Designing Roofs for Life Safety and Sound Isolation

Explore expert insights on how to specify sound-rated automatic smoke vents
[ Page 2 of 5 ]        
Sponsored by The BILCO Company

Introducing Automatic Smoke Vents

Automatic smoke vents are openings in the roof that are fully insulated and weathertight. When a fire is detected, the smoke vent covers open automatically, allowing the heat, smoke, and gasses produced by the fire to escape the building instead of remaining trapped inside. Once the fire has been extinguished, the covers on the smoke vent can then be reset, and the roof returns once again to a fully insulated, weathertight, and fire-ready structure.

Mechanically opened automatic smoke vents consist of covers, often constructed of aluminum or steel, that provide a fully insulated and weathertight assembly when closed and then open automatically to release smoke and gas in the event of a fire.

Automatic smoke vents are an important and effective fire management tool for buildings with large expanses of unobstructed space, such as convention centers, auditoriums, retail stores, schools, manufacturing facilities, warehouses, casinos, and theaters. In these types of applications, where compartmentalization is a difficult fire-containment strategy to execute, smoke vents enable smoke, heat, and gasses to make a swift exit near the area where the fire originated. This removes these noxious elements from the building more quickly than other solutions and prevents them from spreading throughout the building and causing more damage and destruction. The open smoke vents also perform a life-safety function in allowing more time for people to escape the fire and smoke.

There are two categories of automatic smoke vent solutions found on the market: mechanically opened and gravity opened.

Mechanically opened automatic smoke vents consist of covers that are over-sprung by a mechanical lift assist and held closed by a latching mechanism with a fusible link. The fusible link—the same technology commonly used as the trigger mechanism in fire sprinkler systems and automatic fire doors—melts at a specific temperature, opening the vent doors when temperatures at the ceiling reach a certain level. When released, engineered lift assistance opens the covers and locks them in an open position, allowing the heat, smoke, and toxic fumes gathering at the ceiling to escape the burning building. Once the fire is under control, the vents can easily be reset from the roof level. These automatic smoke vents are the most common solution used for smoke and heat removal in large, single-story, undivided buildings.

Gravity-opened vents, also referred to as melt-out domes, do not open by mechanical means. When the heat from the fire reaches the dome material, it shrinks the material until it falls into the building, providing a hole in the roof through which the heat and gas can escape. There are a few notable disadvantages in selecting this technology over the mechanically opened smoke vents. The melt-out domes cannot be reset or designed to activate in response to a fire alarm or smoke detector. Coordinating operation with a sprinkler system is also difficult due the the melting temperature of the domes. Another item to consider is that the dome material could contribute to the fire as it falls into the building. These melt-out smoke ventilation solutions are not often selected, but they do offer architects a less expensive alternative to mechanically operated smoke vents.

Selecting the Right Mechanically Operated Smoke Vent

Mechanically operated smoke vents are largely preferred over gravity-opened smoke vents due to the number of options available with these products that allows architects to choose automatic smoke vents that best complement the types of spaces they are creating. Size, operation, activation mode, cover material, and acoustic control are a few of the features and functionalities that architects can specify to tailor the smoke vent solution to the specific needs of a project.

Size

Automatic smoke vents are available in single-leaf and double-leaf models and in aluminum or prime-painted steel construction. The single-leaf solutions are ideal for smaller, more confined areas like stairwells or elevator shafts. Double-leaf smoke vents are designed for larger areas, and size and spacing are based on building volume and contents. Generally, smoke ventilation requirements for large areas are dictated by fire codes, and usage is coordinated with local fire-protection authorities.

Activation Mode

Standard automatic smoke vents are automatically activated by a fusible link that melts when the temperature at the ceiling reaches 165 degrees Fahrenheit. Different fusible-link release temperatures are available to allow coordination with sprinkler-head activation temperatures. Products can also be specified with an electric latching mechanism that allows the smoke vents to be activated by a fire alarm or smoke-detection system. This feature can be especially useful in applications where the smoke vents are more centrally located because it ensures that the vents will be open, even if a fire starts in an area of the building that is not near the smoke vents.

On-Demand Operation

Sometimes it is nice or necessary to open the automatic smoke vent when there is no fire. For example, there are times that spaces could benefit from more natural ventilation, such as manufacturing environments or stage areas to exhaust heat from overhead lighting. Automatic smoke vents can be equipped so that they can be opened and closed on demand as well. This on-demand control is typically delivered through hand winch or electric motor operation. Rigging can be installed at the floor level, which allows an operator to use a manual winch to open and close the vents. Motorized smoke vents are opened and closed with the simple push of a button.

Translucent Polycarbonate Covers

While most automatic smoke vents feature covers that are solid aluminum or steel, translucent polycarbonate covers can be selected to allow daylight into the building, while the covers remain closed and weathertight. This change in material effectively transforms the smoke vent into a fire-responsive skylight designed to resist ultraviolet (UV) degradation and offer superior thermal performance. Allowing daylight into the floor plan can reduce energy costs and create a more pleasant working environment for building occupants. These solutions are often specified in manufacturing facilities, large atriums, and even schools.

Acoustical Sound-Rated Covers

One of the latest innovations in automatic smoke vent design is improved acoustical sound-rating performance. These acoustical smoke vents guard against noise intrusion when the cover is in the closed position, preventing outside noise from traveling into the interior, while supporting fire and life-safety codes in the event of a fire. The enhanced acoustic performance is the result of a new smoke vent design that incorporates mineral wool insulation and high-density sound-mat material within the unit’s covers and curb. In addition, the acoustical smoke vents feature a heavy-gauge steel construction, custom weather gasketing, gas-spring lift assistance, and a center-mounted gas traction spring to ensure reliable and controlled smoke vent operation. These sound-rated solutions are ideal for concert halls, theaters, and other interior applications that require limited noise intrusion from the outside.

Smoke vents with sound-rated acoustical covers (shown here) feature mineral wool insulation and high-density sound-mat material in the unit’s cover that prevent outside noise from traveling into the interior.

“As a general rule, sound is managed or modified by the mass of the material or assembly through which it passes. The more mass an object has, the greater its ability will be to reduce noise energy. When you look at wall assemblies of the building envelope, they are often comprised of several layers—the finished exterior surface, metal studs, framing, drywall, insulation, an air cavity. These are often sufficient to manage sound, but the doors and windows are relatively lightweight in comparison, and these elements are often the weak spots that are vulnerable to sound penetration. The same concept applies to a roof—the greater the mass of the structure, the better it will manage sound. For example, a concrete roof deck will, in most cases, block sound better than a metal roof deck. Openings in the roof, like the openings that are created for smoke vents, could provide a path that would allow sound to travel into the interior. Architects concerned with sound control will want to make sure that the smoke vent products they select have enough mass in the lid to be comparable to the roof in which they are installed,” Merck says.

Today, sound-rated smoke vents have achieved industry-high sound ratings of STC 50 and OITC 46 to effectively match the sound-rating performance of roof assemblies without penetrations. To fully understand what this means, we will need to briefly explore the basics of sound and the common metrics used to describe the soundproofing effectiveness of a material or assembly.

The Basics of Sound

When looking to control the transmission of sound, both the frequency and decibel level of the sound must be considered. The frequency describes how fast the sound vibrations are moving and is measured in Hertz (Hz). The hearing range of the human ear is generally defined as 20–20,000 Hz. Sounds are typically divided into high-, middle-, and low-frequency categories. Low-frequency sounds are often experienced as a rumble. Bass, thunder, or a tuba are all examples of low-frequency sounds. Middle-frequency sounds often fall in the 500–2000 Hz range and include sirens and human speech. High-frequency sounds start at 2000 Hz. Birds chirping or music from a piccolo are good examples of high-frequency sounds.

The topic of frequency is important when discussing sound management because sounds at different frequencies move through materials differently. While higher frequencies may be effectively absorbed by a certain material, lower frequencies may easily pass through it. In general, very-high- and very-low-frequency sounds are more difficult to attenuate.

The volume of sound is also an important quality to understand when managing environmental noise. Measured in decibels (dB), soft sounds, like a whisper, typically register at 15–20 dB, while a jet engine creates sound at 150 dB. It is important to note that the decibel scale is not linear, but logarithmic. This means that doubling the decibel reading of a sound, say from 15 dB to 30 dB, does not double the noise. Instead, for every 10 dB increase in sound level, the human ear will perceive the loudness as doubled. For example, a sound at the 60 dB level is perceived to be twice as loud as a 50 dB level.

The common environmental sounds that a building facade must be designed to manage can be quite loud. A blog post published by the American National Standards Institute (ANSI) titled "How Loud Is Construction Site Noise?" shared that most construction sounds fell in the range of 80–90 dB. For context, prolonged exposure to sounds over 85 dB or higher can cause damage to the ear. Automobile traffic is one of the main contributors to noise pollution and can generate a lot of noise inside a building. According to CityandStateNY.com, traffic in midtown Manhattan creates between 70 and 85 dB of noise. The sound of a motorcycle can reach 88 dB and trains 100 dB.

Understanding the frequency and decibel level of the environmental noise found at a project site is the first step in creating a building envelope that can effectively manage that noise.

 

[ Page 2 of 5 ]        
Originally published in Architectural Record
Originally published in August 2020

Notice

Academies