Assuming that through the use of double-facade walls it is possible to decrease the building’s demand for air conditioning and mechanical ventilation, as well as to have net energy gains from solar energy for its heating, the overall energy balance of the building should improve. Few published studies on the subject, however, fail to wholly support this thesis.

K. Daniels published a study which compares the energy cost of an office building with a given set of parameters describing its internal physical environment, dependent on the chosen model of facade [2]. Altogether, 5 options were taken under consideration:

• Double facade with a box structure and insulating glass in the interior wall — no.1

• Double facade with a box structure, insulating glass and a thermal filling in the interior wall — no.2

• Single facade with insulating glass and a thermal filling in the interior wall — no.3

• Single facade with a glass Luxguard Natural — no.4

• Single facade with a glass Climasol Neutral — no.5

The presented cumulative diagram (Fig.6) sums the different elements of energy cost (heating, cooling, lighting etc.) and shows that the difference between two best results for a double facade

(2) and a single facade (5) is not more than 8% (sunshades were not taken into consideration).

D. Richards published a study on comparative cumulated energy expenditures of buildings with different types of external walls, including double facades [9]. In comparison to a single facade with single glazing and without external insulation (under today’s thermal insulation standards — discontinued), energy savings from using double facades reach 75%. When single glazing was replaced with low emission insulation, these gains fall to 50%. Should an additional external system of protection from the sun’s radiation be used, same levels of energy consumption are recorded (the difference is within 2%). In the case of buildings which use external shading systems (low buildings), much less expensive single facades can be a serious competitor for double facades. Comparisons should then be made in other areas, such as protection from the noise or the aesthetics of both solutions.

4. Conclusions

The analysis of utility value of double facades is related to their ability to take advantage of natural weather conditions (sun and wind). It points to the following advantages and shortcomings of their application in buildings (tab.1):

This enumeration allows us to conclude that double facades largely meet the requirements of interactive walls. Such was the dominant view emerging from publications dating from mid 1990s, when such solutions were at the top of their popularity. However, the experience from exploitation of these double-facade buildings over the next 10 years do not allow for such one-sided opinions.

The main problem is that the advantages stipulated above can be attained only with a proper design process, which requires much higher inputs of knowledge, experience and cost that in the case of other types of walls. It also necessitates interdisciplinary research involving simulation, testing and comparison of different types of solutions to choose optimal options. In the case of many constructed buildings, these were deliberately omitted [11], but other advantages came to the fore: acoustic properties of double facades (good insulation from external noise even when windows are opened) and their architectural value. These buildings are characterized by above-the-average energy consumption and offer a number of discomforts for their users, including overheating in summer. Buildings that took advantage of interactive double facade walls (such as the Commerzbank head office in Frankfort, RWE office building in Essen, Post-Office in Bonn) have better exploitation characteristics, even though these are slightly worse than planned. Therefore, even a careful process of designing is prone to error. The functioning of double facades is dependent on many elements and feedbacks with other elements of the building (buffer zones, installation systems), which are hard to precisely assess beforehand. It also requires important outlays for cleaning, maintenance and repair. Their effectiveness also depends on the conscientiousness of their users and rationality of their behaviour. Furthermore, each building should be treated separately and optimized for its shape and location on site.

Perhaps technological development will allow for the design process to be both more simple and effective. However, it may also be that progress in combining glass with nanomilimeter layers, which improves its utility value, will allow to construct a wall, in which control over internal environment of the building will take place within the microscale of the glass pane. The sense of creating multilayer, complex structures, such as double facades, could then be seriously questioned.

Refercences

[1] Compagno, „Intelligent Glass Facades“, Birkhauser Publishers, Basel-Bostn-Berlin1997.

[2] K. Daniels, „Low-Tech Light-Tech High-Tech Building in the Information Age“, Birkhauser Publishers Basel-Boston-Berlin 1999.

[3] K. Daniels, „The technology of Ecological Building”, Birkhauser Verlag Basel-Boston-Berlin 1997.

[4] Double-Skin Walls”, collective research of Ecole Polytechnique Federate de Lausanne, Institut de Technique du Batiment, Departement d’Architecture 1999.

[5] G. Gogolok,„Sciany oslonowe dwupowlokowe” [Double-layer facade walls], conference materials Nowoczesne lekkie sciany oslonowe i okna” [Modern light walls and windows], Mrqgowo 1999.

[6] V. Lerum „High Performance Building”, John Wiley&Sons, New Jersey 2008.

[7] R. D.Lieb, E. Oesterle, “’Das Dusseldorfer Stadttor’ Exercising an Integrated Approach to Double Facade Planning”, 1999.

[8] Oesterle, Lieb, Lutz, Heusler, „Double Skin Facades, Integrated Planning”, Prestel Munich-London-New York 2001.

[9] D. Richards, “An Integrated Approach to Sustainability”, “Dimensions of Sustainability”, E&FN SPON 1998.

[10] K. Zielonko-Jung “Podwojne elewacje szklane we wspolczesnej architekturze” [Double glass facades in contemporary architecture], Ph. D. thesis, Faculty of Architecture, Warsaw Polytechnic, Warszawa 2003.

[11] K. Zielonko-Jung „Uwarunkowania technologiczno-ekologiczne stosowania scian dwupowlokowych — tendencje rozwojowe” [Technological and environmental determinants of double-facade walls — development trends], unpublished research, Faculty of Architecture, Warsaw Polytechnic, Warszawa 2005.

i Published research grant financed (2007-2009) by the Ministry of Science and Higher Education, Fusion of traditional and advanced technologies as a new environmental orientation in contemporary architecture”.