Aims: The building sector accounts for a large share of global final energy use and greenhouse gas emissions, and improving building energy performance is regarded as a principal pathway toward sustainable development. In Iran, Topic 19 of the National Building Regulations (2020 edition) provides the main regulatory framework for managing building energy consumption, yet the extent to which its requirements align with international standards and rating systems remains insufficiently examined. This study aims to comparatively evaluate the energy performance of a two-storey, 480 m² office building in Kashan against Topic 19, the Swiss Minergie standard, and the Japanese CASBEE rating system, to identify the capacities and shortcomings of Topic 19 relative to international benchmarks, quantify the relationship between climatic parameters and energy consumption under base and optimized conditions, assess the preliminary economic feasibility of the proposed measures, and propose practical strategies for near-zero-energy office buildings in Iran's hot and arid climate.
Materials & Methods: The research is based on dynamic energy simulation of a representative two-storey office building with a net floor area of 480 m² (240 m² per floor), comprising open-plan offices, management rooms, a meeting hall and service spaces. The building is located in Kashan (33°59′N, 51°26′E, 940 m above sea level), classified as BWh (hot and arid) under the Köppen-Geiger system, with a July maximum temperature of 41.9°C, a January minimum of 1.3°C, mean annual precipitation of about 115.2 mm and mean relative humidity of about 35.6%. Hourly weather data were compiled into an EPW file using ten years of synoptic station records (WMO code 40785). Three scenarios were modelled: a base case with uninsulated walls (U=1.89 W/m²K) and roof (U=1.5 W/m²K), single-glazed windows (U=5.7 W/m²K), a 75%-efficiency boiler, a split air conditioner (COP=2.5) and infiltration of 0.8 ACH; an optimized case applying Topic 19 requirements, including wall and roof U-values of 0.48 and 0.35 W/m²K, Low-E windows (U=2 W/m²K), a 90%-efficiency condensing boiler, an inverter split unit (SEER=4.2), infiltration reduced to 0.5 ACH, a 15° reorientation toward the optimal south-facing axis and 0.8 m deep horizontal shading on the south façade; and a renewable-integrated case adding a 40 kWp rooftop photovoltaic array of 114 polycrystalline panels (228 m²). The window-to-wall ratio was 30% on the south and east façades and 20% on the north and west. The model was developed in SketchUp, transferred through OpenStudio to EnergyPlus 23.2, and simulated with a six-minute time step. Validation included an annual energy balance error below 0.5%, sensitivity testing of wall U-value and infiltration rate, and comparison of EUI with comparable office buildings in hot and arid climates. Sequential parametric analysis quantified the contribution of individual measures, while Pearson correlation analysis related monthly climatic parameters to heating, cooling and total energy consumption. The outdoor-air requirement was checked against ANSI/ASHRAE Standard 62.1. A preliminary economic assessment estimated incremental investment costs and simple payback periods (SPP) using 2023 (1402) reference prices, a feed-in tariff of 22,000 IRR/kWh, a retail electricity tariff of 5,472 IRR/kWh and a natural-gas price of 5,000 IRR/m³. Finally, the optimized scenarios were assessed against the CASBEE for Office (2018) checklist and Minergie Standard (2020).
Findings: Annual energy consumption fell from 75,823 kWh in the base case to 58,301 kWh in the Topic-19-optimized case, a reduction of 23.11%, with the largest savings occurring in July and August, the months of peak cooling demand. Sequential parametric analysis showed that optimal orientation (7.03%) and envelope thermal insulation (5.08%) contributed the most to this reduction, followed by efficient heating and cooling systems (4.52%), Low-E windows (3.36%) and horizontal shading (3.12%); because the method is order-dependent, these shares should be read as indicative rather than fixed contributions. The rooftop photovoltaic system generated 40,451 kWh per year, lowering net grid energy demand to 17,850 kWh and yielding a net energy use intensity of 37.19 kWh/m²/year, close to the range reported for near-zero-energy office buildings. Pearson correlation analysis indicated a significant negative relationship between mean monthly temperature and heating consumption (r=−0.59 in the base case, r=−0.579 in the optimized case) and a significant positive relationship between mean and maximum monthly temperature and cooling consumption (r≈0.76 in both cases), while relative humidity correlated negatively with cooling demand, more strongly in the base case (r=−0.687) than in the optimized case (r=−0.659). Checking the reduced infiltration rate (0.5 ACH) against ANSI/ASHRAE 62.1 showed that natural infiltration together with the assumed mechanical supply meets only about 79% of the required outdoor-air rate for the building's occupancy, a roughly 21% shortfall that could raise indoor CO₂ concentrations above 1,000 ppm during peak occupancy, indicating that airtightening measures need to be paired with controlled mechanical ventilation with heat recovery. The preliminary economic assessment showed that the Topic-19 envelope, glazing, HVAC, shading and infiltration measures require an incremental investment of about 774 million Tomans and reduce annual consumption by 17,522 kWh, but because EnergyPlus outputs do not separate the resulting electricity and gas savings, their financial benefit and payback period could not be quantified; the 40 kWp photovoltaic system, by contrast, requires an investment of about 1,000 million Tomans, generates 40,451 kWh/year and, assuming all generated electricity is sold at the feed-in tariff, yields an estimated annual revenue of about 89 million Tomans and a simple payback period of about 11 years and 3 months. Under CASBEE for Office, the building scored 3.5, 3 and 2.5 out of 5 on indoor environmental quality, service quality and outdoor environmental quality respectively, and 4, 3 and 3.5 on energy, resources and materials, and site environment, yielding Q=50, L=37.5, a BEE index of 1.33 and a B+ (Good) rating. Under the Minergie Standard, by contrast, the building failed every applicable criterion: wall and roof U-values of 0.48 and 0.35 W/m²K exceeded the 0.2 W/m²K limit, window U-value of 2 W/m²K exceeded the 1.2 W/m²K limit, mandatory mechanical ventilation with heat recovery was absent, no blower-door air-tightness test was carried out, and fossil-free energy supply was only partially achieved, so no certification level was obtained despite the marked improvement in net EUI.
Conclusion: The findings indicate that Topic 19 offers an effective framework for reducing office building energy consumption in Iran's hot and arid climate and, combined with photovoltaic generation, can bring performance close to advanced international near-zero-energy benchmarks, as reflected in the B+ CASBEE rating. The contrast between the two rating outcomes stems from their different logics: CASBEE is a relative, weighted index (BEE=Q/L) that rewards proportional gains in environmental quality and load reduction, whereas Minergie is a threshold-based standard with absolute, mandatory limits on envelope transmittance, mechanical ventilation with heat recovery and air-tightness testing, so a low net EUI alone does not guarantee compliance. The economic assessment further shows that, of the measures studied, only the photovoltaic system has a directly quantifiable payback (about 11.3 years) under 2023 reference prices, while the financial return on the Topic-19 envelope and system upgrades could not be isolated from the available simulation output; this preliminary analysis excludes discount rates, inflation, maintenance costs and the time value of money, so it should be read as indicative rather than a full life-cycle cost assessment. The reduction in infiltration recommended by Topic 19 also creates a fresh trade-off, since the resulting outdoor-air supply falls short of ANSI/ASHRAE 62.1 requirements, underscoring that airtightening without mechanical ventilation with heat recovery is an incomplete solution. Revising Topic 19 to tighten wall and roof U-value limits, to require or incentivize heat-recovery mechanical ventilation, and to introduce air-tightness testing is recommended as a means of narrowing the gap with Minergie and moving office buildings in similar climates closer to near-zero-energy performance. Because the results derive from a single case study validated only through internal energy-balance and sensitivity checks rather than measured field data, and the CASBEE scoring reflects the authors' self-assessment rather than an independent certification, broader generalization requires further work on more diverse building samples, measured performance data, independent CASBEE assessment, and a comprehensive life-cycle cost analysis.