by sadia_badhon | September 4, 2019 10:08 am
by Bert Slone
What constitutes a ‘mission critical’ building? How do the various parts of the building enclosure, particularly the roof, present challenges for insulating materials? What considerations should be taken into account when conducting cost/benefit analysis for mission critical buildings? Inspired by building science, insights from commercial construction in Europe, and feedback from architects, cellular glass is being re-introduced as an approach for insulating mission critical enclosures, specifically in commercial rooftop applications.
As every building has a mission or purpose to fulfill, a mission critical building for this article will be any facility with high-value processes, where a disruption in operations would impose significant human productivity or financial costs. Today’s mission critical buildings protect functions ranging from data processing and food manufacturing to national security. Research laboratories are carrying out scientific work where any interruption could discredit findings. Seamless communication transmission is vital for businesses involved in cyber security. Historic documents and ancient artifacts cannot be replaced should a leak occur in a roof. In mission critical buildings, a disruption in operations can threaten security, financial performance, productivity, and even human life. When viewed through this lens, it becomes evident many projects could be classified as mission critical.
The identification of mission critical building occupancies is addressed in Chapter 16 of the International Building Code (IBC). Table 1604.5 ‘Risk Category Designations’ identifies the risk category of buildings and classifies various code-defined occupancies housed in structures. For example, buildings housing critical communications, or functions essential to public welfare (e.g. water, electricity), or hospitals serving patients unable to egress a building on their own, all fall into the higher-value categories. These buildings are designed with additional factors of safety, or ‘redundancy’ to make them more ‘fail resistant,’ as well as to protect against weather and water.
Redundancy supports reliability
Across all types of mission critical facilities reliability is a common denominator. Redundant systems that provide a back-up level of protection against failure are a key component of mission critical enclosures. In such buildings, the consequences of failure are costly, and thus, a high level of risk-benefit analysis is required. Architects must compare the economic cost of a building failure with the higher price of protective materials and systems.
The costs associated with a failure in the future should also be considered. The passage of time eventually stresses building materials, such as the roofing membrane protecting against moisture intrusion. Future remediation may be prohibitive in terms of logistics and economics. For example, in congested urban areas, the pathways and staging areas necessary to access and renovate or re-roof a building may be restricted, thereby, increasing the cost of the renovation. Hence, specifying a more expensive but durable building material at the outset of a project could be cost-effective in the long run.
In all buildings, the roof is fundamental in protecting against water intrusion. In mission critical facilities, a leak in the roofing membrane can present very devastating consequences in terms of lost time and money. Therefore, a resilient, reliable, and redundant system is needed to protect in the inevitable event of a roofing membrane failure.
Long used in Europe’s commercial buildings, cellular glass is well-suited to deliver redundancy and back-up protection for commercial roofs. While architects with considerable tenure in the industry may recall working with cellular glass in the 1970s and ’80s, the introduction of foam plastics in the late 1980s shifted the nature of materials specified for commercial roofs. Foam plastics became the de facto standard for commercial roofs over the past three decades. Recent innovations in the cellular glass manufacturing process have resulted in an increased R-value from 3.66 to 4 per inch. The re-launch of cellular glass in the United States is reintroducing architects to a once-popular insulating material.
Although cellular glass is being relaunched in the United States, there are thousands of existing assemblies throughout the nation that include rigid cellular glass with a variety of membranes. Conversations with architects show that the U.S. market is embracing the concept, similar to European architects’ longstanding affinity for cellular glass and its performance properties.
Cellular glass insulation has a unique chemistry, and a special manufacturing process is used to produce it (Figure 1). Sand is the primary ingredient used in the manufacturing of cellular glass, along with limestone, soda ash, etc. The ingredients are melted into a molten glass, which is then cooled. The cooled glass, called cullet, is mixed with carbon black and crushed into a fine powder. The powdered mixture is then heated in an oven to cause a chemical reaction. This results in a glass matrix of insulating ‘bubbles’ similar to how baking soda makes a cake to rise. The finished product comprises of millions of completely sealed glass cells offering a number of performance benefits. ASTM C552, Standard Specification for Cellular Glass Thermal Insulation, defines the basic physical property requirements of cellular glass insulation. Performance benefits associated with cellular glass as an insulating material are noted below.
Impervious to moisture
The 100 percent glass nature of cellular glass makes it impervious to moisture and therefore makes it a suitable material to protect against water infiltration in liquid or vapor form. As a high performance material, cellular glass is always used in conjunction with a high-quality membrane. Therefore, even if a large enough point of impact crushes a layer of brittle foam glass, the membrane provides an additional layer of protection.
As a glass, it neither contains fuel content, nor burns, spreads fire, or presents a fire risk in the structure.
Cellular glass insulation consists of pure glass with a low coefficient of thermal movement, comparable to concrete and steel. This level of stability means there is no warping, dishing, or shrinking of the insulation, even as the temperature fluctuates over seasons. Used in commercial building roofs, cellular glass provides a stable foundation for the roofing membrane, minimizing the stress arising from the constant flexing that causes a material to deteriorate over time. From a flexibility and ease of installation viewpoint, cellular glass can also be tapered to address various drainage objectives and is incredibly easy to cut.
From an installation perspective, cellular glass is typically seated in an asphalt base and joints are also sealed with the same. Unlike foam plastic, cellular glass is entirely resistant to heat and tolerates solvents and bitumen. Cellular glass can also be sealed to functionally serve as a secondary waterproofing membrane. The key benefit for mission critical buildings is, as a roofing membrane degrades over time, the cellular glass provides a secondary layer to stop water infiltration.
The lowest compressive strength for cellular glass is in the range of 345 to 483 kPa (50 to 70 psi), and can be much higher without deflection under load. Contrastingly, the compressive strength of foam plastics typically tops out in the 345 to 483 kPa range. The compressive strength range allows cellular glass to support high loads without deflection or movement, even under sustained stress. Applications well-suited for cellular glass insulation include rooftop plazas that support heavy equipment, traffic, and parking.
Resistance to acid and chemicals
Naturally inert, cellular glass is unaffected by chemically aggressive agents or environments where these exist. The pure glass composition does not react with organic solvents and acids.
Cellular glass delivers ecological benefits as it is manufactured using more than 60 percent recycled glass. Completely inorganic, cellular glass does not contain ozone-depleting propellants, flame retardants, or binders, and is free of volatile organic compounds (VOCs).
Resistant to infestation
As an inert insulating material, cellular glass does not support the growth of microorganisms or bacteria and cannot rot. It is resistant to pests such as insects and vermin.
Cellular glass for roof replacements
In addition to new construction, cellular glass is a long-lasting solution for roof replacements. The James W. Jardine Water Filtration Plant adjacent to the Navy Pier in Chicago, Illinois, provides an example of cellular glass used in a re-roofing project. In 2014, the 50-year-old graveled coal tar pitch roof was demolished and replaced with 712,000 board feet of cellular glass insulation that protected the thermoplastic membrane. Of significant note is that the demolition was not the result of the failure of cellular glass, but rather the deck support structure had deteriorated from years of exposure to treatment tanks within the building and the roof had to be removed to replace the structure. The resulting roof stands up to the Windy City’s weather extremes, as well as chemical agents, sunlight, and the acidic deposits of flocks of birds congregating on the rooftop. The Eugene Sawyer Water Purification Plant, also in Chicago, will be going through the same reconstruction beginning later this year.
Solutions beyond the roof
The properties that make cellular glass an insulating material for commercial roofing applications also address performance issues confronting architects in other parts of the enclosure, including walls and below-grade areas. For example, the compressive strength of cellular glass coupled with its R-value allows the material to serve as a connector between the roof layer and the continuous insulation (ci) on the wall, thus making the insulation jacket continuous (Figure 2). The cellular glass provides not only the strength of a concrete block for weight-bearing purposes, but also the R-value required to meet insulating needs, which is impossible with concrete. The insertion of cellular glass interrupts the thermal bridge efficiently.
Cellular glass’s strength and thermal properties help it deliver both loadbearing and thermal-bridging mitigation in below-slab applications. These below-grade performance requirements cannot be achieved with foam plastics.
The performance benefits cellular glass brings to the walls of the enclosure also apply to below-grade areas. For example, brick veneer placed on a bearing shelf in order to transfer dead load into the foundation has been accepted as a necessary thermal bridge to accommodate load resolution into the foundation. There is a ci path and a load-resolution path in the enclosure. The two paths cannot cross without a ‘structural insulation’. The load path must always take priority. However, cellular glass offers the necessary strength to provide a structural solution permitting the ci path to cross over the load-resolution path. Figure 3 (page 40) shows how cellular glass insulation crosses the load resolution path, connecting the ci to provide uninterrupted insulation, and avoiding a thermal bridge from the brick veneer into the foundation.
The evolution of energy codes in the United States over the past 30 to 40 years have largely helped to improve the energy efficiency of the building envelope. Today’s internal loads are more efficient, and insulation R-value levels in walls and roofs are better. Thus, previously overlooked areas of thermal inefficiency are now receiving new scrutiny—and the industry is embracing micro-efficiencies to be able to deliver continuous improvements in energy efficiency. Precision improvements are becoming more important in energy-saving endeavors and cellular glass used in the enclosure can help optimize these efforts.
The installation of the material on a jobsite should also be considered. At a time when demand for construction labor is high, cellular glass insulation does not require any special skill set. It may be installed using hot asphalt, cold-applied asphalt adhesives, or two-part polyurethane adhesives—all methods familiar to roofing contractors.
As noted earlier, architects should perform a cost-benefit analysis to compare the potential costs of a failure against the higher cost of a building material. The performance benefits of cellular glass come with a cost premium compared to foam plastic insulation options. If an architect is specifying building materials purely based on material cost, cellular glass will not be an option. However, if an architect is looking for materials value addition—including the confidence, peace of mind, and potential savings that come with avoiding a roof leak in a mission critical building—cellular glass is suitable.
The functionality of cellular glass can replace other layers in the construction system such as vapor retarders, moisture barriers, and radon-control components. Cellular glass may also enhance longevity and deliver environmental attributes desired by building owners and occupants. Used under loadbearing slab applications, the rigidity of cellular glass might even make it possible to install a thinner or less heavily reinforced concrete slab.
From the commercial rooftop, throughout the enclosure’s walls, and in below-grade applications, cellular glass is providing architects with a proven option for insulating mission critical buildings.
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