full saturation is called the dew point.If you continue to cool the air below the dew point, then the limiting elasticity of water vapor will decrease, and the excess amount of water vapor actually present in the cooled air will condense, that is, turn into a droplet-liquid state.
In cold weather, prolonged thaws can occur when a mass of warm humid air invades. When mixed with cold air, it gradually cools, condenses moisture, which leads to the formation of fog.
Thermal insulation materials used in building envelopes have the ability to absorb moisture in a vaporous state from the surrounding air. This phenomenon is called sorption. Although the penetration of water vapor deep into the material occurs rather slowly and depends on the density of the material and the air temperature, over time, not
only the heat-shielding properties of the insulation, but also the durability and appearance of the enclosing structures may decrease.
Water vapor contained in the indoor air of a building. Water vapor is constantly generated in the interior of a building as a result of human activities (cooking, washing, bathing, washing floors, etc.). Humidity is especially high in newly built or renovated buildings. New designs can at times have exceptionally high moisture content due to structural moisture. The higher the temperature and the more efficient the ventilation, the faster the drying process of the structure takes place.The water vapor contained in the air inside the building, during diffusion and convective transfer, penetrates the wall structure and, cooling down to a temperature below the dew point, condenses. The amount of generated moisture is the higher, the greater the difference in temperatures outside and in the interior, therefore, in cold weather, moisture accumulates quite intensively in the wall. It should be understood that the moisture of the internal air can also pass into the wall structure together with air flows through various types of cracks, cracks and leaking joints and seams.
In order for the wall to not lose its heat-insulating ability and structural strength from year to year, it is necessary that all moisture that accumulates in the thickness of the wall in winter and summer should go outside.
The most reliable protection against water vapor is especially important in buildings with high humidity rooms: swimming pools, computer rooms, etc. Steam protection must be given special attention when building in areas with extremely cold climates (even with normal indoor humidity). The negative consequences of this phenomenon can be prevented either by using various design techniques (first of all, the device of ventilated gaps), or by including vapor barrier materials in the wall structure (from inside the room).
Soil moisture. In the absence of waterproofing, ground and sediment water in the foundation of the building can rise to the basement under the influence of capillary forces. In the event of an inadequate insulating material between the plinth and the wall structure, moisture can rise into the wall structure itself.
Wind. Wind currents, meeting an obstacle in the form of a building on the road, bypass it, as a result, areas of positive and negative pressure are formed around the building. Wind loads that increase along the height of the building must be taken into account when calculating the enclosing structures.
The effect of wind on buildings and residential buildings is quite strong. When the wind flow approaches the building, it begins to exert pressure on the part of the facade that faces it.As a result, on this side of the building, a zone of increased pressure or wind support is formed, in which cold air more intensively begins to penetrate through walls, windows, joints, cracks into residential premises, intensively cooling them. This phenomenon is called infiltration.
Having rounded the building, the wind flow continues to move, forming an area of reduced pressure or wind suction on the opposite side of the building. As a result of this, a significant pressure difference arises from the two fire sides of the house, which promotes the penetration of cold air into the room, more intensive air movement inside the house from the windward side to the opposite side. All this contributes to the formation of drafts that erode the heat from the rooms and contribute to a decrease in the temperature of the internal air and a sharp increase in heat losses in winter.
It should be noted that a strong wind, especially a hurricane, has a significant effect on the state of the building envelope and can lead to a significant decrease in their operational properties up to destruction.
Solar radiation. Different materials have different sensitivity to solar radiation. For example, solar radiation has practically no effect on ceramic tiles, as well as on materials made of metals without polymer coatings applied to them. On the other hand, paint and varnish coating materials are subject to very significant deterioration, which manifests itself in the form of paint cracking on the facade.Some materials do not change their physical properties, but they lose their visual appeal, for example, they fade (paints and some polymer coatings).
Therefore, when choosing a facing material for use in southern areas, you should make sure that it has sufficient lightfastness.
Outside air temperature and temperature differences. The design temperature of the outside air in the field of construction in the cold season significantly affects the choice of a constructive solution for the enclosing structures and the materials used. For thermotechnical calculations of enclosing structures, the average outside air temperatures are used: the average temperature of the coldest five-day period, the average temperature of the coldest day, the absolute minimum outside air temperature The average temperature of the coldest day is always lower than the average temperature of the coldest five-day period. The differences between the calculated outdoor temperatures need to be known in order to choose the right materials for thermal insulation of buildings. Heat loss by the enclosing structure occurs unevenly even during the day. At night, when the air is coldest, the temperature of the outer surface of the wall decreases as much as possible, and gradually the wall begins to cool down in thickness. The cooling rate of the structure depends on the ability to absorb and release heat. For enclosing structures with high thermal inertia, the design temperature of the outside air is taken equal to the average temperature of the coldest five-day period. The period of 5 days is adopted because its duration is sufficient for the low temperature of the external air, established during this period, to cause the maximum decrease in temperature on the inner surface of the wall.To cool down the enclosing structure of low inertia, one day is enough, therefore, for their heat engineering calculation, the average temperature of the coldest day is taken
In the form of enclosing structures, the external walls function in a rather tough regime, experiencing the influence of temperature differences. As a rule, the inner surface of the walls has a temperature close to that of the room. At the same time, the temperature of the outer surface varies within a fairly wide range from very significant negative values (on a winter, frosty night) to values close to 100 ° C (on a summer, sunny day). The temperature of the outer surface of the wall, at the same time, can be non-uniform due to the different illumination by the sun of different parts of it. But, as you know, all materials are subject to thermal stretching and compression to one degree or another. Therefore, in order to avoid deformations and destruction, it is very important that the materials "working" in a single structure have similar coefficients of thermal expansion, or appropriate technical solutions would be applied to ensure their joint work. Frequent, sometimes daily, temperature drops from plus to minus can carry a serious danger to wall materials. This tends to occur in areas with mild and humid winters. Therefore, in such climatic zones, it is necessary to pay close attention to such an important characteristic of materials as water absorption.With high water absorption at (positive temperatures) moisture penetrates and accumulates in the porous structures of the material, and with negative ones it freezes and, expanding, deforms the very structure of the material. As a result, there is a progressive destruction of the material, leading to the formation of cracks.
Chemically aggressive substances in the air. As a rule, in large cities or near large enterprises in the atmosphere, there is a fairly high concentration of chemically aggressive substances, for example, hydrogen sulfide and carbon dioxide. Therefore, for all elements of the building envelope in such areas, it is necessary to use materials that are resistant to chemicals present in the air.
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