The distinction between the interface height and the neutral plane is important when assessing the potential for RFD. While the two terms are often used interchangeably to describe the boundary between the hot smoke and cool air, they are in fact distinct terms. The interface height refers to the level of the interface between the hot and cold zones within an enclosure, while the neutral plane refers to the horizontal plane of net-zero pressure difference between the interior and exterior of a compartment at an opening. Above the neutral plane, pressurized gases and smoke flow out of the enclosure; below the neutral plane, a negative pressure allows air to flow into the enclosure, as shown in Figure 15.

As the fire develops within an enclosure, the interface height (i.e., the boundary between the heated smoke and the cooler air) will lower, and may be observed through thermal imaging or the corresponding smoke stratification and neutral plane. Therefore, a high interface height could indicate that the fire is in the early stages of development, with dependant factors being ceiling area/height, distance to the fire, vent openings/heights and building geometry.

A sudden rise in the layer could indicate that ventilation has occurred. Gradual lowering of the layer often indicates a build-up of hot fire gases, a situation that could progress to flashover, if the appropriate controls are not put into action. A sudden lowering of the layer could indicate a rapid intensification of the fire, as shown in Figure 16.

Both terms effectively describe a boundary, and taken together, they provide an enhanced understanding of fire development in an enclosure that aids in the selection of appropriate fire ground strategies and risk levels. A descending neutral plane and interface layer must be recognized and communicated to ensure safe operations.

It must be stressed that an observed neutral plane at an opening may correspond to the interface height within a hallway or room. However, the neutral plane may not be representative of the interface height within the fire room in a complex building layout (e.g., plenum). This may result in firefighters not perceiving the severity of fire conditions that will soon be overhead or banking down on them.