Intumescent Paint Over XPS
If you do not provide the thermal and ignition barriers specifically mentioned in the code then you will need to get specific written approval from your local code official. The sprayed on intumescent paint protection is an exception to the code and needs written approval to be considered acceptable.
Here's the exact code exerpt. Notice how it does not allow it to be left uncovered and/or painted without said approval. Also note that the foil facings on the foam are not considered approved thermal/ignition barriers as they are not listed in the approved methods noted below.
SECTION 2603 - FOAM PLASTIC INSULATION
2603.4 Thermal Barrier. Except as provided for in Sections 2603.4.1 and 2603.9, foam plastic shall be seperated from the interior of the building by an approved thermal barrier of 1/2-inch (12.7mm) gypsum wallboard or equivalent thermal barrier material that will limit the average temperature rise of the unexposed surface to not more than 250degF (102degC) after 15 minutes of fire exposure, complying with the standard time-temperature curve of ASTM E 119 or UL 263. The thermal barrier shall be installed in such a manner that it will remain in place for 15 minutes based on FM 4880, UL1040, NFPA 286 or UL 1715. Combustible concealed spaces shall comply with Section 717.
2603.4.1 Thermal barrier not required. The thermal barrier specified in Section 2603.4 is not required under the conditions set forth in Sections 2603.4.1.1 through 2603.4.1.13.
(Sections 2603.4.1.1 -> 2603.4.1.5 and 2603.4.1.7 -> 2603.4.1.13 don't pertain to this topic)
2603.4.1.6 Attics and Crawl Spaces. Within an attic or crawl space where entry is made only for service of utilities, foam plastic insulation shall be protected against ignition by 1 1/2-inch-thick (38mm) mineral fiber insulation; 1/4-inch-thick (6.4mm) wood structural panel, particleboard or hardboard; 3/8-inch-thick (9.5mm) gypsum wallboard, corrosion-resistant steel having a base metal thickness of 0.016 inch (0.4mm) or other approved material installed in such a manner that the foam plastic insulation is not exposed. The protective covering shall be consistent with the requirements for the type of construction.
2603.9 Special approval. Foam plastic shall not be required to comply with the requirements of Sections 2603.4 through 2603.7 where specifically approved based on large-scale tests such as, but not limited to, NFPA 286 (with the acceptance criteria of Section 803.2), FM 4880, UL 1040 or UL 1715. Such testing shall be related to the actual end-use configuration and be performed on the finished manufactured foam plastic assembly in the maximum thickness intended for use. Foam plastics that are used as interior finish on the basis of special tests shall also conform to the flame spread requirements of Chapter 8. Assemblies tested shall include seams, joints, and other typical details used in the installation of the assembly and shall be tested in the manner intended for use.
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