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Trends in Exterior Curtain Walls
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Issue 24: Trends in Exterior Curtain Walls

By William E. Fitch, P.E.

A "curtain wall" is a wall that encloses the space within a building but does not support the roof or other floors, typically on a modern high-rise. One of the most significant developments in modern high-rise architectural design was the curtain wall. A curtain wall cladding system is a continuous vertical "curtain" of glass, stone or other material that does not provide structural support, and is supported by the floor slab. A curtain wall is, however, required to keep out the wind, rain, heat and cold.

By the very nature of materials used, curtain wall systems typically do not have any level of fire resistance rating and are generally utilized on buildings where there is no requirement for such. The major fire safety issue that must be addressed with curtain wall systems is fire spread up the building from floor to floor. This can take two paths: spreading up the inside of the wall due to inadequate fire stopping at the floor-wall interface; and spreading up the exterior surface on combustible facing materials.

Code Requirements

Interior Fire Spread
The basic requirement for controlling spread through the interior is that the opening where the floor meets the curtain wall must be sealed with a material or system with fire-resistance equal to that required for the floor.

The primary evaluation method is an ASTM E23071 test. This test utilizes NFPA 285's2 two-story apparatus with a wall representative of that used in common construction practice. In this test, the front is closed and the perimeter joint system to be evaluated is placed between the wall and the second story floor. Burners expose both the exterior face of the wall and the underside of the joint to the fire.


ASTM E2307 Test Apparatus1

 

Buildings expand, contract and move in response to live loads, wind, earthquakes, temperature, freeze-thaw cycles and other forces. Accordingly, this test standard also contains a provision for cycling the joint system prior to fire testing.

 

Exterior Fire Spread

To control fire spread up the exterior face of a curtain wall, the code addresses two factors: fire separation distance as a function of ease of ignition of the wall covering; and flame spread up the face from a fire breaking out a window.

Combustible exterior wall coverings are required to be tested for ignition resistance per NFPA 268.3 This test standard exposes a 4 ft. x 8 ft. (1.2 m x 2.4 m) test specimen to a 12.5 kW/m2 heat flux from a large (3 ft. x 3 ft. – 0.9 m x 0.9 m) radiant panel for 20 minutes to see if the material ignites. If the material passes - i.e., does not ignite - it can be used on the face of a building with a fire separation distance of 5 feet (1.5 m) or less.

If it does ignite, the material can be tested at a reduced heat flux until a passing level is determined. That heat flux level then determines how far the fire separation distance must be.

Metal Composite Materials
The codes also have specific provisions for certain materials. Metal Composite Materials (MCM) typically consist of two sheets of metal separated by a plastic core. This core is not foamed and, therefore, is not regulated by the foam plastic sections of the code, although the requirements do somewhat parallel the foam plastic provisions.

If the MCM has a flame spread index of 25 or less, a smoke developed index of 450 or less, and a thermal barrier is placed between the MCM and the interior of the building, it can be used without limits. Flame spread and smoke developed indices are determined per ASTM E84,4 NFPA 2555 or UL 723.6 If the MCM passes large scale tests, such as UL 10407 or UL 1715,8 and it passes NFPA 2852 for fire propagation, the thermal barrier can be eliminated.

If the MCM has a flame spread index of 75 or less and a smoke developed index of 450 or less, then further restrictions apply. On a building up to 40 ft. (12 m) high and with more than 5 feet (1.5 m) fire separation distance, there is no limit. If the fire separation distance is less than five feet (1.5 m) the material is limited to 10 percent of the wall area. On a building up to 50 feet (15 m) in height, the MCM must have an ignition temperature of 650°F (340°C) or greater and is limited to sections not more than 300 square feet (28 m2) with 4 foot (1.2 m) separation between sections.

Foam Plastic Insulation
Curtain wall systems, such as exterior insulation and finish systems (EIFS), that incorporate foam plastics fall under the requirements for foam plastics. This means the foam plastic must be separated from the interior of the building by a thermal barrier. The foam plastic and all coatings or facings must have a flame spread/smoke developed index of 25/450 or less. The curtain wall system must also pass the NFPA 285 multi-story test for flame propagation and the potential heat per unit area of any installation cannot exceed the potential heat per unit area of the test assembly.

Finally, unless the system incorporates a thermal barrier on the exterior face, it must also pass the NFPA 268 radiant panel ignition test.

 

 New materials

The combination of testing requirements detailed above provides a reasonable means of evaluating existing and new materials and systems. The key is that they have to be evaluated.

The question is what constitutes a new material that requires testing? Numerous EIFS systems have been tested, some including large decorative sections such as cornices. Yet an apparently untested polyurethane resin coated expanded polystyrene (EPS) foam was used around the top of the Monte Carlo hotel and casino in Las Vegas leading to $100 million in damages when the material was ignited by workmen.9

A number of manufacturers are working on EIFS-type systems that use a noncombustible insulation instead of EPS. Others are developing thermal barrier coating systems.

A unique material, Greenpix, has been developed which incorporates photovoltaic cells into the glass curtain wall. Greenpix behaves like an organic system, absorbing solar energy during the day and then generating light from the same power that evening. See Figure 1.


Figure 1 - GreenPix - Zero Energy
Media Wall

 

The polycrystalline photovoltaic cells are laminated within the glass of the curtain wall and placed with changing density on the entire building's skin. The density pattern increases the building's performance, allowing natural light when required by interior program, while reducing heat gain and transforming excessive solar radiation into energy for the media wall. But, the question to ask is how, if at all, does this affect the fire performance?

As with any new development in building materials or systems, it is critical to consider not only the specific code requirements but the as-built, in-use performance. This is the advantage of the two-story firestopping and flame spread tests. They evaluate the materials and systems in a realistic arrangement that, while more costly, provide much greater assurance of safer performance in the field.

William Fitch is with Phyrefish Enterprises.

  1. ASTM E2307, "Standard Test Method for Determining Fire Resistance of Perimeter Fire Barrier Systems Using Intermediate-Scale, Multi-story Test Apparatus," ASTM International, West Conshohocken, PA, 2004.
  2. NFPA 285, "Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Non-Load-Bearing Wall Assemblies Containing Combustible Components," National Fire Protection Association, Quincy, MA, 2006.
  3. NFPA 268, "Standard Test Method for Determining Ignitibility of Exterior Wall Assemblies Using a Radiant Heat Energy Source," National Fire Protection Association, Quincy, MA, 2007.
  4. ASTM E84, "Standard Test Method for Surface Burning Characteristics of Building Materials," ASTM International, West Conshohocken, PA, 2008.
  5. NFPA 25, "Standard Method of Test of Surface Burning Characteristics of Building Materials," National Fire Protection Association, Quincy, MA, 2006.
  6. UL 723, "Standard for Test for Surface Burning Characteristics of Building Materials," Underwriters Laboratories, Northbrook, IL, 2008.
  7. UL 1040, "Standard for Fire Test of Insulated Wall Construction," Underwriters Laboratories, Northbrook, IL, 2007.
  8. UL 1715, "Standard for Fire Test of Interior Finish Material," Underwriters Laboratories, Northbrook, IL, 2008.
  9. Illia, T., "Las Vegas Strip Resort Fire Fueled By Unapproved Resins," Engineering News Record, September 5, 2008.

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