Vertical Foam Insulation: Defeating the Firestop

Vertical Foam Insulation: Defeating the Firestop

George Howard’s Training Notebook

A relatively new and popular method of insulating private and small commercial structures is defeating the concept of firestopping. It involves the application of foam insulation boards to the exterior of the structure’s framing or sheathing before new siding materials are installed.

Foam insulation often leads to rapid extension of fire because it bypasses the inherent firestopping capability of straight platform frame construction, which was used in most commercial and residential structures built during the past 40 years or so. With its top and bottom plates, this type of construction is extremely effective in containing the vertical extension of fire within the enclosure walls.

The vertical studs, the top and bottom plates, and the exterior wood sheathing help hold in check a fire that is extending into the void space behind the interior wall covering. Eventually, the wood sheathing might burn through. This would allow the fire to extend to the outside of the structure and to the upper floors. However, this is a slow extension process, and an aggressive fire attack might prevent extension from occurring at all.

By contrast, foam insulation often leads to rapid extension of fire. Because of this, you must be aware of the hazards associated with foam insulation and take certain precautions when handling such an incident.

Siding materials such as vinyl and aluminum are popular today. Before this siding is installed, styrofoam panels are nailed to the exterior surface of the house, either directly over the wood sheathing or over existing asbestos or wood shingles. These panels can be anywhere from three eighths of an inch to two inches thick, but the thinner panels are used more often. The boards are used both as an insulation and a leveling material. After the foam is applied, the siding panels are installed. The resulting foam core poses some serious firefighting problems.

If the fire burns through to this core, it may completely bypass the top and bottom plates that provide firestopping and quickly reach the structure’s upper floors relatively undetected. This kind of extension is rapid and disastrous. As the foam is consumed, the siding panels may loosen and fall. Vinyl panels may even ignite and melt.

Cutaway diagram of typical foam insulation installed on wood frame construction. Sheathing may or may not be used.

Fire may extend into the core through ducts, windows, and doors. Fire may also reach the core through gaps, holes in the sheathing, and sheathing burn-through.

In another application of the foam, usually involving the thicker boards, the panels may be hung directly on the vertical studs, without the protection of the sheathing. The siding material is then applied directly to the styrofoam, using special nails. As a result, the fire can impinge immediately upon the insulation board.


When you arrive at an incident, it may not be immediately noticeable that foam insulation is in use and that fire may already be extending into the core and up the side of the structure. If there is newly installed siding on the structure, you should assume that the building contains foam insulation, and act accordingly in order to prevent the fire’s extension:

  • Immediately pull off siding and insulation from around windows and doors of the involved room.
  • Pull off any siding where you suspect a core fire.
  • Look for buckled, peeling, bubbled, or melted siding. These are all reliable indicators that there may be a core fire present.

Studies have shown that foam insulation results in higher interior temperatures, making the wood studs and sheathing burn faster and with greater intensity. Thus, you may be faced with a greater volume of fire than you anticipated, and you run the risk of serious thermal and steam burns. In addition to the flames, you should expect highly toxic smoke in the area. Wear your selfcontained breathing apparatus at all times, and continue its use throughout the overhaul stage.


If you are in doubt about the construction of a private dwelling or commercial structure fire to which you respond, you should assume the core is present and take appropriate actions to prevent extension.

Of course, the best way to handle an incident at a foam-insulated structure is with effective preplanning. Before an incident occurs, familiarize yourself with new construction sites in your area. Determine if foam insulation is present and how it is being applied. You should also be aware of vendors and suppliers who may have a large amount of material in storage. This way, you’ll be better prepared to handle a rapid-extension fire at a foaminsulated structure.

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