A photovoltaic module generates electrical power when installed on a roof. A photovoltaic module preferably has an upper transparent protective layer, and a photovoltaic layer positioned beneath the upper transparent protective layer, the photovoltaic layer comprising a plurality of electrically interconnected photovoltaic cells disposed in an array. A rigid substrate layer is preferably positioned beneath the photovoltaic layer. A plurality of inserts is configured to be fixedly attached to (i) a bottom surface of the rigid substrate and (ii) a surface of a roof. The plurality of inserts is preferably disposed in an array, each foam insert having a substantially triangular-shaped cross section when viewed from a side orthogonal to a line of a roof downward slope.