What typically causes undesirable infiltration in buildings?

Undesirable infiltration in buildings primarily occurs due to structural gaps and openings, which allow outside air to enter the building unnecessarily. Cracks around doors and windows are common sources of this problem, as they provide pathways for air leaks. These cracks can arise from various factors like settling of the building, poor installation, or deterioration of materials over time. When these gaps are present, they can compromise indoor air quality, energy efficiency, and overall comfort by allowing unconditioned air to flow in and conditioned air to escape.

In contrast, the other choices, while related to building performance, do not directly cause infiltration. For example, improperly designed HVAC systems might lead to issues like inadequate heating or cooling, but they don't create physical openings for air to infiltrate. Open vents can serve specific purposes for ventilation or exhaust but do not necessarily contribute to undesired infiltration unless they are located inappropriately. Insufficient insulation might affect energy efficiency, but it primarily relates to thermal resistance rather than air leaks. Thus, identifying and sealing cracks and openings is crucial for managing infiltration effectively in buildings.