Concealed Often Equals Confusion – Part 2

April 3, 2025

Concealed Often Equals Confusion – Part 2 on Concealed Spaces in NFPA 13

By Jon Nisja

In a prior blog, it was identified that concealed spaces can be a confusing section of NFPA 13 to interpret and apply. That blog looked at some of the spaces where NFPA 13 allows fire sprinklers to be omitted. In this edition, other concealed space options will be examined. Some of these provisions are fairly common while others deal with very specific construction features and likely will rarely apply.

Review of Concealed Spaces in NFPA 13

NFPA 13 starts with the premise that fire sprinklers should be installed in all spaces in a building and generally requires fire sprinklers in a 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.

When addressing sprinkler omissions in concealed spaces, there may be a corresponding increase in the design area (to 3,000 sq. ft.) that is adjacent to the combustible concealed space.

  • NFPA 13 – 2025 edition – Section 20.10.1
  • NFPA 13 – 2022 edition – Section 19.2.3.1.5.1
  • NFPA 13 – 2019 edition – Section 19.3.3.1.5.1
  • NFPA 13 – 2016 edition – Section 11.2.3.1.5.1

Option #9 – Composite Wood Joists Option 2 (limited open space)

This section of NFPA 13 allows fire sprinklers to be omitted in concealed spaces in composite wood joist construction when the space below the joist is filled with non-combustible insulation (up to the bottom of the joist) and where the volume of the space is 160 cubic feet or less and separated or blocked off from other spaces. The NFPA 13 Handbook contains a diagram of such an arrangement.

  • NFPA 13 – 2025 edition – Section 9.2.1.9
  • NFPA 13 – 2022 edition – Section 9.2.1.9
  • NFPA 13 – 2019 edition – Section 9.2.1.9
  • NFPA 13 – 2016 edition – Section 8.15.1.2.8

Option #10 – Above Small Isolated Compartments

NFPA 13 exempts small bathrooms from fire sprinkler protection under certain circumstances:

  • They are located in dwelling units (residential occupancies like homes, apartments, or hotels),
  • They are 55 square feet or less in size,
  • They have walls and ceilings of non-combustible or limited combustible materials with a 15-minute thermal barrier (such as ½-inch gypsum), including the space between the stud walls and shower or tub enclosures.

Often these bathrooms will have lower ceilings or will have soffits above the tub or shower or above the vanity allowing the installation of fans, lights, sprinkler piping, or mechanical equipment. Since NFPA 13 often exempts the room itself from sprinklers, it follows that the concealed spaces above these rooms can be exempt from sprinklers.

  • NFPA 13 – 2025 edition – Section 9.2.1.10
  • NFPA 13 – 2022 edition – Section 9.2.1.10
  • NFPA 13 – 2019 edition – Section 9.2.1.10
  • NFPA 13 – 2016 edition – Section 8.15.1.2.9

Option #11 – Small Soffits

This provision omits fire sprinklers in soffits or horizontal enclosures that:

  • Have noncombustible or limited combustible finishes,
  • Are 160 cu. ft. or less in volume, and
  • Can contain ductwork, electrical, plumbing, or mechanical equipment.

Figure 1 – Soffit exemption

The size (160 cu. ft.) is consistent with the size of other exempt areas. It is assumed that this volume of space would not allow a fire to grow to sufficient size to negatively impact the building. Here are examples of the size of the spaces that are exempt:

  • 2 ft. by 2 ft. by 40 ft. long
  • 3 ft. by 3 ft. by 17.7 ft. long
  • 4 ft. by 4 ft. by 10 ft. long

This option did not appear in editions of NFPA 13 prior to 2022.

  • NFPA 13 – 2025 edition – Section 9.2.1.11
  • NFPA 13 – 2022 edition – Section 9.2.1.11
  • NFPA 13 – 2019 edition – not shown as an option
  • NFPA 13 – 2016 edition – not shown as an option

Options 12 through 14 get a little technical and a basic knowledge of some fire protection engineering principles and terminology is required. They are intended to have materials that inhibit fire from spreading across their surfaces, but they must meet some strict testing or listing requirements. Due to the strict testing and listing requirements, contractors are encouraged to have these options reviewed by a fire protection engineer prior to their use.

Option #12 – Rigid Materials

The first paragraph allows spaces with rigid materials with a flame spread rating of 25 or less to forego fire sprinkler protection. Flame spread ratings are primarily intended for materials that are placed on walls and ceilings. Many construction materials meant for walls and ceilings have a flame spread rating. Materials with a flame spread of 0-25 are given a Class A rating (the highest-class rating). The criteria outlined here are stricter than a standard flame spread rating test. Fire spread is limited to 10 ½ feet beyond the burners in the Steiner Tunnel Test apparatus for 20 minutes.

Figure 2 – Steiner Tunnel Test Apparatus (photo by Jon Nisja)

The second paragraph contains similar criteria but from a slightly different test. It measures the relative surface burning characteristics of the product or material for a 30-minute period of time.

Figure 3 – Flame spread rating on plywood (photo by Jon Nisja)

Few, if any, wood products can meet a Class A flame spread rating (0-25) so, if wood is used, it would likely require some sort of additional treatment.

  • NFPA 13 – 2025 edition – Section 9.2.1.12
  • NFPA 13 – 2022 edition – Section 9.2.1.12
  • NFPA 13 – 2019 edition – Section 9.2.1.11
  • NFPA 13 – 2016 edition – Section 8.15.1.2.10

Option #13 – Fire-Retardant-Treated (FRT) Lumber

This option exempts fire sprinklers when FRT wood is used to construct the concealed space. Wood products can be treated during their manufacturing process to make them FRT. This is typically done by drawing fire retardant chemicals into the wood through a pressure vacuum process. There are no paints, stains, varnishes, or other after-market products that can make normal wood into FRT lumber.

The commentary for this section of NFPA 13, however, suggests that fire-retardant coatings can be considered. These tend to be intumescent products that are applied during construction and will swell up and form an insulation barrier when exposed to heat to protect the wood. It should be noted that commentary language is not part of the actual standard itself and has not always been vetted by the standard’s technical committee.

  • NFPA 13 – 2025 edition – Section 9.2.1.13
  • NFPA 13 – 2022 edition – Section 9.2.1.13
  • NFPA 13 – 2019 edition – Section 9.2.1.12
  • NFPA 13 – 2016 edition – Section 8.15.1.2.11

Option #14 – Combustible Insulation

This section allows noncombustible concealed spaces to be surfaced with exposed combustible insulation and not be protected with sprinklers. The insulation is limited to a heat release rate of 1,000 Btu per sq. ft. This is roughly the heat energy from one ounce of burning gasoline. While that is certainly hot, the fire is unlikely to significantly spread as the space, other than the insulation, is noncombustible.

  • NFPA 13 – 2025 edition – Section 9.2.1.14
  • NFPA 13 – 2022 edition – Section 9.2.1.14
  • NFPA 13 – 2019 edition – Section 9.2.1.13
  • NFPA 13 – 2016 edition – Section 8.15.1.2.12

Option #15 – Space Between Attic Insulation and Ceiling

Where a fire sprinkler-protected attic has an insulation barrier but a concealed space is formed below the attic to which a lower ceiling is attached, the interstitial space does not need sprinkler protection.

Figure 4 – Sprinklers in attic above insulation and concealed space

  • NFPA 13 – 2025 edition – Section 9.2.1.15
  • NFPA 13 – 2022 edition – Section 9.2.1.15
  • NFPA 13 – 2019 edition – Section 9.2.1.14
  • NFPA 13 – 2016 edition – Section 8.15.1.2.13

Option #16 – Vertical Pipe Chases

Small vertical pipe chases, such as those seen behind bathroom and kitchen walls, are not required to have fire sprinklers installed. To use this option, the pipe chases must:

  • Be under 10 sq. ft. in size,
  • Contain no sources of ignition,
  • Have fire-blocking if the piping penetrates more than one story,
  • Older editions of the standard required that the piping be noncombustible or filled with water so that the pipe didn’t burn or melt but that requirement was removed starting with the 2019 edition.

Here are the applicable sections from NFPA 13:

  • NFPA 13 – 2025 edition – Section 9.2.1.16
  • NFPA 13 – 2022 edition – Section 9.2.1.16
  • NFPA 13 – 2019 edition – Section 9.2.1.15
  • NFPA 13 – 2016 edition – Section 8.15.1.2.14

Option #17 – Exterior Columns of Canopies

This option allows sprinklers to be omitted from exterior columns that are supporting canopies when the columns:

  • Are less than 10 sq. ft. in size,
  • Are formed by studs or wood joist construction,
  • Support canopies that are fully sprinkler-protected.

Here are the applicable sections from various editions of NFPA 13:

  • NFPA 13 – 2025 edition – Section 9.2.1.17
  • NFPA 13 – 2022 edition – Section 9.2.1.17
  • NFPA 13 – 2019 edition – Section 9.2.1.16
  • NFPA 13 – 2016 edition – Section 8.15.1.2.15

Option #18 – Ceilings Below Joists (Sprinklers Above)

Sprinklers can be omitted from concealed spaces that:

  • Are created by suspending a noncombustible or limited combustible ceiling below wood structural members,
  • Have insulation filling the gaps between the bottom of the wood members,
  • Have sprinkler protection above the insulation within the trusses or joists,
  • The insulation heat content is limited to 1,000 Btu/ft.

Figure 5 – Example of a concealed space below wood trusses with insulation and sprinkler protection above

  • NFPA 13 – 2025 edition – Section 9.2.1.18
  • NFPA 13 – 2022 edition – Section 9.2.1.18
  • NFPA 13 – 2019 edition – Section 9.2.1.17
  • NFPA 13 – 2016 edition – Section 8.15.1.2.16

Option #19 – Ceilings Below Joists (Small Chords and Insulation)

Sprinklers can be omitted from concealed spaces that:

  • Are created by suspending a noncombustible or limited combustible ceiling below wood joists or composite wood joists with a maximum nominal chord width of 2 inches,
  • Have noncombustible insulation filling the joist spaces,
  • Have a maximum gap of 2 inches at the top of the insulation, and
  • Have facing of noncombustible or limited combustible materials covering the bottom chord of the wood structural members that area secured in place.

Figure 6 – Interstitial space below joist space with insulation in space above

Here are the applicable sections from NFPA 13:

  • NFPA 13 – 2025 edition – Section 9.2.1.19
  • NFPA 13 – 2022 edition – Section 9.2.1.19
  • NFPA 13 – 2019 edition – Section 9.2.1.18
  • NFPA 13 – 2016 edition – Section 8.15.1.2.17

Have a Question About Concealed Spaces in NFPA 13? Contact the NFSA Today!

The sections of NFPA 13 allowing sprinklers to be omitted from concealed spaces can be very specific and 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 additional spaces where NFPA 13 allows sprinklers to be omitted.

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