Architectural Record BE - Building Enclosure

Window Sprinklers as an Alternative to Fire-Rated Glass

Architects now have a choice in how to achieve fire-rated partition designs
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Sponsored by TYCO
By Peter J. Arsenault, FAIA, NCARB, LEED AP

Design Criteria For Window Sprinkler Installations

While likely assumed by most design professionals, the AC and ESR for window sprinklers specifically requires that any use of window sprinklers must be developed with plans and details prepared by a registered design professional. In particular, one of the conditions of use is that “the registered design professional must provide the code official with documentation outlining the basis of compliance with the criteria specified by the IBC for a code modification in accordance with Section 104.10 or for an alternative method of construction in accordance with Section 104.11.” These sections provide the basis for use of the AC and ESR in this case, but the design professionals are tasked with providing the basis and design coordination.

Illustration of a window sprinkler in action in case of fire.

Image courtesy of Johnson Controls

The proper design of buildings using window sprinklers is meant to provide time-rated protection in the event of a fire.

In any project where window sprinklers are used, both architects and sprinkler system engineers need to work together to assure that the glass wall portion and the window sprinkler portion are coordinated and consistent with the AC and ESR specific requirements. Fortunately, most of these requirements are fairly straightforward and identified in the different sections of the ESR. Essentially, these requirements become design criteria for a particular building project and are summarized in some basic categories as follows.

General Building Design Factors

Window sprinklers, as approved by an ICC ESR, are intended to provide an alternative for up to a 2-hour fire-resistance-rated non-load-bearing interior fire barrier assembly. That means they can be a used with window panels as a substitute to non-load-bearing fire-rated partitions, such as metal stud and gypsum board assemblies, masonry, or other common partition types per IBC Sections 707 (fire barrier assemblies) and 708 (fire partition assemblies). However, they specifically, and logically, cannot be used as firewalls, which are defined by the IBC as “a fire-resistance-rated wall having protected openings, which restricts the spread of fire and extends continuously from the foundation to or through the roof, with sufficient structural stability under fire conditions to allow collapse of construction on either side without collapse of the wall,” (IBC Section 202). This is quite reasonable given the nonstructural nature of glass partitions.

Some other general criteria include:

  • Exit passageways: Window sprinklers and glass partitions may not be used as fire barriers in exit passageways, horizontal exits, or exit enclosures unless the entire building is fully sprinklered in accordance with Chapter 9 of the IBC. Since most window sprinkler installations are located in buildings that are fully sprinklered, this requirement can readily be accommodated.
  • Combustible materials and pony walls: All combustible materials must be kept a minimum of 2 inches away from the glass. This can be achieved by the use of a 36-inch-high “pony wall” that has a fire rating equal to the code-required rating of the partition. The rationale behind this requirement is that the pony wall would be thick enough on either side of the glass (assumed centered) to provide at least a 2-inch-thick barrier between any combustible items (furniture, etc.) and the face of the glass. The 36-inch height of the pony wall helps assure that separation. Logically, the pony wall must be constructed and shown to meet the required fire rating for the partition and must be accepted by the code official.
  • Exterior walls: Although not commonly done, window sprinklers are allowable for use on an exterior wall assembly that requires a fire-resistance rating per IBC Section 705. However, zero-lot line installations are not intended here since window sprinklers used in exterior walls require a building fire separation distance greater than 5 feet under the 2006 IBC and 10 feet in subsequent versions.
  • Installation and related data: Manufacturers installation instructions and other data that were the basis for an ESR must be followed for all aspects of the building design as applicable.
  • Hazardous areas: The use of window sprinklers is not allowed in locations that contain materials that present deflagration or detonation hazards as defined in the code.

Window Design Factors

Having defined the general locations in the building where windows are intended to be used, there are some specific but readily achievable criteria that need to be followed regarding the glazing system used.

  • Glazing: Glass must be at least ¼ inch (nominal) thickness. The glass types allowed are heat-strengthened or tempered glass. Either single- or dual-pane glazing panels are acceptable.
  • Glass size: Exposed glass must not exceed 13 feet in height above the pony wall. There is no width restriction on the glass as long as the window sprinkler spacing is followed (discussed in next section). Also, the glass is required to be completely vertical and cannot be sloped or curved.
  • Glazing frames: Framed glazing needs to be provided with an elastomeric seal around the entire perimeter. Unframed glazing is allowed provided the spaces between the glass panels are filled with silicone sealant.
  • Horizontal mullions: Intermediate horizontal mullions are not permitted because they will interfere with the uniform distribution of water over the surface of the glazing. Therefore, the glass needs to be full height with only vertical separations.
  • Operability: Window sprinklers are only allowed for nonoperable, fixed glazing.
  • Openings and penetrations: Glazing assemblies using window sprinklers are not permitted to incorporate penetrations of any type. Further, any opening in the assembly (i.e., doors) must be treated separately and carry its own fire-resistance rating as defined by the IBC for the particular partition condition and occupancy.

Sprinkler System Design Factors

With the building and window details determined, the following criteria shall apply to the design of the building sprinkler system in general and for the window sprinklers in particular:

  • NFPA 13 requirements: Since this is still an automatic fire-protection system, the demand and hazard criteria shall be per NFPA 13. Further, all piping must be designed, sized, and installed in accordance with NFPA 13.
  • Water pressure limits: Hydraulic calculations must show a maximum pressure of 175 psi for windows separated by vertical mullions and 70 psi for unframed windows.
  • Water flow time: Automatic water supply systems must have the capability to supply water to the glazing assembly equal to the time of the rating (i.e., up to 2 hours).
  • Window sprinkler head locations: Sprinklers must be located on both sides of the glazing. Further, the location of the sprinkler heads in relation to the glass face and the top of window must be as indicated in the AC and ESR documents. For vertical sidewall applications, that means the horizontal distance between the glass and the window sprinkler must be a minimum of 4 inches and a maximum of 12 inches, while the vertical distance between the top of the glass and the bottom of the window sprinkler should be approximately 3 inches (sprinkler below the top of glass). For horizontal sidewall installations, the distance between the glass and the window sprinkler must be no less than ½ inch and no more than 4 inches, while the center of the window sprinkler head needs to be approximately 2 inches below the top of the glass.
  • Window sprinkler head spacing: The maximum horizontal distance between window sprinkler heads is 8 feet with a minimum spacing of 6 feet.
Diagram of spacing of window sprinklers based on AC and ESR publications.

Image courtesy of Johnson Controls

The spacing of window sprinklers for windows with or without vertical mullions is dictated by AC and ESR publications as determined by testing and review.

Since this is only a summary of typical conditions, it is of course best to consult the original AC and ESR documents when working on a specific project. Note that such documents are updated from time to time, just like the IBC, so be sure to use the most current versions. For window sprinklers, the AC were first approved in 2007 and subsequently approved again in 2013. ESRs are issued and renewed on a regular basis and include some requirements for quality control by manufacturers in order to remain valid.

Cost Comparison: Window Sprinklers and Fire-rated Glass

In every construction or renovation project, cost is always an important consideration. Of course, since so many things in a building are interrelated, a change in one area of the design and construction can have implications on other parts. Nonetheless, there are some fundamental things that are readily observable when comparing the use of window sprinklers versus the use of fire-rated glass. The most obvious one is the lower cost of standard tempered or heat-strengthened glass compared to intumescent fire-rated glass—a difference on the order of 70 percent when looked at on a square foot basis of material alone. In addition, standard glass can often be quicker and easier to procure, install, and seal compared to fire-rated glass, thus saving on labor and management costs as well. One impact on the cost of related materials providing support for the glass might come from the fact that standard glass is typically lighter in weight than fire-rated glass, thus reducing the dead load on framing systems and even the structural floor systems. Finally, in the context of a full commercial building covered by the IBC, sprinkler work is likely already part of the project’s budget, so the cost of adding the extra window sprinkler piping and heads can be fairly nominal.

In order to illustrate some of the cost differences, let’s take the example of a glazed window assembly in an academic building that is 8 feet wide by 13 feet tall (maximum size allowed by AC).

  • Fire-rated glass cost: According to an architect who was recently involved in such a project, he found that the uninstalled cost per square foot of fire-rated glass was approximately $90 per square foot. For our 8-by-13-feet example, that equates to 104 square feet of glass for a total cost of $9,360 for each glass panel.
  • Window sprinklers with standard glass cost: When the architect looked at standard tempered glass, he found that could be budgeted for a much lower $25 per square foot, uninstalled. In our 104-square-feet example, that equates to $2,600 for the glass. In addition, the cost of two window sprinklers (one on each side of the glass) needs to be accounted for. According to quotes received, that would be worth about $925 for the two, also uninstalled. Combining the glass cost with the window sprinkler cost, the total comes to $3,525 for each glass panel.
  • Cost difference: For this example, the difference between the two fire-rated systems shows the window sprinkler solution to be $5,835 less expensive, or a savings of more than 62 percent. Of course, that is just for one window panel - multiple windows means greater dollar savings producing greater results for the total project. So, carrying our example out further, four window panels would calculate to $37,440 for fire-rated glass vs. $14,100 for a window sprinkler system, resulting in $23,340 cost savings for the project. For 12 windows, fire-rated glass would cost $112,320 compared to $42,300 for window sprinklers, resulting in project cost savings of $70,020.

Overall, even without factoring in some of the related installation costs of time and labor, plus some potential weight savings, it seems apparent that the larger the project and application, the greater the value provided by the window sprinkler option compared to fire-rated glass.


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Originally published in Architectural Record