Experimental study of air cavity thermal performance of opaque ventilated facades under extreme wind conditions: case study Baku

1. INTRODUCTION

Today Azerbaijan fully satisfies its internal energy needs with its own resources, but lack of a law on energy-efficiency of buildings slows down the widespread usage of energy-saving and energy-efficient measures in the construction sector (1(1) Akbarova, S., Mammadov, N. (2018). Multi-disciplinary Energy Auditing of Educational Buildings in Azerbaijan: Case Study at a University Campus. IFAC, International Federation of Automatic Control. International Conference https://www.sciencedirect.com/journal/ifac-papersonline, 51(30), 311-315.). According to statistical data, in Azerbaijan, energy consumption by the construction sector accounts for more than 47% of the final energy balance (2(2) Energy balance of Azerbaijan. (2017). Report of State Statistical Committee of the Republic of Azerbaijan. https://www.stat.gov.az/source/balance_fuel/?lang=en.). Therefore, one of the ways to improve this situation is to use ventilated facade systems due to their better thermal performance characteristics in comparison to conventional facade systems (3(3) Sierra-Pérez, J., Boschmonart-Rives, J., Gabarrell, X. (2015). Comparative combinations of façade-building systems and thermal insulation materials for different climatic conditions: an environmental assessment. Journal of cleaner production. 113. doi: 10.1016/j.jclepro.-5(5) Bienvenido-Huertas, D., Rodríguez-Álvaro, R., Moyano, J., Marín, D., Rico, F. (2019). A comparative study of the methods to assess the thermal transmittance in opaque walls in the Mediterranean winter. Informes de la Construcción, 71 (554), e288. doi:10.3989/ic.62542.). The facades of most buildings in the country do not meet the thermal-technical requirements of existing construction codes and regulations, heat losses through them are significant and make up to 20-28% of the total losses (6(6) Akbarova S. (2018). Trends of Energy Performance Certification of Buildings in Azerbaijan. International Journal of Engineering and Technology (UAE), 7(3.2), 563-566. doi:10.14419/ijet.v7i3.2.14590. https://www.sciencepubco.com/index.php/ijet/article/view/14590 , 7(7) Mammadov, N., Akbarova, S., Rzayeva N. (2018). Building energy auditing is a tool to improve their energy efficiency. Ingenieurtag_2018. Brandenburq Technical University, Germany.210-219. https://opus4.kobv.de/opus4-btu/frontdoor/deliver/index/docId/4724/file/4_Ingenieurtag_2018.pdf. ), therefore additional effective measures are required to insulate the building enclosure. Although the implementation of external insulation of facades is quite a long-term and laborious process the use of ventilated facades systems allows to make new and reconstructed buildings close to energy-saving ones in their thermal characteristics (8(8) Halawa, E., Ghaffarianhoseini, A., Ghaffarianhoseini, A., Trombley, J., Hassan, N., Baig, M. (2017). A review on energy conscious designs of building façades in hot and humid climates: Lessons for (and from) Kuala Lumpur and Darwin. Renewable and Sustainable Energy Reviews. 82, 2147-2161. doi:10.1016/j.rser.2017.08.061.-10(10) Tusnina, V. (2016). To the problem of bearing capacity and operational reliability of suspended ventilated facade Procedia Engineering, 153, 799-804.). Since these systems have significant potential for energy saving, a study of their thermal performance is required, especially during long-term operation in difficult climate conditions, such as significant wind loads and high humidity (11(11) Ibanez-Puy, M., Vidaurre-Arbizu, M., Saristan-Fernandez, J., Martin-Gomez, C. (2017). Opaque ventilated facades: thermal and energy performance review. Renewable and Sustainable Energy Reviews. 79, 180-191.-13(13) Shameri, M., Alghoul, A., Sopian, K., Zain, M., Elayeb, O. (2011). Perspectives of double skin façade systems in buildings and energy saving. Renew Sustain Energy. 15, 1468-1475. doi:10.1016/j.rser.2010.10.016.), which is typical for climate conditions in Baku, where the referent building is located.

A widespread use of opaque ventilated facades (OVF) makes research of their thermal performance relevant because there is a need for an objective assessment of their energy efficiency at the operational stage. Modern facade systems with ventilated air cavity have become popular subject of science-intensive studies in an overall facade construction (14(14) Peci López, F., Ruiz de Adana Santiago, M. (2015). Sensitivity study of an opaque ventilated façade in the winter season in different climate zones in Spain. Renew Energy, 75(C), 524-533. doi:10.1016/j.renene.2014.10.031.). At the same time, it is necessary to be aware of the fact that the better a façade deals with all the environmental factors the lower energy demand of the building is (11(11) Ibanez-Puy, M., Vidaurre-Arbizu, M., Saristan-Fernandez, J., Martin-Gomez, C. (2017). Opaque ventilated facades: thermal and energy performance review. Renewable and Sustainable Energy Reviews. 79, 180-191.). There is a significant number of articles that study OVF systems in terms of their energy and thermal performance (11(11) Ibanez-Puy, M., Vidaurre-Arbizu, M., Saristan-Fernandez, J., Martin-Gomez, C. (2017). Opaque ventilated facades: thermal and energy performance review. Renewable and Sustainable Energy Reviews. 79, 180-191., 15(15) Iribar-Solaberrieta, E., Escudero-Revilla, C., Odriozola-Maritorena, M., Campos-Celador, A., García-Gáfaro, C. (2015). Energy performance of the opaque ventilated facade. Energy Procedia, 78, 55-60. doi:10.1016/j.egypro.2015.11.114.-18(18) Peci López, F. (2012) Numerical and experimental analysis of the energy saving and potential of ventilation of opaque ventilated facades. (Tesis doctoral ). Universidad de Córdoba.). These articles study air cavity in terms of its ability to transfer heat (19(19) Guerrero-Rubio, J., Sendra, J.J., Fernández-Agüera, J., Oteiza, I. (2017). Test cell data-based predictive modelling to determine HVAC energy consumption for three façade solutions in Madrid. Informes de la Construcción, 69(548), e225, doi: 10.3989/id.54794.-21(21) Mingottia, N., Chenvidyakarna, T., Woodsb, A.W. (2011). The fluid mechanics of the natural ventilation of a narrow-cavity double-skin facade. Building and Environment, 46, 807-823.). There are studies dedicated to experimental measurements of the air velocity in the ventilated cavity (22(22) Balter, J., Pardal, C., Paricio, I., Ganem, C. (2019). Air cavity performance in opaque ventilated façades in accordance with the Spanish Technical Building Code. Architecture, City and Environment, 13 (39), 211-232. doi:10.5821/ace.13.39.6487., 23(23) Falk, J., Sandin, K. (2012). Ventilated rain screen cladding: Measurements of cavity air velocity estimation of air change rates and evaluation of driving forces. Building and Environment. 59, 164-176. doi:10.1016/j.buildenv.2012.08.017.). Nevertheless, not enough attention has been given to study of naturally ventilated opaque facade systems with closed joints in regions with high wind speed in winter and with low or no solar radiation (24(24) Palyvos, A. (2008). A survey of wind convection coefficient correlations for building envelope energy systems’ modeling. Applied Thermal Engineering. 28(8-9), 801-808., 25(25) Seferis, P., Strachan, A., Dimoudi, A., Androutsopoulos, A. (2011). Investigation of the performance of a ventilated wall. Energy and Buildings. 43(9), 2167-2178.). It is well-known that the wind impact is crucial in determining facade`s thermal performance. Primarily this is due to modern trends in construction such as an increased number of floors in a building. In Baku average wind speed at an altitude of 56 m can be 14 m/s in Baku (1(1) Akbarova, S., Mammadov, N. (2018). Multi-disciplinary Energy Auditing of Educational Buildings in Azerbaijan: Case Study at a University Campus. IFAC, International Federation of Automatic Control. International Conference https://www.sciencedirect.com/journal/ifac-papersonline, 51(30), 311-315.).

Some studies do not take into account the effects of wind on the thermal characteristics of air cavity (19(19) Guerrero-Rubio, J., Sendra, J.J., Fernández-Agüera, J., Oteiza, I. (2017). Test cell data-based predictive modelling to determine HVAC energy consumption for three façade solutions in Madrid. Informes de la Construcción, 69(548), e225, doi: 10.3989/id.54794., 26(26) Balocco, C. (2002). A simple model to study ventilated facades energy performance. Energy and Buildings. 34 (5), 469-475. doi:10.1016/S0378-7788(01)00130-X., 27(27) Lorente, S. (2002). Heat losses through building walls with closed, open and deformable cavities. International Journal of Energy Research. 26, 611-632. doi:10.1002/er.807.), although wind as a major force of ventilation of air cavity which causes considerable heat losses (11(11) Ibanez-Puy, M., Vidaurre-Arbizu, M., Saristan-Fernandez, J., Martin-Gomez, C. (2017). Opaque ventilated facades: thermal and energy performance review. Renewable and Sustainable Energy Reviews. 79, 180-191.). In addition, the intensity of heat and moisture transfer of OVF depends on the air velocity in the cavity, which is defined by many parameters: internal characteristics of the cavity; number, location, materials, cross-sectional area, thermal conductivity of the fastening elements; aerodynamic resistance in the cavity; external climate influences (28(28) Peci Lopez, F., Jensen, R.L., Heiselberg, P., Ruiz de Adana Santiago, M. (2012). Experimental analysis and model validation of an opaque ventilated facade. Building and Environment, 56, 265-275., 29(29) Katunska, J., Bullova, I., Špaková, M. (2017). Analysis of Air Flow in the Ventilated Insulating Air Layer of the External Wal. Journal of Civil Engineering. doi:10.1515/sspjce-2016-0015.). Therefore, the development methods for calculating cavity thermal performance is a complicated engineering problem (30(30) Katunska, J., Bullová, I., Špaková, M. (2017). Analysis of Air Flow in the Ventilated Insulating Air Layer of the External Wall. Journal of Civil Engineering. doi:10.1515/sspjce-2016-0015., 31(31) Katunska, J., Bullová, I. (2017). Air Flow in the Ventilated Insulating Air Layer. MABD, At Mutenice, Czech Republic, Conference Paper.).

The purpose of this article is to carry out an on-site measurement to assess the air velocity in the ventilated cavity which depends on the height on extended areas of the facade when wind speed is more than 7 [m/s]. Calculations were computed in line with local construction standards and used for further thermal performance study of the cavity. The referent facade is operating. Obtained results are proposed to be applied to calculated-experimental control of OVF for solving practical problems on the operational stage to fulfill the multi-disciplinary energy audit of the buildings and their further certification (32(32) Mammadov, N., Akbarova, S. (2017). New Methodology of Multi- Disciplinary Energy Auditing of Buildings in Azerbaijan. International Symposium On Innovative Technologies In Engineering And Science. Sakarya University, Turkey, Academic Platform. 210-219. http://isites.info/PastConferences/ISITES2017/ISITES2017/papers/A10-ISITES2017ID46.pdf ).

2. METHODS

2.3. Case study

 ⌅

The object of this full-scale experimental study is north orientated facade of the main building of Azerbaijan University of Architecture and Construction (Figure 1). It is located in the most weathered part of Baku amphitheater under prolonged wind conditions. The facade has been operated for more than 11 years.

Figure 1.  Case study building: north (measured) facade.

The main masonry of the facade is a hollow brick on a cement- sand mortar with internal lime-sand plaster. There is also a continuous mineral wool insulation layer. A lightweight cladding of composite smooth panels (height 1.8 [m] and width 0.7; 1; 1.2 [m]) are fastened to the main masonry by steel substructure- steel fastening bolts. The height of the northern facade is 55,8 [m]. Light-colored composite panels absorb approximately 20% of the total amount of solar radiation falling on a vertical surface. Between the insulation layer and cladding panels, there is a slightly- ventilated air cavity with a thickness about 4-7 [cm] which is continuous along the height. A hydro- windproof membrane on the thermal insulation surface is missing.

At the base and in the upper part of OVF there are inlet and outlet slots in nearly equal sizes for ventilation of the air cavity. The seal between the facing plates is dense and airtight. The thermal characteristics of the facade layers from internal to external are given in Table 2. Constructive scheme of the cross-section of OVF is illustrated below in Figure 2. In the immediate vicinity of the investigated facade, there are no high-rise buildings which can affect the wind speed.

Measurements of the air velocity in the cavity had been carried out by the multi-function measuring instrument Testo 435 in seven different points through the open joints of the cladding panels. The points have a diameter of about 15 [mm] and they are located along the windward side of the facade oriented to the north. Measurements had been carried out during January and February in the morning and in the afternoon (Figure 1, red dots). Measurements of the wind speed have been carried out at the same height with measuring instrument Testo 417. The average external air temperature was about (2-11) [⁰С], the temperature difference between the outside air and the air cavity was about (1-2.5) [⁰С]. Data of the average values of measured air velocity and wind speed are given in Table 3.

Table 2.  Thermal characteristics of the facade materials.

No	Materials	ρ [kg/m3]	d [cm]	λ [W/(m·0С)]	R [(m2·0С)/W]
1	Lime- sandplaster	1600	2	0.70	0.029
2	Hollow brick	800	40	0.52	0.77
3	Thermal insulation	150	5	0.044	1.14
4	Steel fastening bolts	7800	-	47	0.1
5	Air cavity	1.25	5	-	0.1
6	Composite panel	-	2.1	-	≈0.06

Figure 2.  Constructive scheme of vertical cross- section of OVF: 1- internal lime-sand plaster; 2- main masonry; 3- thermal insulation; 4- ventilated air cavity; 5- facing panel; 6- fixing substructure.

Engineering calculations of OVF thermal characteristic values were carried out in accordance with (34(34) SP 50.13330. (2012). (Russian acronym, Code of Construction). Thermal performance of the buildings. Appendix L. In Russian., 35(35) SNiP 23-02-2003. (2003). (Russian acronym, Construction Norms and Regulations). Thermal performance of the buildings. In Russian.). Calculations were made assuming that heat flow through OVF system is stationary, there is structural thermal heterogeneity, as well as solar radiation is lack.

The total thermal resistance to heat transfer of facade is determined by the formula:

 [1]

R_si, R, r_s, R_c, R_p, R_se stand for interior surface thermal resistance, thermal resistance from inner surface to the air cavity, thermal resistance of air cavity, thermal resistance of facing panel, exterior surface thermal resistance, respectively, [(m²·⁰С)/W], r _s = coefficient of thermal heterogeneity of wall assembly for steel fastening bolts (34(34) SP 50.13330. (2012). (Russian acronym, Code of Construction). Thermal performance of the buildings. Appendix L. In Russian.-37(37) SNiP 2.01.07-85*. (1985). (Russian acronym, Construction Norms and Regulations). Loads and exposures. In Russian.).

Per (35(35) SNiP 23-02-2003. (2003). (Russian acronym, Construction Norms and Regulations). Thermal performance of the buildings. In Russian.), in line with ensuring wall energy efficiency, the total thermal resistance, R _T , must be equal or higher than R _reg :

 [2]

 [3]

 

where: D number of degree-days of the heating season, [°С·days]:

 [4]

 

where: t _int =20, t _ext =5.1 internal and external temperatures, respectively, [⁰C];

Z _ht =119 number of days with average temperature less than +8 [⁰C].

For the climate conditions in Baku: D=1773 [⁰С·days], R _reg =2.02 [(m²·⁰С)/W]. According to the calculated results: (R _tot ≈2.11)≥(R _reg =2.02), that means the energy efficiency of the referent wall assembly. Thermal resistance of the air cavity is calculated by formula:

 [5]

h- heat transfer coefficient of air cavity, [W/(m²·⁰С)].

Heat transfer in the air cavity occurs due to convection and radiation, so heat transfer coefficient includes two components- convective heat transfer coefficient hh _c , [W/(m²·⁰C)], and radiative heat transfer coefficient h _r , [W/(m²·⁰C)]:

 [6]

Varies factors affect the value of convective heat transfer coefficient: cavity geometry, air viscosity, air specific heat and so forth. But it mainly depends on the air velocity in the cavity, V _c [m/s]. Convective heat transfer coefficient is determined by the Frank formula (34(34) SP 50.13330. (2012). (Russian acronym, Code of Construction). Thermal performance of the buildings. Appendix L. In Russian., 35(35) SNiP 23-02-2003. (2003). (Russian acronym, Construction Norms and Regulations). Thermal performance of the buildings. In Russian., 37(37) SNiP 2.01.07-85*. (1985). (Russian acronym, Construction Norms and Regulations). Loads and exposures. In Russian.):

 [7]

Depending on speed of wind the convective heat transfer coefficient has been broadly investigated by the numbers of scientists and it is shown in (24(24) Palyvos, A. (2008). A survey of wind convection coefficient correlations for building envelope energy systems’ modeling. Applied Thermal Engineering. 28(8-9), 801-808.). Radiative heat transfer coefficient, h _r , [W/(m²·⁰C)], depends on the intensity of solar radiation and air temperature in the cavity. Radiative heat transfer coefficient equals:

 [8]

С₁, С₂, С₀ radiation coefficients of the insulation surface, inside surface of cladding and black-body surface, respectively 4.5, 3.9, 5.76; t _c air temperature in the cavity, it is about (1.5-2.5) [⁰С] more than exterior temperature for claddings with low thermal conductivity.

The air movement in the cavity occurs under the influence of wind pressure and buoyancy. In general, cavity air velocity should be calculated using the formula below:

 [9]

V ₁₀ average speed of wind, [m/s], on the terrain according to the meteorological data on the height H=10, [m];

L height difference between the air cavity inlet and outlet, [m];

Σ _i ξ _i sum of the local aerodynamic drag coefficients,

ξ=5.5 for input where airflow is bent;

ξ=4.5 for output where airflow is sharply bent;

(t _c -t _ext ) difference between the mean air temperature in the cavity and the mean external temperature that is taken between (1.5-2.5) [⁰С];

k coefficient of the wind speed change, based on building’s height, H, [m] (37(37) SNiP 2.01.07-85*. (1985). (Russian acronym, Construction Norms and Regulations). Loads and exposures. In Russian.):

 [10]

The speed of wind can be calculated based to any building’s height, H, more than 10 [m] using the formula (38(38) Gagarin, V., Guvernjuk, S., Kozlov, V., Ledenev, P., Tsykanovsky, E. (2010). Results of researches of properties of hinged facade systems with the ventilated air layer in the frame of the grant of the russian fundamental researches fund «Aerothermophysics of nontight bodies in low speed air streams». Construction sciences. 3, 261-278.):

 [11]

β local terrain exponent, coefficient, that depends on the area type, β=0.4, β=0.28, β=0.16 for urban area with the buildings height H>25[m], H=10-25[m], H<10 [m] respectively, (37(37) SNiP 2.01.07-85*. (1985). (Russian acronym, Construction Norms and Regulations). Loads and exposures. In Russian.).

3. RESULTS

Results of calculated convective heat transfer coefficients and thermal resistances of the air cavity based on the experimental data and engineering equations [1]-[11] are given in Table 3 and Table 4 respectively.

Radiative heat transfer coefficient was calculated by using formula [8] and it is h _r =2.7 [W/(m²∙⁰С)]. It remains constant along the facade height since it depends mainly on the air temperature in the cavity. According to the results of full-scale measurements and formula [6] cavity heat transfer coefficient is: h=10-12.32 [W/(m²·°C)]. Next empirical equation allows to approximate a full-scale experimental values of the cavity air velocity depending on the wind speed according to the façade height:

 [12]

Analysis of experimental and calculated results show that the higher wind speed causes greater cavity air velocity along the facade height (Table 3, 4). Figure 3 illustrates that an increase in cavity air velocity contributes to increases in convective heat transfer.

Figure 4 shows that the measured thermal resistance of the cavity decreases with height and does not exceed 0.1 [(m²·⁰С)/W]. In the middle part of the cavity between 14-36 [m] of height, thermal resistance has a smaller gradient of change, unlike zones next to the air inlet and outlet openings, which demonstrates the influence of openings on change in the thermal resistance of the cavity.

Figure 3.  Convective heat transfer coefficient, h_c, in the air cavity based on experimental data, Table 3:

Figure 4.  Cavity thermal resistance, R_c based on experimental data, Table 3:

Figure 5.  Cavity air velocity results, V_c, based on:

4. DISCUSSIONS

The purpose of this article was to study the thermal characteristics of an air cavity of a naturally ventilated facade system with closed joints under extreme wind conditions in the winter period. The study compared thermal characteristics of the air cavity which were based on calculations compliant with the construction standards on the base of the full-scale experiment.

The results of the full-scale experiment on measuring cavity air velocity and wind speed and subsequent engineering calculations allow us to perform a more accurate calculation on the thermal parameters of the air cavity: convective heat transfer coefficient and thermal resistance. The study has shown that the results obtained based on experimental data are approximately the same as the ones based on engineering-calculation. The more accurate match cannot be expected because of the many random factors that can influence the results of the experiment.

Analysis of the results of the full-scale experimental study on the thermal performance of the air cavity of ventilated façade with the height up to 60 m under the speed of wind more than 7 [m/s] shows that:

Results that were concluded in this article are consistent with the ones in the studies referenced and cited in these articles (5(5) Bienvenido-Huertas, D., Rodríguez-Álvaro, R., Moyano, J., Marín, D., Rico, F. (2019). A comparative study of the methods to assess the thermal transmittance in opaque walls in the Mediterranean winter. Informes de la Construcción, 71 (554), e288. doi:10.3989/ic.62542., 19(19) Guerrero-Rubio, J., Sendra, J.J., Fernández-Agüera, J., Oteiza, I. (2017). Test cell data-based predictive modelling to determine HVAC energy consumption for three façade solutions in Madrid. Informes de la Construcción, 69(548), e225, doi: 10.3989/id.54794., 22(22) Balter, J., Pardal, C., Paricio, I., Ganem, C. (2019). Air cavity performance in opaque ventilated façades in accordance with the Spanish Technical Building Code. Architecture, City and Environment, 13 (39), 211-232. doi:10.5821/ace.13.39.6487., 38(38) Gagarin, V., Guvernjuk, S., Kozlov, V., Ledenev, P., Tsykanovsky, E. (2010). Results of researches of properties of hinged facade systems with the ventilated air layer in the frame of the grant of the russian fundamental researches fund «Aerothermophysics of nontight bodies in low speed air streams». Construction sciences. 3, 261-278.). This confirms the reliability and reasonableness of the results.

5. CONCLUSIONS

In Baku, the enclosure structures of building operate in fairly hard operation mode, experiencing the effects of constant strong wind, significant humidity and temperature drops (1(1) Akbarova, S., Mammadov, N. (2018). Multi-disciplinary Energy Auditing of Educational Buildings in Azerbaijan: Case Study at a University Campus. IFAC, International Federation of Automatic Control. International Conference https://www.sciencedirect.com/journal/ifac-papersonline, 51(30), 311-315.). Therefore, in order to decrease energy demand in buildings and avoid deformation and damage, it is very important that materials, combined into a common structure, would be investigated for long-term functionality and reliability during their operational stage (14(14) Peci López, F., Ruiz de Adana Santiago, M. (2015). Sensitivity study of an opaque ventilated façade in the winter season in different climate zones in Spain. Renew Energy, 75(C), 524-533. doi:10.1016/j.renene.2014.10.031.). The findings of the conducted full-scale experiment confirm the necessity of a comprehensive experimental-numerical solution for the structures of ventilated facades.

Since the cavity influences significantly the thermal properties of OVF, this should be considered when calculating the energy efficiency of buildings (5(5) Bienvenido-Huertas, D., Rodríguez-Álvaro, R., Moyano, J., Marín, D., Rico, F. (2019). A comparative study of the methods to assess the thermal transmittance in opaque walls in the Mediterranean winter. Informes de la Construcción, 71 (554), e288. doi:10.3989/ic.62542., 25(25) Seferis, P., Strachan, A., Dimoudi, A., Androutsopoulos, A. (2011). Investigation of the performance of a ventilated wall. Energy and Buildings. 43(9), 2167-2178.). Consideration of the impact of wind speed on the cavity thermal regime only improves the accuracy of thermal calculations and corrects them. Some of the practical applications, based on the results of this full-scale experimental study of the thermal characteristics of air cavity on a ventilated facade under significant wind effects can be applied when calculating temperature distribution over the cavity`s height, the intensity of moisture transfer and heat transfer along the facade. Obtained results complement calculated-experimental control of heat and energy indicators of the building during when carrying out its energy audit and further certification. The results of field studies show that for using OVF in conditions with strong wind effects it is necessary to improve and develop the existing calculating methods. This will result in increased functional characteristics, operational reliability as well as durability of OVF.

In Azerbaijan, application of OVF is growing rapidly. As such they contribute to an increase in energy efficiency of buildings. It is important that local specialists continue improving the methodology of complete and accurate thermal engineering calculation of OVF based on the national standards, combined with the full-scale experiments and theoretical studies. Thus, by applying of the results of field measurements in combination with calculations based on the use of standard indicators solves the problem of determining the actual heat- engineering characteristics of external enclosing structures of buildings in actual conditions of their operation. This article does not claim a complete solution to the problem to study the thermal performance of air cavity of OVF. This study was limited by air cavity thickness and values of wind speed. The authors are confident that the search for optimal methods to calculate the thermal performance of OVF is necessary and will be further developed.