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Musings of an Energy Nerd

Thermal Barriers and Ignition Barriers for Spray Foam

The code is complicated, poorly written, subject to interpretation, and often unenforced

Image 1 of 2
Deciphering code requirements for thermal barriers and ignition barriers requires painful study. Code requirements are multi-layered and poorly written, and the fire safety tests that support the code are Byzantine in their complexity.
Image Credit: Image #1: Rick Duncan / Spray Polyurethane Foam Alliance
Deciphering code requirements for thermal barriers and ignition barriers requires painful study. Code requirements are multi-layered and poorly written, and the fire safety tests that support the code are Byzantine in their complexity.
Image Credit: Image #1: Rick Duncan / Spray Polyurethane Foam Alliance
The infamous SwRI 99-02 crawl space test showing the baseline assembly used for comparison purposes. The baseline assembly is constructed with Kraft-faced fiberglass batts, with the paper facing exposed to the flame source.
Image Credit: Image #2: American Chemisty Council / Center for the Polyurethanes Industry

Do building codes require spray foam insulation to be protected with a layer of drywall or a comparable barrier for fire safety? There is no simple answer to the question, for several reasons. The first reason is that the code is complicated.

The second reason is that the code is poorly written.

The third reason is that the code is subject to interpretation by local code officials.

And the fourth reason is that even when the code clearly requires spray foam to be protected with a thermal barrier or an ignition barrier, many code officials don’t bother to enforce the code.

Unprotected spray foam can be a fire hazard

Exposed spray foam can be hazardous. According to a guide for spray foam installers, “Thermal Barriers and Ignition Barriers for the Spray Polyurethane Foam Industry,” “When exposed to fire sources, such as trash fires, welding arcs, cutting torches, or red-hot metal, unprotected SPF [spray polyurethane foam] can ignite and may result in a flash fire.”

If a house that includes spray foam ever burns down, lawyers will want to know whether the spray foam was installed according to code — so builders who don’t pay attention to ignition barriers are taking on liability.

Distinguishing an ignition barrier from a thermal barrier

Before delving into this issue, we need to learn some code jargon.

The building code has two different terms to describe barriers to protect insulation from catching fire. The first term is “thermal barrier.” (The ill-chosen phrase is code-speak. Most builders and scientists use the term “thermal barrier” to refer to insulation; unfortunately, code writers use words in a way that differs from most speakers of English. So for the purposes of this article, forget what you know to be true, and try to accept the code jargon.)

In building code documents, the term “thermal barrier,” for all intents and purposes, means 1/2-inch drywall. (We’ll provide a somewhat more technical definition later.)

The second term is “ignition barrier.” This term refers to a less stringent barrier than a thermal barrier; it’s a lower bar. There are several different materials that meet the definition of an ignition barrier.

When is a thermal barrier required?

The requirement calling for spray foam insulation to be protected by a thermal barrier is found in Section R316.4 of the 2009 International Residential Code (IRC). In most cases where spray spray polyurethane foam is installed in a house, the foam must be separated from living spaces by a layer of 1/2-inch drywall or a material that has been approved as equivalent in fire resistance to 1/2-inch drywall.

There are a few exceptions. In the following cases, spray foam doesn’t have to be protected by a thermal barrier:

  • When the spray foam is used as roofing (2009 IRC section R316.5.2);
  • When the spray foam is covered by concrete or masonry that is at least 1 inch thick (2009 IRC section R316.5.1);
  • When the spray foam is installed in a roof assembly and is protected on the interior side with a layer of tongue-and-groove boards or plywood measuring at least 15/32 inch thick (2009 IRC R316.5.2);
  • When the spray foam in installed on the interior of rim joists and sill plates, as long as the thickness of the spray foam is 3 1/4 inches or less (2009 IRC section R316.5.11);
  • When the spray foam is installed in an attics or crawl space, as long as these spaces aren’t used for storage, and as long as entry to the space is made only for repairs, maintenance, or service of utilities, and as long as the spray foam in this type of space is protected by an ignition barrier (2009 IRC section R316.5.3 and R316.5.4).

If you are hoping to use the attic exception in order to avoid installing a thermal barrier, be careful. It the attic has flooring, or if the attic is used for storage, you still need a thermal barrier over the foam. According to “Thermal Barriers and Ignition Barriers for the Spray Polyurethane Foam Industry,” a thermal barrier is required “whenever the attic or crawl space is used or could be used as an auxiliary living space or for storage. Criteria for such space include easy of entry and presence of flooring. Attics and crawlspaces having access doors, pull-down or fixed stairs, or flooring … would fall into this category.”

What is a thermal barrier?

In section R316.4, the IRC defines a thermal barrier as 1/2-inch gypsum wallboard (a prescriptive thermal barrier) or a material that is equal in fire resistance to 1/2-inch gypsum wallboard. The required test to determine whether a material is equivalent to 1/2-inch drywall is the 15-minute fire test; for this reason, this type of thermal barrier is sometimes called a 15-minute barrier.

Examples of materials that have passed the 15-minute fire test and are therefore considered equivalent to 1/2-inch drywall include certain types of spray-applied cementitious materials, spray-applied cellulose materials, and cement-based plaster.

According to one source, 3/4-inch plywood installed as subflooring is “generally accepted” as a thermal barrier even though it is not listed as a prescriptive thermal barrier. This argument is supported by referencing Table 721.6.2(1) (“Time Assigned to Wallboard Membranes”) in the International Building Code. If you plan to use 3/4-inch plywood as a thermal barrier, you should check with your local building inspector before proceeding. [P.S. For more information on the use of 3/4-inch plywood as a thermal barrier, see Comment #19 by Kohta Ueno, below.]

In addition to a prescriptive thermal barrier (1/2-inch drywall) and an approved equivalent (for example, cement-based plaster), there is a third type of thermal barrier: one that has been tested as an “alternate assembly.” The “alternate assembly” method is the one used to get approval for some types of intumescent coatings. If a builder uses a product that was approved as part of “alternate assembly” testing, then the product has to be installed in exactly the same manner as the assembly that was tested in the laboratory.

When is an ignition barrier required?

An ignition barrier is a less stringent barrier than a thermal barrier. Ignition barriers are discussed in two sections of the 2009 IRC: sections R316.5.3 and R316.5.4.

If your attic or crawl space meets the requirements mentioned above — if it’s only accessed for repairs or maintenance; there is no easy access to the space; it isn’t used for storage; it has no floor — then you can get away with installing an ignition barrier instead of a thermal barrier.

In a comment posted on the Journal of Light Construction website, James Morshead noted, “Attics and crawl spaces are usually considered ‘occupied’ when they are used for storage. Areas used for storage require a thermal barrier between the foam and the occupied space and therefore an ignition barrier will not be sufficient.”

One criterion used by building inspectors to determine whether an attic or crawl space is “occupied” or not is the size of the hatch or door. If you expect to use an ignition barrier to cover spray foam, entry to the space should be by a hatch or small door, not a full-sized door. If the area with spray foam is entirely inaccessible — if you can’t get there without cutting through a layer of drywall — then you don’t need an ignition barrier. (The drywall is already the thermal barrier.)

What is an ignition barrier?

In sections R314.5.3 and R314.5.4, the 2009 IRC defines an ignition barrier as one of six permissible materials: 1 ½-inch-thick mineral fiber insulation; ¼-inch-thick wood structural panels (e.g., plywood); 3/8-inch particleboard; ¼-inch-thick hardboard; 3/8-inch-thick gypsum board; or corrosion-resistant steel having a base metal thickness of 0.016 inch. Presumably, code officials will also approve thicker versions of any of the six listed materials as ignition barriers.

The Infamous Crawl Space Test

Up until 2009, many brands of spray foam were approved for installation in crawl spaces and attics without the use of an ignition barrier, based on an infamous test that was at best ill-advised and at worst deliberately misleading. The test was the crawl space fire test (SwRI 99-02) developed by the Southwest Research Institute.The Southwest Research Institute convinced code officials that the test was legitimate. The key document that paved the way for this fiasco was an April 11, 2000 letter written by Brian Gerber, a senior structural engineer at the ICBO Evaluation Service, the precursor to the International Code Council Evaluation Service (ICC-ES). For reasons that remain unclear, Gerber ruled that spray foam could be used “without an interior covering on exterior walls of attics and crawl spaces” if the foam could be shown to ignite in a fire test no sooner than exposed “3½-inch-thick kraft-faced fiberglass batt insulation.”There was a major problem with Gerber’s ruling — the ruling that formed the basis for the infamous SwRI 99-02 test. The test used, as a baseline for comparison, an illegal assembly: fiberglass batts with exposed kraft facing. As most builders know, IRC R320.1 prohibits kraft facing, which is highly flammable, from being left exposed. Needless to say, spray foam manufacturers found it quite easy to pass this test. It isn’t hard to pass a fire safety test when all you have to do is show that your product is better than exposed asphalt-impregnated kraft paper.After the SwRI 99-02 test was held up for ridicule in a few articles, including one I wrote for the November 2007 issue of Energy Design Update, spray foam manufacturers back-pedaled, realizing that they had egg on their faces. As an industry publication, “Know the Code: Using Spray Foam Insulation In Attics and Crawl Spaces,” noted, “Selection of an appropriate code-complying baseline is important to the credibility of the test.” To clean up the mess, the ICC-ES convened a committee to modify the fire testing section of the Acceptance Criteria for Spray-Applied Foam Insulation (AC 377). The new language was approved in June 2009.Spray foam manufacturers were given until the end of 2009 to test their products with a more realistic test than one that compared spray foam to fiberglass batts with exposed kraft facing.

To get a material approved as equivalent to a prescriptive ignition barrier, the material must meet the “specific approval” criteria set forth in IRC section 314.6. (This section lists the type of fire testing that is required.) To get an assembly approved as an alternate assembly, the assembly must be tested by a laboratory according to criteria set out by the ICC Evaluation Service in Acceptance Criteria 377, Appendix X.

To meet the requirements for an ignition barrier, most builders choose one of four approaches:

  • They install a prescriptive ignition barrier like mineral fiber insulation (blown-in fiberglass or mineral wool);
  • They install cellulose insulation;
  • They install an approved assembly consisting of spray foam covered with an intumescent coating (a protective barrier installed like paint);
  • They choose a spray foam that has passed fire safety tests as is, without any intumescent coating or protective barrier.

After the spray foam industry and code officials created new fire safety tests (“the modified 286 test”) for intumescent coatings, spray foam manufacturers discovered that some closed-cell spray foams passed the tests as is, without any intumescent coating at all. Although it’s much harder for open-cell spray foam to pass this type of test, a few types of open-cell spray foam have been reformulated so that they pass fire safety tests without any intumescent coating.

More information on intumescent coatings

An intumescent coating is a type of paint that bubbles up when exposed to flame or heat. The bubbled-up paint acts as a flame-resistant barrier to delay ignition. Most manufacturers of spray foam sell intumescent coatings that have been approved for use with their brands of foam insulation.

Technically speaking, an intumescent coating is not a thermal barrier or an ignition barrier. But some assemblies have been approved to allow intumescent coatings to be used as part of approved alternate assemblies that can be substituted for a thermal barrier or an ignition barrier.

In a comment posted on the Home Energy Pros website, Pat Dundon, and insulation contractor from Windsor, New York, noted, “If the intumescent [coatings] are tested on a foam product, then that system passes the thermal barrier [or ignition barrier] requirement. These systems are brand-specific. You need an ICC-ESR report indicating the specific combination of a brand of foam with a brand of coating. Many foam manufacturers have relabeled a product called DC-315 as their proprietary product [and] then tested that with their foam to get an ESR for the combination, but if you use the coating from a Demilec bucket on a Lapolla foam, you technically do not have a code-compliant product, even if both manufacturers relabeled DC-315.”

In his JLC article on ignition barriers, Jon Vara makes the same point: “Approval of a given intumescent coating with a given foam doesn’t mean it’s also approved for use with other, similar foams, although advertisements may imply that it does. Look for an ICC-ES report that references the specific coating and the specific foam you’ll be using. … Don’t exceed the thickness or density of the foam specified in the test report. If a non-barrier foam was tested and approved at a thickness of 6 inches and a density of .5 lb. [per cubic foot], that approval no longer applies if the thickness is increased to 8 inches. (You can, however, apply the foam at a lesser thickness than that specified in the report.)”

Remember, if you are using an intumescent coating as an alternative to an ignition barrier, it can only be used in a crawl space or attic — and only if the space is accessed by a hatch or small door, and is never used for storage.

Assembly tests that allow intumescent coatings to be used as approved alternates to an ignition barrier or thermal barrier require the coating to be applied at a specific thickness. This raises the question: “How do you measure the thickness of an intumescent coating?” The answer, of course, is that almost nobody ever tries to measure it. One source noted drily, “Accurately measuring thin coating application on textured SPF surfaces is difficult.”

Exposed spray foam in crawl spaces and attics

Some spray foams have been approved for use in crawl spaces or attics without being protected by an ignition barrier; these products have passed an assembly test (not a material test) for fire safety. These types of foam generally cost more than spray foams that require an intumescent coating.

To get this approach accepted by a local code official, you have to install your spray foam in exactly the same way as the foam was installed in the assembly test.

To learn more about the uses and limitations of these types of spray foam — foam that can be left exposed in an attic or crawl space — refer to information from spray foam manufacturers. The following brands of spray foam may, in some applications, be installed in a crawl space or attic without an intumescent coating:

Spray foam thickness limitations

For those who want to delve deeper into the realm of fire safety testing for spray foam — and it’s hard to imagine that many casual readers would — it’s worth exploring the strange world of the tests that govern the maximum permissible thickness of spray foam installations. It’s an Alice-in-Wonderland world, and you can read all about it here: Maximum Thickness Restrictions For Spray Foam.

Here’s the short version: required material tests for fire safety can’t be performed on materials that are thicker than 4 inches. So if an installer wants approval for spray foam installed at a thickness that exceeds 4 inches, the only route is via an assembly test. That means that spray foam installers who want to install spray foam that is more than 4 inches thick have to follow the exact specifications used in the assembly test that was used to gain approval for the chosen installation.

It also means that some brands of spray foam haven’t been approved for installation at the thickness that builders may desire. Caveat emptor.

As usual, enforcement is spotty

If you’ve read this far, you probably know at least as much about ignition barriers and thermal barriers as many code officials. That’s no surprise: in many areas of the country, the code is unenforced. In these areas, some spray foam contractors have been installing spray foam in attics and crawl spaces without any ignition barriers.

As Allison Bailes noted in his article on ignition barriers, “At least in my part of the U.S. (the Southeast), enforcement of this part of the building codes is spotty. Some jurisdictions are sticklers about it and some don’t even know about it.”

Martin Holladay’s previous blog: “Residential Commissioning.”

Click here to follow Martin Holladay on Twitter.


  1. Malcolm Taylor | | #1

    Our Code
    Very interesting blog Martin.

    "If a house that includes spray foam ever burns down, lawyers will want to know whether the spray foam was installed according to code — so builders who don’t pay attention to ignition barriers issue are taking on liability."

    I know you are addressing the majority of your readers when you refer to "the code", but our codes in Canada place the responsibility for ensuring that their structures meet code on the owner, not the builder. Something worth knowing when you put your faith in your architect or contractor.

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to Malcolm Taylor
    I'm not a lawyer, so I can't claim any special knowledge about U.S. liability laws.

    However, in many cases, the plaintiff's lawyer throws a wide net, alleging that almost everyone is somehow liable -- and in the end, the lawyer hopes to snare someone with deep pockets (either a wealthy individual or an insurance company).

  3. Jeff Carroll | | #3

    Metal SIPs
    Would the metal that encloses metal SIPs serve as an thermal barrier? Though it would require the use of conduit for electrical runs, I've wondered if anyone merely finishes these surfaces on the inside without drywall. I know that it has been done on the outside.

  4. Aj Builder, Upstate NY Zone 6a | | #4

    Martin, I haven't seen spray
    Martin, I haven't seen spray foam companies coating spray foam for fire such as an unvented roof in an attic with limited access hatch. Codes call for access hatches. Which code to break? I know of both being broken.

    And without reading thru again... what about spray foam on attic floor? Cover with ___" of cellulose and then legal, how many inches?

  5. User avater GBA Editor
    Martin Holladay | | #5

    Response to Jeff Carroll (Comment #3)
    Q. "Would the metal that encloses metal SIPs serve as an thermal barrier?"

    A. It isn't a prescriptive thermal barrier, but it may have been approved as an equivalent thermal barrier if the manufacturer of the SIP contracted for the necessary testing. Contact the manufacturer of the SIP and ask whether the required testing has been performed.

  6. User avater GBA Editor
    Martin Holladay | | #6

    Response to A.J. Builder (Comment #4)
    A. J.,
    You wrote, "I haven't seen spray foam companies coating spray foam for fire such as an unvented roof in an attic with limited access hatch."

    It doesn't surprise me to learn that the code isn't being enforced in your area. In spite of the lack of code enforcement, however, builders are still required to follow the code. If a homeowner wants to challenge a builder for building a house that doesn't meet code, the fact that the local inspector didn't catch the code violation won't get the builder off the hook.

    Q. "Codes call for access hatches. Which code to break?"

    A. I advise you not to break the code. If you are required to include an access hatch, you are also required to protect spray foam insulation with an ignition barrier or a thermal barrier.

    Q. "What about spray foam on attic floor? Cover with X inches of cellulose, and then legal? How many inches?"

    A. Cellulose can be used as an ignition barrier; for more information, see Cellulose Fire Blocking and Ignition Barrier Capabilities. Curiously, that document does not state the minimum number of inches required for cellulose to comply with ignition barrier requirements. According to Dana Dorsett, the answer is 3 inches. I suggest that you confirm that thickness with the manufacturer of the cellulose product you are using before adopting this approach.

  7. John Williams | | #7

    I've seen quite a bit of spray foam
    I've inspected some homes that have had spray foam installed and it would seem that the GC's are unaware of the thermal barrier requirements. The spray foam companies don't bother to tell them when they bid the job. In this area there are many homes that want the rustic look of tongue and groove knotty pine and the contractors want to install it without the required thermal barrier.

    The SPF contractors don't tell the GC about the barrier because the cost of the foam plus the thermal barrier makes that option more expensive than fiberglass and tongue and groove.

    The contractors scream when they find out because they never included sheetrock in their bids.

    There was a great write up in a Canadian publication titled "Polyurethane spray foam, solid gasoline" that was pretty scary.

    There are only several jurisdictions rigorously enforcing this part of the code, or that's what I hear from the contractors.

  8. Aj Builder, Upstate NY Zone 6a | | #8

    Building codes that are
    Building codes that are unclear suck.

  9. Andy Kosick | | #9

    I just have to point out that there's an elephant in the room in terms of ignition barriers. Can anyone honestly think of a single accessible space in house that will not eventually be used for storage? The only deterrent I've seen is a crawlspace so disgusting no one would think of going in it. Attics are always game, no floor, no problem.

  10. User avater GBA Editor
    Martin Holladay | | #10

    Response to Andy Kosick
    Some building inspectors may agree with you, so it certainly makes sense for builders to discuss this issue with the local building department.

    Builders of energy-efficient homes who want to discourage homeowners' attempts to use unconditioned attics for storage sometimes omit a conventional access hatch. Instead, access to the attic is via a small exterior door on the gable end of the house; this type of access door can only be reached with an extension ladder.

    This method reduces, but does not eliminate, the chance that homeowners will use this type of attic for storage.


  11. User avater
    Leigha Dickens | | #11

    tongue-and-groove ceiling
    The code directly addressed tongue-and-groove wood ceilings and spray foam in section R316.5.2: "The thermal barrier specified in section R316.4 is not required when foam plastic in a roof assembly or under a roof covering is installed in accordance with the code and the manufacturer's installation instructions and is separated from the interior of the building by by tongue-and-groove wood planks or wood structural panel sheathing in accordance with Section R803, not less than 15/32 thick bonded with exterior glue and identified as exposure 1..."

    I really don't know what kind of fire testing, etc, this came from, but it is pretty clear that foam behind tongue-and-groove is OK by code without a thermal barrier. Not clear whether an ignition barrier is required instead, but I have interpreted that to mean that a cathedral ceiling with tongue-and-groove would not need it, while an unvented attic with limited access over a tongue-and-groove ceiling still would.

  12. User avater GBA Editor
    Martin Holladay | | #12

    Response to Leigha Dickens
    Thanks for pointing out the provisions of section R316.5.2. I have edited the article to include a reference to this code section.

  13. Susie Derkins | | #13

    Response to Leigha and Martin
    Hi Martin and Leigha,

    As far as tongue-and-groove wood planks go - would you interpret the code provisions to apply to walls as well as ceilings?

    I am thinking of having my interior basement walls insulated with spray foam (they are bare concrete right now). I am planning to build some interior stud walls first so that after the insulation I can create some finished space. I want to know if I have at least the *option* not to use drywall but to instead use tongue-and-groove boards. (or anything else - what else might meet the criteria?)

    From my understanding, one way to have that option would be to have the installers also apply intumescent paint (so I would not need an additional "thermal barrier"). But that paint is costly and I would be interested to know if I can proceed without it.

    Thanks for your insights.

  14. User avater GBA Editor
    Martin Holladay | | #14

    Response to Susie Derkins
    The exception provided by Section R316.5.2 of the code clearly applies to roof assemblies, not walls.

    But there are countless examples of muddy language in the code, and the final arbiter is always your local code inspector. My interpretation is irrelevant to your dilemma; the only person who can answer your question is your local inspector.

  15. Luis Rivera | | #15

    thermal barrier on a building base
    Hi Martin,
    In a building base which is not meant to be as a crawling space, nor for storage, but is insulated with SPF, do I need to use a thermal or an ignition barrier?

  16. User avater GBA Editor
    Martin Holladay | | #16

    Response to Luis Rivera
    I don't know what you mean by a "building base." Do you mean a basement?

    If it is accessible, you probably need a thermal barrier. If it is hard to access and not used for storage, you may be able to get away with an ignition barrier. If you have any doubts, just ask your local building inspector (the local authority from your building department).

  17. Luis Rivera | | #17

    Thermal/igniton barrier
    No, is not accesible, nor a basement... is the base for an electrical substation control building. For what yuo say, probably I don't need to use the barrier, but I'll check with the code official.

  18. Richard Beyer | | #18

    See page 37 of this BASF presentation

    Check foam products ICC-ES Report for tested and accepted alternatives

  19. Kohta Ueno | | #19

    Update on 3/4-inch plywood as a Thermal Barrier
    I was recently looking at this topic in more detail for a project--it appears that one of the amendments to the 2015 I-Code now explicitly allows 3/4" (okay, 23/32") wood structural panel as an interior thermal barrier. See page 180 in this writeup of the accepted revisions in the 2015 I-Codes: (screenshot of the page below).

  20. User avater GBA Editor
    Martin Holladay | | #20

    Response to Kohta Ueno
    Thanks very much for sharing the information. I have edited my article to include a reference to your comment.

  21. Nelson Nasmily | | #21

    Need to Rock the Lid?
    Hi All,

    I'm trying to add drywall as a thermal barrier in a basement. A previous homeowner seems to have used a diy spray foam kit on the walls and the foam is currently exposed. I'm happy to sheetrock and fire tape the walls, but for various reasons I'd rather not sheetrock the ceiling. Of course, this will not close off the spray foam completely because of the open joist bays. The only other option I can think of to seal those gaps is to add a dorky little drywall rectangle in each bay to seal it off. Is there a smarter solution or are those my two options?

    Thanks in Advance,

    First Time Long Time

    1. GreenerLiving | | #22

      Hi Nelson,

      While it's likely you may have already found a solution (I'm late to this thread party), we have had success with using Roxul in these situations. In fact, the code inspector in this case recommended it. Of course, as Martin and others have said, best to check with your local official.



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