Driving through small towns, you can often see still preserved monuments of the socialist era: the buildings of rural clubs, palaces, old shops. The 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 insulation in the walls, and in small quantities. Ceilings made of thin ribbed slabs also did not contribute to the conservation of heat in the building.
When choosing materials for constructions of designers of the USSR era, thermal conductivity was of little interest. The industry produced enough bricks and slabs, fuel oil consumption for heating was practically unlimited. Everything has changed in a matter of years. “Smart” combined boiler rooms with multi-tariff metering devices, thermal coats, recuperation ventilation systems in modern construction are the norm, not a curiosity. However, the brick, although it absorbed many modern scientific achievements, as it was building material No. 1, remained so.
The phenomenon of thermal conductivity
In order to understand how the materials differ in thermal conductivity, it is enough on a cold day on the street to put your hand alternately on the metal, brick wall, wood and, finally, on a piece of foam. However, the properties of materials to transfer thermal energy are not necessarily bad.
The thermal conductivity of brick, concrete, wood is 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, to pots, pans and other utensils. Good thermal conductivity ensures that energy will be spent as intended - to heat the cooked food.
What is the measured 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 vibrated in the crystal lattices of solids. If a metal rod placed in a vacuum is heated on one side, the metal atoms, having received a part of the energy, will begin to vibrate in the lattice nests. This vibration will be transmitted from atom to atom, due to which energy will gradually be distributed evenly throughout the mass. For some materials, for example, for copper, this process takes seconds, while for others, it takes hours to heat evenly “spread” throughout the entire volume. The higher the temperature difference between the cold and hot areas, the faster the heat transfer. By the way, the process will accelerate with increasing contact area.
The thermal conductivity (x) is measured in W / (m ∙ K). It shows how much thermal energy in watts will be transmitted through one square meter with a temperature difference of one degree.
Solid ceramic brick
Stone buildings are strong and durable. In stone castles, garrisons withstood the siege, sometimes lasting for years. Buildings made of stone are not afraid of fire, stone is not subject to decay processes, due to which the age of some structures exceeds a thousand years. However, builders did not want to depend on the random shape of the cobblestone. And then a ceramic clay brick appeared on the stage of history - the oldest building material created by human hands.
The thermal conductivity of ceramic bricks is not constant; under laboratory conditions, absolutely dry material gives a value of 0.56 W / (m ∙ K). However, the actual operating conditions are far from laboratory, there are many factors that affect the thermal conductivity of building material:
- humidity: the drier the material, the better it holds heat;
- thickness and composition of cement joints: cement conducts heat better, too thick joints will serve as additional freezing bridges;
- structure of the brick itself: sand content, firing quality, pores.
Under real operating conditions, the coefficient of thermal conductivity of the brick is taken in the range of 0.65 - 0.69 W / (m ∙ K). However, every year the market grows with previously unknown materials with improved performance.
Porous ceramics
A relatively new building material. Hollow brick differs from a full-bodied counterpart in lower material consumption in production, lower specific gravity (as a result, lower costs for loading and unloading 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 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 pores with liquid concrete.
Silicate brick
Burnt 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 a brick that can withstand the atmospheric effects for hundreds of years is expensive.
One of the solutions to the problem was a new material made from a mixture of sand and lime in a steam bath at 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 bearing, and its task is only to retain heat as much as possible, a slit brick with a coefficient of 0.4 W / (m ∙ K) is used. The thermal conductivity of a solid brick is naturally higher to 1.3 W / (m ∙ K), but its strength is an order of magnitude better.
Gas silicate and foamed concrete
With the development of technology, it has become possible to produce foam materials. With reference to a brick it is gas silicate and the made foam 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 gas silicate bricks is only 0.08 - 0.12 W / (m ∙ K).
Foamed concrete keeps heat a little worse: 0.15 - 0.21 W / (m ∙ K), but the structure of it is more durable, it is able to carry a load 1.5 times more than that which can be "trusted" with gas silicate.
Thermal conductivity of different types of bricks
As already mentioned, the thermal conductivity of bricks in real conditions is very different from the table values. The table below shows not only the thermal conductivity values for different types of this building material, but also their structures.
Thermal conductivity reduction
Currently, in construction, the conservation of heat in a building is rarely trusted in one type of material. It is possible to reduce the thermal conductivity of a brick, saturating it with air pockets, making it porous, to a certain limit. The airy, overly lightweight porous building material will not be able to hold even its own weight, not to mention its use in the creation of multi-storey structures.
Most often, a combination of building materials is used to warm buildings. The task of some is to ensure the strength of structures, its durability, while others guarantee the preservation of heat. This solution is more rational, in terms of both construction technology and economics. Example: the use of only 5 cm of polystyrene foam or foam in the wall gives the same effect for the conservation of thermal energy as the "extra" 60 cm of foam concrete or gas silicate.