The heat transfer of building envelopes is a complex process, including convection, heat conduction, and radiation. All of them occur together with the predominance of one of them. The heat-insulating properties of the fencing structures, which are reflected through heat transfer resistance, must comply with current building codes.
How is the heat exchange of air with enclosing structures
In construction, regulatory requirements are set for the amount of heat flow through the wall and its thickness is determined through it. One of the parameters for its calculation is the temperature difference outside and inside the room. The basis is the coldest time of the year. Another parameter is the heat transfer coefficient K - the amount of heat transferred in 1 s through an area of 1 m 2 , with a temperature difference of the external and internal environment of 1 º. 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 will penetrate less if the thickness of the fence is greater.
Convection and radiation from the outside and from the inside also affect the leakage of heat from the house. Therefore, behind the batteries, reflective screens made of aluminum foil are installed on the walls. Similar protection is also done inside ventilated facades from the outside.
Heat transfer through the walls of the house
External walls make up the maximum part of the area of the house and through them energy losses reach 35-45%. The 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, for building bricks K = 0.81 W / (m 2 · ), for concrete K = 2.04 W / (m 2 · ), for plywood K = 0.18 W / (m 2 · ), and for polystyrene boards K = 0.038 W / (m 2 · ).
In the calculations, a value inverse to the coefficient K is used - the heat transfer resistance of the building envelope. It is a standardized value and should not be lower than a certain set value, since heating costs and living conditions depend on it.
The coefficient K is influenced by the moisture content of the building envelope. In 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 fence deteriorate. A wet brick wall allows up to 30% more heat than dry walls. Therefore, the facade and roofs of houses are trying to veneer with materials on which water is not held.
Heat loss through walls and joints of openings is highly dependent on wind. The supporting structures are breathable, and air passes through them from the outside (infiltration) and from the inside (exfiltration).
Facing buildings
The outer lining of ventilated facades is installed with a gap in which air circulates. It does not affect the heat transfer resistance of the walls, but it resists wind load well, reducing infiltration. Air can enter the junction of window and door frames with wall openings. Because of this, the heat transfer resistance of the windows in the extreme sections decreases. In these places, effective insulation is placed that prevents the outflow of heat along the shortest path. The heat transfer resistance of the walls and windows in the interface will be minimal, and condensation will not form on the double-glazed window if you place the frames in the middle of the slope.
The necessary protective properties and energy saving are achieved by the use of heat-insulating multilayer panels, which protect the entire facade of the house from the outside and from the inside. Ventilated facade systems are installed at any time of the year and in any weather. Due to the additional warming, “cold bridges” are eliminated and the comfort of living is increased.
Heat loss through the floors of the first floor
Through the floor, heat loss reaches 3-10%. Builders care little about their insulation, leaving cracks. In the best case, they are cosmetic terminated with cement mortar. If the temperature of the floor surface is lower than in the room by 2 º, then the basement thermal insulation is poorly executed.
Heat loss through the roof
The losses of heat through the roof in one- and two-story houses are especially great. They reach 35%. Modern heat-insulating materials can reliably protect the ceiling and roof from the action of the external environment and heat loss from the inside.
How heat transfer resistance is determined
In the physical sense, the heat transfer resistance of the building envelope characterizes the level of its heat-insulating properties and is found from the ratio
- R = 1 / K (m 2 · about C / W).
The protective properties of a wall are determined by the processes of temperature exchange on its outer and inner surfaces, as well as in the thickness of the material. For complex fencing, the total heat transfer resistance will be:
- 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 in , R n - internal and external interaction with air.
Reduced heat transfer resistance
In practice, the structures are heterogeneous and contain layer attachment elements and other bonds forming “cold bridges”. The heterogeneity of the structures can significantly reduce the heat transfer resistance of the entire structure. Therefore, it leads to a certain average value of 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 elements of the building are taken into account in terms of the reduced heat transfer resistance. The arrows in the picture show how a heat-conducting concrete floor draws heat out.
Reduced heat transfer resistance determined after determining all the main areas of action of different heat fluxes. After that, in accordance with GOST 26254-84, the calculation is made according to 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 - area of the characteristic n-th zone;
R 0 n — heat transfer resistance of the characteristic n-th zone.
Thus, the actual heat fluxes through a complex structure lead to uniform heat transfer through its projection.
According to GOST R 54851-2011, the specific heat flux through the building envelope is determined from the expression:
- q = (t int - t n ) / R 0 ' ,
where t vn and t n are the air temperature in the room, selected according to GOST 30494, and the temperature outside, defined as the average for the coldest five-day week.
Infrared technology allows you to determine where the heat transfer resistance is reduced. The picture shows "cold bridges" where there is a large heat loss. The temperature in the blue zone is 8 º lower than the rest.
Heat loss through window openings
Windows occupy a small part of the surface of the house, but even with double-glazed windows, thermal protection is 2-3 times weaker than against walls. Modern energy-saving windows in terms of temperature protection approach the properties of the walls.
Each glass unit has its own operational characteristics. In the first place among them is the reduced heat transfer resistance, depending on the value of which each product is divided into classes.
The lowest class - D2 - is represented by single-layer double-glazed windows with a glass thickness of 4 mm (R 0 ' = 0.35 - 0.39 m ° C / W). If the window has a heat transfer resistance of double-glazed windows below the minimum values given, then it is not classified. As temperature protection increases , energy-efficient windows reduce light transmission.
The highest class of heat transfer resistance - A1 - is represented by 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 some walls made of building materials.
The heat transfer resistance of double-glazed windows depends on the following factors:
- the ratio of the glazing area and the entire block;
- sectional dimensions of the sash and frame;
- material and design of the window unit;
- characteristics of a double-glazed window;
- seal quality between sash and frame.
When calculating the heat transfer resistance of windows and balcony doors, it is necessary to take into account the influence of the edge zone, since condensation may occur at the junction of the double-glazed window with the window profile.
During installation, you should also pay attention to the quality of the seal of the openings. Through a thermographic device, you can see how the cold penetrates the house through the upper and right parts of the door (picture below).
No matter how effective the double-glazed windows are, with the free passage of air between the frames and walls, all their advantages will be lost.
The choice of windows together with balcony doors for each region is carried out in accordance with the required value of heat transfer resistance R 0 ' and climatic conditions determined by the number of degree-days of the heating period.
Conclusion
Normalized resistance to heat transfer of walls and windows allows the construction of energy-efficient buildings and structures. When calculating the temperature characteristics of walls, it is necessary to take into account the inhomogeneous properties of structural elements. To maintain the microclimate, reliable protection of all parts of the house from the cold is needed. This allows you to make modern heaters.