Fire-Rated Spray Foam Insulation: A Practical Guide for Safer Homes
Fire rated spray foam insulation is a code-tested polyurethane foam that slows flames to a crawl while sealing air leaks for top-tier energy savings. Fire safety and energy efficiency rarely share the spotlight; yet both come together in this guide, turning complex code requirements into clear, actionable steps.
Insulate smarter; buy once and apply with confidence. This closed-cell fire rated insulation outperforms untreated foams, highlights the features that matter most, and provides step-by-step tips any motivated DIYer can follow to fix leaks without hiring a crew. By the end, homeowners can compare options, calculate true costs, and leave with a plan that saves energy, reduces smoke risk, and meets code from day one.
What Is “Fire-Rated” Spray Foam?
Fire-rated spray foam is insulation that passes the ASTM E84 surface-burn test at 1,000°F for ten minutes. ASTM E84 is a tunnel test that exposes a 25-foot sample to direct flame and measures how far fire and smoke spread along its surface. A product earns Class A when its Flame-Spread Index stays between 0 and 25 and its Smoke-Developed Index stays at or below 450.
This classification is especially relevant when installing fire rated insulation foam in areas with elevated risk. Products labeled as fire resistant insulation foam are formulated to meet these standards while providing strong thermal and air sealing performance.
What are the Fire Rating Classifications?
The table below shows how ASTM E84 groups materials into Classes A, B, and C by listing their flame-spread and smoke limits along with a plain-language note on fire growth speed.
|
Rating |
Flame-Spread Index |
How Fast Does Flame Travel? |
Smoke-Developed Index* |
|
Class A |
0 – 25 |
Very slow (similar to concrete or gypsum board) |
≤ 450 |
|
Class B |
26 – 75 |
Moderate (flame moves in seconds, produces more smoke) |
≤ 450 |
|
Class C |
76 – 200 |
Fast (ignites like bare pine; heavy smoke) |
≤ 450 |
*ASTM E84 sets the same ≤ 450 smoke limit for Classes A, B, and C.
A typical Class A spray foam, such as a closed-cell fire rated insulation foam, shows a Flame Spread Index (FSI) of about 15 and a Smoke Developed Index (SDI) of 130, whereas untreated polyurethane foam can reach roughly 90 FSI and 600 SDI, allowing fire and smoke to travel nearly four times faster across its surface, according to testing carried out at the Underwriters Laboratories (UL) Fire Research Laboratory.
How Does Standard Spray Foam Differ from Class A Fire-Rated Foam?
The table contrasts a fully untreated, open-cell spray foam with a Class A spray foam. If a foam is modified enough to post a Flame-Spread Index below 76, it is no longer “untreated” and shifts into Class B.
|
Feature |
Untreated Spray Foam (typical) |
Class A Spray Foam |
|
Flame-Spread Index |
76 – 200 (Class C; values below 76 indicate the foam has some fire-retardant additives and is Class B) |
0 – 25 |
|
Smoke-Developed Index |
> 450 (fails code limit) |
≤ 450 |
|
Char Layer |
Thin, brittle |
Thick, tight |
|
Typical Use |
Draft sealing, basic insulation |
High-risk walls and ceilings |
|
Code Approval in Living Areas |
Often prohibited or needs extra coatings |
Passes with a listed 15-min barrier |
Note: A few “standard” foams are lightly treated and achieve a Flame-Spread Index between 26 and 75. Those products are classified as Class B, not the fully untreated class shown above.
Where Does Fire-Rated Foam Matter Most?
Installing fire rated insulation foam is especially important in these high-risk areas:
- Garage walls and ceilings next to the living rooms.
- Basement ceilings under bedrooms.
- Utility rooms with panels, boilers, or dryers.
- Attic knee walls near chimneys or recessed lights.
These are prime locations where both fire retardant insulation foam spray and soundproofing with fire rated insulation foam can offer dual benefits: safety and quieter interiors.
Which Building Codes Call for Class A Foam?
Below is a concise list of the three model references that shape most local spray-foam rules. Each item notes the core requirement and why it matters.
- International Building Code (IBC): Foam plastic exposed to an occupied room must have a Class A surface rating or be covered by a thermal barrier that holds fire for fifteen minutes, typically ½-inch drywall. This rule applies to commercial buildings and any residential project outside the IRC scope.
- International Residential Code (IRC): In one- and two-family homes, spray foam must also sit behind a fifteen-minute thermal barrier unless the product passes a full-scale room test such as NFPA 286. The goal is to give occupants extra time to escape if fire reaches the wall or ceiling cavities.
- NFPA 286 Room-Corner Test: This laboratory procedure burns a mock room built with foam and its coverings to check whether the assembly avoids flash-over. When a product passes, many jurisdictions allow limited exposed use, for example, in attics or crawl spaces.
What Does It Cost and Save?
Insulating a 1,000 sq ft area to R-11 means adding enough material to reach an insulation value of 11 on the R-value scale, where a higher number blocks heat better. In practical terms, this requires about 2 inches of closed-cell spray foam (approximately R-6 per inch) or 3 inches of open-cell foam (approximately R-3.6 per inch). Choosing Class A spray foam for the job raises the budget only slightly because these cans cost roughly 2 to 5 dollars more each. For most projects, the added expense is 200 to 300 dollars in total, a small price that buys fire-retardant additives, certified testing, several extra minutes for safe escape during a fire, and potential insurance savings of 5 to 15 percent.
How to Apply Class A Foam in Five Steps?
Follow the sequence below to achieve a safe, code-compliant installation.
- Prepare surfaces: Remove dust, open a window or set a fan for airflow, and put on goggles, chemical-resistant gloves, and a respirator.
- Warm the cans: Keep both product and work area between 18 °C and 32 °C (65 °F to 90 °F) for proper expansion.
- Shake and spray: Agitate each can for the time stated on the label, then apply the foam in two-inch layers at a steady pace.
- Watch the rise: Pause after each pass and stop when the foam reaches its full height without bulging.
- Add a thermal barrier: Within twenty-four hours, cover the cured foam with one-half-inch drywall or another code-listed coating, unless the product is approved for exposed use by an NFPA 286 room test.
Final Thoughts
Class A spray foam resists flame spread, reduces smoke, and satisfies building-code requirements in the areas that need protection most, yet it adds only a small cost to a project. OneClickDIY stocks fire rated spray foam insulation kits with step-by-step application guides and fast shipping, so you can upgrade your home’s fire safety this weekend.
Frequently Asked Questions (FAQ)
Why should I choose Class A foam when the smoke limit is the same as Class B and C?
While the maximum smoke limit (SDI ≤ 450) is the same for all classes, the key difference is the Flame-Spread Index (FSI). Class A foam has an FSI of 0-25, meaning it spreads flames extremely slowly, similar to concrete. As the comparison table shows, untreated foams can be Class C and spread fire nearly four times faster. This slow spread is the critical life-safety feature you are investing in.
For a complete overview of what to look for when purchasing, see our Spray Foam Insulation Buying Guide: Everything You Need to Know.
The article gives a 5-step guide. Is there a more detailed walkthrough for a first-time DIYer?
Absolutely. The 5-step guide in this article provides a quick overview for a compliant installation. For a much more in-depth look at the entire process, including surface preparation, safety equipment, and pro tips, we have a complete guide.
You can find it here: DIY Spray Foam Insulation: Step-by-Step Application & Safety Guide.
Is the thermal barrier mentioned in the building codes always required, even for Class A foam?
Yes, in almost all cases. As noted in the International Building Code (IBC) and International Residential Code (IRC) sections, a 15-minute thermal barrier (like ½-inch drywall) is required over foam plastic in occupied spaces. The Class A rating applies to the foam's surface, but the thermal barrier provides an essential, additional layer of protection for the entire assembly. An exception is only possible if the product passes a special room-scale test like NFPA 286.
To understand this requirement fully, read our detailed article: What Is a Thermal Barrier?.
Does the fire-retardant chemistry in Class A foam affect its R-value?
No, the fire-retardant additives are designed to provide fire resistance without compromising the foam's primary function as an insulator. You still get the high R-value per inch that spray foam is known for, which is crucial for creating an effective thermal envelope and preventing thermal bridging.
Learn more about insulation performance in our guide: What Is the R-Value of Spray Foam Insulation and Why Does It Matter?.
The cost section mentions open-cell and closed-cell foam. Which type is usually fire-rated, and what’s the difference?
Both open-cell and closed-cell foam can be manufactured to achieve a Class A fire rating. The choice between them depends on your project's needs (e.g., moisture resistance, structural rigidity, R-value per inch). The cost example in the article shows how both types can be used to reach a target R-value.
For a detailed comparison, see: How Do Open-Cell and Closed-Cell Spray Foam Differ?.
This article focuses on walls and ceilings. Can I use this foam for specific areas like basement rim joists?
Yes, Class A fire-rated foam is an excellent choice for insulating and air-sealing high-risk or critical areas like rim joists. Because rim joists are common locations for air leaks and sit on top of the foundation, using a high-quality, fire-rated foam is a smart decision for both energy efficiency and safety.
Find out more about this specific application here: Rim Joist Insulation With Spray Foam: A Complete Guide.