## 2 Methodology
### Layout of the Farmhouse  
The living lab is in Tianjin's Ninghe District, at 117\({}^{\circ}\)22\({}^{\prime}\)32\({}^{\prime}\) east longitude and 39\({}^{\circ}\)19\({}^{\prime}\)3\({}^{\prime\prime}\) north latitude, which are a typical representative city in the cold area of northern China, and the calculated heating time is 118 days. The outdoor temperature and solar radiation intensity during the heating season from 1 November 2020 to 28 February 2021 are shown in Figure 1.  
As shown in Figure 1, the coldest month in this location is January, which has an average temperature of \(-\)2.9 \({}^{\circ}\)C. The number of sunlight hours is 178.4 h, the sunshine guarantee rate is 24.78% and the average solar radiation is 224.95 W/m\({}^{2}\). Although the  
Figure 1: Outdoor weather conditions from 1 November 2020 to 28 February 2021.  
outdoor temperature is relatively low, solar energy resources are quite abundant, which is a better place for the application and development of passive solar structures [17].  
The farmhouse for this research was built in the 1980s and is a traditional single-story brick-concrete rural building in northern China. Figure 2 depicts the farmhouse's structural schematic. The farmhouse faces 11\({}^{\circ}\) west-south, and the south facade is clear, with enough sunlight space between the other buildings. Bedroom 2 is the experimental room for the living lab. The dimensions of the room are 4.36 m long, 3.00 m wide and 2.65 m high. An attached sunspace has been developed on the south side of the space, unlike the other room. The appearance of the living lab is shown in Figure 3.  
Due to poor thermal performance of the original enclosure structure, its envelope structure has undergone internal thermal insulation transformation. The roof is a 20\({}^{\circ}\) tile roof + 60 mm thick EPS board ceiling; the ground is 60 mm thick EPS board + 10 mm concrete + ceramic floor; the east and west interior walls are 240 mm thick brick walls + 60 mm EPS board; the north exterior wall and the south public wall are both 370 mm thick brick walls + 60 mm EPS plate. The public wall's windows are 5 + 22 + 5 double-layer hollow glass, the solar radiation transmittance of this kind of glass is more than 80%. And the windows' skeleton uses 70 series bridge-cutoff aluminum alloys with size of 2.00 m \(\times\) 1.50 m. There are 0.60 m \(\times\) 1.00 m flat active interior window in the west and the remainder are fixed windows [18]. The heat transfer coefficient of the envelope structure before and after the transformation is shown in Table 1.  
Figure 3: Real view of the farmhouse with attached sunspace.  
Figure 2: Structural schematic of the living lab.  
The attached sunpace has a depth of 1.2 m, a width of 2.0 m, an 8-degree roof slope, and a height of 2.5 m on the south facade. The ground is 270 mm red brick + cement; on the east and west sides, there are 250 mm thick brick walls with gray cement coating, with a heat transfer coefficient of 1.10 W/(m\({}^{2}\cdot\)K); the south facade and top are \(5+22+5\) double-layer hollow glass with 70 series bridge-cutoff aluminum alloy as the skeleton, with a heat transfer coefficient of 1.448 W/(m\({}^{2}\cdot\)K).