In this module, you’ll learn the building science behind one of the most important goals of a healthy home: keeping it dry. Mold requires four ingredients to grow—oxygen, food, spores, and water. Of these, moisture is the only one we can reliably control. This lesson dives deep into all the ways water enters a building, including outdoor sources like rain and groundwater, and indoor sources like plumbing leaks, humidity, and wet materials. You’ll explore how water travels as a liquid and as vapor, how it’s driven by forces like capillary suction, hydrostatic pressure, and condensation, and how critical concepts like thermal bridging and vapor permeability come into play. You’ll also be introduced to your home’s “control layers”—water, thermal, vapor, and air—and how they work together to prevent mold growth and create a healthy, resilient building envelope. With this foundation, you’ll be prepared to make smart, climate-appropriate design decisions that keep your home dry from the ground up.

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