Hemp has attracted considerable attention as a potential building material, particularly among those searching for alternatives to conventional construction products with substantial environmental footprints. One of the most frequently discussed examples is hempcrete, a material made from hemp and a mineral binder. It is often described as carbon-negative, highly insulating and capable of creating healthier indoor environments. These claims are intriguing, but the science behind hempcrete is more nuanced than the headlines suggest. Understanding what the material actually does, and what it cannot do, reveals a surprisingly sophisticated approach to sustainable construction.