What does the term 'pressure boundary' refer to?

The term 'pressure boundary' specifically refers to the area dividing conditioned and unconditioned air within a building. This boundary is critical in building science as it dictates how energy and air move between different areas of the structure.

When a building is designed, the pressure boundary helps determine where the conditioned air (which is heated or cooled) exists and how it interacts with the outside environment. Properly managing this boundary prevents unwanted air infiltration and exfiltration, thereby ensuring energy efficiency and comfort within the conditioned spaces. This distinction is essential for understanding HVAC performance and the overall thermal dynamics of a building.

The other choices, while related to different aspects of building performance and building science, do not accurately define 'pressure boundary'. For example, the area separating indoor from outdoor air does not specifically address the conditioned and unconditioned areas that the pressure boundary signifies. Similarly, insulation and exterior walls have relevant roles in energy efficiency and building integrity, but they do not encapsulate the concept of pressure differential and airflow management as directly as the division between conditioned and unconditioned air.