Fig. 3 Comparative analysis of primary indicators (a) M1, M2, M3, (b) H1, H2, H3
In order to further evaluate the level of low carbonization of each household, this study analyzed the secondary indicators of each household, as shown in Fig 4a, a total of eight secondary indicators in M2 have a weighted evaluation score greater than M1 and M3, which all contribute to the assessment of low carbonization. Checking the research situation of M2, we found that the windows used in the building are aluminum windows, which have very good heat insulation effect and play a good role in reducing heat loss, and the exterior wall construction is made of Sanchi wall, which is better than both M1 and M3, so the building has higher heat insulation effect and plays an important role in LCI. In the case of energy use, M2 uses more biomass, mainly tree branches, and biomass has an important role in low carbon, and these two indicators also show the importance of ”energy use” and ”envelope”, which have a significant impact on improving LCI [22]. M3 is a ”high-carbon” case with the lowest evaluation score, and the analysis of its score can help to improve the level of low carbonization in the future. Generally, because M3 did not carry out envelope renovation and energy renovation, the energy used is mainly bulk coal and cellular coal, which is used a lot and causes relatively serious pollution. For M3, the indicators with good weighted scores are ”per capita annual household income” and ”window-to-wall ratio”, which shows that M3 has a good per capita annual income level and has the economic ability to use clean energy. The ”window-to-wall ratio” also lays a certain foundation for future envelope renovation and does not require too many changes in windows and doors.
Through the study of the renovated H1-H3, it was found that before the renovation as M1-M3, the walls of the buildings did not have good insulation materials, and they could only rely on burning large amounts of coal for heating and as a source of energy consumption such as domestic hot water and cooking in winter. However, in recent years, through the transformation of the envelope structure and the change of energy utilization, the insulation layer has been added to the exterior walls of residential buildings, and the insulation material is mainly of the polystyrene board type, which has a good thermal insulation effect and guarantees an effective improvement of the indoor thermal environment [23]. The energy used is also cleaner and low-carbon, and the reduction in the amount of coal combustion has improved the indoor air quality and changed the original ”dirty, messy, and poor” situation. In terms of ”energy use”, biomass energy resources are abundant in the study village, and the renovated buildings have increased the use of new biomass stoves and solid-formed biomass pellets, and in recent years have increased the use of solar photovoltaic power generation, which can supplement household electricity and reduce the use of coal compared with no energy renovation. In terms of ”envelope”, the addition of wall insulation systems has improved indoor temperatures, and windows have been replaced with aluminum and plastic windows from the original wooden windows or old-fashioned steel windows. The ”layout design” aspect, as an indicator with limited changes during renovation, was also selected with different regional conditions in mind, and although it did not change much in the case study, it will play its importance in a broader range of uses in the future [24]; the research site selected for this paper was Zhejiang Province, so ”site rationality”, ”building orientation arrangement”, and ”geographic location distribution” have not changed, and ”living area per capita ” had some changes before and after the renovation, but this paper only focuses on residential buildings, and the renovation did not have an impact.