Thermal conductivity coefficients of various materials

In the modern world, an important aspect of a private home is its energy efficiency. That is, the ability to spend a minimum amount of energy to maintain a comfortable climate in the house. To spend less energy, you need to take care of reducing energy losses.

Thermal conductivity of materials is the ability of a material to retain heat in cold weather and keep cool in summer.

Heat capacity is the amount of heat absorbed (released) by a body in the process of heating (cooling) per 1 kelvin.

Density is the ratio of the mass of a body to the volume occupied by this body.

Thermal conductivity of building materials

The design of energy-efficient house technologies should be carried out by specialists, but in real life everything may be different. It happens that home owners, for a number of reasons, are forced to independently select materials for construction. They will also need to calculate thermal parameters on the basis of which thermal insulation and insulation will be carried out. Therefore, you need to have at least a minimal understanding of building heating engineering and its basic concepts, such as thermal conductivity coefficient, in what units it is measured and how it is calculated. Knowing these “basics” will help you properly insulate your home and heat it economically.

What is thermal conductivity

Thermal conductivity of a brick wall: without insulation;
with insulation on the outside; with insulation inside the house; In simple terms, thermal conductivity is the transfer of heat from a hotter body to a less hot one. Without going into details, all physical materials and substances can transmit thermal energy.

Every day, even at the most primitive everyday level, we are faced with thermal conductivity, which manifests itself in each material differently and to a very different extent. For example, if you stir boiling water with a metal spoon, you can get burned very quickly, since the spoon heats up almost instantly. If you use a wooden spatula, it will heat up very slowly. This example clearly shows the difference in thermal conductivity between metal and wood - for metal it is several times higher.

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Coefficient of thermal conductivity

To evaluate the thermal conductivity of any material, the thermal conductivity coefficient (λ) is used, which is measured in W/(m×℃) or W/(m×K). This coefficient indicates the amount of heat that can be conducted by any material, regardless of its size, per unit time over a certain distance. If we see that some material has a high coefficient value, then it conducts heat very well and can be used as heaters, radiators, and convectors. For example, metal heating radiators in rooms work very efficiently, perfectly transferring heat from the coolant to the internal air masses in the room.

If we talk about the materials used in the construction of walls, partitions, roofs, then high thermal conductivity is an undesirable phenomenon. With a high coefficient, the building loses too much heat, to retain which it will be necessary to build rather thick structures indoors. And this entails additional financial costs.

The thermal conductivity coefficient depends on temperature. For this reason, reference literature indicates several coefficient values ​​that change with increasing temperatures. Operating conditions also affect heat conductivity. First of all, we are talking about humidity, since as the percentage of moisture increases, the coefficient of thermal conductivity also increases. Therefore, when carrying out this kind of calculations, you need to know the real climatic conditions in which the building will be built.

Heat transfer resistance

Thermal conductivity coefficient is an important characteristic of any material. But this value does not accurately describe the thermal conductivity of the structure, since it does not take into account the features of its structure. Therefore, it is more appropriate to calculate the heat transfer resistance, which is essentially the reciprocal of the thermal conductivity coefficient. But unlike the latter, the calculation takes into account the thickness of the material and other important design features.

During construction, as a rule, multilayer structures are used, such as frame or SIP houses. One of these layers is an insulating material that maximizes the value of thermal resistance. Each layer of such a structure has its own resistance and must be calculated based on the thermal conductivity coefficient and the thickness of the material. By summing the resistance of all layers, we get the total resistance of the entire structure.

It is important to note that the air gaps that are located in the partition structure and do not communicate with the outside air significantly increase the overall heat transfer resistance.

Modern construction trends include the use of synthetic materials such as EPS PIR boards and Izolon as insulation, which have excellent characteristics, are convenient and easy to install.

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Thermal conductivity, density and heat capacity coefficients have been calculated for almost all building materials. Below is a table with information about the coefficients for all materials that can be used in the construction of buildings. Even just looking at these data, it becomes clear how different the thermal conductivity of building materials is and how much the coefficient values ​​can differ. To simplify the choice of material for the buyer, manufacturers indicate the value of the thermal conductivity coefficient in the passport for their product.

Table of thermal conductivity, heat capacity and density of materials

Wood, metals and glass

Wood enjoys well-deserved popularity among Russian builders. It is used to make lining, plywood and even parquet boards. Metal is necessary for the construction of the roof and reinforcement frame, and glass takes its place in the frames of the window openings. Thermal conductivity is presented in table form:

Necessity of calculations

Why is it necessary to carry out these calculations, is there any benefit from them in practice? Let's take a closer look.

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Assessing the effectiveness of thermal insulation

Different climatic regions of Russia have different temperature conditions, so each of them has its own standard indicators of heat transfer resistance. These calculations are carried out for all elements of the structure in contact with the external environment. If the structural resistance is within normal limits, then you don’t have to worry about insulation.

If thermal insulation of the structure is not provided, then you need to make the right choice of insulating material with suitable thermal characteristics.

Heat loss

Heat losses at home
An equally important task is to predict heat losses, without which it is impossible to properly plan a heating system and create ideal thermal insulation. Such calculations may be necessary when choosing the optimal boiler model, the number of radiators required and their correct placement.

To determine heat losses through any structure, you need to know the resistance, which is calculated using the temperature difference and the amount of heat lost from one square meter of the enclosing structure. And so, if we know the area of ​​the structure and its thermal resistance, and also know for what climatic conditions the calculation is being made, then we can accurately determine the heat losses. There is a good calculator for calculating heat loss at home (it can even calculate how much money will be spent on heating, approximately of course).

Such calculations in a building are carried out for all building envelopes interacting with cold air flows, and then summed up to determine the total heat loss. Based on the obtained value, a heating system is designed that should fully compensate for these losses. If the heat losses are too large, they entail additional financial costs, and not everyone can afford this. In this situation, you need to think about improving the thermal insulation system.

Separately, we need to talk about windows, for which the heat transfer resistance is determined by regulatory documents. There is no need to do the calculations yourself. There are ready-made tables in which resistance values ​​are entered for all types of window and balcony door structures. Thermal losses of windows are calculated based on the area, as well as the temperature difference on different sides of the structure.

The calculations above are suitable for beginners who are taking their first steps in designing energy-efficient homes. If a professional gets down to business, then his calculations are more complex, since many correction factors are additionally taken into account - insolation, light absorption, reflection of sunlight, heterogeneity of structures, location of the house on the site, and others.

How to calculate wall thickness

In order for the house to be warm in winter and cool in summer, it is necessary that the enclosing structures (walls, floor, ceiling/roof) must have a certain thermal resistance. This value is different for each region. It depends on the average temperatures and humidity in a particular area.

Thermal resistance of enclosing structures for regions of Russia

In order for heating bills not to be too high, it is necessary to select building materials and their thickness so that their total thermal resistance is not less than that indicated in the table.

Calculation of wall thickness, insulation thickness, finishing layers

Modern construction is characterized by a situation where the wall has several layers. In addition to the supporting structure, there is insulation and finishing materials. Each layer has its own thickness. How to determine the thickness of insulation? The calculation is simple. Based on the formula:

Formula for calculating thermal resistance

R—thermal resistance;

p—layer thickness in meters;

k is the thermal conductivity coefficient.

First you need to decide on the materials that you will use during construction. Moreover, you need to know exactly what type of wall material, insulation, finishing, etc. will be. After all, each of them makes its contribution to thermal insulation, and the thermal conductivity of building materials is taken into account in the calculation.

First, the thermal resistance of the structural material (from which the wall, ceiling, etc. will be built) is calculated, then the thickness of the selected insulation is selected based on the “residual” principle. You can also take into account the thermal insulation characteristics of finishing materials, but usually they are a plus to the main ones. This is how a certain reserve is laid down “just in case.” This reserve allows you to save on heating, which subsequently has a positive effect on the budget.

An example of calculating the thickness of insulation

Let's look at it with an example. We are going to build a brick wall - one and a half bricks long, and we will insulate it with mineral wool. According to the table, the thermal resistance of walls for the region should be at least 3.5. The calculation for this situation is given below.

1. First, let's calculate the thermal resistance of a brick wall. One and a half bricks is 38 cm or 0.38 meters, the thermal conductivity coefficient of brickwork is 0.56. We calculate using the above formula: 0.38/0.56 = 0.68. A wall of 1.5 bricks has this thermal resistance.
2. We subtract this value from the total thermal resistance for the region: 3.5-0.68 = 2.82.
   This value must be “added” with thermal insulation and finishing materials. All enclosing structures will have to be calculated
3. We calculate the thickness of mineral wool. Its thermal conductivity coefficient is 0.045. The thickness of the layer will be: 2.82 * 0.045 = 0.1269 m or 12.7 cm. That is, to ensure the required level of insulation, the thickness of the mineral wool layer must be at least 13 cm.

If the budget is limited, you can take 10 cm of mineral wool, and the missing amount will be covered with finishing materials. They will be inside and outside. But, if you want your heating bills to be minimal, it is better to use the finishing as a “plus” to the calculated value. This is your reserve during the lowest temperatures, since thermal resistance standards for enclosing structures are calculated based on the average temperature over several years, and winters can be abnormally cold

Therefore, the thermal conductivity of building materials used for finishing is simply not taken into account

Avoid getting wet

These are porous, vapor-permeable insulation materials; all steam passing through them will condense inside their layer at the dew point. To remove it, you need to ventilate on one side and prevent steam from entering on the other. Therefore, the insulation is separated from the steam source by a vapor barrier.

In floors on joists, the insulation is separated from the ground by a continuous double roofing material flooring - a conventional vapor barrier under the house. First, roofing material is rolled onto the compacted soil, gluing the joints and wrapping it onto the base. Now the humidity of the insulation will always be the same as in the house.

On the attic floor it’s the same thing - first a polypropylene vapor barrier is placed on the floor, and insulation is placed on top of it.

Main characteristics of insulation

Let us first provide the characteristics of the most popular thermal insulation materials, which you should first pay attention to when choosing. Comparison of thermal conductivity insulation should be made only on the basis of the purpose of the materials and room conditions (humidity, presence of open fire, etc.). We have further arranged in order of importance the main characteristics of insulation

We have further arranged in order of importance the main characteristics of insulation.

Comparison of building materials

Thermal conductivity. The lower this indicator, the less thermal insulation layer is required, which means that insulation costs will also be reduced.

Moisture permeability. The lower permeability of the material to moisture vapor reduces the negative impact on the insulation during operation.

Fire safety. Thermal insulation should not burn or emit toxic gases, especially when insulating a boiler room or chimney.

Durability. The longer the service life, the cheaper it will cost you during operation, since it will not require frequent replacement.

Environmentally friendly. The material must be safe for humans and the environment.

Application technology

The two components are fed into the mixing tank. There they are mixed under pressure and sprayed onto the surface to be treated using a gun. After a few seconds, the mixture sharply increases in volume and quickly hardens.

Method of applying polyurethane foam

Important! To apply polyurethane foam, special equipment and personal protective equipment are required. Therefore, it is better to entrust this process to professional construction organizations.

Polyurethane foam is a quality material in all respects. Savings of time and money can be 50-70% compared to using traditional insulation. Work can be carried out all year round. Technologies do not stand still, so insulating building structures using polyurethane foam will become cheaper and more reliable.

Cement-sand

Depending on the strength of the coating, the proportions of sand to cement are selected - 1:4 or 1:3. It also depends on the brand of cement and sand fraction. This solution is practically not elastic, so it is used for mineral surfaces as a base coating, and not for sealing cracks and cracks. With a layer density of 1800 kg/m3, the thermal conductivity coefficient of the plaster will be 1.2.

This is a material for finishing interior surfaces of premises. Its use is suitable if the ambient temperature ranges from +5 to +25 degrees. The thermal conductivity of gypsum plaster also depends on the density of its application and possible additives. Typically, the thermal conductivity coefficient of gypsum plaster with a material density of 800 kg/m 3 is 0.3.

Brief instructions for installing slag wool

Considering that this insulation can react critically to moisture, it is not recommended to install it on the facade of a building. Also, do not attach slag wool to a metal frame. If you plan to insulate vertical or inclined surfaces, then use wooden sheathing. The heat insulator installation diagram is as follows:

• We prepare wooden beams measuring 50x50 or 50x100 millimeters. We select the thickness and width taking into account the width of the insulation.
• We attach the waterproofing to the surface using construction staples, with an overlap of 10 centimeters.
• In order not to unnecessarily cut slag wool and not raise harmful dust from microparticles of fibers, it is recommended to install the lathing in increments to match the width of the mat. Usually it is about 50 centimeters.
• The slabs must fit tightly into the holes between adjacent beams and be laid end-to-end.
• The insulation does not require additional fastening.
• We place a vapor barrier on top of the slag wool. We also attach it with an overlap and glue the joints with special tape.

On top of this structure, you can install additional sheathing for further wall cladding. During work, make sure that the slag wool does not come into contact with metal elements. You also need to be careful and avoid exposed areas of insulation. Firstly, it may get wet. Secondly, slag wool generates dust and will create an unfavorable microclimate in the room. Watch a video about the production of stone wool:

Thermal conductivity of polystyrene foam from 50 mm to 150 mm is considered thermal insulation

Expanded polystyrene boards, colloquially referred to as polystyrene foam, are an insulating material, usually white. It is made from thermally expanded polystyrene. In appearance, the foam is presented in the form of small moisture-resistant granules; during the melting process at high temperatures, it is smelted into one whole, a slab. The sizes of the granule parts are considered to be from 5 to 15 mm. The outstanding thermal conductivity of 150 mm thick foam is achieved due to a unique structure - granules.

Each granule has a huge number of thin-walled micro-cells, which in turn increase the area of ​​contact with air many times over. We can say with confidence that almost all polystyrene foam consists of atmospheric air, approximately 98%, in turn, this fact is their purpose - thermal insulation of buildings both outside and inside.

Everyone knows, even from physics courses, that atmospheric air is the main insulator of heat in all thermal insulation materials; it is in a normal and rarefied state, in the thickness of the material. Heat-saving, the main quality of polystyrene foam.

As mentioned earlier, polystyrene foam is almost 100% air, and this in turn determines the high ability of polystyrene foam to retain heat. This is due to the fact that air has the lowest thermal conductivity. If we look at the numbers, we will see that the thermal conductivity of polystyrene foam is expressed in the range of values ​​from 0.037 W/mK to 0.043 W/mK. This can be compared with the thermal conductivity of air - 0.027 W/mK.

While the thermal conductivity of popular materials such as wood (0.12 W/mK), red brick (0.7 W/mK), expanded clay (0.12 W/mK) and others used for construction is much higher.

Therefore, polystyrene foam is considered to be the most effective material among the few for thermal insulation of external and internal walls of a building. Residential heating and cooling costs are significantly reduced through the use of polystyrene foam in construction.

The excellent qualities of polystyrene foam boards have found their application in other types of protection, for example: polystyrene foam, which also serves to protect underground and external communications from freezing, due to which their service life increases significantly. Polystyrene foam is also used in industrial equipment (refrigerators, refrigerators) and in warehouses.

What affects the ability of polystyrene foam to conduct heat?

To clearly understand what thermal conductivity is, let’s take a piece of material one meter thick and an area of ​​one square meter. Moreover, we heat one side of it and leave the other cold. The difference between these temperatures should be tenfold. By measuring the amount of heat that transfers to the cold side in one second, we obtain the thermal conductivity coefficient.

Why is polystyrene foam capable of retaining both heat and cold well? It turns out that it's all about its structure. Structurally, this material consists of many sealed polyhedral cells ranging in size from 2 to 8 millimeters. They have air inside – it makes up 98 percent and serves as an excellent heat insulator. Polystyrene accounts for 2% of the volume. And by weight, polystyrene is 100%, because air, relatively speaking, has no mass.

It should be noted that the thermal conductivity of extruded polystyrene foam remains unchanged over time. This distinguishes this material from other foam plastics, the cells of which are filled not with air, but with another gas. After all, this gas has the ability to gradually evaporate, and the air remains inside the sealed polystyrene foam cells.

When buying polystyrene foam, we usually ask the seller what the density of this material is. After all, we are accustomed to the fact that density and the ability to conduct heat are inextricably linked with each other. There are even tables of this dependence, with which you can choose the appropriate brand of insulation.

However, nowadays they have come up with an improved insulation, which contains graphite additives. Thanks to them, the thermal conductivity of polystyrene foam of various densities remains unchanged. Its value is from 0.03 to 0.033 watts per meter per Kelvin. So now, when purchasing modern, improved XPS, there is no need to check its density.

Marking of polystyrene foam whose thermal conductivity does not depend on density:

Building materials with minimal package transformer

According to research, dry air has a minimum thermal conductivity value (about 0.023 W/m°C).

From the point of view of using dry air in the structure of a building material, a structure is needed where dry air resides inside numerous closed spaces of small volume. Structurally, this configuration is represented in the form of numerous pores inside the structure.

Hence the logical conclusion: a building material whose internal structure is a porous formation should have a low level of CFC.

Moreover, depending on the maximum permissible porosity of the material, the thermal conductivity value approaches the value of the thermal conductivity of dry air.

The creation of a building material with minimal thermal conductivity is facilitated by a porous structure. The more pores of different volumes are contained in the structure of the material, the better CTP can be obtained

In modern production, several technologies are used to obtain the porosity of a building material.

In particular, the following technologies are used:

• foaming;
• gas formation;
• water sealing;
• swelling;
• introduction of additives;
• creating fiber scaffolds.

It should be noted: the thermal conductivity coefficient is directly related to properties such as density, heat capacity, and temperature conductivity.

The thermal conductivity value can be calculated using the formula:

λ = Q / S *(T1-T2)*t,

Where:

• Q – amount of heat;
• S – material thickness;
• T1, T2 – temperature on both sides of the material;
• t – time.

The average value of density and thermal conductivity is inversely proportional to the value of porosity. Therefore, based on the density of the structure of the building material, the dependence of thermal conductivity on it can be calculated as follows:

λ = 1.16 √ 0.0196+0.22d2 – 0.16,

Where: d – density value. This is the formula of V.P. Nekrasov, demonstrating the influence of the density of a particular material on the value of its CFC.

Environmental friendliness of slag

The porous structure of the slag retains heat well.
The material in question is industrial waste. Understanding whether slag is harmful as insulation is important already at the beginning of designing a house.

The insulation technology and places where the backfill is used do not provide for direct contact with humans. Dust and gaseous emissions do not penetrate into the rooms, so they are not able to cause harm to health.

When purchasing, you must require a safety certificate. Some slags emit radioactive background.

Criteria for choosing slag wool

When choosing slag wool, you should first of all pay attention to material from well-known manufacturers that has good reviews. Under no circumstances buy insulation from little-known brands at dubious points of sale, where they cannot provide you with the entire list of documents, certificates and licenses for the product. In addition, consider these recommendations:

1. The highest quality mineral fiber insulation is offered by German manufacturers. Only they have the most picky certification bodies that will not release low-quality or potentially dangerous products onto the market.
2. Check with the seller in which direction the fibers of the insulation are located. When placed vertically, the slag wool will store heat well and absorb sound. If it is chaotic, it will be more durable and withstand dynamic loads.
3. Check the GOST of the product on the packaging if the slag is domestically produced. Its presence guarantees the quality of the product.
4. Choose the material that best suits your needs. The density of slag wool can be different, and the scope of its use depends on this. The density of 75 kilograms per cubic meter is suitable for insulating roofs and attics. Material with a density of 125 kg/m3 is used on floors, ceilings, and interior walls.