Thermal conductivity coefficient of materials. What does thermal conductivity depend on??

Understanding a material’s thermal conductivity is essential when building or remodeling a house. The term "thermal conductivity" describes a material’s capacity to transfer heat. This characteristic has a big impact on how well a building regulates its temperature, which has an impact on comfort and energy efficiency.

The thermal conductivities of different materials vary. For instance, metals with good heat conductivity, such as copper and aluminum, find use in applications requiring heat transfer. However, because of their low heat conductivity, materials like foam and wood are perfect for insulation.

A material’s heat conductivity is determined by a number of variables. The main factors are the material’s structure and composition. For example, materials with molecules packed closely together tend to conduct heat more effectively. In addition, a material’s thermal conductivity can be influenced by variables like temperature, density, and moisture content.

Choosing the appropriate materials for construction and renovation based on their thermal conductivity can have a significant impact. It contributes to the construction of comfortable and energy-efficient buildings. Knowing these qualities will help you make wise decisions that are good for the environment and your pocketbook.

Material Thermal Conductivity (W/m·K)
Brick 0.6 – 1.0
Concrete 1.2 – 1.8
Glass Wool 0.03 – 0.04
Wood 0.1 – 0.2
Polystyrene 0.03 – 0.04
Steel 50.2

Coefficient of thermal conductivity

The thermal conductivity coefficient (λ), expressed in W/(m×℃) or W/(m×K), is used to assess the thermal conductivity of any material. This coefficient shows how much heat can be transferred over a given distance in a unit of time by any material, regardless of size. When a material exhibits a high coefficient of conductivity, it is highly suitable for use as convectors, heaters, or radiators. For instance, metal heating radiators in rooms function incredibly well, precisely transferring heat from the coolant to the room’s interior air masses.

High thermal conductivity is a bad thing when it comes to the materials used to build roofs, walls, and partitions. A high coefficient means that the building loses too much heat, which will require the construction of fairly thick interior structures. Also, there will be extra financial outlay for this.

The coefficient of thermal conductivity is temperature dependent. Because of this, the reference literature lists a number of coefficient values that vary as temperature rises. Heat conductivity is also influenced by operating conditions. We are first discussing humidity because the coefficient of thermal conductivity rises with an increase in the percentage of moisture. Consequently, you must be aware of the actual climatic conditions in which the building will be constructed in order to perform this type of computation.

Table of thermal conductivity of materials

The values of the thermal conductivity coefficient for a few common building materials are displayed in the table below.

Material Coeff. heat. W/(m2*K)
Alabaster slabs 0.470
Aluminum 230.0
Asbestos (slate) 0.350
Fibrous asbestos 0.150
Asbestos cement 1,760
Asbestos cement slabs 0.350
Asphalt 0.720
Asphalt in floors 0.800
Bakelite 0.230
Concrete on crushed stone 1,300
Concrete on sand 0.700
Porous concrete 1,400
Solid concrete 1,750
Thermal insulating concrete 0.180
Bitumen 0.470
Paper 0.140
Light mineral wool 0.045
Heavy mineral wool 0.055
Cotton wool 0.055
Vermiculite sheets 0.100
Woolen felt 0.045
Construction gypsum 0.350
Alumina 2,330
Gravel (filler) 0.930
Granite, basalt 3,500
Soil 10% water 1,750
Soil 20% water 2,100
Sandy soil 1,160
The soil is dry 0.400
Compacted soil 1,050
Tar 0.300
Wood – boards 0.150
Wood – plywood 0.150
Hardwood 0.200
Chipboard 0.200
Duralumin 160.0
Reinforced concrete 1,700
Wood ash 0.150
Limestone 1,700
Lime-sand mortar 0.870
Iporka (foamed resin) 0.038
Stone 1,400
Multilayer construction cardboard 0.130
Foamed rubber 0.030
Natural rubber 0.042
Fluorinated rubber 0.055
Expanded clay concrete 0.200
Silica brick 0.150
Hollow brick 0.440
Silicate brick 0.810
Solid brick 0.670
Slag brick 0.580
Siliceous slabs 0.070
Brass 110.0
Ice 0°C 2,210
Ice -20°С 2,440
Linden, birch, maple, oak (15% humidity) 0.150
Copper 380.0
Mipora 0.085
Sawdust – backfill 0.095
Dry sawdust 0.065
PVC 0.190
Foam concrete 0.300
Polystyrene foam PS-1 0.037
Polyfoam PS-4 0.040
Polystyrene foam PVC-1 0.050
Foam resopen FRP 0.045
Expanded polystyrene PS-B 0.040
Expanded polystyrene PS-BS 0.040
Polyurethane foam sheets 0.035
Polyurethane foam panels 0.025
Lightweight foam glass 0.060
Heavy foam glass 0.080
Glassine 0.170
Perlite 0.050
Perlite-cement slabs 0.080
Sand 0% moisture 0.330
Sand 10% moisture 0.970
Sand 20% humidity 1,330
Burnt sandstone 1,500
Facing tiles 1,050
Thermal insulation tile PMTB-2 0.036
Polystyrene 0.082
Foam rubber 0.040
Portland cement mortar 0.470
Cork board 0.043
Cork sheets are lightweight 0.035
Cork sheets are heavy 0.050
Rubber 0.150
Ruberoid 0.170
Slate 2,100
Snow 1,500
Scots pine, spruce, fir (450…550 kg/cube.m, 15% humidity) 0.150
Resinous pine (600…750 kg/cube.m, 15% humidity) 0.230
Steel 52.0
Glass 1,150
Glass wool 0.050
Fiberglass 0.036
Fiberglass 0.300
Wood shavings – stuffing 0.120
Teflon 0.250
Paper roofing felt 0.230
Cement boards 1,920
Cement-sand mortar 1,200
Cast iron 56.0
Granulated slag 0.150
Boiler slag 0.290
Cinder concrete 0.600
Dry plaster 0.210
Cement plaster 0.900
Ebonite 0.160

Because it indicates how well a material conducts heat, the thermal conductivity coefficient of a material is an important consideration in building and renovation. The composition, structure, density, and temperature of the material are some of the variables that affect this property. Comprehending these interdependencies facilitates the appropriate selection of materials for effective thermal management in buildings, guaranteeing both energy efficiency and comfort. Understanding how heat is transferred by various materials can help with decisions about insulation, building design, and energy-saving techniques, all of which lead to more economical and environmentally friendly building methods.

Table of thermal conductivity of thermal insulation materials

The minimum required value for the thermal conductivity of walls, floors, and roofs is determined for each region. This will help you keep your home warmer in the winter and cooler in the summer. The "pie" made up of the walls, floors, and ceilings is composed of various materials, and their thickness is considered to ensure that the final amount is within your region’s recommended range, or even slightly higher.

It is important to consider that certain materials—not all—conduct heat much more effectively in humid environments when selecting materials. The thermal conductivity for this condition is used in the calculations if it is possible for such a situation to arise during operation for an extended length of time. The table provides the thermal conductivity coefficients of the primary insulating materials.

The thermal conductivity material code is named VT/(m · ° C).

In a dry state With normal humidity With high humidity
Woolen felt 0.036-0.041 0.038-0.044 0.044-0.050
Stone mineral wool 25-50 kg/m3 0.036 0.042 0, 045
Stone mineral wool 40-60 kg/m3 0.035 0.041 0.044
Stone mineral wool 80-125 kg/m3 0.036 0.042 0.045
Stone mineral wool 140-175 kg/m3 0.037 0.043 0.0456
Stone mineral wool 180 kg/m3 0.038 0.045 0.048
Glass wool 15 kg/m3 0.046 0.049 0.055
Glass wool 17 kg/m3 0.044 0.047 0.053
Glass wool 20 kg/m3 0.04 0.043 0.048
Glass wool 30 kg/m3 0.04 0.042 0.046
Glass wool 35 kg/m3 0.039 0.041 0.046
Glass wool 45 kg/m3 0.039 0.041 0.045
Glass wool 60 kg/m3 0.038 0.040 0.045
Glass wool 75 kg/m3 0.04 0.042 0.047
Glass wool 85 kg/m3 0.044 0.046 0.050
Expanded polystyrene (foam plastic, EPS) 0.036-0.041 0.038-0.044 0.044-0.050
Extruded polystyrene foam (EPS, XPS) 0.029 0.030 0.031
Foam concrete, aerated concrete with cement mortar, 600 kg/m3 0.14 0.22 0.26
Foam concrete, aerated concrete with cement mortar, 400 kg/m3 0.11 0.14 0.15
Foam concrete, aerated concrete with lime mortar, 600 kg/m3 0.15 0.28 0.34
Foam concrete, aerated concrete with lime mortar, 400 kg/m3 0.13 0.22 0.28
Foam glass, crumbs, 100 – 150 kg/m3 0.043-0.06
Foam glass, crumbs, 151 – 200 kg/m3 0.06-0.063
Foam glass, crumbs, 201 – 250 kg/m3 0.066-0.073
Foam glass, crumbs, 251 – 400 kg/m3 0.085-0.1
Foam block 100 – 120 kg/m3 0.043-0.045
Foam block 121-170 kg/m3 0.05-0.062
Foam block 171 – 220 kg/m3 0.057-0.063
Foam block 221 – 270 kg/m3 0.073
Ecowool 0.037-0.042
Polyurethane foam (PPU) 40 kg/m3 0.029 0.031 0.05
Polyurethane foam (PPU) 60 kg/m3 0.035 0.036 0.041
Polyurethane foam (PPU) 80 kg/m3 0.041 0.042 0.04
Cross-linked polyethylene foam 0.031-0.038
Vacuum 0
Air +27°C. 1 atm 0.026
Xenon 0.0057
Argon 0.0177
Airgel (Aspen aerogels) 0.014-0.021
Slag 0.05
Vermiculite 0.064-0.074
Foam rubber 0.033
Cork sheets 220 kg/m3 0.035
Cork sheets 260 kg/m3 0.05
Basalt mats, canvases 0.03-0.04
Tow 0.05
Perlite, 200 kg/m3 0.05
Expanded perlite, 100 kg/m3 0.06
Linen insulating boards, 250 kg/m3 0.054
Polystyrene concrete, 150-500 kg/m3 0.052-0.145
Granulated cork, 45 kg/m3 0.038
Mineral cork on a bitumen basis, 270-350 kg/m3 0.076-0.096
Cork flooring, 540 kg/m3 0.078
Technical cork, 50 kg/m3 0.037

A portion of the data comes from standards (SNiP 23-02-2003, SP 50.13330.2012, SNiP II-3-79* (Appendix 2)) that specify the properties of specific materials. The websites of the manufacturers provide information on the materials that are not listed in the standards. Since there are no set standards, they could differ greatly between manufacturers; therefore, when making a purchase, be sure to consider the qualities of each material.

Thermal conductivity coefficient for metals and non-metallic solids

Heat is transferred into metals through the electron gas, which makes all metals excellent heat conductors. Conversely, materials with a fibrous structure, ionic, and covalent bonds are good thermal insulators—that is, they conduct heat poorly. In order to fully answer the question of what thermal conductivity is, it should be noted that heat can only be transferred in a vacuum by electromagnetic radiation because this process depends on the presence of a substance, whether it be through convection or conduction.

The values of the thermal conductivity coefficients in J/(s*m*K) for a few metals and non-metals are listed below:

  • steel – 47-58 depending on the steel grade;
  • aluminum – 209.3;
  • bronze – 116-186;
  • zinc – 106-140 depending on purity;
  • copper – 372.1-385.2;
  • brass – 81-116;
  • gold – 308.2;
  • silver – 406.1-418.7;
  • rubber – 0.04-0.30;
  • fiberglass – 0.03-0.07;
  • brick – 0.80;
  • wood – 0.13;
  • glass – 0.6-1.0.

As a result, metals have thermal conductivities that are two to three orders of magnitude higher than those of insulators, providing a startling illustration of what low thermal conductivity really means.

In many industrial processes, thermal conductivity is crucial. In some processes, the goal is to reduce thermal conductivity by using heat-insulating materials and reducing the contact area; in other processes, the goal is to increase it by using good thermal conductors and increasing the contact area.

Wood thermal conductivity table

Unofficially, wood is considered an elite building material when it comes to houses. And the high cost and environmental friendliness are not the only reasons for this. The lowest coefficients of thermal conductivity are found in wood. Furthermore, the breed has a direct impact on these values. Cedar and cork have the lowest coefficients among building species, at just 0.095 W/(m√C). Constructing homes using the latter is highly costly and challenging. However, because of its excellent sound insulation properties and low heat conductivity, cork is valued for use in flooring. The strength and thermal conductivity tables for different rocks are shown below.


What does thermal conductivity depend on??

Thus, as we’ve already seen, the intensity of heat transfer through a particular material is characterized by the thermal conductivity coefficient, or λ (lambda).

For instance, gases have the lowest thermal conductivity while metals have the highest. Heat is effectively transmitted by all electrical conductors, including copper, aluminum, gold, and silver. On the other hand, heat is retained by electrical insulators, such as rubber, plastic, and wood.

What other factors besides the material itself can influence this indicator? Take the temperature, for instance. In contrast to metals, insulating materials’ thermal conductivity rises with temperature while decreasing for metals. Impurities may also have an impact. The thermal conductivities of dissimilar metal alloys are typically lower than those of their alloying elements.

Generally speaking, a substance’s porosity, density, and structure are the primary determinants of its thermal conductivity. As a result, if a manufacturer makes a low lambda value at a low material density claim, it is typically a publicity stunt and has nothing to do with reality.

The influence of moisture on the thermal conductivity of building materials

Once more, it is evident from real-world examples of the application of building materials that moisture negatively impacts their longevity. It has been observed that the CTP value increases with the amount of moisture that the building material is exposed to.

They make different attempts to keep moisture out of the materials used in construction. Considering the rise in the coefficient for wet building materials, this measure is entirely appropriate.

It is not hard to defend such a claim. A building material’s structure is affected by moisture in two ways: it humidifies the air within its pores and replaces some of the surrounding air.

A notable increase in the material’s thermal conductivity is evident when one considers that water’s thermal conductivity coefficient parameter is 0.58 W/m°C.

A more detrimental consequence should be mentioned as well: water entering the porous structure gets frozen and turns into ice.

As a result, assuming the ice thermal conductivity parameters of 2.3 W/m°C, it is simple to compute an even larger increase in thermal conductivity. an increase in water’s thermal conductivity parameter of about four times.

It is important to carefully consider the possibility of some building materials freezing, which would increase thermal conductivity. This is one of the reasons for switching from winter to summer construction.

This clarifies the specifications for protecting insulating building materials from moisture during construction. After all, there is a direct relationship between the quantitative humidity and the level of thermal conductivity.

An additional point that seems equally important is the opposite, which occurs when the building material’s structure is heated significantly. The temperature that is too high also causes a rise in thermal conductivity.

The molecules that comprise the building material’s structural basis have more kinematic energy, which is the cause of this.

It is true that some materials belong to a class whose structure, when heated to a high degree, improves its thermal conductivity characteristics. Metal is one of these materials.

If most common building materials change their thermal conductivity in an increasing direction when heated to high temperatures, then heating a metal to high temperatures has the opposite effect, causing the metal’s CTP to decrease.

Building materials with minimum CTP

Studies show that dry air has a minimum value of thermal conductivity of roughly 0.023 W/m °C.

From the perspective of using dry air in the construction material’s structure, a design that allows dry air to reside inside the closed, multiple spaces of a small volume is required. From a structural perspective, the image of multiple pores inside the structure presents such a configuration.

Thus, it follows logically that building materials with a porous internal structure should be used in the small level of CTP.

Furthermore, the value of thermal conductivity approaches the value of dry air’s thermal conductivity based on the maximum allowable porosity of the material.

A construction material with low thermal conductivity is produced in part by the porous structure. The better the CFC can be obtained, the more pores of various volumes are contained in the material’s structure.

To achieve porosity in building materials, a number of technologies are employed in modern production.

The following technologies are specifically utilized:

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

Note that density, heat capacity, and temperature conductivity are all directly correlated with the thermal conductivity coefficient.

The formula below can be used to determine the thermal conductivity value:

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

The porosity value is inversely proportional to the average density and thermal conductivity values. Thus, the following formula can be used to determine how much thermal conductivity depends on the density of the building material’s structure:

Λ is equal to 1.16 − 0.0196+0.22d2 – 0.16.

Where: the value of density, d. This is formula B.P. Nekrasov, which shows how a material’s density affects the CFC value of that material.

Application of thermal conductivity in practice

All materials are typically separated into two categories in construction: structural materials and thermal insulation. Despite having the highest thermal conductivity, structural raw materials are nonetheless utilized to build fences, walls, and ceilings. The table of thermal conductivity of building materials indicates that, for walls composed of reinforced concrete, the structure’s thickness should be approximately 6 meters to minimize heat exchange with the surrounding air. In this scenario, the structure will end up being enormous, cumbersome, and expensive.

What thickness of various materials will have the same thermal conductivity coefficient is a good example.

As a result, extra heat-insulating materials should be given special consideration when building. Buildings constructed of wood or foam concrete may not require a layer of thermal insulation, but even in these cases, the structure’s thickness must be at least 50 cm.

Must be aware of! Thermal conductivity values of thermal insulation materials are minimal.

How to determine heat loss

The primary building components that allow heat to escape are:

  • doors (5-20%);
  • gender (10-20%);
  • roof (15-25%);
  • walls (15-35%);
  • windows (5-15%).

A thermal imager is utilized to ascertain the extent of heat loss. Yellow and green indicate reduced heat loss, while red denotes the most challenging areas. The blue highlights indicate the areas with the fewest losses. A quality certificate is given to the material after the thermal conductivity value is measured in a lab.

The following factors determine the value of thermal conductivity:

  1. Porosity. Pores indicate heterogeneity of structure. When heat passes through them, cooling will be minimal.
  2. Humidity. A high level of humidity provokes the displacement of dry air by droplets of liquid from the pores, which is why the value increases many times over.
  3. Density. Higher density promotes more active particle interactions. As a result, heat exchange and temperature balancing proceed faster.

A thorough understanding of a material’s thermal conductivity is necessary to make wise decisions when building or renovating. The degree to which a material can conduct heat is determined by this property, which has an immediate impact on a building’s comfort and energy efficiency. Metals and other materials with a high thermal conductivity transfer heat rapidly, whereas insulation and other materials with a low thermal conductivity slow down the transfer of heat and help to maintain a steady interior temperature.

The thermal conductivity of a material is influenced by various factors. The material’s composition is important; porous, lighter materials usually have lower thermal conductivity than dense, lighter materials. Since most materials conduct heat more effectively at higher temperatures, temperature also matters. Furthermore, a material’s ability to conduct heat is greatly influenced by its moisture content; generally speaking, wet materials conduct heat more effectively than dry ones.

To get the appropriate thermal performance, it’s critical to take thermal conductivity into account when choosing materials for building or remodeling. Selecting the appropriate materials can result in substantial energy savings as well as increased comfort. Through comprehension and utilization of this information, constructors and remodelers can produce more economical, eco-friendly, and cozy living environments.

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