The  foam types are distinguished according to the expansion ratio (E) in accordance with DIN EN 1568.

Low expansion foam  (ER > 1: 5 to <1: 20)

Medium expansion foam  (ER > 1: 20 to <1: 200)

High expansion foam  (ER > 1:200)

Low expansion
with up to 8 times expansion is produced from protein foam concentrates, e.g. FLUOR-FOAMOUSSE.
with up to 20 times expansion is produced from synthetic foam concentrates, e.g. MOUSSOL-APS.

Characteristics:
"wet" foam with fine bubbles of relatively high weight and small volume. Low expansion foam has a good flow ability, is resistant to flames and heat, gas tight and prevents re-ignition. Depending on the foam concentrate and induction rate low expansion foam adheres well to vertical objects. Its fire extinguishing effect mainly is due to its separating and cooling properties and - if it is an AFFF - additionally to film formation.
Low expansion foam is produced in low expansion foam branch pipes, foam monitors or in stationary/mobile foam installations. Screens or downstream upgrading sections, e.g. discharge knees, which may be installed, have the effect that the water/foam concentrate mixture is expanded even better with the air drawn in and a more "stiff" foam with finer bubbles is thus produced.

Medium expansion foam
with up to 200 times expansion is mainly produced from synthetic foam concentrates, e.g. STHAMEX f-15.

Characteristics:
depending on the expansion ratio, a "damp" foam with fine bubbles or a "dry" foam with big bubbles of low weight and high volume is produced. Its fire extinguishing property is due especially to the separating, cooling and repression effects.
Medium expansion foam is produced in medium expansion foam branch pipes. The water/foam concentrate mixture is swirled in the foam branch pipe with the air which is drawn in, under pressure hits a mesh inside the branch pipe and is additionally expanded. In this way, a lot of foam is produced within a short while which can be built up high.

High expansion foam
with over 200 times expansion is produced, for example, from the synthetic foam concentrate STHAMEX-K at a low water/ foam concentrate consumption.

Characteristics:
extremely high foam volume and low weight. High expansion foam contains a great proportion of air and are therefore a particularly “dry” foam with big bubbles. Due to its large volume and low weight, high expansion foam can not be "thrown".
Its fire extinguishing property mainly is due to the separating, insulating and suppression effects. These extinguishing effects are supported by a relatively high deterioration rate of the foam as minute drops of water are released which in turn evaporate immediately due to the high combustion temperatures and form the 1,700-fold volume of steam. The ambient air is thinned and cooled accordingly. High expansion foam is produced in high expansion foam generators.

Inside-air foam systems
are new high expansion foam systems used in enclosed areas, for example, in warehouses or ships' engine rooms. To produce foam, inside-air foam Systems use the up to 1,000 °C hot, soot laden combustion gases released during a fire. Special high expansion foam concentrates are used along with these Systems which are particularly resistant against combustion gases and high temperatures.