Today the issue of rational use of energy resources is very acute. Ways to save heat and energy are being worked out continuously in order to ensure energy security of economic development as a country., and each individual family.
Creation of efficient power plants and thermal insulation systems (equipment, which provides the greatest heat transfer (example, steam boilers) and, vice versa, from which it is undesirable (melting furnaces)) impossible without knowledge of the principles of heat transfer.
Approaches to thermal protection of buildings have changed, increased requirements for building materials. Every house needs insulation and heating systems. Therefore, the calculation of thermal conductivity is important in the thermal calculation of enclosing structures.
The concept of thermal conductivity
Thermal conductivity - this is such a physical property of the material, in which the thermal energy inside the body passes from the hottest part of it to the colder. The value of thermal conductivity shows the degree of heat loss in residential areas. Depends on the following factors:
- the density of the subject: increases with its increase;
- structures: for example, wood with transverse fibers has high thermal resistance, than with longitudinal;
- porosity the higher the value, the lower the average density;
- the nature of the voids still: materials with connected pores have high thermal conductivity, with closed fine-grained pores - smaller;
- humidity: dry items are less conductive;
- temperature - heat transfer decreases with increasing;
- pressure - the rate increases with increasing pressure.
You can quantify the property of objects to transmit thermal energy using the coefficient of thermal conductivity. It is very important to make the right choice of building materials, insulation to achieve maximum heat transfer resistance. Miscalculations or unwise savings in the future can lead to a deterioration of the indoor microclimate, humidity in the house, wet walls, stuffy rooms. And most importantly - to the high cost of heating.
For comparison, the table below shows the thermal conductivity of materials and substances.
Table 1
| Materials and substances | aluminum | steel | stainless steel | concrete | air | water | Particleboard | roofing material | cardboard | rubber | polyethylene | glass |
| Thermal conductivity coefficient | Two hundred and twenty-one | Fifty-eight | 17,5 | 1,5 | 0,02 | 0,6 | 0,15 | 0,17 | 0,18 | 0,04 | 0,3 | 0,7 |
Metals have the highest values, low - heat-insulating objects.
Classification of building materials and their thermal conductivity
Thermal conductivity of reinforced concrete, brickwork, expanded clay concrete blocks, commonly used for the construction of enclosing structures, differs in the highest normative indicators. In the construction industry, wooden structures are used much less often.
Depending on the value of thermal conductivity, building materials are divided into classes:
- structural and heat-insulating (from 0,210);
- heat-insulating (to 0,082 - A, from 0,082 to 0,116 - B, etc.).
Efficiency of multilayer constructions
Density and thermal conductivity
Currently there is no such building material, high bearing capacity which would be combined with low thermal conductivity. Construction of houses on the principle of multilayer structures allows:
- meet the design standards for construction and energy saving;
- leave the dimensions of the enclosing structures within reasonable limits;
- reduce material costs for the construction and maintenance of the facility;
- achieve durability and maintainability (example, when replacing one sheet of mineral wool).
Combination structural material and thermal insulation allows to provide durability and to reduce losses of thermal energy to optimum level. Therefore at design of walls at calculations each layer of a future protecting design is considered.
It is also important to consider the density in the construction of the house and its insulation.
Density of matter is a factor, which affects its thermal conductivity, the ability to retain the main insulator - air.
Calculation of wall insulation thickness and
The calculation of wall thickness depends on the following indicators:
- density;
- estimated thermal conductivity;
- heat transfer resistance coefficient.
According to established norms, the value of the heat transfer resistance of the external walls must be at least 3,2? W / m • ° C.
Calculation wall thickness of reinforced concrete and other structural materials presented in the table 2. Such building materials have high load-bearing characteristics, they are durable, but as thermal protection they are ineffective and require irrational wall thickness.
Table 2
| Indicator | Concrete, mortar-concrete mixes | |||
| Ferroconcrete | Cement-sand mortar | A complex solution (cement-lime-sand) | Lime-sandy solution | |
| density, kg / cu. m | Two thousand five hundred | One thousand eight hundred | One thousand seven hundred | One thousand six hundred |
| thermal conductivity, W /(m • ° С) | 2,04 | 0,93 | 0,87 | 0,81 |
| wall thickness, m | 6,53 | 2,98 | 2,78 | 2,59 |
Structural and thermal insulation materials are able to withstand quite high loads, at the same time significantly increase the thermal and acoustic properties of buildings in wall enclosing structures (table 3.1, 3.2).
Table 3.1
| Indicator | Structural and heat-insulating m-ly | |||||
| Pemzobeton | Keramzitobeton | Polystyrene concrete | Foam and aerated concrete (foam and gas silicate) | Clay brick | Silicate brick | |
| density, kg / cu. m | Eight hundred | Eight hundred | Six hundred | Four hundred | One thousand eight hundred | One thousand eight hundred |
| thermal conductivity, W /(m • ° С) | 0,68 | 0,326 | 0,2 | 0,11 | 0,81 | 0,87 |
| wall thickness, m | 2,176 | 1,04 | 0,64 | 0,35 | 2,59 | 2,78 |
Table 3.2
| Indicator | Structural and heat-insulating m-ly | |||||
| Slag brick | Silicate brick 11-type | Silicate brick 14-type | Pine (transverse arrangement of fibers) | Pine (longitudinal arrangement of fibers) | Glued plywood | |
| density, kg / cu. m | One thousand five hundred | One thousand five hundred | One thousand four hundred | Pyatsot | Pyatsot | Six hundred |
| thermal conductivity, W /(m • ° С) | 0,7 | 0,81 | 0,76 | 0,18 | 0,35 | 0,18 |
| wall thickness, m | 2,24 | 2,59 | 2,43 | 0,58 | 1,12 | 0,58 |
Thermal insulation building materials can significantly increase the thermal protection of buildings and structures. Table data 4 show, what the lowest values of the thermal conductivity have polymers, mineral wool boards from natural organic and inorganic materials.
Table 4
| Indicator | Heat-insulating m-ly | ||||||
| PPT | PT polystyrene concrete | Mothers are mineral wool | Heat-insulating plates (PT) from mineral wool | Fiberboard (Particleboard) | Hell | Gypsum sheets (plasterboard) | |
| density, kg / cu. m | Thirty-five | Three hundred | One thousand | One hundred ninety | Two hundred | Hundred and fifty | One thousand and fifty |
| thermal conductivity, W /(m • ° С) | 0,39 | 0,1 | 0,29 | 0,045 | 0,07 | 0,192 | 1,088 |
| wall thickness, m | 0,12 | 0,32 | 0,928 | 0,14 | 0,224 | 0,224 | 1,152 |
The values of the thermal conductivity tables of building materials are used in the calculations:
- thermal insulation of facades;
- general building insulation;
- insulating materials when installing a roof;
- technical insulation.
The task of choosing the optimal materials for construction, of course, provides an integrated approach. However, even such simple calculations in the early stages of design can determine the most suitable materials and their quantity.