The foam is left exposed on the walls of what the plans show as a finished recreation room, and the contractor produces a report showing a flame spread index of 25 and a smoke-developed index of 300 for the product. What should the inspector require?
- ANothing more, because flame spread and smoke indices in that range let the foam remain permanently exposed in a habitable space
Those indices only qualify the foam as an acceptable material; they never let it stay exposed. The code requires a separation between foam plastic and the building interior no matter how favorably the material tests.
- BThat a Class I vapor retarder be applied over the foam before finish work continues, since the foam face is the vapor control layer
A vapor retarder is a moisture control layer, not a fire separation. Closed-cell foam at this thickness already retards vapor, and a sheet over it does nothing about the ignition and smoke hazard the code is addressing.
- That an approved thermal barrier such as half-inch gypsum board be installed between the foam and the interior of the basement
- DThat the foam be removed entirely, because spray-applied plastic foam may not be used on the interior of a foundation wall
Foam plastic is expressly permitted on foundation walls; the code regulates how it is covered, not whether it may be used. Requiring removal treats a covering violation as if it were a material prohibition.
Why C is correct
Foam plastic must be separated from the building interior by an approved thermal barrier, and half-inch gypsum board is the reference assembly. Passing the 75 flame spread and 450 smoke limits qualifies the product for use but does not authorize leaving it exposed. Because this basement is a finished habitable room rather than an attic or crawl space entered only to service utilities, the reduced ignition barrier option is unavailable and gypsum board or an equivalent barrier is required.
What this question is testing
The item tests whether the candidate treats surface burning indices and the thermal barrier as separate requirements, and whether the candidate can identify when the reduced ignition barrier allowance for attics and crawl spaces applies.
On the job
Unfinished basements are the place this gets missed most, because the owner insulates now and finishes later, and the foam sits open for years. Inspectors also see it in attics and crawl spaces, where an ignition barrier is permitted instead, but only where the space is entered solely to service utilities and there is no storage. Once the owner puts boxes in that attic, the reduced barrier argument gets much weaker. Ask what the space will be used for, not only what it is called on the plan.
Memory technique
Half-inch gypsum is the yardstick. Anything else has to prove it performs as well.
Exam tip
Foam plastic questions usually hinge on covering it, not on testing it. Look for the thermal barrier.
Where to look it up
IRC Chapter 3, Section R316 for foam plastic, with R316.4 on the thermal barrier and R316.5 on the specific-use exceptions.