Selection of low-e windows in retrofit of public buildings by applying multiple criteria method COPRAS: A Lithuanian case

https://doi.org/10.1016/j.enbuild.2005.08.005Get rights and content

Abstract

Calculations of building retrofit effectiveness have shown that the replacement of original windows with new ones is not as effective in terms of heat energy saving as are the insulation of a roof, walls and other improvements because the investments are large and take a long time to be repaid. However, in addition to energy saving, window replacement improves the indoor climate of the building, its interior and architectural appearance as well as its market value. The sequence of building operations determines when the replacement of windows should be done. When financial resources are limited, managers of public buildings often begin the renovation of a building's envelope with the replacement of windows. The client faces some problems in choosing among the great variety of windows to satisfy his/her needs, especially with respect to the cost-quality relationship. The method of multiple criteria complex proportional assessment (COPRAS) developed by the authors aims at solving the above-mentioned problems. The solutions based on multicriteria analysis allow for a more rational and realistic assessment of customer's needs as well as cutting down window renewal costs. In this paper, practical example (key-case) of selecting a contractor for the replacement of windows in the main building of Vilnius Gediminas Technical University (VGTU) is presented as a part of its retrofit multivariant design.

Introduction

The authors of this paper are participating in the project Framework 6 “Bringing Retrofit Innovation to the Application of Public Buildings” (BRITA in PuBs) (see http://www.brita-in-pubs.com). Public buildings may be of different types, such as colleges, cultural centers, nurseries, student halls of residence, and churches, etc. Public buildings are most suitable for achieving the market's penetration of innovative and effective retrofit solutions to improve energy efficiency and implement renewables, with moderate additional costs. With their help it will be easier to reach groups of differing age and social origin. Public buildings can also be used as engines to heighten awareness and sensitize society on energy conservation.

In all European countries it can be observed that general opinion on the realization of energy saving measures differs greatly between residential and non-residential buildings as well as between new and existing buildings. Most European countries have succeeded in reducing their energy consumption in new dwellings by more than 50% without increasing the building cost and, therefore, energy efficiency concept has reached greater acceptance by the building's owners.

Development is quite different for the non-residential buildings. Energy consumption problems are secondary to the improvement of comfort. Decisions to retrofit a building are often made because of dissatisfaction regarding the comfort level. Therefore, the primary goal is to improve these comfort conditions. Decision makers are often skeptical about measures that aim to reduce energy consumption because they are afraid that these measures could limit the effect of improving the indoor climate and increase costs. The improvement of comfort as the primary aim can be achieved alongside energy saving.

It is, therefore, particularly important to demonstrate the effective solutions of public building retrofit and to make them visible to the wider public. One such project that represents the renovation of a main building at VGTU is considered in this paper.

Retrofit evaluation of a building is quite difficult to undertake because a building and its environment are complex systems (embracing technical, technological, ecological, social, comfort, aesthetic and other aspects), where all subsystems influence the efficiency of performance and where interdependence between the subsystems plays a significant role. Coherent and effective retrofit scenarios are commonly designed on the basis of knowledge of the degradation state of the building and its obsolescence. A systematic method, based on multicriteria analysis and a constructivist approach are also helpful in designing retrofit scenarios [4].

A methodology for rating buildings or retrofit scenarios according to criteria, such as use of energy for heating, cooling and other appliances, impact on the environment, indoor climate and cost has been suggested [17].

The methodology of a multivariant design and multiple criteria analysis of a building's retrofit enables users to develop thousands of alternatives and to evaluate their economic, technical and architectural, aesthetic and comfort aspects [8].

For the energy saving measures to be profitable, the value of energy saved over the building's life will need to be greater than the capital investment and optimization would involve the selection of all measures with the savings-to-investment ratio (SIR) being more than one [5], where:SIR=PresentvalueofthetotallifetimeenergysavingInvestmentcost.

The retrofit implementation of the benefits of some energy saving measures is associated not only with the reduction of energy consumption, but also with the improvement of the condition of the building elements, as well as with durability and the market value of a building [12]. Building retrofit also improves the indoor climate in the building, its interior and architectural appearance as well as raising its market value [22]. In terms of real estate market value, the effective retrofit may be described by the following market value ratio (MVR)MVR=MvaMvbCr,where Mva is the market value of building after retrofit; Mvb, the market value of building before retrofit; Cr, building retrofit costs. If the implement into retrofit has an MVR greater than 1, it can be regarded as being cost effective with respect to real estate market value. When real estate market is active, the replacement of windows is effective in terms of MVR which is greater than 1.

The efficiency of retrofit implements in the main VGTU building is calculated by determining their SIR (see Fig. 1). As shown in the figure, the most efficient renovation procedures include the upgrading of the heating unit, the reduction of glazed area and the provision of wall and roof insulation. The replacement of windows is not as effective in terms of heat energy saving because it is an expensive and slowly repaid investment (with SIR = 0.66). However, the windows of the VGTU building are old, unsafe, not opening & shutting and poorly insulated. In cold seasons, great heat losses are experienced due to poor insulation and sometimes there are draughts in the spaces. In general, the existing windows do not satisfy standard requirements, their thermal transmittance being Uw = 2.5 W/m2 K.

The renovation of old windows is not reasonable from both technical and economical perspectives. Therefore, the most effective retrofit solution would be the replacement of the original windows with new ones. The building considered was designed and constructed 34 years ago, in the soviet times, when energy saving was not an acute problem. Buildings were designed with large glazed surfaces. Thus, the glazed area of the main VGTU building makes up 27% of the total enclosure surface of the building. Moreover, building renovation is usually done in the following order: first, windows are renovated or replaced and, then, walls are insulated. Therefore, windows should be renovated or replaced prior to carrying out insulation work. As shown by infrared thermovision photographs that were used to determine heat losses of a building, the latter are mostly caused by poorly insulated low-quality windows. Their replacement by low-e windows can cut heating expenses up to 30%.

Section snippets

Various types of windows and the problem of making the proper choice

Windows, like doors, walls and the roof, make up an essential part of a building. These also play an important role in forming the façade and the interior of a building. A residential or public building can hardly be imagined without windows. However, about 30% of heat is lost, if windows are poorly insulated.

The main function of windows in a building is to pass light through to the interior and to decorate the building. However, windows are the thinnest elements transmitting cold, heat, noise,

Contractor selection using COPRAS, a multicriteria decision making method

Selection of a retrofit contractor is a decision characterized by multiple objectives. Owners want to minimize the likely cost of projects, but they also want contractors to maintain schedules as well as to achieve acceptable quality standards [6]. Contractor selection is often based on the lowest bid that is made. However, the choice can be ineffective in terms of quality standard and lifetime. Contractors trying to win a contract often reduce the bid price. This may result in many changes,

Conclusions

In order to realize an effective selection of low-e windows in a building's retrofit, it is necessary to carry out an exhaustive investigation of all possible solutions. The efficiency level of a particular window's replacement alternative depends on a large number of factors, including cost of the project, energy savings gained after renovation, tentative pay-back time, thermal transmittance of the profile and double glazing unit, emission ability of low emissive glass coating, weighed sound

Acknowledgements

This work was carried out in the research project Framework 6 “Bringing Retrofit Innovation to the Application of Public Buildings” (BRITA in PuBs), funded by the EU (Contract No. TREN/04FP6EN/SO7.31038/503135). The authors would like to acknowledge this institution for their support in this research.

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