More About Insulating Plastics in Exterior Wall Construction

Quality construction is more than just meeting code. This article looks at issues related to Foam Plastic Insulation (FPI) that go beyond the code and the simple sustainable goal of increasing energy efficiency.

Regarding Sustainability
As with any plastic, FPI is inherently flammable. Manufacturers formulate the FPI with fire-retardant additives which go a long way to improving their performance in fire situations. The sustainable problem is that the fire retardants usually contain bromine. The type of bromine used in FPI, Hexabromocyclododecane (HBCD), is known to be somewhat persistent, bioaccumulative, and toxic to human beings. It's considered a borderline case requiring more research subsequent to more regulation in Europe. The call is out to look for replacements. You can find an articles addressing the issue here on BuildingGreen.com.

Regarding Fire
Fire-retardant additives go a long way to improving the performance of plastics in fire situations as quantified by ASTM E 84. It's conceivable you could light a match to a sample of insulation and see absolutely no fire propagation. But what about real building fires? Large-scale fire tests such as NFPA 286, FM 4880, UL 1040, and UL 1715 expose 20-30 foot high corner assemblies to a burning crib that is more akin to a real fire, and observations and measurements are taken. In these tests, the fire-retardant additives have negligible effect.

And it's borne out by experience. In a 21-year recent study, FM Global determined that 80 percent of the damage in fires involving plastic construction materials was caused by FPI. In May 2008, they issued new guidelines for cavity walls (Data Sheet 1-12) as follows:

Protect new exterior cavity walls using one of the following methods:

1. Use FM Approved Class 1, expanded glass insulation in cavity walls, or
2. Use FM Approved Class 1, foil-faced polyisocyanurate insulation in cavity walls, or
3. Use combustible insulation over a noncombustible substrate, but eliminate the air space so the thermal barriers are in direct contact with the insulation on both surfaces, or
4. Use combustible insulation over a noncombustible substrate in conjunction with fire stops to divide the wall cavities into areas not exceeding 2000 ft2 (186 m2).

In all cases, ensure the exterior wall veneer and the substrate for the insulation are noncombustible. Do not directly attach combustible insulation to wall studs.

What's not immediately obvious to those who are not familiar with the entire Data Sheet is that non-combustible insulation (such as mineral wool) is the preferred insulation.

In existing buildings, the FM Global recommendation is to install sprinklers around the perimeter to control the fire should the exterior wall somehow be compromised and the insulation catch on fire.

Regarding Thermal Barriers
Thermal barriers are also a tricky thing. If the barrier is not in direct, continuous contact with the plastic, then there is no telling how the assembly will perform in a fire. Therefore, 1/2-inch Type X gypsum board or 3/4-inch FR-treated plywood should only be used as a thermal barrier when in direct contact with a smooth insulation such as an insulating board. For spray-applied polyurethane, a thermal barrier that conforms to the contours should be used (such as Carboline Pyrocrete L/D or W.R. Grace Monokote Type Z-3306, or 1/2-inch Portland cement plaster on metal lath). Metal (0.016 in. steel or 0.32 in. aluminum) is effective as a thermal barrier in factory-formed sandwich panels because it allows a properly-formulated core to char, protecting the rest of the core.