A building-integrated solar energy system is disclosed that comprises an evacuated closed-loop conduit network circulating a working fluid through a solar thermal collector and at least one heat usage device, wherein the effective entirety of the surfaces of the closed-loop conduit network are in contact with the working fluid such that phase change occurs whenever heat energy is added by the solar thermal collector or removed by a heat usage device. The working fluid is adiabatically isolated and contained in a low pressure environment within the closed-loop conduit network. The full surface contact and low-pressure isolation of the working fluid dramatically reduces temperature differentials and energy losses, allowing for highly efficient and cost-effective heat collection and distribution.