Building science is simply applying what we know about physics and other sciences to buildings, with a focus on moisture and heat.
As such, building science touches on concepts from material science: the engineering discipline that studies the physical and chemical properties of materials (e.g. brick, plywood, insulation, drywall, anything buildings are made out of that might interact with moisture and heat in ways that may need to be considered), thermodynamics: dealing with methods of heat transfer, phase changes, pressure dynamics, and mechanical engineering: which gets into all the nitty-gritty of airflows and motors and equipment that generates heat, cools air, or produces or removes moisture, and even touches some on health and biology with respect to indoor air quality and mold growth (a natural consequence of moisture.)