# BEST--Building Energy-Saving Tool for Sustainable Residential Buildings
## 4 Results  
The matrix provided by the tool identifies nine insulation solutions for the roof: three non-ventilated external insulation systems (applied to the upper surface), four ventilated roof solutions, and two internal insulation options (applied to the underside). For the vertical envelope, six solutions are proposed: two external thermal insulation composite systems, two ventilated facade types, and two internal insulation options. Given that the building is not subject to any heritage or urban planning constraints, all insulation solutions can be applied externally. The solutions selected for the case study correspond to those with the simplest installation processes that also result in the lowest steady-state thermal transmittance values for both the roof and vertical envelope. The tool allows users to download technical datasheets (Supplementary File S3), which include:  
* The code of the roof and vertical wall typologies, the historical period in which they were most prevalent, and the stratigraphy to which the proposed solution is applicable;
* The code of the selected insulation solution;
* The type, thickness, and thermal conductivity of the proposed insulating material;
* A drawing of the insulation stratigraphy, along with a description of its individual layers;
* Thermo-physical properties of the existing building components (walls and roof) and the proposed insulation layers;
* Calculated performance values under both winter and summer conditions;
* Surface condensation risk assessment (isotherm analysis);
* Interstitial condensation risk assessment (Glaser diagram--critical month);
* A radar chart comparing key performance parameters, including steady-state transmittance (U-value), time lag (\(\Phi\)), internal surface periodic thermal capacity (Cip), dynamic transmittance (Yie), and insulation thickness;
* Verification of compliance with regulatory limits for both heating and cooling seasons. For the case study, three different retrofit interventions aimed at improving the thermal performance of the opaque envelope were selected: INT.1, INT.2, and INT.3, the latter being a combination of the first two. * INT.1: The first energy retrofit intervention involves external insulation of the roof using solution IE.01.b, which consists of 10 cm thick rigid polyurethane foam panels with a thermal conductivity of 0.023 W/mK. This measure reduces the steady-state U-value to 0.20 W/m\({}^{2}\)K, complying with the maximum transmittance limits set out in Annex E--Thermal Insulation Requirements, which refers to the UNI EN ISO 6946 standard [44] for calculating the thermal transmittance of opaque structures (excluding thermal bridges). * As shown by the results, this intervention requires an investment of \(\xi\)14,148.40, achieves a CO\({}_{2}\) emissions reduction of 21.18 kg/m\({}^{2}\).year, a yearly energy cost saving of \(\xi\)713.00, and yields a positive 20-year NPV of \(\xi\)3546.00, corresponding to a Payback Time (PBT) of 19 years and 10 months. The PBT does not account for interest, inflation, or variations in gas prices. * INT.2: The second intervention targets thermal insulation of the vertical envelope using an ETICS system identified as SC.01, consisting of 12 cm thick rigid hydrophobic glass wool panels with low vapor permeability and a thermal conductivity of 0.032 W/mK. This reduces the steady-state U-value to 0.233 W/m\({}^{2}\)K, also in compliance with Annex E.
* According to the results, this intervention requires an investment of \(\xi\)16,395.32, achieves a CO\({}_{2}\) emissions reduction of 35.55 kg/m\({}^{2}\).year, a yearly saving of \(\xi\)1193.00,and yields a positive 20-year NPV of \(\ell\)1343.54, corresponding to a PBT of 13 years and 9 months. * INT.3: The third intervention combines both previous strategies (INT.1 + INT.2).