## 4 Discussion  
### Effectiveness of Proposed Indicators  
The study demonstrated that multivariate microclimate indicators can effectively identify surface moisture risks in historic porous masonry, though their diagnostic power varied with environmental and material contexts. Dew point depression proved to be highly sensitive to condensation risk, particularly in unheated, high-humidity environments like San Romerio (temperature below 2.5 \({}^{\circ}\)C for 46.39% of hours). However, its utility was limited in thermally buffered spaces (e.g., heated San Vittore Mauro), where surface temperatures may still permit condensation. Relying solely on DPD calculations can therefore be misleading as they do not account for localized surface temperatures or the heterogeneous thermal properties of historic building materials. These variations can produce cold spots and condensation even when the overall indoor air dew point does not suggest a significant risk. Non-destructive techniques have revealed that the moisture distribution on historic building facades is closely linked to surface temperature differences [58; 59]. During gravimetric testing at Sant'Ignazio Church in May 2010, the internal environment exhibited an average RH of 75%, a mixing ratio of 6.8 g/kg, and a dew point depression (DPD) ranging between 3.5 \({}^{\circ}\)C and 13 \({}^{\circ}\)C. The moisture content, calculated as W% = (wet weight \(-\) dry weight)/dry weight, was measured to be 4.6% in Sample 2 (where the data logger was installed, as shown in Figure 13) for the lime mortar. According to experiments conducted in the Mobile Testing Unit (MTU) laboratory at the ABC Department of Politecnico di Milano, porous historic materials typically reach an equilibrium moisture content of 4-5% when the relative humidity exceeds 55%. While higher RH levels do not pose critical degradation risks when temperatures remain above 0 \({}^{\circ}\)C, the test conditions warrant special consideration. When measured after several rainy days with a sustained RH of 75%, Sample 2's moisture content reached 4.6%. These results partially validate the DPD findings at Sant'Ignazio, though additional testing is required for a comprehensive verification. Notably, dew point depressions as low as 3 \({}^{\circ}\)C may critically affect already deteriorated surfaces, particularly lime-based mortars. While the DPD (3.5-13 \({}^{\circ}\)C) suggested a low condensation risk from air temperature alone, gravimetry detected moisture retention (4.6%), likely due to localized cold surfaces and salt contamination. This confirms DPD's limitation without surface-temperature data. The evaluation of critical temperature-humidity (T-RH) zones revealed seasonal threats. High RH levels (>60%) at elevated temperatures (>25 \({}^{\circ}\)C) were recorded during 23% of July and August in Lake Como sites (e.g., San Fedele) and were associated with observed salt dissolution (Figure 14). Furthermore, the summer RH increased in multiple cases (San Perpetua, Masegra Castle, Pievani Palace, etc. ), suggesting that even in warmer  
Figure 13: (**a**) Location of gravimetric samples in Sant’Ignazio Church, (**b**) surface degradation in altar (Sample 2 area), (**c**) efflorescence and moisture-related degradation in altar. months, moisture remains a critical concern, necessitating continuous monitoring to prevent material degradation. Models and experimental data suggest that omitting temperature considerations may result in underestimating moisture risks--particularly in materials sensitive to combined temperature and humidity variations [60]. Thus, a comprehensive assessment incorporating T-RH zones, RH fluctuation indices, and diurnal temperature variability can reveal cycles related to moisture degradation. It should be noted that heated buildings (Besta Palace, San Vittore, and San Fedele) avoided these extremes but required careful post-heating monitoring to prevent rebound humidity.