Energy-Efficiency Retrofitting Strategies for Existing Residential Building Envelope System—A Case Study in China

With the rapid development of urbanization, the Chinese government has put equal emphasis on construction and retrofitting. But those projects did not achieve optimal effect because of the lack of targeted and systematic design guidance system. In this study, it first analyzes existing retrofitting methods and sorts into five retrofitting types as a basis, and then, captures the combinations and permutations of retrofitting methods and materials by parts and layers to build a database. After that, it combines different kinds of approaches by hierarchical matrix method to conclude the most efficient strategy. This study also selects typical residential buildings built between 1980 and 2000 in cold climate area of Northeast China as the research objects to test the integrity and effectiveness. As the results of this paper, it provides systematic guidance and multiple performance-based retrofitting strategies of the existing residential envelope system, which can improve indoor thermal comfort with low energy consumption.

Based on above problems, existing residential buildings built between the 1980 and 2000 in Northeast China are regarded as research objects, because of the intact and unified masonry construct, low construction standards, high energy consumption, and poor thermal performance. This study first captures the combinations and permutations of retrofitting methods and materials by parts and layers to build a database. Then, it focuses on combining the individual component retrofitting approaches in simulation tool to solve the compatibility of the whole envelope system and improve energy efficiency in retrofitting project. The result of this study can provide designers and related practitioners with new ideas and effective design-related decision-making strategies.
Literature review, field investigation and analysis, comparative analysis, and case analysis are also included. Figure 1 summarizes the structure of this paper.

Background of Residential Building Retrofitting
The existing research includes five parts: theory, approach, effect evaluation, retrofitting component, and construction process. John Habraken [3] first articulated Open Building Theory which is the basis of Four Layers [4] and Six Shearing Layers of Change [5]. The development of Brand's Shearing Layers is the Adaptive Future [6] which conducted a set of surrounding layers that explain how a building and its constituent parts will change over time. Fan Yue [7] conducted research on the improvement of existing residential quality from functionality, comfort and locality.
Studies on retrofitting approach mainly based on case study by summarizing methods and technologies, such as summarizing the design methods, processes and funding sources [8]; constructing an information-based auxiliary system for residential pathology diagnosis and repairing [9]; establishing a double-layer interface and consensus interval technologies [10]. Although researches on retrofitting approaches which summarized by the practical cases can be a reference, it lacks pertinence and specificity model which have not formed a comprehensive and systematic strategy database for the complexity and diversity of existing buildings.
Most of the research on the retrofit effect focuses on the energy consumption reduction of individual components [11] [12]. For example, Cui [13] compared energy reduction of two conditions from the energy-saving renovation of windows during a year. Zhu [14] researched on the development of windows and balconies with structural waterproof and thermal insulation properties Module.
Without overall coordination with other parts in the entire enclosure system, those strategies can only improve the energy-saving effect at a certain extent.
Energy consumption of air conditioning and maintaining indoor thermal comfort accounts for about 50% of the total energy consumption [15]. Therefore, energy-retrofitting of the envelope system is an effective measure to improve indoor comfort and physical performance [16] such as improving insulation, reducing thermal loss, and solving cold bridges. A large number of studies have shown that it can effectively reduce energy consumption and improve energy-efficiency [17] [18] by improving the insulation performance of wall [19] [20], door and window [21], roof [22] and corridor.
In terms of the construction process and effect evaluation, Häkkinen [23] proposed to use the computer-aided simulation for performance evaluation before carrying out construction transformation, which avoided the uncertainty of detection during operation.
It is shown that most of the existing residential building retrofitting studies based on practical cases and focus on individual component, lacking a comprehensive retrofitting approaches system for the entire envelope system. It is necessary to build a functionality database of envelope system that is suitable for different building conditions of the huge amount of housing stock to reduce energy consumption of maintaining thermal comfort.

Retrofitting Parts and Layers
According to the hierarchical approach, the envelop system is divided into five layers, including wall, window, roof, balcony, floor, each of which can be divided into more specific layers. For example, the wall and the roof can be divided into exterior surface layer, performance layer, structural layer and interior surface layer hierarchically. The floor includes interior surface layer, performance layer and structural layer ( Figure 2).

Retrofitting Strategy
Based on the different function of every layer, retrofitting approach can be cata-  (Table 1), which can be regarded as the basis to construct the hierarchical retrofitting strategies system. For example, the performance layer is embodied in improving the thermal insulation and waterproof performance of the envelop system.  The five retrofitting approaches can be applied to different layers of the existing envelope system.

1) Wall
The wall is one of the most important components of the envelope system. It can be divided into four levels from outside to inside: exterior finish, performance layer, structural layer, and interior finish. Improving the performance of wall system can effectively reduce energy consumption that required to maintain the indoor thermal environment. It can be retrofitted by three approaches: Add-in, Replace and Add-on.
Add-in includes cavity insulation and internal insulation, applying for maintaining the original façade and monumental buildings, but it may affect indoor space and occupants living. Replace and Add-on can replace or repair the existing exterior finish to protect the performance layer ( Figure 3), such as the combination of performance layer and exterior surface layer transforms to form external insulation or "ventilated vertical surface".

2) Window
As the transitional part between indoor and outdoor, the thermal loss directly caused by window accounts for about 23% -27% of the total amount [25], and it affects the indoor thermal comfort, lighting, ventilation and sound insulation.
The performance of the window is related to the performance of the frame, glass, heat transfer coefficient K, shading, air tightness, water tightness, and wind resistance.
The retrofitting approaches are Add-in, Replace and Add-on, including structure and materials. Add-in includes glazing upgrading and additional windows. Glass upgrading can improve performance by reinstalling high performance glass, such as Low-E glass and vacuum glass; additional windows are attaching single/double-glazed windows to the inside of existing windows to form the box window [26] [27]. Replace includes replacing the entire window with the double/three-layer window [28] of better performance and enlarging window.
Add-on is to add fixed or adjustable sunshades to control solar gain and reduce cooling load according to different facade orientations.

3) Roof
The roof is an enclosure component that isolates adverse external environmental factors form the top of the building. It can be divided into exterior finish the key retrofitting layers. The retrofitting approaches include Add-in, Wrap-it, Replace, and Add-on. The performance layer is divided into slope roof insulation and flat roof insulation according. The slope roof insulation uses Add-in and Add-on to add an insulation layer between/above/below the rafters.
Add-in is to add loose materials between the top beams and slab to form a thermal insulation layer. Wrap-it adapts the transformation of flat roof insulation, including three methods: upright insulation, inverted insulation, removal and replacement of roof panels and roof systems. It can also use both Wrap-it and Add-on to combine performance layer and exterior surface layer [29], such as roofing green plants and flat slopes.

4) Balcony
The balcony is an extension of the internal space of the building. It can be di-  (Figure 4(a)). If there is no underground space, a new performance layer is installed at the bottom of the slab by adding insulation materials (Figure 4(b)).

Retrofitting Material
There are five materials types: thermal insulation materials, glass, frames, sealants,   [32] is about 10 times higher than that of the traditional polystyrene board of the same thickness, and the space restriction is very low, which is very effective in rebuilding projects.
Glass plays a key role in building performance, which provides a visual and light connection between outdoor and indoor and improves the quality of the indoor environment. The measurement index is the heat transfer coefficient K value and is influenced by the number of glazing layers, the depth of the cavity, the filled gas and the coating. The glass can be used in retrofitting includes: heat-absorbing glass, heat reflecting glass, multi-layer glass, Low-E glass, vacuum glass [33].

Case Study
Hierarchical matrix editing method is introduced to catalogue design approaches, emphasizing the hierarchical progress in retrofitting process. It firstly formulates the hierarchical retrofitting database from the individual component retrofitting approaches. Then, transform the individual component system to the overall retrofitting strategies of the envelope system by layers, realizing the integration of technologies from single to diverse.

Case Introduction
This study has made field surveys and data collection work about existing resi- The hierarchical matrix editing method is introduced to solve the compatibility of individual components of different layers. Firstly, building-21 is divided into five layers by part ( Figure 5). Then, a basic information database is established to form the first hierarchical editing layer (Table 3), including location, original information, current statue information, plan and et al.

Retrofitting Strategy
Based on the first hierarchical editing layer, collecting the corresponding retrofitting approaches of each layer to form the second hierarchical editing layer (Table 4).     Then, carry a comparison study with the integrity and effectiveness of the combination of individual components retrofitting approaches to form different strategies. The most efficient strategy can be selected according to simulation results to form the third level matrix editing ( Table 5). The three hierarchical systems proceed in sequence, systematically formulating an efficient retrofitting strategy to the systematically overall system.

Conclusions
This paper first divides the envelope system into five parts and several structural layers, and then, sorts out existing residential retrofitting approaches into five types by parts. The hierarchical method is introduced to form a systematical database of existing building retrofitting strategy. On the one hand, it can improve indoor thermal comfort with lower energy consumption. On the other hand, it expands the basic information data of the existing residential envelope system in the cold climate area in China. The database can help designers make retrofitting-related decision and choose retrofitting strategies at early-design-stage efficiently. It also establishes a system for the evaluation and comparison of the building performance.
However, this study only investigated a few typical cities in cold climate area, and the scope of the study needs to be expanded. Therefore, follow-up studies will adopt a variety of data collection methods to more accurately collect the current status of the envelope system, and carry out more interdisciplinary research to improve accuracy. Simulation tools are also introduced to make evaluation and comparison.