Which of the following is a common ventilation defect in unventilated crawl spaces?

Openings to the exterior not sealed is a common ventilation defect in unventilated crawl spaces because these unsealed openings can allow for unwanted outside air infiltration. This not only contributes to moisture problems, which can lead to mold and wood rot, but it also disrupts the intended airflow within the crawl space. The presence of unsealed openings can significantly affect the temperature and humidity levels in the crawl space, creating an environment that is prone to various structural issues and pest infestations.

In contrast, while blocked vents can also be a problem, they do not apply specifically to unventilated crawl spaces, as those spaces would inherently lack proper ventilation mechanisms. Excess air flow may not be applicable in a situation where ventilation is inadequate or non-existent, while negative pressure zones typically occur in spaces attempting to draw air in from outside, which is again not a defining characteristic of unventilated crawl spaces. Therefore, unsealed openings present a clear and direct ventilation defect that can exacerbate other issues commonly found in crawl spaces lacking proper ventilation.