## 2 Methods
### RHTC Calculation Theory  
The RHTC is mainly determined by the actual long-wave radiation heat transfer between a building external surface and the outside environment, including the two parts of long-wave radiation heat transfer between the building external surface and the sky or the ground. However, considering that the sky and ground radiation conditions are hard to determine, the surface temperature of the outside environment is usually replaced by the air temperature around the envelope within the margin of error so as to simplify calculations in the field of thermal engineering in China. The long-wave radiation heat transfer between the building external surface and the sky or the ground would not be reflected in the RHTC calculation, but this does not mean that there is a problem in RHTC calculation theory. In the simplification process, radiation heat transfer \(q_{\rm ra}\) between the envelope surface and the surrounding environment can be calculated through the following equation [38]:  
\[q_{\rm ra}=C_{\rm b}\varepsilon_{\rm wa}\left[\left(\frac{T_{\rm w}}{100} \right)^{4}-\left(\frac{T_{\rm a}}{100}\right)^{4}\right] \tag{8}\]  
\(\varepsilon_{\rm wa}\) is system emissivity and can be expressed as:  
\[\varepsilon_{\rm wa}=\frac{1}{1/\varepsilon_{w}+F_{1}/F_{2}(1/\varepsilon_{ \rm a}-1)} \tag{9}\]  
where \(F_{1}\) and \(F_{2}\) are the envelope surface area (m\({}^{2}\)) and atmosphere radiation area (m\({}^{2}\)), respectively, and it is quite obvious that \(F_{1}\) is much less than \(F_{2}\). Therefore, \(\varepsilon_{\rm wa}\) is equal to \(\varepsilon_{w}\).  
Thus, Equation (8) can be rewritten as Equation (10):  
\[q_{\rm ra}=\frac{5.67\varepsilon_{w}\left[\left(\frac{T_{\rm w}}{100}\right)^ {4}-\left(\frac{T_{\rm a}}{100}\right)^{4}\right]}{T_{\rm w}-T_{\rm a}}(T_{ \rm w}-T_{\rm a})=h_{r}(T_{\rm w}-T_{\rm a}) \tag{10}\]  
\[h_{r}=\frac{5.67\varepsilon_{w}\left[\left(\frac{T_{\rm w}}{100}\right)^{4}- \left(\frac{T_{\rm a}}{100}\right)^{4}\right]}{T_{\rm w}-T_{\rm a}}=5.67 \varepsilon_{w}\theta \tag{11}\]  
\[\theta=\frac{\left(\frac{T_{\rm w}}{100}\right)^{4}-\left(\frac{T_{\rm a}}{100 }\right)^{4}}{T_{\rm w}-T_{\rm a}}\approx 4\times 10^{-8}T_{\rm m}^{3}=4 \times 10^{-8}\left(\frac{T_{\rm w}+T_{\rm a}}{2}\right)^{3} \tag{12}\]  
For general building materials, the surface emissivity is usually from 0.85 to 0.95 [38]. Thus, the range of RHTCs under different \(T_{\rm m}\) values can be calculated, and the results are listed in Table 2. It can be observed that \(\theta\) barely changes when \(T_{\rm m}\) varies within a certain range. It should be noted that the actual sky radiation conditions were not taken into consideration in the analysis, owing to the RHTC processing method in this study. According to the RHTC calculation theory, it can be known that RHTC depends on the average temperatures of the envelope surface and air. Furthermore, the tropical island region has its own unique climatic features, such as small daily temperature range,  
Figure 3: Steps of the naphthalene sublimation experiment.  
small annual temperature range, and unclear four seasons, together with the characteristics of RHTC and \(T_{\rm m}\) shown in Table 2. Thus, it is reasonable for RHTC to be treated as a constant. Additionally, considering that the annual average temperature in Xisha is more than 28 \({}^{\circ}\)C, the RHTC in this study is determined as 5.8 W/(m\({}^{2}\cdot\)K).