Optimizing photovoltaic electric generation and roof insulation in existing residential buildings

Refurbishing existing buildings to reduce energy use is a priority worldwide to reduce greenhouse gas emissions. Millions of buildings around the globe have old roofs that are poorly insulated, but with large roof surface areas that could potentially provide significant renewable energy generation. Often, photovoltaic panels are simply added onto existing buildings regardless of thermal integrity. However, due to weathering, roofs frequently need repairs over their useful life. We evaluate a comprehensive methodology using EnergyPlus and TRNSYS simulation tools to evaluate how best to combine solar electric generation and improved insulation to achieve cost reductions, improve efficiency and renewable energy utilization at the time roofs are accessed. Advantages of combining efforts into a single intervention has not been fully explored in building energy research. We further incorporate important impacts that PV stand-off arrays have on roof thermal performance from shading and long-wave irradiance to skies. These influences have important comfort implications for poorly insulated structures with increasing summer heat waves associated with climate-related warming. They also have strong interaction with installed roof insulation levels. With an example analysis in Milan, Italy, we considered three typical residential building types in wide-use around the world (single-family, multi-family, apartment complex) with different geometries, insulation levels and roof constructions. We evaluated two options: re-roofing (roof in need of repair/replacement) and refurbishment (energy intervention for roof improvement). We optimized roof insulation levels via state-of-the-art building energy simulations considering both energy and documented costs. PV electric generation was quantified in detail, also exploring how PV array roof shading affects roof thermal performance, an influence not previously considered. This is particularly important for uninsulated buildings where upper floors can experience excessive summertime heating. Both heating and cooling needs are considered to determine the optimal roof intervention and what savings- and related costs- can be obtained depending on differing parameters. We found combining appropriate insulation with PV can provide a cost-effective option to reduce net primary energy use in residential buildings. Savings from insulation alone varied from 3% (apartment complex) to 17% (single-family). When adding PV systems with the roof initially uninsulated, net savings range from 55% (apartment complex) to 80% (single-family). Shading from stand-off PV arrays reduced summer cooling loads by 17% in uninsulated apartment complexes and provided large predicted improvements in comfort to upper floor dwellings next to exposed roofs.