High expansion foam is used for controlling a fire that may be escaping sprinkler control. It has the ability to overcome a well-established fire. Like gas extinguishments, to do its job hi-ex foam must fill the entire fire compartment to totally submerge the fire, and continue refilling at a rate adequate to totally replace the foam broken down by the fire. It can be used as a backup for sprinkler failure only if adequate reliability factors are built into design of the system.