Building Science

Sustainable and Resilience—Are they related, the same, or opposites?

By Thomas J Taylor

October 08, 2018

The words sustainable and resilience juxtaposed against a glass building.

Most people in the built environment community are aware that using and specifying sustainable and resilient materials and designs is "the right thing to do." There has been and continues to be industry discussions about the environment and climate change; sustainability and resilience are sometimes just assumed to address environmental and climate change concerns in ways that are "good." But sometimes, these words are used without really appreciating whether or not they are essentially the same, different, or in some way linked.

The built environment represents a large consumer of resources, both in terms of materials and energy. The U.S. Energy Information Administration estimates that residential and commercial/industrial buildings consume 40% of the energy used in the U.S. Of that, the majority is used for heating, air conditioning, ventilation, and lighting. Non-energy resource use, including material use, construction waste, operating resources such as water, and end of life demolition waste is far harder to estimate, but is significant. However, the construction market is estimated to be $1.231 trillion for 2018, versus a total gross domestic product of $20.412 trillion. This means the construction market represents around 6% of the U.S. economy.

Residential and commercial/industrial buildings consume 40% of the energy used in the U.S.

Due to the impact of the built environment on both energy use and resource consumption, it's important to reach a better understanding of sustainability and resilience. Are these terms the same, complimentary, or independent? This article will examine each in turn and then compare the two. As will be seen, they are not the same and, sometimes, can conflict in ways that are counter to goals generally regarded as "good."


This adjective generally means:

  • Able to be used without being completely used or destroyed.
  • Involving methods that do not completely use up or destroy natural resources.
  • Able to last or continue for a long time.

Sustainability (the noun) is the capacity for:

  • Human health and well being
  • Economic vitality and prosperity
  • Environmental resource abundance

So, the building designer who wishes to incorporate sustainability into buildings might ask:

  • Are materials safe for humans and the ecosystem?
  • Is this design energy and resource-efficient?
  • Is a material available or will its use today cause a shortage in the future?


This noun generally means:

  • An ability to recover from or adjust easily to misfortune or change.
  • The ability to become strong, healthy, or successful again after something bad happens.

Putting this in terms of the built environment, resilience is the capacity to:

  • Overcome unexpected problems.
  • Continue or rapidly bounce back from extreme events.
  • Prepare for and survive catastrophes.

So, key questions for the building designer are:

  • Can a structure be occupied and functional after a severe storm or some other extreme environmental event?
  • Will occupants be able to function in the absence of utilities?
  • What reduction in occupational capacity is acceptable after an extreme environmental event?

Sustainable versus Resilient

To compare the two terms, it's instructive to examine some design choices. The schematic below shows some examples of choices that represent more or less of the two characteristics:

The following is a discussion of some of the examples shown in each of the quadrants:

Less Sustainable / Less Resilient

In the lower left corner, "Grid Only Power" refers to a building's reliance on grid electricity supply. The national grid gets about 17% of its power from renewable sources such as wind and solar. The remainder is from fossil fuels and nuclear and therefore today's grid power, while improving, can be regarded as less sustainable versus, for example, a grid that was essentially all supplied from renewable resources. From a building use perspective, grid-only power is likely the least resilient choice of energy since a building's power could be cut not just by an event locally but anywhere within the region. Properly designed and installed solar energy with storage, as indicated in the upper-right quadrant of the schematic, would be more resilient.

More Sustainable / Less Resilient

In the upper-left corner, "Zero Waste to the Landfill" typically refers to the production of a material that does not result in any waste being landfilled. Instead, any manufacturing scrap is recycled either back into the product or into other materials. The term can also refer to construction practices that recycle all waste. Eliminating waste by recycling is generally regarded as a sustainable endeavor but in no way does it impact a building's resilience in the face of future extreme events.

Less Sustainable / More Resilient

In the lower-right corner, "Large Reserves of Bottled Water" refers to a planning approach aimed at withstanding a loss of water due to some extreme weather event. Such water reserves could indeed be invaluable during such an outage, but plastic bottles are generally not viewed as being a sustainable choice. It's worth noting that while the loss of water might be viewed as a very unlikely possibility, it occurred in high buildings throughout New York City in the aftermath of Superstorm Sandy in 2012. In tall buildings, the water supply to higher floors and sometimes the entire building is totally reliant on electric pumps. Therefore, loss of grid power quickly leads to a loss of water supply.

More Sustainable / More Resilient

In the upper-right hand corner, "Solar with Storage" refers to the use of solar panels coupled with battery power storage to withstand temporary loss of grid power. Solar power is widely regarded as being more sustainable than fossil-fueled power plants, for example. However, solar power systems typically shut down during a grid outage to prevent the system from attempting to power the neighborhood. The addition of battery storage enables a system that would shut off from the grid and provide uninterrupted power for some time after the loss of grid power. The degree of resilience would depend on many factors including the size of the solar array, amount of battery storage, the power usage within the building, etc.

Sustainable versus Resilient — Conclusion

It should be clear from the discussion so far, that sustainable and resilient are two different considerations and are independent of each other. Simply because a material or design is sustainable doesn't mean that it is resilient and vice versa.

Simply because a material or design is sustainable doesn't mean that it is resilient and vice versa.

The following section examines sustainable and resilient choices for low-slope roofing.

Sustainable Roofing

Discussions about sustainable roofing are mostly about the materials used. Some materials used in the past, such as asbestos insulation, would be regarded as unsustainable given their potential for harm. But, given today's wide array of choices, how is a building professional able to make decisions as to whether a particular material is sustainable or not? There are many opinions and options, but LEED® v.4 provides a common baseline that can be useful in making decisions. LEED® contains a Building Product Disclosure & Optimization (BPD&O) section that encourages:

  • Use of products having an Environmental Product Declaration (EPD), this being an internationally accepted, verified, and published report focusing on the ways in which a product affects the environment throughout its life cycle.
  • A publicly available report from raw material suppliers which includes raw material supplier extraction locations, a commitment to long-term ecologically responsible land use, a commitment to reducing environmental harms from extraction and/or manufacturing processes, and a commitment to meeting applicable standards or programs voluntarily that address responsible sourcing criteria.
  • Disclosure of the material ingredients through documentation such as Health Product Declarations (HPD).

In addition, as part of the LEED® Building Design & Construction — Sustainable Site section, there are credits given for Heat Island Reduction through the use of cool roofing or reflective membranes.


The following table summarizes the status of the major types of low-slope roofing membranes in terms of BPD&O and cool roof status:

BPD&O - Building Product Disclosure & Optimization

EPDM, while a popular membrane, is generally not used as a white version due to its added cost. Also, when the market share of the various membrane types is considered, as in the following chart, it's apparent that TPO should be examined closely for its sustainable characteristics.

The chart shows a projection over the coming years and it is clear that TPO has become the dominant low-slope membrane. Also, as older TPO roofs come up for replacement, it's possible that the long-term trend towards TPO will accelerate. Examining TPO in terms of sustainability shows:

  • TPO typically doesn't contain any "red list" materials.
  • TPO can be obtained with up to 35-year warranties, depending on type, installation, thickness, and manufacturer. Materials with long life cycles are widely regarded as being more sustainable.
  • TPO can potentially be recycled at the end of its useful life as a roof membrane.


Since insulation is used to reduce long-term energy demand for heating and air conditioning, its use is generally considered sustainable. The status of the major insulation materials used in low-slope roofing in terms of BPD&O is shown in the following table:

BPD&O — Building Product Disclosure & Optimization. Certification and documentation depends on individual manufacturer.

As for membranes, it is instructive to examine the market share of each of these materials, as shown:

In examining polyiso more closely, it's apparent that it is generally a good choice especially when the non-halogen version is specified. The material has a low water vapor permeance, is thermoset (i.e., doesn't melt during fires), and isn't negatively affected by solvents.

Balance of System Any roof assembly consists of more than the membrane and insulation. The additional components necessary to complete any roofing system, depending on exact design, are discussed as follows:

Cover Boards — the use of gypsum boards can provide LEED¬Æ Materials and Resources credit. Similarly, high-density polyiso boards, when specified in the non-halogen version can provide LEED¬Æ BPD&O credit.

Adhesives — traditional solvent-based adhesives are still used in many regions, but tightening environmental regulations have led to increased use of water-based and low-VOC (volatile organic content) materials. There are few available with an EPD or HPD, however water-based and low VOC materials are regarded as being more sustainable versus solvent-based materials.

Fasteners — in terms of overall content, fasteners are a very small proportion of a roofing assembly and many might consider them to have an insignificant contribution to sustainability. However, in the case of coated plates that allow inductive heating attachment of the membrane, this enables most of the benefits of fully adhered systems to be achieved without the use of adhesives.

Resilient Roofing Sustainable roofing is mainly focused on material choices:

  • Content / how they are manufactured
  • Life cycle
  • Length of use / Durability

However, resilient roofing is concerned with how those materials are used together in a roof design to better protect against large and severe deviations from normal weather patterns. Such events can be:

  • High Wind / Storms — wind and storms of above-normal expectations require roof assemblies with enhanced wind uplift resistance. This requires care with design details as well as a good understanding of wind load fundamentals and design basics. As high wind events and storms increase in severity, it is worth considering the use of known designs that improve wind uplift resistance. An example would be specifying fully adhered membranes versus mechanical attachment.
  • Hail — data from the U.S. Department of Commerce shows hail damage to property as being economically very significant:

While long-term trends are debatable, there is a sense that a combination of changing climate combined with increased population densities in areas at risk are leading to a long-term trend toward more claims. Studies have shown that more conservative single ply roof assemblies, consisting of fully adhered fleece-back membrane over high-density polyiso cover boards that are also adhered can lower the risk of ice ball impacts. Furthermore, TPO designed for longer-term weather resistance may provide longer-term protection against impact.

  • Loss of Heat — grid power brownouts and outages due to weather-related incidents have been trending upwards, according to the U.S. Energy Information Administration.

Electrical power outages, in turn, lead to a loss of building heat or air conditioning which can reduce or prevent functional business operations in affected buildings. From a low-slope roof design perspective, insulation and air barrier requirements can help reduce energy loss during power outages and thereby make a building more resilient. Insulation can be specified to go beyond code requirements but even seemingly minor changes in installation can improve its efficiency. For example, always making sure that insulation is installed in two layers with staggered joints helps reduce thermal loss through gaps. Also, specifying fully adhered membranes and adhering the top layer of insulation can reduce thermal bridging.

Sustainable Versus Resilient Roofing — in Conclusion

Sustainable roofing is mostly about material choices. Of today's common materials, TPO roofing membranes and non-halogen polyiso insulation are generally accepted as meeting sustainability requirements. For the balance of system materials, non-halogen high-density polyiso and gypsum cover boards also are considered sustainable materials. For securement, low-VOC and water-based adhesives are available as replacements for traditional solvent-based systems.

Resilient roofing enhances the ability of buildings to continue to function after extreme weather events.

Resilient roofing enhances the ability of buildings to continue to function after extreme weather events. It is mainly the roof assembly design that determines resiliency of the roof together with workmanship and quality of installation. As discussed in this article, there are many known designs available to improve on resiliency beyond a basic system that simply meets code requirements. Such systems can include fully adhered systems to increase wind uplift resistance , the use of fully adhered fleece-back membranes and adhered high-density polyiso cover boards to improve protection against impact, and increased insulation to help reduce heat gain/loss during power outages.

As a final note, it may be worth considering that many would argue that climate change might induce more extreme weather challenges than are planned for today. It might be important to consider adaptation strategies to improve resilience of the built environment toward future climate changes. Adaptation will be discussed in a future article. NOTE: This blog is for information purposes only. GAF does not provide professional design services. You should always consult with a design professional to determine whether the roofing system to be installed is suitable for the particular needs of your building.

Note: LEED®—an acronym for Leadership in Energy and Environmental Design™—is a registered trademark of the U.S. Green Building Council.

About the Author

Thomas J Taylor, PhD is the Building & Roofing Science Advisor for GAF. Tom has over 20 year’s experience in the building products industry, all working for manufacturing organizations. He received his PhD in chemistry from the University of Salford, England, and holds approximately 35 patents. Tom’s main focus at GAF is roofing system design and building energy use reduction. Under Tom’s guidance GAF has developed TPO with unmatched weathering resistance.

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Are Hybrid Roof Assemblies Worth the Hype?

How can roofing assemblies contribute to a building's energy efficiency, resiliency, and sustainability goals? Intentional material selection will increase the robustness of the assembly including the ability to weather a storm, adequate insulation will assist in maintaining interior temperatures and help save energy, and more durable materials may last longer, resulting in less frequent replacements. Hybrid roof assemblies are the latest roofing trend aimed at contributing to these goals, but is all the hype worth it?What is a hybrid roof assembly?A hybrid roof assembly is where two roofing membranes, composed of different technologies, are used in one roof system. One such assembly is where the base layers consist of asphaltic modified bitumen, and the cap layer is a reflective single-ply membrane such as a fleece-back TPO or PVC. Each roof membrane is chosen for their strengths, and together, the system combines the best of both membranes. A hybrid system such as this has increased robustness, with effectively two plies or more of membrane.Asphaltic membranes, used as the first layer, provide redundancy and protection against punctures as it adds overall thickness to the system. Asphaltic systems, while having decades of successful roof installations, without a granular surface may be vulnerable to UV exposure, have minimal resistance to ponding water or certain chemical contaminants, and are generally darker in color options as compared to single ply surfacing colors choices. The addition of a single-ply white reflective membrane will offset these properties, including decreasing the roof surface temperatures and potentially reducing the building's heat island effect as they are commonly white or light in color. PVC and KEE membranes may also provide protection where exposure to chemicals is a concern and generally hold up well in ponding water conditions. 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Building Science

Thermal Bridging Through Roof Fasteners: Why the Industry Should Take Note

What is going on here?No, this roof does not have measles, it has a problem with thermal bridging through the roof fasteners holding its components in place, and this problem is not one to be ignored.As building construction evolves, you'd think these tiny breaches through the insulating layers of the assembly, known as point thermal bridges, would matter less and less. But, as it happens, the reverse is true! The tighter and better-insulated a building, the bigger the difference all of the weak points, in its thermal enclosure, make. 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Likewise, The National Energy Code of Canada for Buildings: 2020 addresses thermal bridging of a number of building components, but also explicitly excludes fasteners: "in calculating the overall thermal transmittance of assemblies…fasteners need not be taken into account" (Section Admittedly, point thermal bridges are often excluded because it is challenging to assess them with simple simulation tools.Despite this, researchers have had a hunch for decades that thermal bridging through the multitude of fasteners often used in roofs is in fact significant enough to warrant study. Investigators at the National Bureau of Standards, Oak Ridge National Laboratory, the National Research Council Canada, and consulting firms Morrison Hershfield and Simpson Gumpertz & Heger (SGH), have conducted laboratory and computer simulation studies to analyze the effects of point thermal bridges.Why Pay Attention Now?The problem has been made worse in recent years because changes in wind speeds, design wind pressures, and roof zones as dictated by ASCE 7-16 and 7-22 (see blogs by Jim Kirby and Kristin Westover for more insight), mean that fastener patterns are becoming denser in many cases. This means that there is more metal on average, per square foot of roof, than ever before. More metal means that more heat escapes the building in winter and enters the building in summer. 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Extra insulation beyond the code minimum can be specified to make up the difference.Where Do We Go From Here?Some work remains to be done before we have a computer simulation that more closely aligns with physical experiments on identical assemblies. But, the two methods in our recent study aligned within a range of 0.8 to 6.7%, which indicates that we are making progress. With ever-better modeling methods, designers should soon be able to predict the impact of fasteners rather than ignoring it and hoping for the best.Once we, as a roofing industry, have these detailed computer simulation tools in place, we can include the findings from these tools in codes and standards. These can be used by those who don't have the time or resources to model roof assemblies using a lab or sophisticated modeling software. 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Defending Against Very Severe Hail

Think that your roof doesn't need protection against hail? Think again.Severe hail events are increasing in geographic footprint and are no longer just in hail alley. The geographic region that experiences 1 inch or larger hailstones has expanded to be nearly two-thirds of the United States. Nearly 10 percent more U.S. properties, more than 6.8 million, were affected by hail in 2021 than in 2020. Coinciding with the increase in properties affected by a damaging hail event in 2021, there was also an increase in insurance claims, which rose to $16.5 billion from $14.2 billion in 2020.Figure 1: The estimated number of properties affected by one or more damaging hail events. Source: NOAA, graphed by VeriskAccording to data from Factory Mutual Insurance Company (FM Global), a leader in establishing best practices to protect buildings, the review of client losses between 2016-20, showed that the average wind/hail losses averaged $931,000 per event. 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Loss Prevention Data Sheets provide FM's best advice for new construction and for Data Sheet 1-34, this includes new or reroofing projects on existing buildings. Data Sheet 1-34 provides guidelines to minimize the potential for hail damage to buildings and roof-mounted equipment. FM Global intends that the data sheets apply to its insured buildings; however, some designers use data sheets as design guidelines for buildings other than those insured by FM Global.FM's LPDS 1-34 identifies the hail hazard areas across the United States: Moderate Hail hazard area, Severe Hail hazard area, and Very Severe Hail (VSH) hazard area which are defined by hail size. Note that the VSH area roughly correlates to Hail Alley. Hail Alley receives more hailstorms, and more severe ones, compared to other parts of the country.Figure 2: FM's LPDS 1-34 map outlining the different hail categories: moderate, severe, and very severe. The Very Severe area is most commonly referred to as "Hail alley".The hail hazard areas are divided by hail size, with the Very Severe hail hazard area being the largest hail size of greater than 2 inches. As a result, roofing assemblies have to meet the most stringent hail testing for designation in the Very Severe hazard area.Figure 3: Description of FM Approval hail regions.Even if you are not in hail alley, or one of the states in FM's Very Severe Hail area, hail larger than 2 inches still has the potential to occur throughout the contiguous United States. The National Oceanic and Atmospheric Administration (NOAA) tracks weather events throughout the United States, including hail. NOAA's hail database includes information such as location, date, and magnitude (size) of the hail stone for each event. A sampling of typical data is provided below; note that several states that are outside of FM's VSH zone, had hail events that would qualify as VSH, where hail stones were recorded to be larger than 2-inches in size.Figure 4: Hail events in states that are outside of the VSH area, but qualify as VSH by size.How Do I Design For Very Severe Hail?In order for a roof assembly to achieve a hail rating, the assembly must pass a hail test. FM Approvals designs the hail tests including a different test for each hail hazard area. Hail testing generally includes the use of steel or ice balls that are dropped or launched at roof assemblies in a laboratory setting. Pass criteria vary by the test, but generally visual damage cannot be present to either the membrane or components below. Roof assemblies that pass each individual hail test are FM approved to be installed in each hail hazard area.There are thousands of FM rated assemblies and it can be difficult to choose just one. To start, it is important to note that selection consists of an entire assembly, however consideration of all roof components including the membrane, coverboard, and attachment method each play an important role in how the assembly defends against hail.Membrane selection is critical for Very Severe Hail prone regions. Thicker roof membranes, as well as higher performance grades that will remain pliable under heat and UV exposure over time and will outperform standard grade materials. Fleeceback membranes also provide an added cushion layer that buffers hail impact.Coverboard selection is a critical component of the roof system design. High compressive strength coverboards are an effective means to enhance the performance of the roof system when exposed to hail events. A coverboard will enhance the roof's long term performance by fortifying the membrane when hail strikes as well as providing a firm surface to help resist damage from typical foot traffic. It will also help the roof insulation below withstand damage from hail. While conventional gypsum coverboards and high-density polyiso coverboards provide excellent protection against foot traffic and smaller hail, they are not effective for VSH. Coverboards for VSH systems were originally limited to plywood or oriented strand board (OSB). The use of plywood and OSB is very labor intensive to install as compared to traditional gypsum coverboards, increasing the cost of the installation. Recently, coverboard manufacturers have developed glass mat roof boards which are a reinforced gypsum core with a heavy-duty coated glass mat facer. Not only do these boards provide protection against 2-inch hail and are an important part of VSH assemblies, they are also a FM Class 1 and UL Class A thermal barrier for fire rated assemblies. These boards are 5/8" thick and are 92-96 pounds per 4'x8' board; about 30 percent heavier compared to plywood yet easier to install as they can be scored and cut like a traditional gypsum board.Consideration of roof attachment method is critical for selection of VSH assemblies. Historically, mechanically attached systems were not able to pass the VSH tests; when an ice ball hit the head of the fastener or plate, the result was a laceration in the membrane. To avoid failures of the membrane at the fasteners and plates, the fasteners were traditionally buried in the system; the insulation was mechanically attached and the coverboard and membrane were adhered. This is still a common installation method and as a result, there are a large number of assemblies where the membrane and coverboard are adhered. Additionally, burying the fasteners allows for the installation of a smooth backed membrane. With the development of glass mat coverboards, there are VSH rated assemblies that can be simultaneously fastened (mechanically attached coverboard and insulation) that utilize an adhered fleece-back membrane.Figure 5: VSH systems. Left is simultaneously fastened 60 mil Fleeceback TPO over glass mat VSH roof board and Polyiso Insulation. Right is 60 mil Fleeceback TPO over glass mat VSH roof board adhered in low rise foam ribbons to mechanically attached Polyiso Insulation.Figure 6: A sample of available VSH assemblies.SummaryWhy Should We Design for VSH?Severe hail events are increasing in geographic footprint and storms with hailstones that meet Very Severe Hail criteria are occurring throughout the country. While designing for VSH is a requirement if a building falls within the VSH area and is ensured by FM Global, many owners and designers are opting for roof assemblies that can withstand VSH storms even if they are not insured by FM Global. Material selection, such as coverboard and membrane, are key components to managing this risk. Glass mat coverboards and thicker, higher grade single-ply membranes, such as fleece-back, increase the roof assembly's resistance to damage. Choosing the right roof assembly could be the difference between weathering the storm or significant damage from hail.What are the next steps?Learn about GAF's Hail Storm System Resources, and as always, feel free to reach out to the Building & Roofing Science team with questions.

By Authors Kristin Westover

January 30, 2023

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