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Building Code Reference

Got a CRC R302.10 correction?

Flame spread of insulation

For builders in California, Greater Seattle, the Las Vegas area, the Denver area and Kansas City · CrossBeam Permits

What this code requires, how to fix it, and how cities and counties review it

What this code requires

CRC R302.10 is the California Residential Code section that sets flame spread index (FSI) and smoke development index (SDI) limits for insulation materials and their facings. Any insulation installed in a residence — batt, blown, rigid board, or spray foam — must meet these limits, and R302.10.1 sends foam plastic insulation to Section R303. Foam plastic insulation (spray polyurethane foam, rigid EPS/XPS boards) gets the most scrutiny: left exposed after the contractor finishes the work, it fails R303.4 because it lacks the required thermal barrier separating it from the interior living space. Most single-family and ADU projects that use spray foam in attics, crawl spaces, or on foundation walls get flagged on this section when the plans do not address thermal barrier requirements.

How to fix it

  1. Identify every insulation product on the plans

    Go through your drawings and list every insulation type shown: spray foam (open or closed cell), rigid board (EPS, XPS, polyiso), batt (fiberglass, mineral wool), and any composite panels. Note where each product appears and whether it is in a location where it will be exposed — visible from the living space or accessible crawl space or attic. This inventory is what you will address in the correction response.

  2. Add a thermal barrier over all exposed foam plastic

    For any foam plastic insulation (spray foam or rigid board) that is exposed to the interior of the building, show a thermal barrier on your plans. R302.10.1 sends foam plastic to Section R303, and the standard thermal barrier under R303.4 is 1/2-inch gypsum wallboard (or 23/32-inch wood structural panel) over the foam. Show this on the relevant wall section, attic detail, or foundation detail with a note referencing R303.4. If space constraints prevent drywall — common on rim joists — check whether your foam product has a listed alternative: an intumescent coating tested to NFPA 286 or UL 1715 with a test report the manufacturer can provide. List the product name and test report number on the plans.

  3. Confirm FSI and SDI ratings for insulation facings

    Kraft-faced batts and reflective foil facings are the usual suspects. The flame spread index limit is 25 or less, and smoke development index is 450 or less for insulation installed in accessible locations. Pull the product data sheet for your insulation and confirm these ratings. Add a note to the plans: [Product name], FSI 25 or less, SDI 450 or less, per manufacturer data sheet, conforming to CRC R302.10. If you cannot confirm the rating from your product, switch to an unfaced batt in accessible locations — unfaced fiberglass and mineral wool batts generally meet the limits without issue.

  4. Update attic and crawl space insulation details

    Attics and crawl spaces that are accessible for maintenance get their own scrutiny. If you are running spray foam on the underside of roof sheathing (unvented attic assembly) or on crawl space walls, your section detail needs to call out the protection. Where the space is entered only for repairs or maintenance, R303.5.3 and R303.5.4 allow a listed ignition barrier (such as 3/8-inch gypsum board) or a foam tested per R303.6 instead of a full thermal barrier. Draw a section showing the foam layer and the barrier in front of it, and label the barrier material and thickness.

  5. Address exceptions for specific listed foam products

    Some foam plastic products have tested assemblies that are specifically listed as an alternative to the standard 1/2-inch drywall thermal barrier. These are approved under R303.6, for example specific intumescent or ignition barrier coatings tested per NFPA 286 or UL 1715. If you want to use one, show the listed product name, the applicable test standard, and the manufacturer listing report number on the plans. The plan-checker needs to verify the listing without calling you. Vague notes like approved intumescent coating will not pass — name the product.

  6. Add a general note to the notes sheet

    Put a catch-all note on your general notes sheet: All insulation facings in accessible spaces shall have a flame spread index of 25 or less and smoke development index of 450 or less per CRC R302.10. All exposed foam plastic insulation shall be protected by an approved thermal barrier per CRC R303.4. This covers any insulation locations not shown in detail and signals to the plan-checker that you have addressed the requirement globally.

  7. Attach manufacturer data sheets to the resubmittal

    When you resubmit, attach the product data sheets for any foam plastic products you are using. Include the page showing FSI and SDI ratings and any listed exception testing. Tab these so the plan-checker can find them fast. A corrections cycle runs at least 60 days — anything that makes the reviewer job one minute easier is worth doing.

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Why you got flagged

The most common trigger is spray polyurethane foam (SPF) shown on plans without a specified thermal barrier. Plan-checkers apply R303.4 (through R302.10.1) as requiring that foam plastic not be left exposed to the building interior, and if your drawings show SPF on foundation walls, rim joists, or attic rafters with no 1/2-inch drywall or code-approved intumescent coating called out, the correction goes on the letter. It does not matter that you intend to cover it — if it is not on the plans, you get flagged.

A second scenario involves insulation facings. Kraft-faced batt insulation has a paper facing with its own FSI rating. When insulation is installed in an accessible attic or crawl space and the facing is left exposed toward the living area, that paper or foil facing must meet the flame spread limits. If the plans just say R-19 batt insulation without specifying faced versus unfaced or addressing the facing requirement in an attic or crawl space application, the plan-checker will flag it.

Rigid foam board is a third common trigger. EPS or XPS board used on the interior of a foundation wall or under a slab perimeter is foam plastic governed by R303. When a designer shows rigid foam on a detail without a thermal barrier note, the correction cites R302.10 or R303.4. The fix is usually a layer of 1/2-inch gypsum wallboard, but some jurisdictions also accept specific listed intumescent coatings if the foam manufacturer has a test report.

The official code

2025 California Building Standards Code (Title 24) — California Residential Code, adopted edition via DGS

Read the source →

Common questions

Does CRC R302.10 require a thermal barrier over all spray foam insulation?
Yes, in most cases. Foam plastic insulation including spray polyurethane foam must be separated from the interior of the building by an approved thermal barrier. The standard barrier is 1/2-inch gypsum wallboard. There are exceptions: some foam products carry a listing for an alternative ignition barrier or intumescent coating tested to NFPA 286 or UL 1715. If your foam product has one of those listings, you can use the listed coating instead of drywall — but you have to name the specific product and test report on your plans. A general note saying approved intumescent coating with no product name will not satisfy the plan-checker.
What are the flame spread and smoke development limits for insulation under R302.10?
For insulation facings in accessible locations such as attics and crawl spaces, the limits are a flame spread index (FSI) of 25 or less and a smoke development index (SDI) of 450 or less. Foam plastic is governed by R303 instead: a flame spread index of 75 or less and smoke-developed index of 450 or less (R303.3), plus a thermal barrier (R303.4). The facing on a batt or the surface of a reflective insulation in an accessible attic does need to meet FSI 25 or less and SDI 450 or less. Pull the product data sheet and confirm before specifying.
Does R302.10 apply to rigid foam board on foundation walls?
Yes. Rigid foam board (EPS, XPS, or polyisocyanurate) is foam plastic governed by R303, which R302.10.1 points to, regardless of where it is installed. If it is on the interior face of a foundation wall, in a conditioned crawl space, or on a rim joist, it needs a thermal barrier. For foundation wall applications, the standard fix is 1/2-inch gypsum board over the foam, shown on your foundation section detail. Exterior-only rigid foam below grade that never faces the interior is a different situation — it is not accessible and not within living area, so the thermal barrier requirement does not typically apply to the buried portion.
Can I use an intumescent coating instead of drywall as the thermal barrier?
Yes, but only if the specific intumescent or ignition barrier coating has been tested for that application. The coating needs a test report to NFPA 286, UL 1715, or an equivalent standard, and the test must cover the specific foam substrate and thickness you are using. Get the product name and listing number, then put both on your plans. The plan-checker needs to look up the listing without calling you. A correction for missing thermal barrier is one of the faster ones to clear if you have the right product documentation ready.
Does R302.10 apply to unfaced fiberglass batt insulation?
Unfaced fiberglass batts generally do not trigger R302.10 concerns because the glass fiber itself does not have a facing with a flame spread rating issue. The section becomes relevant when there is a kraft paper or foil facing, or when you are using foam plastic. If you are installing faced batts in an accessible attic or crawl space where the facing is exposed toward the living area, confirm the facing FSI and SDI ratings from the product data sheet. Mineral wool batts are similar — the fiber is non-combustible, so R302.10 is not the concern, but always check the data sheet for the specific product.
What if my spray foam is in a non-accessible crawl space — does R302.10 still apply?
The thermal barrier rule is in R303.4, and R303.5.4 relaxes it for crawl spaces: where crawl space access is required and the space is entered only for repairs or maintenance, a listed ignition barrier (such as 3/8-inch gypsum board) or foam tested per R303.6 can replace the full thermal barrier. If your crawl space has no access point and is permanently sealed, document that clearly on the plans. When in doubt, specify the thermal barrier. It is cheaper to add drywall than to go another 60 days on a corrections cycle.

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