Reax Research Contributes to California’s Approval of Below-Grade Insulation without Flame Retardants

January 16th, 2019 – Sacramento, California

The California Building Standards Commission unanimously approved the use of expanded polystyrene foam without flame retardant chemicals in slab on grade assemblies. This decision was based on the result of independent testing, review by the California Building Standards Commission’s Code Advisory Committee, public comment, and a recommendation for adoption from the Office of the State Fire Marshal to the California Building Standards Commission.

Driving Greater Use of Insulation Below Slab in New Residential Construction

Sounds insignificant right? How much is this application use used in a state where only one climate zone exists which requires below slab insulation?

But the push for enhanced thermal performance in new residential construction may drive greater use of insulation below slab—and is leading to a rapidly increasing use of foam plastic insulation overall. Consumers, builders, and architects are increasingly concerned about health effects associated with construction materials and want greater flexibility in selecting materials without compromising fire safety or their health. The decision is also significant for its novelty, i.e. heretofore no allowance for an exception to the flammability testing requirements for foam has been permitted.

Foam Behind a Thermal Barrier = Safe without Added Flame Retardants

The Commission’s basis for approval, in summary, that non-FR foam sandwiched between a concrete thermal barrier and 37 miles of dirt doesn’t constitute a fire spread hazard or increased risk to building occupants or firefighters. Neither an ignition source nor sufficient oxygen is present below a concrete slab-on-grade to support combustion.

Assembly details matter, foam behind a thermal barrier is safe without added flame retardants. This more closely models the approach of Sweden, Norway, and Spain where the flame retardant free foam has been safely used in construction for decades.

Scientific Fire Investigation

Reax began a scientific investigation of this proposed change in the summer of 2015. A series of reduced and intermediate assembly scale tests on ignition and flame spread behavior of expanded polystyrene foam (EPS) demonstrated an insignificant increase in risk associated with the use of non-fire retarded foam. Those tests were repeated in Fall of 2018 by Oklahoma State University at the request of the California State Fire Marshall in support of proposed code change. The OSU researchers found:

  • When installing below-slab, insulation without flame retardants presents no risk of fire spread to the building and will not endanger occupants or first responders.
  • Adding flame retardants to polystyrene insulation does not significantly reduce peak heat release rates.
  • The time to ignition of flame-retardant free polystyrene was comparable to other combustible materials commonly found at construction sites.

Check out the Phase II report.

Testing evaluated fire performance of the proposed below grade assembly and bench scale assessments of bare foam ignition behavior when exposed to ignition sources that might be found on construction sites, e.g. grinding sparks, welding and brazing slag, and hot surfaces. Intermediate scale measurements of stacked materials provided a relative risk assessment of the foam when compared to other commonly stored materials on construction sites.

David Rich from Reax Engineering will travel to Albuquerque, NM in May as lead proponent of a similar provision in next edition of The International Residential Code.

Residential Code Change Details

For those interested in the code change details, they are bolded (see below) in the relevant exception to California Residential Code Section R316.3 on Surface Burning Characteristics, which reads as follows:

Unless otherwise allowed in Section R316.5, foam plastic or foam plastic cores used as a component in manufactured assemblies used in building construction shall have a flame spread index of not more than 75 and shall have a smoke development index of not more than 450 when tested in the maximum thickness and density intended for use in accordance with ASTM E84 or UL 723.


1: Foam plastic insulation more than 4 inches (102 mm) thick shall have a flame spread index of not more than 75 and a smoke-developed index of not more than 450 where tested at a thickness of not more than 4 inches (102mm) provided that the end use is approved in accordance with section R316.6 using the thickness and density intended for use.

2: Polystyrene foam insulation boards with a maximum thickness of 2 inches (51 mm) when installed below a minimum 3.5-inch thick (89 mm) concrete slab-on-grade.

Section R316.2.1 includes changes in marking and labeling associated with the excepted foam above.

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