The flows of heat, air and moisture through a building enclosure determine every aspect of indoor spaces. The major drivers of these flows are gradients between internal and external environmental conditions. For this reason buildings are designed to control the flows of these quantities to certain extent. Heat can flow through a material by three major mechanisms namely conduction, convection and radiation. Conduction occurs by vibration of molecules in solid objects under the presence of temperature gradient across building components such as wall, windows, roof, and floor. Whereas convection or flow of air occurs when there is a difference in air pressure between interior and exterior. Air flow can occur due to wind effect, mechanical systems, stack effects and/or a combination of one or more of these. Radiation as the name indicates is a phenomenon when particles radiated from surfaces with above absolute zero temperature. There are many mechanisms for the transport of moisture where the dominant ones are convection, diffusion and capillary effect. We will discuss each transport phenomenon in details in other sections.
Figure: Understanding building system.