Foam plastics used in buildings of Types I-IV construction require an assessment of their ability to resist vertical and lateral flame spread.

Step-by-step Design Guide

Before You Begin

  • Determine if your assembly is required to comply with NFPA 285 Fire testing.
  • Download the NFPA 285 Compliance Decision Tree PDF and follow the steps, answering each Yes/No question.
  • If your assembly requires compliance with NFPA 285, use these step-by-step directions as your guide.

Step 1

Identify the Polyiso Product

  • To begin the process, the specifier will need to decide on which polyiso product they are going to use.
  • Because of the proprietary nature of the product, products must qualify individually for use in NFPA 285 assemblies.

Step 2

Code Evaluation Report

  • Locate the polyiso manufacturer’s code evaluation report showing compliance with NFPA 285.
  • Code compliance reports can be obtained from the manufacturer or directly from the code evaluation agency (e.g. DrJ Engineering, IAPMO, ICC-ES, Intertek/ATI, NTA, etc.)

Step 3

Approved Assemblies

  • Locate the manufacturers approved assemblies.
  • These include specifically tested assemblies as well as engineering extensions that are allowed based on the NFPA 285 testing and additional cone calorimeter testing to identify allowable substitutions.
  • These may be located in the code evaluation report or directly from the manufacturer.

Step 4

Design the Assembly

Some manufacturers provide very specific assemblies that are pre-defined. In this case, simply select the assembly that suits the application.

  • Many manufactures publish tables that are used by the specifier to design their own wall assembly.
  • This option provides greater flexibility for the designer to build a variety of code compliant assemblies.

Here is an example of a table that might be used:

  • The column on the left is the wall component categories.
  • Each category represents a layer of the assemblies’ cross section.
  • To use the Table, simply select one item from each of the wall component categories in the right hand column.

Step 5

Verify compliance

  • Be careful to comply with all of the Table notes to be sure the full assembly meets all of the requirements for the products chosen.
  • Watch for any specific details on how the assembly must be constructed.

Note that some categories have “none” as an option. In these cases, that component is optional in the assembly.

Only one option may be chosen from each category and all categories must be used.

Note that the category “fire-stopping at Floor Lines” only has one option; therefore all assemblies must include this fire-stopping.

Note also that some options can only be used with a specific option from another category.


First, select the Base Wall System

We will select option 3: steel studs. Note that this option includes minimum interior and exterior sheathing and bracing requirements.

Next, select Fire-Stopping at Floor Lines

Any approved mineral-fiber-based safing insulation may be used. Thickness of insulation must match stud cavity depth

Next, select Cavity Insulation

We will select "None."

Next, select Exterior Sheathing

We will select "1/2" or thicker exterior gypsum sheathing". Note that we cannot use "none" since the steel stud base wall system requires the exterior sheathing.

Next, select WRB Over Base Wall Surface

We will select option 2 since we are using ½" or thicker exterior sheathing.

Next, select Exterior Insulation

We will select ISO-TITE as it is the only option. Since we have not used spray-foam cavity insulation, and are using exterior sheathing, it is permitted.

Next, select WRB Over Exterior Insulation

We will select “None” since we already have a WRB selected over the exterior sheathing.

Next, select Exterior Cladding

We will select option 1. We cannot select option 2 as we did not select WRB option 3.

Next, check for applicable footnotes

None of the footnotes will impact our selections.

Your wall Assembly is code compliant!