Technical article
Freezing of structures in winter and overheating in summer, the formation of condensation and, as a consequence, a reduction in their service life, high energy consumption of the building are the main results of errors made in thermal calculations. In modern construction, the level of thermal resistance is important parameter enclosing structures along with their load-bearing capacity. The requirements for creating a reliable, environmentally friendly living environment with reasonable energy consumption are formulated by the Research Institute of Building Physics, subordinate to the Ministry of Construction of the Russian Federation Russian Academy architecture and construction sciences" (NIISF RAASN). Since the entry into force of the set of rules developed by him SP 50.13330.2012 “Thermal protection of buildings. Updated edition of SNiP 02/23/2003” approach to determining the reduced resistance of enclosing structures has changed significantly. Now, instead of the usual tabular values of the coefficient of thermal uniformity of enclosing structures, it is necessary to calculate each building envelope separately. What advantages does it give new technique calculation in practice?
As an example of a building envelope, consider the combined roof covering of a residential apartment building. When carrying out calculations in accordance with the methodology for determining the reduced resistance described in SNiP 02/23/2003, we will not find tabulated homogeneity values for these types of structures. Therefore, all that remains is to rely on your intuition and choose these values at random. Or, relying on data from structures with similar values, such as attic floors, the homogeneity value of which is in the range from 0.5 to 0.9.
When solving the problem according to the standards described in Appendix E SP 50.13330.2012, we can accurately determine, based on the specific geometry, the value of the coefficient of thermal homogeneity of the structure or fragment under consideration. For a combined roof covering, we determine the flat, linear and point elements that make up the enclosing structure. Let's list the most common ones. Flat refers to the area of the roof along the surface; linear refers to the area adjacent to different types parapets, roof exits, ventilation shafts, etc., and to the point ones - fastening of insulation and waterproofing. Next, you will need to find the specific geometric indicator of each of the elements present on the roof. By determining: its area for flat ones, its length for linear ones, and the number of pieces for point elements. As a rule, for these types of structures, among the linear elements, the largest specific geometric indicator is those adjacent to the parapet.
Then, it is necessary to calculate the specific heat loss passing through the element. To determine this parameter, you can use ready-made tabular values given in SP 230.1325800.2015, or simulate the node in a specialized program for calculating thermal fields and determine the specific heat loss through the node yourself. The results obtained are entered into a table according to form E2 SP 50.13330.2012 and the reduced heat transfer resistance of the considered fragment of the enclosing structure is calculated using formula E1 SP 50.13330.2012.
Now, using an example, let’s look at the combined roof of a conventional section of a residential apartment building. In calculating the reduced resistance, we will take two elements that have the largest geometric index: the surface area of the roof and the junction with the uninsulated parapet. We do not take into account the remaining elements in the calculation.
Initial data for calculation:
The roof surface area is 263 m2;
The length of the connections to the parapet is 101 m;
Conditional heat transfer resistance of a homogeneous part of the roof is 5.526 m 2 * 0 C/W;
Thermal resistance of the insulation layer on the wall is 3 m 2 * 0 C/W;
Thermal conductivity of the parapet base is 0.6 W/m2 * 0 C;
Thermal resistance of the insulation layer on the coating slab is 5 m 2 * 0 C/W;
There is no additional insulation of the parapet.
We will carry out the calculation using the available parameters and enter the results into Table 1 (a form similar to table E2). The values of specific heat losses through the parapet are taken based on the data in Table G.42 SP 230.1325800.2015.
Table 1
The reduced resistance for such a design will be equal to R pr = 2.978 m 2 * 0 C/W. And the value of the thermal homogeneity coefficient is r=0.54.
Example 1: Temperature fields of the junction with the parapet. Option 1.*
Let's make adjustments to the original data. Let's reduce the thermal conductivity of the base to 0.2 W/m2 * 0 C and add insulation 500 mm high on the parapet. The values of specific heat losses through the parapet are taken based on the data in Table G.47 SP 230.1325800.2015.
Let's correct table 1.
Now the reduced resistance for the same design will be equal to R pr = 3.973 m 2 * 0 C/W. And the coefficient of thermal uniformity is r=0.72.
Example 2: Temperature fields of the junction with the parapet. Option 2.*
Thus, by making minor changes to the design of the junction with the parapet and without changing the thickness of the main insulation, we obtain an increase in the value of the reduced resistance by 33% relative to the original value.
Based on the foregoing, we can conclude: the more detailed and rational, not only from the point of view of load-bearing capacity, but also from the point of view of heating engineering, all components are worked out, the less heat the building will lose through the enclosing structures, and the higher the efficiency of using insulation in such designs.
From TECHNONICOL, you can order a complete thermal engineering calculation of a building, according to the SP 50.13330.2012 methodology, or a calculation of a specific unit to determine heat loss and meet sanitary and hygienic requirements.
Figure H.1 - Diagrams of heat-conducting inclusions in enclosing structures
H.1 CALCULATION OF THERMAL HOMOGENEOUSITY COEFFICIENT BY FORMULA (12)
OF THIS CODE OF RULES
Table H.1 - Determination of coefficient
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Coefficient at (Figure H.1) |
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Note - Designations are adopted according to Figure H.1. |
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Calculation example
Determine the reduced heat transfer resistance of a panel with effective insulation (expanded polystyrene) and steel cladding of an industrial building.
Initial data
Panel size 6x2 m. Structural and thermal characteristics of the panel:
thickness of steel cladding 0.001 m, thermal conductivity coefficient 
;
thickness of polystyrene foam insulation 0.2 m, thermal conductivity coefficient 
.
Beading the sheet material along the extended sides of the panel leads to the formation of a heat-conducting inclusion of type IIb (Figure H.1), having a width = 0.002 m.
Calculation procedure
Heat transfer resistance away from the switch and along the heat-conducting switch:
The value of the dimensionless parameter of the heat-conducting inclusion according to Table N.2
0.002·58/(0.2·0.04)=14.5.
Table N.2 - Determination of the coefficient
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#G0Heat-conducting connection diagram according to Figure H.1 |
Values of the coefficient at (according to Figure H.1 |
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Using Table N.2, we determine by interpolation the value
0,43+[(0,665-0,43)4,5]/10=0,536.
Coefficient , according to formula (13)
The coefficient of thermal uniformity of the panel according to formula (12)
Reduced heat transfer resistance according to formula (11)
H.2 CALCULATION OF THERMAL HOMOGENEOUSITY COEFFICIENT BY FORMULA (14)
OF THIS CODE OF RULES
Calculation example
Determine the reduced heat transfer resistance of a single-module three-layer reinforced concrete panel with flexible connections to the window opening of a large-panel residential building of series III-133.
Initial data
The 300 mm thick panel contains outer and inner reinforced concrete layers, which are connected to each other by two hangers (in the piers), a strut located in the lower zone of the window sill, and spacers: 10 at the horizontal joints and 2 in the window slope area (Figure N. 2).
1 - spacers; 2 - loop; 3 - pendants;
4 - concrete thickenings (=75 mm of internal reinforced concrete layer); 5 - strut
Figure H.2 - Construction of a three-layer panel with flexible connections
In #M12293 0 1200037434 4120950664 4294967273 80 2997211231 403162211 2325910542 403162211 2520Table H.4#S shows the calculated parameters of the panel.
In the area of hangers and hinges, the inner concrete layer has thickenings that replace part of the insulation layer.
Calculation procedure
The fencing design contains the following heat-conducting inclusions: horizontal and vertical joints, window slopes, thickening of the internal reinforced concrete layer and flexible connections (suspensions, struts, struts).
To determine the influence coefficient of individual heat-conducting inclusions, we will first calculate the thermal resistance of individual sections of the panel using formula (7):
in the thickening zone of the internal reinforced concrete layer
along the horizontal joint
along a vertical joint
thermal resistance of the panel away from heat-conducting inclusions
Conditional resistance to heat transfer away from heat-conducting inclusions
Since the panel has a vertical axis of symmetry, the following values are determined for half of the panel.
Let's determine the area of half the panel without taking into account the window opening
Panel thickness =0.3 m.
Let's determine the area of influence zones and the coefficient for each heat-conducting inclusion of the panel:
for horizontal joint
2,95/3,295=0,895.
According to table N.3 =0.1. Area of influence zone according to formula (15)
for vertical joint
Table H.3 - Determination of the influence coefficient
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#G0Type of heat-conducting connection |
Influence factor |
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Without adjacent internal fences |
With adjacent internal fences |
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Without ribs |
With ribs thickness, mm |
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Without ribs |
With ribs thickness, mm: |
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Flexible connections with diameter, mm: |
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Notes |
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1 The table shows - thermal resistances, respectively, panels outside the heat-conducting inclusion, joint, thickening of the internal reinforced concrete layer, determined by formula (8); - distances, m, from the longitudinal axis of the window frame to its edge and to inner surface panels. |
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2 Intermediate values should be determined by interpolation. |
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According to table N.3 =0.375. Area of influence zone according to formula (15)
;
for window slopes at = 0.065 m and = 0.18 m, according to table H.3 = 0.374. The area of influence of half of the window opening, taking into account the corner sections, is determined by formula (16)
for concrete thickenings of the internal reinforced concrete layer in the suspension and hinge area at =1.546/3.295=0.469 according to table M.3*=0.78. The total area of the zone of influence of the suspension and hinge thickenings is found using formula (17)
for suspension (rod diameter 8 mm) according to table N.3 = 0.16, area of influence zone according to formula (17)
for a strut (rod diameter 8 mm) according to table N.3 = 0.16, according to formula (17)
for spacers (rod diameter 4 mm) according to table N.3 =0.05.
When determining the total area of the influence zone of five spacers, it should be taken into account that the width of the influence zone from the joint side is limited by the edge of the panel and is 0.09 m. According to formula (18)
Let's calculate using formula (14)
The reduced heat transfer resistance of the panel is determined by formula (11)
Table H.4
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#G0Layer material |
Layer thickness, mm |
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Away from inclusions |
in the suspension and hinge area |
horizontal joint |
vertical joint |
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Outer reinforced concrete layer | ||||||
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Thermal insulation layer - polystyrene foam | ||||||
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Mineral wool liners | ||||||
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Inner reinforced concrete layer | ||||||
APPENDIX P
Description:
In a number of cases*, the specific consumption of thermal energy in old panel buildings and modern monolithic frame houses with two-layer walls made of aerated concrete and facing brick is practically the same. One of the reasons for this phenomenon is that double-layer wall designs are often overrated in terms of their thermal protection parameters.
A. S. Gorshkov, Ph.D. tech. Sciences, Director of the Scientific and Training Center “Monitoring and Rehabilitation of Natural Systems” of the Federal State Autonomous Educational Institution of Higher Education “St. Petersburg State Polytechnic University”
P. P. Rymkevich, Ph.D. physics and mathematics Sciences, Professor of the Department of Physics of the Federal State Educational Institution of Higher Professional Education "Military Space Academy named after. A.F. Mozhaisky"
N. I. Vatin, Doctor of Technical Sciences Sciences, Professor, Director of the Institute of Civil Engineering, Federal State Autonomous Educational Institution of Higher Education "St. Petersburg State Polytechnic University"
In a number of cases * the specific consumption of thermal energy in old panel buildings and modern monolithic frame houses with two-layer walls made of aerated concrete and facing brick is practically the same. One of the reasons for this phenomenon is that double-layer wall structures are often overrated in terms of their thermal protection parameters. Therefore, a calculation was carried out of the reduced heat transfer resistance of a two-layer wall structure, which showed that its thermal characteristics do not meet not only the required, but also the minimum acceptable regulatory requirements. At the design stage for this constructive solution Usually, a thermal uniformity coefficient of 0.9 is specified, which is overestimated in many cases. In addition, designers use unreasonable values for the thermal conductivity of aerated concrete.
Currently, in the practice of design and construction of buildings with a monolithic reinforced concrete frame and floor-by-floor support of external walls on monolithic or prefabricated monolithic reinforced concrete floors, one of the most common options for filling the outer heat-protective shell is a wall design consisting of two layers (Fig. 1):
– an internal non-load-bearing layer made of masonry from aerated concrete blocks with a thickness of 300–400 mm, depending on the region of construction and its climatic parameters;
– an outer facing layer of face brick one or two bricks thick.
Description of the wall fencing design
In the constructive solution under consideration, the inner layer of the wall fencing performs the function of thermal insulation, the outer one – the function of protection from external climatic influences, ensures the required durability of the facades and forms the architectural appearance of the building. It is believed that this design solution satisfies the thermal protection requirements for most regions of the Russian Federation.
In St. Petersburg, the traditional solution is a wall fence in which the thickness of the aerated concrete layer is 375 mm (Fig. 1a).
Regulatory Requirements
In SNiP 23-02–2003 “Thermal protection of buildings” (hereinafter referred to as SNiP 23-02), three thermal protection indicators are established for buildings:
a) individual elements of the building envelope;
b) sanitary and hygienic, including temperature difference between temperatures internal air and on the surface of enclosing structures and the temperature on the inner surface is above the dew point temperature;
c) specific consumption of thermal energy for heating the building, allowing to vary the values of heat-protective properties various types building envelopes, taking into account the space-planning solutions of the building and the choice of microclimate maintenance systems to achieve the standardized value of this indicator.
Reduced heat transfer resistance R r 0 of enclosing structures should be taken no less than the standardized values 1 R req, determined 2 depending on the degree-day of the heating period (hereinafter referred to as GSOP) of the construction area.
GSOP for residential buildings located in St. Petersburg is 3 4 796 °C day, and the normalized value of the reduced heat transfer resistance for the external walls of residential buildings is 4 3.08 m 2 °C/W. In this case, it is allowed to reduce the normalized value of the reduced heat transfer resistance for the walls of residential and public buildings by 37% if the requirements of SNIP 23-02 are met (clause 5.1).
Thus, in relation to the case under consideration, the minimum permissible value of the reduced heat transfer resistance for the external walls of residential buildings designed in St. Petersburg should not be lower than 6 R min = 1.94 m 2 °C/W.
Purpose and objectives of the study
Reduced heat transfer resistance R r 0 for external walls should be calculated for the facade of the building or for one intermediate floor, taking into account the slopes of the openings without taking into account their fillings 7. Let's look at a specific example of how this requirement is met in practice.
To do this, we will calculate the reduced resistance to heat transfer of the external walls of the intermediate floor of a typical multi-apartment residential building with a monolithic-frame structural design and two-layer external walls (Fig. 1) and compare the obtained value with the standardized value R req and minimum acceptable R min values of the reduced heat transfer resistance of the external walls of a residential multi-apartment building.
Initial data for thermal engineering calculations
Construction area - St. Petersburg.
The purpose of the building is residential.
Design temperature: indoor air tв = 20 °С; outside air t n = –26 °C.
Humidity zone – wet.
The humidity conditions in the building premises are normal.
Operating conditions for enclosing structures – “B”.
Thermal characteristics of materials used in wall fencing:
– cement-sand mortar γ o = 1,800 kg/m 3, λ B = 0.93 W/(m °C);
– brickwork from ordinary clay bricks with cement-sand mortar γ o = 1,800 kg/m 3, λ B = 0.80 W/(m °C);
– masonry made of unreinforced wall blocks made of autoclaved aerated concrete with a density γ o = 400 kg/m 3, λ B = 0.14 W/(m °C).
Boundary conditions:
Calculated heat transfer coefficient:
– inner surface of the wall α int = 8.7 W/(m 2 °C);
– window units α int = 8 W/(m 2 °C);
– outer surface of walls, windows α ext = 23 W/(m 2 °C).
Design diagrams of fragments of external walls are presented in Fig. 2.
Calculation results
The reduced heat transfer resistance of the considered fragments of the building's heat-protective shell was calculated based on the calculation of temperature fields. The essence of the method is to model the stationary process of heat transfer through building envelopes using computer programs 8. The method is intended for assessing the temperature regime and calculating the reduced resistance to heat transfer of building envelopes or their fragments, taking into account the geometric shape, location and characteristics of structural and heat-insulating layers, ambient temperatures, and heat transfer coefficients of surfaces.
The value of the reduced heat transfer resistance of the middle intermediate floorR r 0 was determined based on the calculation of the reduced resistance of a number of sections (fragments) R r 0,i taking into account heat losses through the ends of floor slabs, slopes of window openings and balcony doors (see table), in particular the following fragments:
– blank wall without openings, dimensions: height – floor height h= 3.0 m, width – 1.2 m (Fig. 2a);
– walls with window openings, dimensions: height – floor height h= 3.0 m, width - the distance between the axes of the window openings (Fig. 2b);
– walls with a balcony door, dimensions: height – floor height h= 3.0 m, in width - the distance between the axes of the walls (Fig. 2c).
Reduced heat transfer resistance of the external walls of the middle intermediate floor of an apartment building R r 0 taking into account the areas of wall sections along the facades of the building, calculated using formula (1) (see Calculation formulas), is 1.81 m 2 °C/W.
Having calculated the conditional (without taking into account the influence of heat-conducting inclusions on the thermal uniformity of the walls) heat transfer resistance R 0 of the design solution under consideration (formula (2), Calculation formulas), we obtain 2.99 m 2 °C/W.
Hence the coefficient of thermal homogeneity r, considered in the example of an external wall of a typical intermediate floor, taking into account the slopes of openings without taking into account their fillings, will be equal to 0.61 (formula (3), Calculation formulas).
What affects the coefficient of thermal heterogeneity?
For a similar design solution, an even lower calculated value of the coefficient of thermal uniformity was obtained r = 0,48.
Differences in thermal homogeneity coefficients may be due to differences in the design solutions used in the project, quantitative and quality composition heat-conducting inclusions. Also, the thermal heterogeneity of a wall structure depends on the quality of installation.
In particular, it was noted that based on the results of shooting 15 thermograms, the heat transfer resistance of a two-layer outer wall measured under natural conditions was 1.3–1.5 m 2 °C/W (with the conditional heat transfer resistance of a wall enclosure R 0 = 3.92 m 2 °C/W). It turns out that the actual coefficient of thermal homogeneity may be even less than the calculated value and be according to r= (1.3÷1.5) / 3.92 = 0.33÷0.38.
As one of possible reasons identified discrepancies in the noted poor-quality construction due to receipt of construction site irregularly shaped blocks. Indeed, the presence of cracks, faults, potholes and other product defects can lead to excessive consumption of mortar, which acts as an additional heat-conducting inclusion that is not taken into account in the calculation.
It should be noted that the actual humidity of aerated concrete products during the initial period of operation can significantly exceed the calculated one. In this regard, the thermal conductivity of aerated concrete products may be higher than the calculated values accepted in the project, since the thermal conductivity of the material depends on the mass moisture content.
Based on the calculations obtained, we formulate the following conclusions:
- Reduced heat transfer resistance R r 0 of a two-layer wall structure consisting of an internal self-supporting layer of aerated concrete wall blocks of unreinforced density grade D400 and an outer facing layer of facing ceramic bricks 120 mm thick, calculated based on the calculation of temperature fields for a typical intermediate floor of a multi-apartment residential building, is 1.81 m 2 °C/W.
- The design of the considered wall enclosure (Fig. 1) does not meet the regulatory requirements for thermal protection ( R req = 3.08 m 2 °C/W).
- The design of the wall fence (Fig. 1) does not meet the minimum acceptable requirements for thermal protection ( R min = 1.94 m 2 °C/W).
- Thermal uniformity coefficient r the structure of the outer wall, made of masonry from aerated concrete blocks of density D400 with a facing layer of facing bricks, does not exceed 0.61.
- The actual value of the coefficient of thermal uniformity of the design solution under consideration, taking into account the quality of the products delivered to the site and the quality of their installation, may turn out to be significantly less than the calculated value.
- To ensure regulatory requirements to the level of thermal protection of external walls of buildings as part of a wall enclosure (Fig. 1), one should either increase the thickness of aerated concrete blocks as part of a two-layer wall structure, or use an intermediate layer of thermal insulation materials with a calculated thermal conductivity of no more than 0.05 W/m °C. The thermal insulation layer should be placed between the aerated concrete and the front (cladding) layers.
- In all cases, to effectively remove moisture from the wall structure, a ventilated gap should be provided between the thermal insulation layer and the facing brick, the effective cross-section of which (thickness) should be determined by calculation.
Literature
- Krivoshein A.D., Fedorov S.V. On the issue of calculating the reduced resistance to heat transfer // Engineering and Construction Journal. 2010. No. 8.
- Krivoshein A.D., Fedorov S.V. User’s manual for the “TEMPER” software package for calculating temperature fields of building envelopes. Omsk: SibADI, 1997.
- Sokolov N. A., Gorshkov A. S. Thermal conductivity of building materials and products: level of harmonization of Russian and European building standards // Construction materials, equipment, technologies of the 21st century. 2014. No. 6 (185).
- Gagarin V.G. Thermophysical problems of modern wall enclosing structures of multi-storey buildings // Academia. Architecture and construction. 2009. No. 5.
- Nemova D.V., Spiridonova T.I., Kurazhova V.G. Unknown properties of a known material // Construction of unique buildings and structures. 2012. No. 1.
* Data on the actual energy consumption of residential buildings different years buildings were collected and analyzed by the authors of the article. – Approx. ed..
1 In accordance with the requirements of SNiP 23-02 (clause 5.3).
2 According to SNiP 23-02, table 4.
3 According to the requirements of RMD 23-16–2012 “St. Petersburg. Recommendations for ensuring energy efficiency of residential and public buildings”, table 3.
4 Ibid., table 9.
5 According to the requirements of SNiP 23-02, clause 5.13.
6 See SNiP 23-02, formula (8).
7 According to the requirements of SNIP 23-02, clause 5.6.
8 In our case, the calculation was performed using the TEMPER 3D software package.
Without exception, all walls and coverings (and other types of enclosing structures of buildings and structures) cannot be called isothermal. In other words, the distribution of the temperature field over a cross section perpendicular to the heat flow in the structure does not represent a constant value, due to the presence of all kinds of heat-conducting inclusions (the so-called “cold bridges”), which are almost always present in one form or another in the structure of the fence. Reinforcing steel or composite rods in the ligation of facing masonry can act as heat-conducting inclusions load-bearing structures, cement-sand mortar or glue in masonry, fasteners for thermal insulation materials, corners and junctions of ceilings and coverings. Therefore, such a concept as the reduced resistance to heat transfer of a fence R req is accepted, which is a value equal to the averaged thermal characteristics of a combined (non-uniform in composition) structure, the heat flow in which, under a constant time regime, does not appear to be one-dimensional along the perpendicular section of the structure.Thus, R req is equal to the heat transfer resistance of a single-layer fence of the same unit area, which transmits the same heat flow as in the actual structure with the same temperature gradient between the inner and outer surfaces of the fence. If we ignore the influence of the above heat-conducting inclusions or, as we have already said, “cold bridges” in the design of the fence, then its heat-protective characteristics can be conveniently represented using the concept of conditional heat transfer resistance. After we have defined such concepts as conditional and reduced resistance, we can introduce the definition of the coefficient of thermal uniformity r which is the ratio of the reduced heat transfer resistance to the conditional heat transfer resistance. Thus, r depends on the characteristics of the materials and the thicknesses of the layers that make up the enclosing structure, as well as on the presence of the heat-conducting inclusions themselves. The numerical value of the coefficient r evaluates how effectively the thermal insulation properties of the insulation in the enclosing structure are used and the influence of the presence of thermal insulation inclusions on this. Based on decisions on the design of the fence, the value of the coefficient of thermal uniformity varies from 0.5 to 0.98. If it is equal to 1, this means that there are actually no heat-conducting inclusions, and the efficiency of the layer of thermal insulation material is used to its maximum.
Determination of the coefficient of thermal homogeneity of enclosing structures.
Coefficient value r must be determined using rather labor-intensive calculations using the temperature field method or by conducting thermal conductivity measurements based on experiment. In particular, the coefficient of thermal homogeneity is r can also be calculated according to the instructions contained in SP 23-101-2004 “Design of thermal protection of buildings”. In practice, it is enough to take the value of the coefficient by . If, despite the thermal homogeneity coefficient adopted according to regulatory documents, the fencing design still does not comply with current standards, then the coefficient can be increased by confirming its applied values with calculations.
In the event that the calculated fencing design fails to meet the requirements regulatory documents requirements for the coefficient of thermal uniformity, the use of such a design is subject to revision. It's possible here various options, such as replacing the types and types of materials used in the fencing, reducing the thickness of the joints in the masonry, replacing the connecting steel reinforcement with a composite one, changing the size of the masonry blocks.