Homeliness, Building
Resistance to heat transfer. Resistance to heat transfer of the enclosing structure
Heat transfer of enclosing structures is a complex process involving convection, thermal conductivity and radiation. All of them occur together with the predominance of one of them. The thermal insulation properties of fencing structures, which are reflected through the resistance to heat transfer, must comply with the current building codes.
How does heat exchange of air with enclosing structures take place?
In construction, the regulatory requirements are set for the amount of heat flow through the wall and through it the thickness is determined. One of the parameters for its calculation is the temperature difference between the outside and inside the room. The coldest time of the year is taken as a basis. Another parameter is the coefficient of heat transfer K - the amount of heat transferred per 1 s through an area of 1 m 2 , with a difference in the temperature of the external and internal environment of 1 ºC. The value of K depends on the properties of the material. As it decreases, the heat-shielding properties of the wall increase. In addition, the cold in the room will penetrate less if there is more thickness of the fence.
Convection and radiation from the outside and from the inside also affect the heat leakage from the house. Therefore, behind the batteries on the walls are installed reflecting screens of aluminum foil. Such protection is also done inside the ventilated facades from the outside.
Heat transfer through the walls of the house
The outer walls make up the maximum part of the area of the house and through them the energy losses reach 35-45%. Building materials, of which the enclosing structures are made , have different protection from the cold. Air has the lowest thermal conductivity. Therefore, porous materials have the lowest values of heat transfer coefficients. For example, in building bricks K = 0.81 W / (m 2 С C), for concrete K = 2.04 W / (m 2 С C), for plywood K = 0.18 W / (m 2 · O C), and for polystyrene plates K = 0.038 W / (m 2 · ° C).
In calculations, the inverse of the coefficient K is used, is the resistance to heat transfer of the enclosing structure. It is a normalized value and should not be lower than a certain set value, because it depends on the cost of heating and indoor conditions.
The coefficient K is affected by the humidity of the material of the enclosing structures. The raw material water displaces air from the pores, and its thermal conductivity is 20 times higher. As a result, the heat-shielding properties of the enclosure deteriorate. A wet brick wall flows 30% more heat compared to a dry one. Therefore, the facade and roofs of houses try to be lined with materials that do not hold water.
The loss of heat through the walls and the joints of the openings largely depend on the wind. The supporting structures are air-permeable, and the air passes through them from the outside (infiltration) and from the inside (exfiltration).
Facing of buildings
The external lining of ventilated facades is installed with a gap in which air circulates. It does not affect the resistance to the heat transfer of the walls, but it resists the wind load well, reducing infiltration. Air can penetrate into the junction of window and door frames with wall apertures. Because of this, the resistance to the heat transfer of the windows at the extremities is reduced. In these places, an effective insulation is placed, which prevents the outflow of heat along the shortest path. Resistance to heat transfer of walls and windows at the interfaces will be minimal, and the condensate on the insulating glass will not be formed if you place the frames in the middle of the slope.
The necessary protective properties and energy saving are achieved by using heat-insulating laminated panels, which protect the entire facade of the house from the outside and from the inside. The systems of the hinged ventilated facade are installed at any time of the year and in any weather. Due to additional insulation, "cold bridges" are eliminated and comfort of living is improved.
Heat loss through the floors of the first floor
Through the floor of the floor heat loss reaches 3-10%. Builders care little for their insulation, leaving cracks. At best, their cosmetic sealing with cement mortar. If the temperature of the floor surface is lower than in the room by 2 ºС, then the thermal insulation of the socle is poor.
Heat loss through the roof
Especially large heat loss through the roof in one and two-story houses. They reach 35%. Modern thermal insulation materials can reliably protect the ceiling and roof from the effects of the external environment and heat loss from the inside.
How is the resistance to heat transfer determined?
In the physical sense, the resistance to heat transfer of the enclosing structure characterizes the level of its thermal insulation properties and is found from the relation
- R = 1 / K (M 2 · o C / W).
The protective properties of the wall are determined by the processes of temperature exchange on its outer and inner surfaces, and also in the thickness of the material. For a complex fence, the total resistance to heat transfer will look like:
- R 0 = (R 1 + R 2 + ... + R n ) + R in + R n ,
Where R 1 , R 2 , R n characterize the properties of the individual layers, and R b, R n - internal and external interaction with air.
Resistance to heat transfer
In practice, the structures are inhomogeneous and contain elements for fastening layers and other bonds forming "cold bridges". The heterogeneity of the structures can significantly reduce the resistance to heat transfer of the entire structure. Therefore, it leads to a certain averaged value R 0 ' for an equivalent fence with uniform properties over the entire area. For example, in calculating the thickness of the walls of a building, heat losses in window and door slopes, gates, and individual building elements are taken into account through the amount of reduced resistance to heat transfer. In the picture, the arrows show how the heat-conducting concrete overlap extends heat to the outside.
Resistance to heat transfer Is determined after the determination of all the main areas of action of different heat fluxes. After this, in accordance with GOST 26254-84, the calculation is made by the formula:
- R 0 ' = F / (F 1 / R 01 + F 2 / R 02 + ... + F n / R 0 n ), where:
F is the area of the enclosing structure;
F n - the area of the characteristic n-th zone;
R 0 n is the heat transfer resistance of the characteristic n-th zone.
Thus, the actual heat fluxes through a complex construction are led to uniform heat transfer through its projection.
According to GOST R 54851-2011, the specific heat flux through the enclosing structures is determined from the expression:
- Q = (t вн - t н ) / R 0 ' ,
Where t вн and t н - the air temperature in the room, selected according to GOST 30494, and the outside temperature, defined as the average for the coldest five-day period for the year.
Infrared technology allows you to determine the places where the resistance to heat transfer decreases. The picture shows "cold bridges", where a large loss of heat occurs. The temperature in the blue zone is 8 ° C less than the rest.
Loss of heat through window openings
Windows occupy a small part of the surface of the house, but even for double-glazed windows, heat protection is 2-3 times weaker than that of walls. Modern energy-saving windows by the characteristic of thermal protection approach the properties of walls.
For each glass unit there are performance characteristics. In the first place among them is the reduced resistance of heat transfer, depending on the value of which each product is divided into classes.
The lowest class - D2 - is a single-pane double-glazed windows with a glass thickness of 4 mm (R 0 ' = 0.35 - 0.39 m · ° C / W). If the window has a resistance to heat transfer of insulating glass units below the minimum values given, it is not classified in any way. As the temperature protection increases, energy-efficient windows reduce light transmission.
The highest class of resistance to heat transfer - A1 - represents two-chamber energy-saving windows with inert gas and protective coatings (R 0 ' > = 0.8 m · ° C / W). Their heat-shielding properties are higher than for some walls of building materials.
Resistance to heat transfer of insulated glass units depends on the following factors:
- The ratio of glazing areas to the whole block;
- The sizes of sections of a sash and a frame;
- Material and construction of the window unit;
- Characteristics of the insulating glass unit;
- Quality of seals between the leaf and the frame.
When the resistance to heat transfer of windows and balcony doors is calculated, it is necessary to take into account the influence of the edge zone, since condensate can fall out at the junction of the double-glazed window with the window profile.
The choice of windows with balcony doors for each region is made in accordance with the required value of resistance to heat transfer R 0 ' and climatic conditions, determined by the number of degree-days of the heating period.
Conclusion
Normalized resistance to the heat transfer of walls and windows allow the construction of energy-efficient buildings and structures. When calculating the temperature characteristics of walls, it is necessary to take into account the heterogeneous properties of structural elements. To maintain the microclimate you need a reliable protection of all parts of the house from the cold. This allows modern insulators to be made.
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