Which condition is necessary to set up a house for a blower door test?

To prepare a house for a blower door test, it is necessary to have the storm windows closed. This condition helps ensure that the test accurately measures the air leakage of the building. When the storm windows are closed, it minimizes the potential for external air to enter through these openings, which could skew the results.

In the context of a blower door test, having storm windows closed allows for a proper assessment of the house's built envelope by creating a controlled environment where leaks can be detected without interference from external factors. This setup is crucial for evaluating the home’s energy efficiency and air sealing performance accurately.

The other options would not be suitable for an effective blower door test setup. For example, having windows open or all external doors open would introduce uncontrolled airflow, leading to inaccurate results, while closing interior doors can be important for isolating the test areas within the home. However, the primary condition that specifically contributes to an accurate assessment of the building’s air-tightness is ensuring that storm windows are closed.