What does a pressure measurement of 50 Pa in the attic tell you when the house is depressurized to -50 Pa?

When the house is depressurized to -50 Pa, a pressure measurement of 50 Pa in the attic indicates that the pressure in the attic is significantly higher than the pressure in the house. This suggests that the attic is well-ventilated and likely connected to outdoor air. A pressure measurement of 50 Pa implies that there is an inflow of air from outside, which is characteristic of a ventilated space.

In a well-ventilated attic, the pressure would typically be greater than that of the conditioned space if outdoor air can freely flow in. This situation commonly arises when air is being drawn out of the house into the attic due to the depressurization of the entire structure. The ability for the attic to maintain a positive pressure relative to the house indicates that there is sufficient ventilation, allowing that air exchange to occur.

In contrast, if the attic were completely sealed, there would not be a significant pressure differential, nor would it result in a reading of 50 Pa while the house is at -50 Pa. This pressure differential suggests air leaks might be present elsewhere, but the measurement specifically implies that the attic is actively interacting with the outdoor environment.