#477 – What Are Window Sprinklers and When Are They Used?

Window fire sprinklers play a special role in buildings by supporting passive fire protection systems

Window sprinklers fill a unique niche in the fire protection world. Whereas most sprinklers aim to wet and cool fuel sources to control fire spread, window sprinklers protect the structural integrity of windows and glass walls that are used as part of fire-rated barriers.

In modern architecture, architects and building owners often favor large glass exterior windows and even interior glass walls. Though aesthetically beautiful, this design choice can conflict with building and life safety codes that demand passive fire protection measures.

Fire-rated barriers and partitions are featured in documents like the International Code Council’s International Building Code (IBC)and International Fire Code (IFC), currently in their 2024 edition. These passive fire protection systems help compartmentalize a fire, slowing its spread within a building and protecting occupants’ escape routes and areas of refuge. Standard glass is unsuitable for these purposes, as it shatters in a fire’s heat, and heavy-duty fire-rated glass is extremely expensive.

That’s where window sprinklers come in: they spray water onto “glazing” to keep glass from shattering in a fire, maintaining its integrity as part of a fire-rated barrier.

Read on for an exploration of:

• The characteristics of a window sprinkler
• An intro to fire partitions, fire barriers, fire walls, and the role of window sprinklers
• Considerations for using window sprinklers

QRFS is your window sprinkler source. We’re ready to ship a selection of Viking horizontal and vertical sidewall models, with more coming soon from Tyco, Victaulic, and Reliable. If you need a part we don’t currently stock, contact us, and we’ll help you find a solution!

What is a window sprinkler?

Window sprinklers are special sprinklers, both in appearance and application. Their physical design differs from other models, and they often feature an otherwise uncommon sprinkler orientation: the vertical sidewall. The shape of window sprinklers serves their role as special-application sprinklers designed to throw water sideways onto glass windows and walls, preventing these surfaces from breaking in a fire.

Look closely at window sprinklers, and you’ll quickly see they aren’t like other models. Unlike other types, a window sprinkler has a uniquely flat deflector shape. Additionally, window sprinklers come in two main orientations: horizontal sidewalls or vertical sidewalls, with some concealed versions available.

Rarely used otherwise, vertical sidewall sprinklers are like pendent or upright sprinklers—they protrude downward or upward from supply pipes—but with deflectors designed to throw water sideways like sidewalls. It’s this unique spray pattern that allows window sprinklers to accomplish their special application of protecting glass surfaces.

Most spray fire sprinklers wet and cool a fire’s possible fuel sources to control the fire’s spread and growth. So, window sprinklers’ mission of stopping glass from shattering is unique, which is why NFPA 13 defines them as “special sprinklers,” aka “special application sprinklers:”

From the 2025 edition of NFPA 13

3.3.223.4.18 Special Sprinkler.

A sprinkler that has been tested and listed as prescribed in Section 15.2.

15.2 Special Sprinklers.

15.2.1 Special sprinklers that are intended for the protection of specific hazards or construction features shall be permitted where such devices have been evaluated and listed for performance […]

Window sprinklers’ role in protecting glass from breaking in a fire is important because some glass walls and windows are part of fire-rated assemblies that are designed to stay fireproof for a specific period. Glass isn’t traditionally used for this purpose because of its fragility, though some glass can meet fire-resistance requirements—at significant cost or, when using wired glass, by sacrificing the intended design aesthetic. However, properly employed window sprinklers offer another way to address this challenge.

Window sprinklers transform glass into a fire-rated assembly

The “special application” of window sprinklers is to make a glass wall or large window into a functional fire-rated assembly. Such assemblies relate very little to the design of fire sprinkler systems, which are active fire protection systems, and the standards that govern their installation. Instead, they are passive fire protection measures related to building design and life safety—the purview of codes like the IBC and IFC.

The purpose of fire-rated partitions, barriers, and walls is to:

• Compartmentalize the building
• Minimize the effect of fire on the building structure
• Slow or prevent fire and smoke spread
• Create safe corridors and spaces for occupant escape
• Provide enough time for escape

If you’ve ever thought that the walls and doors of an office building stairwell seem particularly sturdy, passive fire protection is partially why. Stairwells are often dedicated evacuation routes that are protected by fire barriers and fire walls.

Chapter 7 of the IBC lists five types of fire-rated construction elements, but we’ll focus on three of them:

1. Fire partitions (IBC 708) are the least solid of the three. They don’t totally stop the spread of fire, but slow it enough to allow occupants to escape.
2. Fire barriers (IBC 707) are designed to enclose exits and shafts and separate hazardous materials from the rest of a building.
3. Fire walls (IBC 706) are the most comprehensive protection, maintaining their integrity even when structures on either side of a fire wall collapse in fire conditions.

Fire partitions, barriers, and walls all provide time-limited shelter. Depending on the need, IBC calls for resistance ratings up to 4 hours.

Of course, glass isn’t a “traditional” material used as part of a fire-rated assembly. However, as Meyer Fire discusses, some heavy-duty glass with fire-rated glazing can be approved for this use—but it is quite expensive!

Instead of using fire-rated glass, a listed “assembly” that includes properly spaced window sprinklers combined with glass surfaces can meet the requirements for traditional fire-rated assemblies, including ASTM E119-20: Standard Test Methods for Fire Tests of Building Construction and Materials. For example, these data sheets from Tyco and Reliable indicate that their sprinklers meet ASTM E119’s fire test requirements.

As Consulting-Specifying Engineer explains, ASTM E119-20 requires a fire test where one side of a barrier is exposed to a fire, and testers monitor the wall’s integrity on one side while monitoring temperature on the other. By placing ready-to-spray sprinklers on both sides of a glass barrier, the glass can maintain its integrity during ASTM testing and real fire conditions. This diagram illustrates how that works:

This video from Viking shows how a window sprinkler completely coats a window with water to protect it from fire:

Considerations for using window sprinklers

While window sprinklers can be useful for converting glass into a fire-rated barrier, they come with restrictions and practical considerations. Some relate to the rules and requirements in codes and standards, but others have to do with the cost and need for AHJ approval.

Window sprinklers can be expensive

While fire-rated glass is costly, so are window sprinklers. Most non-specialized commercial fire sprinkler heads cost less than $50. But a single vertical sidewall listed window sprinkler can run more than $500.In addition, implementing window sprinklers as a solution requires putting them as a component of a fire sprinkler system’s overall design, which may require running additional branch lines to additional coverage areas.

Rules and limitations from NFPA 13 and the IBC

Window sprinklers used for fire-rated assemblies have two main limitations under IBC rules: they can only be used to create fire partitions and fire barriers (never fire walls), and they can only provide up to 2 hours of protection. Beyond these conditions, NFPA 13 and the IBC both have requirements for using window sprinklers. NFPA 13 says (9.3.15):

• Window sprinklers must be supplied by a wet pipe system (not dry or preaction)
• The glazing must be heat-strengthened, tempered, or ceramic glass
• The glazing must be fixed (i.e., the windows can’t open)
• If the assembly is intended to be protected on both sides, there must be sprinklers on both sides
• The glazing must not be part of a load-bearing wall
• The sprinkler water supply must match the required duration of the assembly protection (E.g., a 2-hour fire barrier needs a water supply that will hold for 2 hours)
• No part of the glazing assembly should interfere with water covering the entire surface, and there must be no obstructions to the sprinkler spray

There are additional IBC requirements, including that the glass must be at least 1/4-inch thick and fewer than 13 feet tall.

The fact that regulations for window sprinklers are scattered between ICC codes and NFPA standards can be confusing. However, the truly complex issue is that this technology is ultimately non-prescriptive, optional, and up to the approval of the authority having jurisdiction (AHJ).

Window sprinklers are exceptions, not the rule

Along with the price tag and code complexity that come with using glass in rated enclosures and barriers, fire protection experts and architects should also understand that window sprinklers are atypical. More specifically, they are considered an “alternative method” for achieving a fire-rated partition or barrier.

As MeyerFire explains, mixing active fire protection systems, such as fire sprinklers, with passive fire-protection systems, such as fire-rated barriers, to achieve a specific purpose is a complicated process that “may not always lead to a desired result.” Therefore, these applications are “limited” and not standard. For example, the IBC explicitly says that, normally, fire-rated materials should be capable of the necessary fire resistance without involving automatic fire sprinklers:

703.4 Automatic Sprinklers

Under the prescriptive fire-resistance requirements of this code, the fire-resistance rating of a building element, component or assembly shall be established without the use of automatic sprinklers. […] However, this section shall not prohibit or limit the duties and powers of the building official allowed by Sections 104.10 and 104.11.

However, while codes and standards outline specific rules for materials, equipment, and design approaches, including prescribing the need for fire-rated barriers, alternatives can be used to achieve the same result. These “alternative materials, design, and methods” require the approval of the authority having jurisdiction (AHJ):

From the 2024 edition of the IBC

104.2.3: Alternative materials, design and methods of construction and equipment.

The provisions of this code are not intended to prevent the installation of any material or to prohibit any design or method of construction not specifically prescribed by this code, provided that any such alternative has been approved.

104.2.3.1: Approval authority.

An alternative material, design, or method of construction shall be approved where the building official finds that the proposed alternative is satisfactory and complies with Sections 104.2.3 through 104.2.3.7 as applicable.

Sections 104.2.3 through 104.2.3.7 of the code outline requirements to submit an alternative proposal to the AHJ in writing, that the alternative “shall comply with the intent of the provisions of [the] code,” that it will meet equivalent performance and safety criteria to what it replaces, and that the alternative has met relevant testing and reporting standards.

Window sprinklers fall into this category of alternative methods, as well as special applications. As MeyerFire emphasizes, nothing in NFPA 13 requires window sprinklers, and the IBC doesn’t even mention them. However, because sprinkler manufacturers test and list window sprinklers according to ASTM and UL standards, they are available as an option with explicit AHJ approval.

Window fire sprinklers have a unique and valuable mission

Window sprinklers have a singular job that sits at the intersection of fire sprinkler standards and building codes, as well as active and passive fire protection.

Like all automatic fire sprinklers, they stand ready to spray water should a fire activate their heat-sensing elements. Yet, unlike most sprinklers, their purpose is not to wet flammable fuel sources to control a fire’s spread. Instead, they’re aimed directly at a noncombustible surface—a glass wall or window. By coating the glass with water, the sprinklers keep it from shattering, maintaining its integrity as part of a fire-rated assembly.

While window sprinklers are a useful tool, they also require careful consideration as part of both a fire sprinkler system and a building’s fire protection and life safety design process.

If you need to purchase window sprinklers, QRFS has you (and your glass surfaces) covered. We carry Viking horizontal and vertical sidewall models ready to ship, with more coming soon from Tyco, Victaulic, and Reliable. If you need a window sprinkler we don’t currently stock, contact us!

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