indoor air pressure

Indoor air pressure is the mean or controlled difference between the air pressure within a building or an enclosed building space, and that of the outside air or the air in surrounding rooms. The value can be positive or negative.

A positive difference means that the enclosed space has higher pressure than its reference surroundings. A negative difference means that its pressure is lower.

Equal pressures give a difference of zero even though both spaces contain air at a substantial absolute pressure. The comparison can use adjacent rooms when their pressure supplies the relevant surrounding condition.