When one or more compartments in a structure are involved in a fire, many important fire indicators must be carefully interpreted to determine the position of a neutral plane for the entire structure. This is a key factor in determining where and how to ventilate a structure fire.

In such fires, neutral planes can still be observed across many of the openings, and might be at different heights in different parts of the structure. In sizing up the entire fire, it is important to observe the clues that can help determine the position of neutral pressure (neutral plane) within the entire structure. Pressure differences determine how the smoke flows through each vent opening. The flow seen at a given opening depends on whether the fire is located above, below or on the same floor as that vent. This can be helpful in determining the location of a fire even when it may not be visible.

In Figure 22, where the fire is below the opening (i.e., the opening is above the neutral plane for the structure), hot gases are likely to flow out of the entire vent. This is the case for the central window on the upper floor of the structure. On the other hand, if the fire is above the opening (i.e., opening is below the neutral plane for the structure), there will likely be very little, if any, flow of smoke from the vent.

Neutral Plane vs. Interface Height

The term "neutral plane" is often used by firefighters to describe both the neutral plane and the interface height. The neutral plane refers to a horizontal plane at any opening between an enclosure and the exterior-although the exterior might be another room in a structure-where the interior and exterior pressure are the same.

At any opening, the neutral plane is the horizontal plane where there is no flow into or out of the enclosure. As has been described above, the neutral plane is dependent on the conditions of each opening, and can vary from one opening to another, even in the same compartment. The neutral plane is an indication of the pressure differences at an opening.

Within the enclosure, the separation between the hot upper layer and the cool lower layer is known as the interface layer, and its height above the floor is known as the interface height. The interface height is an indication of how much of the volume of an enclosure is filled with smoke, and how far the hot upper layer has banked down.

While the interface height and the neutral plane can be at the same elevation, they often differ. The neutral plane will be affected by ventilation (either natural or forced) while the interface height may not be as strongly influenced. It is important for firefighters to understand that, while the neutral plane is an important indication of fire severity, it does not necessarily indicate the height of the smoke within an enclosure or structure. Firefighters need to be aware that the interface height might vary substantially from the neutral plane at a given opening, and is likely to vary from room to room within a structure.