Basic thermal engineering parameters and terms

Thermal conductivity of the materials and construction elements depends mainly on:

   - conductivity of the main material (the metals, for example, conduct heat very well, whereas plastics do not)

  - the amount of closed air (or another gas / or vacuum), and the way it is closed  

The more poorly a material conducts heat – the better insulator it is. In much of the construction elements, the decisive factor is the closed air. For example, the unicameral windows – the glass itself is a good thermal conductor. Thanks to the layer of air between the two glasses, however, there is a greater insulating effect than if the space was filled with glass.

To illustrate the thermal insulation properties of the different materials are used several different variables, which are often used wrong and confused with one another.

Prior to clarifying these concepts is necessary to clarify the basic units of measurement: The thermal losses are measured in watts [W]. The unit watt does not mean amount of heat, but heat flow (1 W = 1 J/s).

A room or an entire house constantly gives heat outside, which theoretically could be described in amount of watts. For example, a single-family house in the winter days could have heat losses of about 6 kilowatts. If additionally for some time the windows are opened, the heat losses increase to 10 kW. However, since rarely the heat losses are being calculated for the entire house, they are being assigned to a particular area. Thus the unit of heat losses is not given for a “house”, but for a square meter [W/m2].

In order to determine the heat losses of a house the temperature difference is also crucial. If the temperature outside and inside is 20°C, then there is no heat loss, no matter how bad the thermal insulation is. However, if the outside temperature is 15°C there is a certain amount of heat loss, and at 10°C – double the amount. This temperature difference in the physics is measured not in °C but in Kelvin [К].

If we want to execute the thermal insulation of the house according to the technical requirements it is needed to combat a number of terms and formulas. Without the appropriate education this task is almost insurmountable. For this reason, here we try to reduce everything to the basic essentials, so that even the non-professional to be able to calculate the thermal protection.



The thermal conductivity represents the ability of the materials to conduct heat through themselves in the presence of a temperature difference on both their opposite surfaces. The speed, with which, when a particular material is heated, it transfers heat from one particle to another, determines the thermal conductivity.

in solid bodies

The thermal conductivity is illustrated easily using a parallelepiped with length [s] and section [А], of which one side is connected with a cold medium, and the opposite – with warm. Всички други страни са възможно най-добре топлоизолирани.

The thermal conductivity λ of a particular material indicates the amount of heat, which passes at a constant temperature for 1s in an area of А=1m2 for one layer of material with thickness S=1m, when the temperature difference between the two edges is 1K. Thus, the unit of thermal conductivity is obtained W/mK.

On the size of the coefficient of thermal conductivity influence:

   -  the bulk density (with its increasing is increased the λ, and therefore its thermal insulation properties are being reduced);

   - the moisture of the material (increasing the amount of moisture in the pores and capillaries of the material displaces from them the air, which has a smaller λ than that of water, which leads to increasing of the λ of the material itself, i.e. it becomes a poor insulator);

   - the temperature (by increasing of the temperature of the materials is also increased their λ – metals are the only exception).

The highest thermal conductivity of all solid bodies has the diamond (2000 ÷ 2500 W/mK), and from the metals – the silver (429 W/mK). As a rule it applies: The material, which conducts well electricity (silver, copper), also conducts heal well. The material, which conducts heat badly (paper, wool) also, conducts electricity poorly.

in liquids and gases

At liquids and gasses the thermal conductivity varies, depending on the currents and turbulence in them.

When the currents and turbulence are prevented, the remaining thermal conductivity of most of the gases is very little. This fact is used in many thermal insulation materials (polystyrene, glass wool, etc.), which consist mainly from air or other gasses, that are prevented from circulation from the surrounding solid material.

Conversely, over fluid liquids (foe example helium-4 below 1,6 К) have almost infinite thermal conductivity.

in vacuum

There is no thermal conductivity in vacuum. The thermal conductivity is carried out only by thermal radiation. It is used, for example, in the thermos in order to reduce the heat transfer. In order to prevent the thermal transfer by thermal radiation, the glass or steel surfaces that insulate the vacuum are made mirror-like.



The thermal conductivity of a particular material indicates the amount of heat, which in the maintenance of a constant temperature passes for 1 second through the surface of 1 m2 and thickness 1m, when the temperature difference between the two ends is 1К. The unit for thermal conductivity is W/mK.

In this value is not taken into account the thickness of the insulating material. Only after the thermal insulation material with a specific lambda is chosen, then the thickness of the insulation layer can be determined (for example 5 cm), and it can be predicted how good the thermal insulation will (and calculate the U-value). The smaller the lambda value is, the better the thermal insulation properties are.

For example, the lambda value cannot be determined for a window. The lambda values are suitable for comparing the characteristics of pure materials such as: air, vacuum, glass, water, iron, wool, grease. The lambda value is the physical quantity of the primary material, as its density or color. This is why there is no sense to be given the lambda value for example of “5 cm of the material”.

The thermal conductivity of the different materials is leading when determining the thickness of the planned exterior wall. From the figure you can see that the insulation board of polystyrene with thickness 1,7 cm insulates as well as a concrete wall with thickness  изолира 91 cm.

In terms of static, a normal brick (17,5 cm) is sufficient for a house, however, because of the thermal insulation, it should be 36,5 cm.


If we compare two exterior walls, the one 17,5 cm + 8,0 cm thermal insulation, and the other 36,5 brick wall, we will establish that in a single-family house from 10m х 10m, the first option saves us about 8 m2.



The heat transmissibility is the property of the materials to give or receive heat from a direct contact with another body.

The coefficient of heat transmissibility indicates the amount of hear, that is transmitted to the wall (or vice versa) from the getting in contact with it air in an area of 1m2 for 1s, if the temperature difference between the surface and the air is 1К. The unit of measurement is W/m2K.

This coefficient describes the ability of a gas or liquid to give or receive energy from/to the surface of a certain material.



The heat absorption is the property of the materials to absorb or give heat in the presence of temperature difference.

The heat absorption describes the ability of a body to accumulate energy in the form of heat. It represents the amount of heat which is absorbed (given) from a body of mass 1kg when changing its temperature with 1К.



The thermal conductance is a material constant, describing the temporal changes in the spatial distribution of the temperature through heat exchange, obtained as a result of a certain decrease in the temperature.

Contrary to the thermal conductivity, the thermal conductance describes not only the stationary behavior like in the thermal exchange. Non-stationary effects, formed, for example, during the transmission in the interior premises of the temperature cycles, occurring due to the day and night variations of the outside temperature, cannot be described only with the use of thermal conductivity.

How hot or warm a body “feels” in the first moment is determined by the thermal conductance, and after a while (when the temperature filed becomes stationary) only by the thermal conductivity.



Practically the heat penetration can be felt when with bare hands we touch different materials with the same temperature. Materials with high coefficient of heat penetration Материали с (such as metals) feel particularly cold if their temperature is lower than that of the skin, and materials with low coefficient of heat penetration (insulating materials, wood) feel warmer even when their temperature is the same as the that of the skin. This is the reason of the false thermal insulation effect of the underlay wallpapers from a few millimeters thick foam. In this way the thermal insulation of an exterior wall changes faintly, but after the application the this kind of wallpapers, the surface of the wall feels warm.



The heat passing is the property of the surrounding building elements to pass through them heat when there is an existing temperature difference in the air from both sides. This includes the heat exchange between the air with higher temperature and the corresponding surface of the surrounding element, thermal conductance of the heat flow through the surrounding element and heat transfer from the surface of the wall to the air with lower temperature.

Heat transfer coefficient

The heat transfer coefficient indicates the amount of heat, which transfers for 1 second through a surface of 1 m2 of material with thickness [s], when the temperature difference between the two surfaces is 1 К. It is dependent on the thermal conductivity of the material and its thickness. The unit of measurement is in W/m2K.

The higher the heat transfer coefficient is, the worse are the thermal insulation properties of the material.




Heat transfer resistance

In order to evaluate a construction material, decisive, however, is not the amount of heat that passes through it (the heat transfer coefficient), but the resistance to the heat transfer it has. Mathematically this resistance represents the reciprocal of the heat transfer coefficient.

The higher the heat transfer resistance is, the better are the thermal insulation properties of the material.


7. U - VALUE

The U-value is the measure of the passage of heat flow through the surrounding element, consisting of one or more layers of material, if on both sides there are different air temperatures. It represents the amount of heat, which flows for 1s between the surrounding element and the bordering air though a surface of 1m2, when the difference in the temperatures is 1К. The unit of measurement of the U-value is W/m2K.

Particularly widespread, the U-value has in the construction, where it is used to determine heat losses when passing through construction elements and it is one of the most important criteria for energy assessment of a building.

The U-value is the measure of “heat permeability” and the thermal insulation properties of the construction elements – for example of the precise glazing or of a specific window. Construction elements with smaller U-values pass less heat than these with bigger U-values.

The U-value of a surrounding construction element depends of the thermal conductivity of the used materials, the thickness of the layers of these materials, the geometry of the building elements (straight wall, curved cylindrical wall, etc.) and the conditions of heat transfer of their surfaces to the individual surrounding fluids (air, water, etc.).



The U-value is the most important measure for describing and assessing the energy performance of a construction element. Contrary to the λ, U-value is the one that can be given for the ready-made product – for example – for a brick, a window, etc. This value is more suitable in the practice, as it relates to ready-to-use materials and construction elements, instead of the raw materials they are made from, like the lambda value. When doubling the thickness it is also double increased the U-value, while the λ remains the same.

Indicatively applies: U-value * 8,4 = Energy loss in liters diesel per m2 for a year

For example: a room (4 external wall) with an area of 15m2 and U-value of 0,6 (equivalent to a brick) 0,6 W/m2K * 15 m2 * 8,4 = 75,6 liters diesel for e year

In Europe after 1 February 2002 in accordance with the adopted regulations for energy efficiency should the annual energy consumption Qp and the transmissible heat losses HT´ to observe some limits. U-value enters into the calculation of the transmissible heat losses, and they enter in the calculation of the annual energy consumption. Additionally, the regulations prescribe limits on heat transfer coefficients for certain construction elements and details when these should be newly installed or replaced.


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