Thermotechnical calculation of enclosing structures: an example of calculation and design. Formula of thermotechnical calculation of enclosing structures

Creating comfortable conditions for living or working is the primary task of construction. A significant part of the territory of our country is located in the northern latitudes with a cold climate. Therefore, maintaining a comfortable temperature in buildings is always relevant. With rising energy tariffs, lower energy consumption for heating comes to the fore.

Climatic characteristics

The choice of wall and roof construction depends primarily on the climatic conditions of the construction area. To determine them, you must refer to SP131.13330.2012 “Construction climatology”. The following quantities are used in the calculations:

• the temperature of the coldest five-day security of 0.92, indicated by Tn;
• average temperature, indicated by Thoth;
• duration is indicated by ZOT.

For an example for Murmansk, the values ​​have the following meanings:

• Tn = -30 deg;
• That = -3.4 deg;
• ZOT = 275 days.

In addition, it is necessary to set the estimated temperature inside the TV, it is determined in accordance with GOST 30494-2011. For housing, you can take TV = 20 degrees.

To perform the heat engineering calculation of building envelopes, the GSOP value (degree-day of the heating period) is preliminarily calculated:
GSOP = (TV - That) x ZOT.
In our example, GSOP = (20 - (-3.4)) x 275 = 6435.

Main characteristics

For the correct choice of materials for enclosing structures, it is necessary to determine what thermotechnical characteristics they should have. The ability of a substance to conduct heat is characterized by its thermal conductivity, is indicated by the Greek letter l (lambda) and is measured in W / (m x deg.). The ability of the structure to retain heat is characterized by its resistance to heat transfer R and is equal to the ratio of thickness to thermal conductivity: R = d / l.

If the structure consists of several layers, the resistance is calculated for each layer and then summed.

Heat transfer resistance is the main indicator of the outdoor structure. Its value should exceed the normative value. Performing the heat engineering calculation of the building envelope, we must determine the economically sound composition of the walls and roof.

Thermal conductivity

The quality of thermal insulation is determined primarily by thermal conductivity. Each certified material undergoes laboratory tests, as a result of which this value is determined for the operating conditions “A” or “B”. For our country, the conditions of operation "B" correspond to most regions. Performing the heat engineering calculation of the building envelope, this value should be used. The values ​​of thermal conductivity are indicated on the label or in the passport of the material, but if they are not, you can use the reference values ​​from the Code of Practice. Values ​​for the most popular materials are listed below:

• Masonry from ordinary brick - 0.81 W (m x deg.).
• Masonry made of silicate brick - 0.87 W (m x deg.).
• Gas and foam concrete (with a density of 800) - 0.37 W (m x deg.).
• Coniferous wood - 0.18 W (m x city).
• Extruded polystyrene foam - 0.032 W (m x deg.).
• Mineral wool plates (density 180) - 0.048 W (m x deg.).

Standard value of heat transfer resistance

The calculated value of the heat transfer resistance should not be less than the base value. The base value is determined according to table 3 of SP50.13330.2012 “ Thermal protection of buildings”. The table defines the coefficients for calculating the basic values ​​of resistance to heat transfer of all building envelopes and types of buildings. Continuing the initiated thermal engineering calculation of enclosing structures, an example of calculation can be represented as follows:

• Rsten = 0.00035x6435 + 1.4 = 3.65 (mx deg / W).
• = 0.00056435 + 2.2 = 5.41 (mx deg / W).
• Rcherd = 0.00045x6435 + 1.9 = 4.79 (mx deg / W).
• Rokna = 0.00005x6435 + 0.3 = 0.62 (mx deg / W).

The thermal engineering calculation of the external building envelope is carried out for all structures that close the “warm” circuit - ground floor or technical flooring, external walls (including windows and doors), combined coating or overlapping of an unheated attic. Also, the calculation must be performed for internal structures, if the temperature difference in adjacent rooms is more than 8 degrees.

Thermotechnical calculation of walls

Most walls and ceilings are multilayered and heterogeneous in design. The thermal engineering calculation of building envelopes of a multilayer structure is as follows:
R = d1 / l1 + d2 / l2 + dn / ln,
where n are the parameters of the nth layer.

If we consider a brick plastered wall, we get the following design:

• the outer layer of plaster is 3 cm thick, the thermal conductivity is 0.93 W (m x deg.);
• full-clay clay brick masonry 64 cm, thermal conductivity 0.81 W (m x deg.);
• the inner layer of plaster is 3 cm thick, the thermal conductivity is 0.93 W (m x deg.).

The formula for the heat engineering calculation of enclosing structures is as follows:

R = 0.03 / 0.93 + 0.64 / 0.81 + 0.03 / 0.93 = 0.85 (mx deg / W).

The obtained value is significantly less than the previously determined base value of the heat transfer resistance of the walls of a residential building in Murmansk 3.65 (m x deg / W). The wall does not meet regulatory requirements and needs to be insulated. For wall insulation we use mineral wool boards with a thickness of 150 mm and a thermal conductivity of 0.048 W (m x deg.).

Having picked up the insulation system, it is necessary to perform a verification thermotechnical calculation of the building envelope. An example of calculation is given below:

R = 0.15 / 0.048 + 0.03 / 0.93 + 0.64 / 0.81 + 0.03 / 0.93 = 3.97 (mx deg / W).

The calculated value obtained is greater than the base one - 3.65 (mx deg / W), the insulated wall meets the requirements of the standards.

The calculation of overlappings and combined coatings is performed similarly.

Thermotechnical calculation of floors in contact with the soil

Often in private homes or public buildings, the floors of the first floors are carried out on the ground. The heat transfer resistance of such floors is not standardized, but at least the floor design should not allow dew to fall. Calculation of structures in contact with the soil is performed as follows: the floors are divided into strips (zones) of 2 meters wide, starting from the outer border. There are up to three such zones; the remaining area belongs to the fourth zone. If the floor structure does not provide for effective insulation, then the heat transfer resistance of the zones is taken as follows:

• 1 zone - 2.1 (m x city / W);
• 2 zone - 4.3 (m x city / W);
• 3 zone - 8.6 (m x city / W);
• 4 zone - 14.3 (mx city / W).

It is easy to see that the farther the floor is from the external wall, the higher its resistance to heat transfer. Therefore, they are often limited to warming the perimeter of the floor. At the same time, the heat transfer resistance of the insulated structure is added to the heat transfer resistance of the zone.
Calculation of resistance to heat transfer to the floor must be included in the general heat engineering calculation of building envelopes. An example of calculating floors on the ground will be considered below. Take a floor area of ​​10 x 10, equal to 100 sq. M.

• The area of ​​1 zone will be 64 square meters.
• The area of ​​2 zones will be 32 square meters.
• The area of ​​3 zones will be 4 square meters.

The average value of the resistance to heat transfer of the floor through the soil:
Rpola = 100 / (64 / 2.1 + 32 / 4.3 + 4 / 8.6) = 2.6 (mx deg / W).

After warming the perimeter of the floor with a polystyrene foam plate 5 cm thick, a strip 1 meter wide, we obtain the average value of the heat transfer resistance:

Rpola = 100 / (32 / 2.1 + 32 / (2.1 + 0.05 / 0.032) + 32 / 4.3 + 4 / 8.6) = 4.09 (mx deg / W).

It is important to note that in this way not only floors are calculated, but also the structures of walls in contact with the ground (walls of a buried floor, a warm basement).

Thermotechnical calculation of doors

In a slightly different way, the base value of the heat resistance of the entrance doors is calculated. To calculate it, you will first need to calculate the heat transfer resistance of the wall according to the sanitary-hygienic criterion (dew drop):
Pst = (Tv - Tn) / (DTn x av).

Here DTn - the temperature difference between the inner surface of the wall and the air temperature in the room, is determined by the Code of Practice and for housing is 4.0.
AB - heat transfer coefficient of the inner surface of the wall, according to the joint venture is 8.7.
The base value of the doors is taken equal to 0.6xRst.

For the selected door design, it is required to perform a verification thermal engineering calculation of the enclosing structures. Example of calculating the front door:

Rdv = 0.6 x (20 - (- 30)) / (4 x 8.7) = 0.86 (m x deg / W).

A door insulated with a 5 cm thick mineral wool slab will correspond to this calculated value. Its heat transfer resistance will be R = 0.05 / 0.048 = 1.04 (mx deg / W), which is more than the calculated one.

Comprehensive requirements

Walls, ceilings, or coatings are calculated to verify item-wise regulatory requirements. The set of rules also established a complete requirement characterizing the quality of insulation of all enclosing structures as a whole. This value is called the "specific heat-shielding characteristic." Not a single thermotechnical calculation of enclosing structures is possible without its verification. An example of a JV calculation is given below.

Cob = 88.77 / 250 = 0.35, which is less than the normalized value of 0.52. In this case, the area and volume are taken for the house with dimensions of 10 x 10 x 2.5 m. Heat transfer resistance is equal to the basic values.

The normalized value is determined in accordance with the joint venture, depending on the heated volume of the house.

In addition to the complex requirements, to compile the energy passport, they also perform the heat engineering calculation of the building envelope, an example of the design of a passport is given in the appendix to SP50.13330.2012.

Uniformity coefficient

All the above calculations are applicable for homogeneous structures. Which in practice is quite rare. To take into account inhomogeneities that reduce the resistance to heat transfer, a correction factor for heat engineering uniformity is introduced - r. It takes into account the change in heat transfer resistance introduced by window and door openings, external corners, inhomogeneous inclusions (for example, jumpers, beams, reinforcing belts), cold bridges , etc.

The calculation of this coefficient is quite complicated, therefore, in a simplified form, you can use the approximate values ​​from the reference literature. For example, for masonry - 0.9, three-layer panels - 0.7.

Effective insulation

Choosing a home insulation system, it is easy to make sure that it is almost impossible to fulfill modern thermal protection requirements without using an effective insulation. So, if you use traditional clay brick, you will need masonry several meters thick, which is not economically feasible. At the same time, the low thermal conductivity of modern heaters based on polystyrene foam or stone wool allows you to limit yourself to a thickness of 10-20 cm.

For example, to achieve a baseline heat transfer resistance of 3.65 (m x deg / W), you would need:

• 3 m thick brick wall;
• masonry of foam concrete blocks 1.4 m;
• mineral wool insulation 0.18 m.