Max Attic Size with Insulation to Avoid Sprinkler?

2/10/2023

We are a structural engineer currently work for an architect on a small assembly hall project; a wedding venue that will seat over 99, which will require sprinkler protection.

The roof structure will be wood scissors trusses, 5’-7” deep at the center – see below.

I mentioned to the architect that they will have to protect the “attic” space also with sprinklers.

In response, the architect said the owners want to finish the ceiling and insulate the attic space.

Where is it written about the maximum cavity size without sprinklers? Is it any cavity?

If so possibly we fill the entire attic space, which is small because of the unique scissors trusses. I don’t think this would be economical or practical.

But, what would the cavity depth have to be (underside of roof deck to top of batt or blown insulation) so that sprinkler protection of that concealed combustible space is not required?

I assume if this space is not sprinkler protected that “attic” compartments would be required. Any guidance on this matter would be appreciated. Thank you.

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16 Comments

sean link

2/10/2023 08:04:59 am

based on the current arrangement you would need to provide sprinkler protection in the attic. To insulate your would have to completely fill. As in no room allowing no room for ventilation of the attic space either.

Henry

2/10/2023 08:14:13 am

I thought NFPA 13 allowed for a 2" gap at the top of the cavity if desired? We have seen this and it allowed for the required ventilation of the roof deck...

Jason

2/21/2025 09:37:52 am

I have a similar situation but the OWWJ are 24" deep with a 6" top chord. They are filling to 4" from the underside of the deck to get the required R value in our cold climate.

It doesn't make sense to install sprinklers in this attic as the sprinklers and pipe will be below the top chord and will be completely covered by the insulation.

I can't find a reference for a situation like this.

Glenn Berger

2/10/2023 08:14:10 am

NFPA 13 provides the guidelines of when ceiling cavity spaces (See 9.2.1 in the 2019 edition).

A potential design solution - Provide a "ceiling" that is attached to the bottom of the structure that will minimize the insulation space and by definition sprinklers can be avoided.

James Evans

2/10/2023 08:29:29 am

We have had the entire attic space filled before in this situation that would be hard. I believe you are referring to the 6" space allowed by NFPA 13, but I do not think you could keep almost 5' of insulation in that space. Depending on how much you want to spend for the fire sprinkler system, you could leave the concealed combustible space unsprinklered and increase your remote area also allowed by NFPA 13. Another option would be to put firewalls in the attic to limit the area of each compartment.

I know that attics cause alot of headaches but in any situation it adds up to $$$$$$

WES

2/10/2023 10:43:52 am

You cannot voluntarily choose to not protect an attic and justify it by increasing your remote area to 3,000 sqft. That's a relatively common misnomer that NFSA (and AFSA I believe) have clarified (and it's not the way NFPA 13 reads either).

NFSA - from October 2020 TechNotes #9:

There is a building with a combustible attic to be equipped with a NFPA 13 sprinkler systems. The owner request is to omit a dry system for the attic. Can we use NFPA 13 (2016) NFPA 13 11.2.3.1.4 (3) and calculate 3,000 SQ FT below the ceiling only and omit the sprinklers in the attic?

The adjacent design area increase of 3,000 sq. ft. can be used, however, the area without sprinklers has to be specifically allowed by the standard. Increasing the adjacent design area does not exempt a space from sprinklers.

Sections 8.15.1.2 and 8.15.6 (special situations) allows sprinklers to be omitted. Section 11.2.3.1.4 (3) (water demand) applies an increased area of operation to the areas adjacent to the spaces where sprinklers are omitted. If an area is exempt from sprinklers, the fire load (or chance of) increases, so, an additional demand (in terms of 3,000 sq ft) is put on the system surrounding the exempt area. Some of the exempted areas in 8.15.1.2 and 8.15.6 are also exempt from the 3,000 sq ft area increase, meaning, no increase of 3,000 sq ft to the adjacent area is required.

The sprinkler system in the attic is required and is only exempted from sprinklers if the space meets 8.15.1.2. For example, if the attic is constructed entirely of fire-retardant treated wood, or is filled entirely of non-combustible insulation, etc. it can omit sprinklers. These two examples are also not required to have the adjacent areas increased to 3,000 sq. ft. (see 11.2.3.1.4 (4) (c) and (f)). These spaces do not support combustion or provide significant material to burn uncontrolled, so, they are not expected to burden an adjacent area to the point of overtaxing the system.

To conclude, the exempt area first has to be permitted and an entire attic is not allowed to be exempt from sprinklers unless some special construction provisions are made. Increasing the design area may or may not be required, however, nothing in the standard allows an increased design area to exempt a space from sprinklers.

Casey Milhorn

2/10/2023 08:38:36 am

8.15.1.2.7 Concealed spaces filled with noncombustible insulation
shall not require sprinkler protection.
8.15.1.2.7.1 A maximum 2 in. (50 mm) air gap at the top of
the space shall be permitted.

Henry is correct, you can fill the space completely with insulation and also leave a 2" air gap at the top.

I am pretty sure you cannot "choose" to not sprinkler a concealed space and double your calculated area as suggested. On that note, don't forget with the bottom side of the scissor truss being more than a 3/12 slope, you will need to add 30% to your design area.

If filling the space completely with insulation is too costly, I would suggest avoiding a dry system if at all possible (for cost and maintenance reasons). See if foam insulation can be sprayed on underside of roof deck and install a wet system in the attic space. You already need piping in that area for your pendants. The extra cost for the sprigs and uprights should be minimal.

Jason Phelps

2/10/2023 09:16:27 am

I would recommend sprinklering the attic or you can compartmentalize with sheetrock draftstopping into 160 ft3 areas.

These requirements were in 8.15 of 2016 NFPA 13.

Filling completely with insulation is better for interior spaces, not on the energy envelope, or between floors.

Yes, NFPA allows for the 2" air gap, for required ventilation, but how will you get cross-ventilation from the eave to the peak? In my experience it is very easy to get it wrong on attic ventilation and you are likely to having building problems down the road, like mold, excess condensation, and potentially dry rot.

Todd E Wyatt

2/10/2023 09:50:41 am

The scoping Code (e.g. 2021 IBC) determines “Where Required” an automatic sprinkler system (ASPS) is required per the Occupancy Classifications (OC) of the building.

The referenced standard (e.g. 2019 NFPA 13) determines HOW “sprinklers shall be installed throughout in accordance with NFPA 13.”

This building would (probably) be classified as a “Group A-2 Assembly” (e.g. banquet hall).

“903.2.1.2 Group A-2” requires an ASPS where one condition exists :
1. The fire area exceeds 5,000 square feet (464 m2).
2. The fire area has an occupant load of 100 or more.
3. The fire area is located on a floor other than a level of exit discharge serving such occupancies.

Since you state the (calculated) Occupant Load (OL) exceeds (100) occupants, an ASPS is required.

IBC defines “attic” as “the space between the ceiling framing of the top story and the underside of the roof.”

“903.3.1.2.3 Attics” includes (4) conditions that require attic protection by an ASPS (see REFERENCES below) :
1. Not applicable
2. Not applicable
3. Applicable?
4. Not applicable

If “903.3.1.2.3 (3)” requires attic protection by an ASPS, then NFPA 13 shall be referenced to HOW this “concealed space” is to be protected (or not protected) by an ASPS.

Lastly, “draftstopping” for this attic space should be reviewed (see REFERENCES below) IF it is determined that an ASPS is NOT required.

REFERENCES
2021 IBC
Section 903 Automatic Sprinkler Systems
903.2 Where Required
Approved automatic sprinkler systems in new buildings and structures shall be provided in the locations described in Sections 903.2.1 through 903.2.12.
903.2.1 Group A
903.2.1.2 Group A-2
An automatic sprinkler system shall be provided throughout stories containing Group A-2 occupancies and throughout all stories from the Group A-2 occupancy to and including the levels of exit discharge serving that occupancy where one of the following conditions exists:
1. The fire area exceeds 5,000 square feet (464 m2).
2. The fire area has an occupant load of 100 or more.
3. The fire area is located on a floor other than a level of exit discharge serving such occupancies.

903.3 Installation Requirements
Automatic sprinkler systems shall be designed and installed in accordance with Sections 903.3.1 through 903.3.8.
903.3.1 Standards
Sprinkler systems shall be designed and installed in accordance with Section 903.3.1.1 unless otherwise permitted by Sections 903.3.1.2 and 903.3.1.3 and other chapters of this code, as applicable.
903.3.1.1 NFPA 13 Sprinkler Systems
Where the provisions of this code require that a building or portion thereof be equipped throughout with an automatic sprinkler system in accordance with this section, sprinklers shall be installed throughout in accordance with NFPA 13 except as provided in Sections 903.3.1.1.1 and 903.3.1.1.2.
903.3.1.1.1 Exempt Locations

903.3.1.2.3 Attics
Attic protection shall be provided as follows:
1. Attics that are used or intended for living purposes or storage shall be protected by an automatic sprinkler system.
2. Where fuel-fired equipment is installed in an unsprinklered attic, not fewer than one quick-response intermediate temperature sprinkler shall be installed above the equipment.
3. Where located in a building of Type III, Type IV or Type V construction designed in accordance with Section 510.2 or 510.4, attics not required by Item 1 to have sprinklers shall comply with one of the following if the roof assembly is located more than 55 feet (16 764 mm) above the lowest level of fire department vehicle access needed to meet the provisions in Section 503.
3.1. Provide automatic sprinkler system protection.
3.2. Construct the attic using noncombustible materials.
3.3. Construct the attic using fire-retardant-treated wood complying with Section 2303.2.
3.4. Fill the attic with noncombustible insulation.
The height of the roof assembly shall be determined by measuring the distance from the lowest required fire vehicle access road surface adjacent to the building to the eave of the highest pitched roof, the intersection of the highest roof to the exterior wall, or the top of the highest parapet, whichever yields the greatest distance. For the purpose of this measurement, required fire vehicle access roads shall include only those roads that are necessary for compliance with Section 503 of the International Fire Code:
4. Group R-4, Condition 2 occupancy attics not required by Item 1 to have sprinklers shall comply with one of the following:
4.1. Provide automatic sprinkler system protection.
4.2. Provide a heat detection system throughout the attic that is arranged to activate the building fire alarm system.
4.3. Construct the attic using noncombustible materials.
4.4. Construct the attic using fire-retardant-treated wood complying with Section 2303.2.
4.5. Fill the attic with noncombustible insulati

Kyle

2/10/2023 11:28:27 am

Note that 903.3.1.2.3 are for sprinkler systems that fall under NFPA 13R.

Jesse

2/10/2023 09:57:47 am

We run into this a lot, and usually end up protecting the space with automatic sprinklers. In one case where the arch wanted to insulate, the fire marshal required plywood sheathing installed to hold the insulation rolls in the joist space - which in effect created another combustible concealed space below that. And thuis required protection.

I doubt it will be feasible or economical to spray foam insulated the entire void. So you may end up with a combination of an insulated deck and sprinkler protection, but at least in that case you likely won't need to provide a dry system.

2/10/2023 03:10:10 pm

Even if they sprayed foam insulation, would that even be considered a "noncombustible insulation"? I believe the NFPA 13 requirement would require the insulation to be considered noncombustible.

Dave

2/10/2023 10:30:25 am

Maybe I am missing something, but if they already have to sprinkler the space below the ceiling,then there already has to be a sprinkler piping network installed and suspended from the structure. So I am not understanding the reluctance to sprinkler the attic space. If it is because wet-system piping sill be below, and the attic is not a conditioned space (such as if the insulation is at the bottom chord) I have often protected the attic space with intermediate-temperature dry upright sprinklers supplied by the wet piping below. Once I had to order them longer than their listing allows; that could be an issue.

Kevin

2/11/2023 11:01:44 am

What the engineer is not telling us, is if they are spray foaming at the top chord or using batt insulation at bottom chord. As Dave is saying, there will be pipe in the attic truss area regarless. The cost is either spray foam to create a conditioned space or a dry pipe fire sprinkler system for the heads below the ceiling in the occupied space. Batt insulation of a wet system in theory sounds good but always ends up in a freeze up. Spray foam is used today on 99% of the jobs we see in NY. The cost savings is to spray foam top chord and end walls with a wet fire sprinkler system protecting the attic and the occupied space below. This is the least expensive fire sprinkler system. We all know dry systems fail at a greater rate than wet systems due to lack of proper maintance. A wet system with sloped roof has a design area of 1950 square feet, compared to as Wes noted a 3000 sq ft design with out sprinklers in attic. That 3000 sq ft is based on a flat ceiling and we would add the additional 30% to the design area. Thats double the water demand, increased pipe size, increased cost.

Anthony

2/15/2023 03:58:39 pm

If you have a sprinkler system in the building already it will most likely be cheaper to install back to back attic heads rather than fill the void with insulation.

2/15/2023 06:21:45 pm

Agreed or at least comparable.

Max Attic Size with Insulation to Avoid Sprinkler?

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