10.5 Foam Plastic Insulation & Interior Finishes (Drywall)

Foam Plastic Insulation & Interior Finishes (Drywall)

Minnesota's 2020 Residential Code incorporates IRC Section R316 (foam plastic) and R702 (interior covering) with statewide amendments. Because foam plastic insulation is a combustible material even when it performs well thermally, the code layers multiple fire-separation requirements on top of the standard R-value rules — and this is where exam candidates most often lose points by confusing "thermal barrier" with "ignition barrier."

1. Flame Spread and Smoke-Developed Limits

All foam plastic insulation installed in a dwelling — rigid board, spray polyurethane, or foam sheathing — must be tested per ASTM E84 or UL 723 and carry a maximum:

• Flame spread index of 75
• Smoke-developed index of 450

This rating alone does not permit foam plastic to be left exposed inside a home. It only establishes that the material is acceptable for use behind a required fire-separation layer.

2. Thermal Barriers: The Interior Separation Requirement

Wherever foam plastic insulation is installed on the interior of a building — for example, spray foam on the underside of roof sheathing in a finished attic, or rigid foam on a basement wall — it must be separated from the habitable space by an approved thermal barrier equal to 1/2-inch gypsum wallboard, or another material demonstrated by testing to limit the average temperature rise of the unexposed surface to not more than 250°F after 15 minutes of fire exposure.

3. Ignition Barriers: Attics and Crawl Spaces

A less stringent but still mandatory layer applies where foam plastic is exposed within an attic or crawl space that is entered for reasons beyond servicing utilities (e.g., storage attics, mechanical rooms). Because a full 1/2-inch gypsum thermal barrier is often impractical to install against a roof deck or foundation wall in these spaces, the code instead permits an ignition barrier, consisting of any one of:

• 1.5-inch-thick mineral fiber insulation
• 0.5-inch-thick structural cellulosic insulating board
• 0.25-inch-thick hardboard
• 0.375-inch-thick particleboard
• 0.375-inch gypsum board
• Corrosion-resistant steel a minimum of 0.016 inch thick
• Any other material tested and listed as an ignition barrier for the specific foam product

Exception (attic/crawl space access-only): An ignition barrier is not required where the attic or crawl space is accessed only to service utilities (not used for storage) and the installed spray foam product has been specifically tested and listed for exposed application without an ignition barrier (commonly certified under AC377 Appendix X). This exception is a frequent trick point on the exam — the exemption depends on both limited access and a specific product listing, not on either condition alone.

4. Exterior Continuous Insulation (Foam Sheathing)

Rigid foam sheathing applied to the exterior of framing as continuous insulation is treated differently because it sits outside the structural wall, not inside occupied space. It must still be covered by an approved exterior wall covering (siding, stucco, or brick veneer) fastened per the manufacturer's or code-prescribed schedule, and it must integrate with — not replace — the water-resistive barrier unless the specific foam product is also listed as a WRB with taped, sealed seams. Where foam sheathing thickness exceeds what standard-length siding nails can penetrate through into framing, longer fasteners or an engineered furring/fastening system rated for the additional foam thickness is required to maintain both structural attachment and weather-tightness.

5. Interior Finishes: Gypsum Board (R702)

Gypsum wallboard thickness and fastening are governed by framing spacing, since sag resistance decreases as spacing increases:

Ceilings at wider 24-inch spacing require the thicker 5/8-inch board specifically to resist visible sag between joists over time — a detail candidates should remember as "wider ceiling spacing needs thicker board," even without memorizing every table cell.

Fastening spacing also tightens for ceilings versus walls: nails are typically spaced no more than 7 inches on center on ceilings and 8 inches on center on walls, while screws allow wider spacing (around 12 inches on center in the field) because their holding power per fastener is greater than a nail's.

6. Water-Resistant Gypsum Board in Wet Areas

Water-resistant gypsum board ("greenboard") may be used as a general wall substrate in bathrooms and kitchens, but it has firm limits:

• It may not be used as a tile backer directly inside tub or shower enclosures — cement board, fiber-cement backer board, or another moisture-rated tile substrate is required there instead.
• It may not be installed over a vapor retarder within a shower or tub enclosure, since trapped moisture behind the board accelerates core deterioration.
• It is unsuitable in areas of sustained high humidity without mechanical ventilation, such as unvented steam shower enclosures.

Exam Takeaway

Two separate fire-protection concepts govern foam plastic — a thermal barrier (1/2-inch gypsum standard) for occupied interior space, and a lighter ignition barrier for attics/crawl spaces, with a narrow access-and-listing exception between them. Layer that distinction on top of standard gypsum board thickness and moisture-substrate rules, and this section becomes a set of predictable, memorizable comparisons rather than a wall of code citations.