👉 Underlayer in the context of a building or structure refers to the bottom layer of material that is below the surface. It could refer to insulation, such as cellulose or foam, which provides thermal protection and helps insulate the interior space beneath it. The underlayer can also include other materials such as drywall, tiles, or brick, depending on the specific construction method used. Understanding the underlayer plays a crucial role in ensuring that the building's overall structure is strong and durable.