The LWF Blog
Fire Safety Engineering for Design – Foam Fire Suppression Systems – Part 267
December 29, 2025 9:55 amLWF’s Fire Safety Engineering blog series is written for Architects, building designers and others in the construction industry to highlight and promote discussion on all topics around fire engineering. In part 266, LWF began to discuss foam fire suppression systems. In part 267, we continue to talk about foam and how it suppresses and extinguishes fire.
A foam system takes a proportion of the foam concentrate, usually 1–3%, and combines it with water at the remaining 99-97%. The resulting foam solution is then aspirated to form foam bubbles which are applied to the surface of the flammable liquid.
The foam works to extinguish the fire in various ways:
- By smothering the fire and preventing air mixing with the flammable vapour
- It suppresses the release of flammable vapour from the fuel
- By separating the flames and heat from the surface of the fuel
- It cools the fuel surface and sources of ignition
The foam must be capable of reacting in certain ways in order to be a successful suppressant:
- It should flow freely
- Form a tough and cohesive blanket
- Suppress flammable vapours
- Seal against hot surfaces
- Be heat resistant
- Resist fuel pickup
- Retain water
- Offer good ‘burn-back resistance’
Foam is unsuitable for live electrical hazards and three dimensional running fuel fires. There are various types of foam concentrate, each formulated for specific purposes.
Fluoroprotein Foam (FP)
FP foam is protein-based with fluorochemical additives. It is best used aspirated and is stable but flows freely and has effective firefighting properties. It has low fuel pickup, provided it is applied gently. Standard FP foam cannot be used on water-soluble risks. FP foam is a thick, stable foam with good burn-back resistance.
Aqueous film-forming Foams (AFFF)
AFFFs were developed specifically for crash fire situations where it is crucial that a fast fire knockdown is performed in order to rescue personnel. The foams are formed with a combination of fluorocarbon surfactants and synthetic foaming agents, which provide tension characteristics capable of producing a thin vapour-sealing film on a hydrocarbon liquid surface. The produced film spreads rapidly to provide fast flame knockdown. AFFFs are formulated to drain foam solution fast from the foam bubble for optimum film formation for rapid fire knockdown.
AFFFs sacrifice some elements of long-term sealability and burnback resistance in order to prioritise fast acting suppression.
Due to the nature of the easily-aspirated foam, AFFFs can be used through non-aspirating equipment, such as conventional sprinkler heads. This means that existing water-deluge systems could be easily converted to foam systems by the simple addition of the appropriate proportioning equipment.
Research has shown that some foams used within firefighting applications can be harmful to humans. These are commonly termed under the umbrella of PFAS (per- and polyfluoroalkyl substances). Existing and proposed systems should consider specialist advice in relation to the chemicals used in systems alongside any action that is required.
In part 268 of LWF’s series on fire engineering we will continue to discuss the different types of foam concentrate used in fire suppression, starting with synthetic detergent. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.
Lawrence Webster Forrest has been working with their clients since 1986 to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact the LWF office on 0800 410 1130.
While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.