Passing through small towns, you can often see the still preserved monuments of the socialist era: the buildings of rural clubs, palaces, old shops. Dilapidated buildings are characterized by huge window openings with a maximum of double glazing, walls made of reinforced concrete products of relatively small thickness. Expanded clay was used as a heater in the walls, and in small quantities. The thin ribbed slab ceilings also did not help keep the building warm.
When choosing materials for structures, designers of the USSR era had little interest in thermal conductivity. The industry produced enough bricks and slabs, the consumption of fuel oil for heating was practically not limited. Everything changed in a matter of years. "Smart" combined boiler houses with multi-tariff metering devices, thermal coats, recuperative ventilation systems in modernconstruction is already the norm, not a curiosity. However, brick, although it has absorbed many modern scientific achievements, as it was the No. 1 building material, it has remained so.
The phenomenon of heat conduction
In order to understand how different materials are from each other in terms of thermal conductivity, on a cold day outside, it is enough to put your hand alternately on metal, brick wall, wood and, finally, on a piece of foam. However, the properties of materials to transmit thermal energy are not necessarily bad.
The thermal conductivity of bricks, concrete, wood are considered in the context of the ability of materials to retain heat. But in some cases, heat, on the contrary, must be transferred. This applies, for example, pots, pans and other utensils. Good thermal conductivity ensures that energy is used for its intended purpose - to heat the food being cooked.
What is measured the thermal conductivity of its physical essence
What is heat? This is the movement of the molecules of a substance, chaotic in a gas or liquid, and vibrating in the crystal lattices of solids. If a metal rod placed in a vacuum is heated on one side, the metal atoms, having received part of the energy, will begin to vibrate in the nests of the lattice. This vibration will be transmitted from atom to atom, due to which the energy will gradually be distributed evenly over the entire mass. For some materials, such as copper, this process takes seconds, while for others, it will take hours for the heat to evenly “spread” throughout the volume. The higher the temperature difference betweencold and hot areas, the faster the heat transfer. By the way, the process will speed up with an increase in the contact area.
The thermal conductivity (x) is measured in W/(m∙K). It shows how much heat energy in Watts will be transferred through one square meter with a temperature difference of one degree.
Full ceramic brick
Stone buildings are strong and durable. In stone castles, garrisons withstood sieges that sometimes lasted for years. Buildings made of stone are not afraid of fire, the stone is not subject to decay processes, due to which the age of some structures exceeds a thousand years. However, the builders did not want to depend on the random shape of the cobblestone. And then ceramic bricks made of clay appeared on the stage of history - the oldest building material created by human hands.
Thermal conductivity of ceramic bricks is not a constant value; in laboratory conditions, absolutely dry material gives a value of 0.56 W / (m∙K). However, real operating conditions are far from laboratory ones, there are many factors that affect the thermal conductivity of a building material:
- humidity: the drier the material, the better it retains heat;
- thickness and composition of cement joints: cement conducts heat better, too thick joints will serve as additional freezing bridges;
- the structure of the brick itself: sand content, firing quality, presence of pores.
In real conditions of operation, the thermal conductivity of a brick is taken within 0,65 - 0.69 W / (m∙K). However, every year the market grows with previously unknown materials with improved performance.
Porous ceramics
Relatively new building material. A hollow brick differs from a solid counterpart in lower material consumption in production, lower specific gravity (as a result, lower costs for loading and unloading operations and ease of laying) and lower thermal conductivity.
The worst thermal conductivity of a hollow brick is a consequence of the presence of air pockets (the thermal conductivity of air is negligible and averages 0.024 W/(m∙K)). Depending on the brand of brick and the quality of workmanship, the indicator varies from 0.42 to 0.468 W / (m∙K). I must say that due to the presence of air cavities, the brick loses its strength, but many in private construction, when strength is more important than heat, simply fill all the pores with liquid concrete.
Silicate brick
Baked clay building material is not as easy to manufacture as it might seem at first glance. Mass production produces a product with very dubious strength characteristics and a limited number of freeze-thaw cycles. Making bricks that can withstand the weather for hundreds of years is not cheap.
One of the solutions to the problem was a new material made from a mixture of sand and lime in a steam "bath" with a humidity of about 100% and a temperature of about +200°C The thermal conductivity of silicate brick is very dependent on the brand. It, just like ceramic, is porous. When the wall is not a carrier, and its task is only to retain heat as much as possible, a slotted brick with a coefficient of 0.4 W / (m∙K) is used. The thermal conductivity of a solid brick, of course, is higher up to 1.3 W / (m∙K), but its strength is an order of magnitude better.
Aerated silicate and foamed concrete
With the development of technology, it has become possible to produce foam materials. In relation to bricks, these are gas silicate and foamed concrete. The silicate mixture or concrete is foamed, in this form the material hardens, forming a finely porous structure of thin partitions.
Due to the presence of a large number of voids, the thermal conductivity of a gas silicate brick is only 0.08 - 0.12 W / (m∙K).
Foamed concrete holds heat a little worse: 0.15 - 0.21 W / (m∙K), but buildings made of it are more durable, it is able to carry a load 1.5 times more than what can be "trusted" gas silicate.
Thermal conductivity of different types of bricks
As already mentioned, the thermal conductivity of a brick in real conditions is very different from the tabular values. The table below shows not only the thermal conductivity values for different types of this building material, but also structures made from them.
Decrease in thermal conductivity
Currently, in construction, the preservation of heat in a building is rarely trusted to one type of material. reducethe thermal conductivity of a brick, saturating it with air pockets, making it porous, can be up to a certain limit. An airy, overly light porous building material cannot even support its own weight, let alone use it to create multi-story structures.
Most often, a combination of building materials is used to insulate buildings. The task of some is to ensure the strength of structures, its durability, while others guarantee the preservation of heat. Such a decision is more rational, from the point of view of both construction technology and economics. Example: using only 5 cm of foam or foam plastic in the wall gives the same effect for saving thermal energy as "extra" 60 cm of foam concrete or gas silicate.
