The lowermost seal on an exterior door, typically made of rubber, silicone, felt, or similar materials, prevents air and water infiltration. This component creates a barrier between the door and the threshold, closing the gap that would otherwise exist. For instance, a sweep attached to the bottom of a door effectively blocks drafts.
A properly functioning seal at the base of an exterior door plays a vital role in energy efficiency by minimizing heat loss during colder months and reducing the strain on cooling systems in warmer weather. This lower seal also protects against moisture intrusion, which can lead to damage such as wood rot, mold growth, and insect infestation. Historically, simple felt or brush-like materials were used, but advancements in material science have led to more durable and effective options. This evolution reflects a growing understanding of building science and the importance of a tight building envelope.