Concealed Often Equals Confusion: Understanding Concealed Spaces in NFPA 13

By Jon Nisja

One of the more confusing sections of NFPA 13 deals with concealed spaces. The National Fire Sprinkler Association (NFSA) has their Expert of the Day (EOD) program where members can request informal interpretations or opinions on how to apply the codes and standards. Questions dealing with concealed spaces in NFPA 13 are one of the leading EOD topics.

NFPA 13 starts with the premise that fire sprinklers should be installed in all spaces in a building. It then continues to add a number of exemptions. Since there are several provisions in NFPA 13 dealing with concealed spaces, this blog is the first of a few explaining these requirements.

What is Considered a Concealed Space in NFPA 13?

For several editions of NFPA 13, concealed spaces were located in the “special situations” section of NFPA 13. Prior to the 2002 edition, they were listed as exceptions to a requirement for fire sprinkler protection in concealed spaces of combustible construction. One could argue that concealed spaces are not really “special” as they are present to some degree in many buildings.

NFPA 13 generally requires fire sprinklers in concealed space where there are exposed combustible materials in a quantity that could cause serious fire damage in a building. One of the risks of concealed spaces is that the fire can burn undetected for extended periods of time.

Terminology

In order to understand and apply the concealed space requirements, a review of some terminology is probably in order. While a more detailed analysis can be found in the August 22, 2023 blog post entitled “Combustible, Non-Combustible, or Limited Combustible Materials … That is the Question”, for this blog, simplified definitions will be provided:

• Combustible means wood or plastics are present.
• Non-combustible means the materials are metal, clay, or masonry products.
• Limited combustible means that gypsum is being used to cover any wood or meets the requirements of Section 4.9.2 of NFPA 13 (2025 edition).

Figure 1 – Limited combustible construction

Photo 1 is an example of a combustible concealed space.

Photo 1 – Example of a combustible concealed space

The following sections outline various options for concealed spaces. Some of the applications are fairly common while others deal with very unique construction features.

Option #1 – Nothing to Burn

If the space is of non-combustible or limited combustible construction, has no combustible material in it, and there is no access to the space, fire sprinklers are not required. This space is allowed to have small openings like what is seen as return air for a plenum.

Example: Steel bar joists supporting a non-combustible roof deck material with a solid non-combustible gypsum ceiling suspended below.

• NFPA 13 – 2025 edition – Section 9.2.1.1
• NFPA 13 – 2022 edition – Section 9.2.1.1
• NFPA 13 – 2019 edition – Section 9.2.1.1
• NFPA 13 – 2016 edition – Section 8.15.1.2.1

Option #2 – Little to Burn with no Access

If the space is of non-combustible or limited combustible construction and it has very few combustibles in it, sprinklers are generally not required. NFPA 13 assumes that there will be limited amounts of plastic plumbing piping, cable and wires, or non-structural wood (like wooden mounting for an electrical panel or control box) and the presence of these items would not trigger fire sprinklers. Unfortunately, the point at which the amount of combustibles is excessive is not well-defined and this becomes a judgment call.

Example: Steel bar joists supporting a non-combustible roof deck material with a suspended acoustical ceiling tile assembly with some low-voltage wiring and plastic plumbing drain pipe.

• NFPA 13 – 2025 edition – Section 9.2.1.1
• NFPA 13 – 2022 edition – Section 9.2.1.1
• NFPA 13 – 2019 edition – Section 9.2.1.1
• NFPA 13 – 2016 edition – Section 8.15.1.2.1

Option #3 – Little to Burn with Limited Access

Spaces of non-combustible or limited combustible construction that have limited access and don’t allow storage or occupancy don’t require fire sprinklers. These spaces can have building equipment, such as electrical or HVAC equipment as long as they are not fuel-fired equipment (such as gas furnaces or water heaters). The allowance for non-fuel-fire equipment was added in the 2022 edition of NFPA 13.

Example: Steel bar joists supporting a non-combustible roof deck material with a solid non-combustible gypsum ceiling suspended below yet having an access panel or small scuttle hole to enter the space.

• NFPA 13 – 2025 edition – Section 9.2.1.2
• NFPA 13 – 2022 edition – Section 9.2.1.2
• NFPA 13 – 2019 edition – Section 9.2.1.2
• NFPA 13 – 2016 edition – Section 8.15.1.2.2

Option #4 – Small Wood Stud or Joist Spaces

Fire sprinklers can be omitted in small spaces (less than 6 inches) between wooden studs or joists. The reason for the 6-inch measurement is a practical one: it is very difficult to work with tools in a space that small.

Figure 2 is an example of this type of application

• NFPA 13 – 2025 edition – Section 9.2.1.3
• NFPA 13 – 2022 edition – Section 9.2.1.3
• NFPA 13 – 2019 edition – Section 9.2.1.3
• NFPA 13 – 2016 edition – Section 8.15.1.2.3

Option #5 – Small Spaces with Bar Joists

Fire sprinklers can be omitted in small spaces having bar joists where there is 6-inches or less distance between the ceiling and the roof or floor deck. Similar to Rule #4 above, the 6-inch distance is a limitation based on being able to work in a space that small.

• NFPA 13 – 2025 edition – Section 9.2.1.4
• NFPA 13 – 2022 edition – Section 9.2.1.4
• NFPA 13 – 2019 edition – Section 9.2.1.4
• NFPA 13 – 2016 edition – Section 8.15.1.2.4

Option #6 – Ceilings Below Wood Joists or Solid Wood Members

Where ceilings are attached to or within 6-inches of solid wood members or wood joists, fire sprinklers can be omitted again based on the limited space to be able to install or service sprinklers.

Figure 3 is an example of applying Section 9.2.1.5 of NFPA 13.

• NFPA 13 – 2025 edition – Section 9.2.1.5
• NFPA 13 – 2022 edition – Section 9.2.1.5
• NFPA 13 – 2019 edition – Section 9.2.1.5
• NFPA 13 – 2016 edition – Section 8.15.1.2.5

Option #7 – Composite Wood Joists Option 1

In case where a ceiling is attached to composite wood joists directly or using metal channels (up to 1-inch in depth) and where the volume of the space is limited to 160 cubic ft., fire sprinklers can be omitted. When metal channels are used for attaching the ceiling to the bottom of the composite wood joists, a minimum of 3 ½-inches of batt insulation is required to be in the bottom of the joist channels.

• NFPA 13 – 2025 edition – Section 9.2.1.6
• NFPA 13 – 2022 edition – Section 9.2.1.6
• NFPA 13 – 2019 edition – Section 9.2.1.6
• NFPA 13 – 2016 edition – Section 8.15.1.2.6

Figure 4 showing I-joists where insulation can be used with metal channel

Option #8 – Fill with Noncombustible Insulation

If the space is filled with noncombustible insulation up to 2-inches from the top of the space, fire sprinklers are not required. Common examples of noncombustible insulation are fiberglass, mineral wool, or “rock wool”. Since the paper backing on rolled insulation is combustible, it is best to use unfaced insulation rolls. Foam insulation products, including expanding spray foams, polystyrene, polyurethane, polyisocyanurate, and phenolics, are combustible and often give off very toxic gases when heated.

Photo 2 shows a concealed space filled with insulation (Photo courtesy of Dan Kaiser)

• NFPA 13 – 2025 edition – Section 9.2.1.7
• NFPA 13 – 2022 edition – Section 9.2.1.7
• NFPA 13 – 2019 edition – Section 9.2.1.7
• NFPA 13 – 2016 edition – Section 8.15.1.2.7

Option #9 – Wood Joists Filled with Noncombustible Insulation

Similar to Rule #7 above, noncombustible insulation can be used to fill the spaces of wood joist construction to exempt sprinklers in the space. The insulation must fill the space up to the bottom edge of the top of the joist supporting the floor deck or roof above. This option first was allowed in the 2019 edition of NFPA 13.

Figure 5 shows a combustible concealed space with I-Joists mostly filled with insulation in lieu of sprinkler protection

• NFPA 13 – 2025 edition – Section 9.2.1.8
• NFPA 13 – 2022 edition – Section 9.2.1.8
• NFPA 13 – 2019 edition – Section 9.2.1.8
• NFPA 13 – 2016 edition – This provision did not exist

Whether or not sprinklers are required in concealed spaces can be confusing. The critical considerations are often whether combustible materials exist (or could exist) and whether there are enough combustibles to pose a fire risk to the building should a fire originate in a concealed space. This blog reviewed some of the spaces where NFPA 13 allows sprinklers to be omitted. Future blogs will review additional concealed space exemptions.

Do you Have a Question About Concealed Spaces in NFPA 13? Contact the NFSA Today.

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About the Author

Jon Nisja is a Data and Fire Protection Specialist with the National Fire Sprinkler Association (NFSA). Prior to working for NFSA, Jon served as a fire marshal since 1985 in two communities and as a state fire safety supervisor for the Minnesota State Fire Marshal Division. From 2005-2022, he supervised the State Fire Marshal’s fire protection, training, and data teams. He is the past president of both the Fire Marshals Association of Minnesota and the International Fire Marshals Association. Jon can be contacted at nisja@nfsa.org.