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Ciencia de la construcción

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

08 de octubre de 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."

sustentable

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?

Resilience

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.

Membranas

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.

Aislantes

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 long-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

Thomas J Taylor, PhD trabaja para GAF como asesor de Ciencias de la Construcción y Techado. Tom tiene más de 20 años de experiencia en la industria de productos para la construcción y durante todo ese tiempo ha trabajado para compañías de fabricación. Obtuvo su PhD en Química en la Universidad de Salford, Inglaterra, y tiene aproximadamente 35 patentes. El enfoque principal de Tom en GAF es el diseño del sistema de techos y la reducción del consumo energético en las construcciones. Bajo la guía de Tom, GAF ha desarrollado la TPO con una resistencia inigualable a los factores climáticos.

¿Alguna vez has pensado en que los productos de construcción pueden reducir las emisiones de dióxido de carbono de tu edificio? When considered over their useful life, materials like insulation decrease total carbon emissions thanks to their performance benefits. Read on for an explanation of how this can work in your designs.What is Total Carbon?Total carbon captures the idea that the carbon impacts of buildings should be considered holistically across the building's entire life span and sometimes beyond. (In this context, "carbon" is shorthand for carbon dioxide (CO2) emissions.) Put simply, total carbon is calculated by adding a building's embodied carbon to its operational carbon.Total Carbon = Embodied Carbon + Operational CarbonWhat is Embodied Carbon?Embodied carbon is comprised of CO2 emissions from everything other than the operations phase of the building. This includes raw material supply, manufacturing, construction/installation, maintenance and repair, deconstruction/demolition, waste processing/disposal of building materials, and transport between each stage and the next. These embodied carbon phases are indicated by the gray CO2 clouds over the different sections of the life cycle in the image below.We often focus on "cradle-to-gate" embodied carbon because this is the simplest to calculate. "Cradle-to-gate" is the sum of carbon emissions from the energy consumed directly or indirectly to produce the construction materials used in a building. The "cradle to gate" approach neglects the remainder of the embodied carbon captured in the broader "cradle to grave" assessment, a more comprehensive view of a building's embodied carbon footprint.What is Operational Carbon?Operational carbon, on the other hand, is generated by energy used during a building's occupancy stage, by heating, cooling, and lighting systems; equipment and appliances; and other critical functions. This is the red CO2 cloud in the life-cycle graphic. It is larger than the gray CO2 clouds because, in most buildings, operational carbon is the largest contributor to total carbon.What is Carbon Dioxide Equivalent (CO2e)?Often, you will see the term CO2e used. According to the US Environmental Protection Agency (EPA), "CO2e is simply the combination of the pollutants that contribute to climate change adjusted using their global warming potential." In other words, it is a way to translate the effect of pollutants (e.g. methane, nitrous oxide) into the equivalent volume of CO2 that would have the same effect on the atmosphere.Today and the FutureToday, carbon from building operations (72%) is a much larger challenge than that from construction materials' embodied carbon (28%) (Architecture 2030, 2019). Projections into 2050 anticipate the operations/embodied carbon split will be closer to 50/50, but this hinges on building designs and renovations between now and 2050 making progress on improving building operations.Why Insulation?Insulation, and specifically continuous insulation on low-slope roofs, is especially relevant to the carbon discussion because, according to the Embodied Carbon 101: Envelope presentation by the Boston Society for Architecture: Insulation occupies the unique position at the intersection of embodied and operational carbon emissions for a building. Insulation is the only building material that directly offsets operational emissions. It can be said to pay back its embodied carbon debt with avoided emissions during the building's lifetime.A Thought Experiment on Reducing Total CarbonTo make progress on reducing the total carbon impact of buildings, it is best to start with the largest piece of today's pie, operational carbon. Within the range of choices made during building design and construction, not all selections have the same effect on operational carbon.When making decisions about carbon and energy reduction strategies, think about the problem as an "investment" rather than a "discretionary expense." Discretionary expenses are easier to reduce or eliminate by simply consuming less. In the example below, imagine you are flying to visit your client's building. Consider this a "discretionary expense." The input on the far left is a given number of kilograms of carbon dioxide equivalent (CO2e) generated for the flight, from the manufacturing of the airplane, to the fuel it burns, to its maintenance. The output is the flight itself, which creates CO2 emissions, but no durable good. In this case, the only CO2 reduction strategy you can make is to make fewer or shorter flights, perhaps by consolidating visits, employing a local designer of record, or visiting the building virtually whenever possible. Now consider the wallpaper you might specify for your client's building. It involves a discretionary expenditure of CO2e, in this case, used to produce a durable good. However, this durable good is a product without use-phase benefits. In other words, it cannot help to save energy during the operational phase of the building. It has other aesthetic and durability benefits, but no operational benefits to offset the CO2 emissions generated to create it. Your choices here are expanded over the previous example of an airplane flight. You can limit CO2 by choosing a product with a long useful life. You can also apply the three Rs: reduce the quantity of new product used, reuse existing material when possible, and recycle product scraps at installation and the rest at the end of its lifespan. In the final step in our thought experiment, consider the insulation in your client's building. As before, we must generate a certain amount of CO2e to create a durable good. In this case, it's one with use-phase benefits. Insulation can reduce operational energy by reducing heat flow through the building enclosure, reducing the need to burn fuel or use electricity to heat and cool the building. The good news is that, in addition to the other strategies considered for the flight and the wallpaper, here you can also maximize operational carbon savings to offset the initial embodied carbon input. And, unlike the discretionary nature of some flights and the often optional decision to use furnishings like wallpaper, heating and cooling are necessary for the functioning of almost all occupied buildings.Based on this example, you can consider building products with operational benefits, like insulation, as an "investment." It is appropriate to look at improving the building enclosure and understanding what the return on the investment is from a carbon perspective. As the comparison above demonstrates, if you have a limited supply of carbon to "invest", putting it into more roof insulation is a very smart move compared to "spending" it on a discretionary flight or on a product without use-phase carbon benefits, such as wallpaper.This means we should be careful not to measure products like insulation that save CO2e in the building use-phase savings only by their embodied carbon use, but by their total carbon profile. So, how do we calculate this?Putting It to the TestWe were curious to know just how much operational carbon roof insulation could save relative to the initial investment of embodied carbon required to include it in a building. To understand this, we modeled the US Department of Energy's (DOE) Standalone Retail Prototype Building located in Climate Zone 4A to comply with ASHRAE 90.1-2019 energy requirements. We took the insulation product's embodied energy and carbon data from the Polyisocyanurate Insulation Manufacturers Association's (PIMA) industry-wide environmental product declaration (EPD).To significantly reduce operational carbon, the largest carbon challenge facing buildings today, the returns on the investment of our building design strategies need to be consistent over time. This is where passive design strategies like building enclosure improvements really shine. They have much longer service lives than, for example, finish materials, leading to sustained returns.Specifically, we looked here at how our example building's roof insulation impacted both embodied and operational carbon and energy use. To do this, we calculated the cumulative carbon savings over the 75-year life of our model building. In our example, we assumed R-30 insulation installed at the outset, increased every 20 years by R-10, when the roof membrane is periodically replaced.In our analysis, the embodied CO2e associated with installing R-30 (shown by the brown curve in years -1 to 1), the embodied carbon of the additional R-10 of insulation added every 20 years (too small to show up in the graph), and the embodied carbon represented by end-of-life disposal (also too small to show up) are all taken into account. About five months after the building becomes operational, the embodied carbon investment of the roof insulation is dwarfed by the operational savings it provides. The initial and supplemental roof insulation ultimately saves a net of 705 metric tons of carbon over the life of the building.If you want to see more examples like the one above, check out PIMA's study, conducted by the consulting firm ICF. The research group looked at several DOE building prototypes across a range of climate zones, calculating how much carbon, energy, and money can be saved when roof insulation is upgraded from an existing baseline to current code compliance. Their results can be found here. Justin Koscher of PIMA also highlighted these savings, conveniently sorted by climate zone and building type, here.Support for Carbon Investment DecisionsSo how can you make sure you address both operational and embodied carbon when making "carbon investment" decisions? We've prepared a handy chart to help.First, when looking at lower-embodied-carbon substitutions for higher-embodied-carbon building materials or systems (moving from the upper-left red quadrant to the lower-left yellow quadrant in the chart), ensure that the alternatives you are considering have equivalent performance attributes in terms of resilience and longevity. If an alternative material or system has lower initial embodied carbon, but doesn't perform as well or last as long as the specified product, then it may not be a good carbon investment. Another consideration here is whether or not the embodied carbon of the alternative is released as emissions (i.e. as part of its raw material supply or manufacturing, or "cradle to gate" stages), or if it remains in the product throughout its useful life. In other words, can the alternative item be considered a carbon sink? If so, using it may be a good strategy.Next, determine if the alternative product or system can provide operational carbon savings, even if it has high embodied energy (upper-right yellow quadrant). If the alternative has positive operational carbon impacts over a long period, don't sacrifice operational carbon savings for the sake of avoiding an initial embodied product carbon investment when justified for strategic reasons.Last, if a product has high operational carbon savings and relatively low embodied carbon (lower-right green quadrant), include more of this product in your designs. The polyiso roof insulation in our example above fits into this category. You can utilize these carbon savings to offset the carbon use in other areas of the design, like aesthetic finishes, where the decision to use the product may be discretionary but desired.When designing buildings, we need to consider the whole picture, looking at building products' embodied carbon as a potential investment yielding improved operational and performance outcomes. Our design choices and product selection can have a significant impact on total carbon targets for the buildings we envision, build, and operate.Click these links to learn more about GAF's and Siplast's insulation solutions. Please also visit our design professional and architect resources page for guide specifications, details, innovative green building materials, continuing education, and expert guidance.We presented the findings in this blog in a presentation called "Carbon and Energy Impacts of Roof Insulation: The Whole[-Life] Story" given at the BEST6 Conference on March 19, 2024 in Austin, Texas.References:Architecture 2030. (2019). New Buildings: Embodied Carbon. https://web.archive.org/web/20190801031738/https://architecture2030.org/new-buildings-embodied/ Carbon Leadership Forum. (2023, April 2). 1 - Embodied Carbon 101. https://carbonleadershipforum.org/embodied-carbon-101/

¿Cómo pueden los armados de techos contribuir a los objetivos de eficiencia energética, resiliencia y sostenibilidad de un edificio? La selección deliberada de materiales aumentará la solidez del armado, incluida la capacidad de soportar una tormenta; el aislante adecuado ayudará a mantener la temperatura interior y a ahorrar energía; y los materiales más duraderos pueden tener una mayor vida útil, lo que causará que los reemplazos sean menos frecuentes. 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. The combination of an asphaltic base below a single-ply system increases overall system thickness and provides protection against punctures, which are primary concerns with single-ply applications.Pictured Above: EverGuard® TPO 60-mil Fleece-Back MembraneOlyBond 500™ AdhesiveRUBEROID® Mop Smooth MembraneMillennium Hurricane Force ® 1-Part Membrane AdhesiveDensDeck® Roof BoardMillennium Hurricane Force ® 1-Part Membrane AdhesiveEnergyGuard™ Polyiso InsulationMillennium Hurricane Force ® 1-Part Membrane AdhesiveConcrete DeckPictured Above: EverGuard® TPO 60-mil Fleece-Back MembraneGAF LRF Adhesive XF (Splatter)RUBEROID® HW Smooth MembraneDrill-Tec™ Fasteners & PlatesDensDeck® Prime Gypsum BoardEnergyGuard™ Polyiso InsulationEnergyGuard™ Polyiso InsulationGAF SA Vapor Retarder XLMetal DeckWhere are hybrid roof assemblies typically utilized?Hybrid roof assemblies are a common choice for K-12 & higher education buildings, data centers, and hospitals due to their strong protection against leaks and multi-ply system redundancy. The redundancy of the two membrane layers provides a secondary protection against leaks if the single-ply membrane is breached. Additionally, the reflective single-ply membrane can result in lower rooftop temperatures. The addition of a reflective membrane over a dark-colored asphaltic membrane will greatly increase the Solar Reflectance Index (SRI) of the roof surface. SRI is an indicator of the ability of a surface to return solar energy into the atmosphere. In general, roof material surfaces with a higher SRI will be cooler than a surface with a lower SRI under the same solar energy exposure. A lower roof surface temperature can result in less heat being absorbed into the building interior during the summer months.Is a hybrid only for new construction?The advantage of a hybrid roof assembly is significant in recover scenarios where there is an existing-modified bitumen or built-up roof that is in overall fair condition and with little underlying moisture present. A single ply membrane can be installed on top of the existing roof system without an expensive and disruptive tear-off of the existing assembly. The addition of the single-ply membrane adds reflectivity to the existing darker colored membrane and increases the service life of the roof assembly due to the additional layer of UV protection. Additionally, the single-ply membrane can be installed with low VOC options that can have minimum odor and noise disturbance if construction is taking place while the building is occupied.Is the hybrid assembly hype worth it?Absolutely! The possibility to combine the best aspects of multiple roofing technologies makes a hybrid roof assembly worth the hype. It provides the best aspects of a single-ply membrane including a reflective surface for improved energy efficiency, and increased protection against chemical exposure and ponding water, while the asphaltic base increases overall system waterproofing redundancy, durability and protection. The ability to be used in both new construction and recover scenarios makes a multi-ply hybrid roof an assembly choice that is here to stay.Interested in learning more about designing school rooftops? Check out available design resources school roof design resources here. And as always, feel free to reach out to the Building & Roofing Science team with questions.This article was written by Kristin M. Westover, P.E., LEED AP O+M, Technical Manager, Specialty Installations, in partnership with Benjamin Runyan, Sr. Product Manager - Asphalt Systems.

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. A range of codes and standards are beginning to address this problem, though it's important to note that there is often a time lag between development of codes and their widespread adoption.What Is the Industry Doing About It?Long in the business of supporting high-performance building enclosures, Phius (Passive House Institute US) provides a Fastener Correction Calculator along with a way to calculate the effect of linear thermal bridges (think shelf angles, lintels, and so on). By contrast, the 2021 International Energy Conservation Code also addresses thermal bridging, but only considers framing materials to be thermal bridges, and actually pointedly ignores the effects of point loads like fasteners in its definition of continuous insulation: "insulation material that is continuous across all structural members without thermal bridges other than fasteners and service openings" (Section C202). 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 3.1.1.7.3). 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. By making our buildings more robust against wind uplift to meet updated standards, we are in effect making them less robust against the negative effects of hot and cold weather conditions.So, how bad is this problem, and what's a roof designer to do about it? A team of researchers at SGH, Virginia Tech, and GAF set out to determine the answer, first by simplifying the problem. Our plan was to develop computer simulations to accurately anticipate the thermal bridging effects of fasteners based on their characteristics and the characteristics of the roof assemblies in which they are used. In other words, we broke the problem down into parts, so we could know how each part affects the problem as a whole. We also wanted to carefully check the assumptions underlying our computer simulation and ensure that our results matched up with what we were finding in the lab. The full paper describing our work was delivered at the 2023 IIBEC Convention and Trade Show, but here are the high points, starting with how we set up the study.First, we began with a simple 4" polyisocyanurate board (ISO), and called it Case A-I.Next, we added a high-density polyisocyanurate cover board (HD ISO), and called that Case A-II.Third, we added galvanized steel deck to the 4" polyiso, and called that Case A-III.Finally, we created the whole sandwich: HD ISO and ISO over steel deck, which was Case A-IV.Note that we did not include a roof membrane, substrate board, air barrier, or vapor retarder in these assemblies, partly to keep it simple, and partly because these components don't typically add much insulation value to a roof assembly.The cases can be considered base cases, as they do not yet contain a fastener. We needed to simulate and physically test these, so we could understand the effect that fasteners have when added to them.We also ran a set of samples, B-I through B-IV, that corresponded with cases A-I through A-IV above, but had one #12 fastener, 6" long, in the center of the 2' x 2' assembly, with a 3" diameter insulation plate. These are depicted below. The fastener penetrated the ISO and steel deck, but not the HD ISO.One visualization of the computer simulation is shown here, for Case B-IV. The stripes of color, or isotherms, show the vulnerability of the assembly at the location of the fastener.What did we find? The results might surprise you.First, it's no surprise that the fastener reduced the R-value of the 2' x 2' sample of ISO alone by 4.2% in the physical sample, and 3.4% in the computer simulation (Case B-I compared to Case A-I).When the HD ISO was added (Cases II), R-value fell by 2.2% and 2.7% for the physical experiment and computer simulation, respectively, when the fastener was added. In other words, adding the fastener still caused a drop in R-value, but that drop was considerably less than when no cover board was used. This proved what we suspected, that the HD ISO had an important protective effect against the thermal bridging caused by the fastener.Next, we found that the steel deck made a big difference as well. In the physical experiment, the air contained in the flutes of the steel deck added to the R-value of the assembly, while the computer simulation did not account for this effect. That's an item that needs to be addressed in the next phase of research. Despite this anomaly, both approaches showed the same thing: steel deck acts like a radiator, exacerbating the effect of the fastener. In the assemblies with just ISO and steel deck (Cases III), adding a fastener resulted in an R-value drop of 11.0% for the physical experiment and 4.6% for the computer simulation compared to the assembly with no fastener.Finally, the assemblies with all the components (HD ISO, ISO and steel deck, a.k.a. Cases IV) showed again that the HD ISO insulated the fastener and reduced its negative impact on the R-value of the overall assembly. The physical experiment had a 6.1% drop (down from 11% with no cover board!) and the computer simulation a 4.2% drop (down from 4.6% with no cover board) in R-value when the fastener was added.What Does This Study Tell Us?The morals of the study just described are these:Roof fasteners have a measurable impact on the R-value of roof insulation.High-density polyisocyanurate cover boards go a long way toward minimizing the thermal impacts of roof fasteners.Steel deck, due to its high conductivity, acts as a radiator, amplifying the thermal bridging effect of fasteners.What Should We Do About It?As for figuring out what to do about it, this study and others first need to be extended to the real world, and that means making assumptions about parameters like the siting of the building, the roof fastener densities required, and the roof assembly type.Several groups have made this leap from looking at point thermal bridges to what they mean for a roof's overall performance. The following example was explored in a paper by Taylor, Willits, Hartwig and Kirby, presented at the RCI, Inc. Building Envelope Technology Symposium in 2018. In that paper, the authors extended computer simulation results from a 2015 paper by Olson, Saldanha, and Hsu to a set of actual roofing scenarios. They found that the installation method has a big impact on the in-service R-value of the roof.They assumed a 15,000-square-foot roof, fastener patterns and densities based on a wind uplift requirement of 120 pounds per square foot, and a design R-value of R-30. In this example, a traditional mechanically attached roof had an in-service R-value of only R-25, which is a 17% loss compared to the design R-value.An induction-welded roof was a slight improvement over the mechanically attached assembly, with an in-service value of only R-26.5 (a 12% loss compared to the design R-value).Adhering instead of fastening the top layer of polyiso resulted in an in-service R-value of R-28.7 (a 4% loss compared to the design R-value).Finally, in their study, an HD polyiso board was used as a mechanically fastened substrate board on top of the steel deck, allowing both layers of continuous polyiso insulation and the roof membrane to be adhered. Doing so resulted in an in-service R-value of R-29.5, representing only a 1.5% loss compared to the design R-value.To operationalize these findings in your own roofing design projects, consider the following approaches:Consider eliminating roof fasteners altogether, or burying them beneath one or more layers of insulation. Multiple studies have shown that placing fastener heads and plates beneath a cover board, or, better yet, beneath one or two layers of staggered insulation, such as GAF's EnergyGuard™ Polyiso Insulation, can dampen the thermal bridging effects of fasteners. Adhering all or some of the layers of a roof assembly minimizes unwanted thermal outcomes.Consider using an insulating cover board, such as GAF's EnergyGuard™ HD or EnergyGuard™ HD Plus Polyiso cover board. Installing an adhered cover board in general is good roofing practice for a host of reasons: they provide enhanced longevity and system performance by protecting roof membranes and insulation from hail damage; they allow for enhanced wind uplift and improved aesthetics; and they offer additional R-value and mitigate thermal bridging as shown in our recent study.Consider using an induction-welded system that minimizes the number of total roof fasteners by dictating an even spacing of insulation fasteners. The special plates of these fasteners are then welded to the underside of the roof membrane using an induction heat tool. This process eliminates the need for additional membrane fasteners.Consider beefing up the R-value of the roof insulation. If fasteners diminish the actual thermal performance of roof insulation, building owners are not getting the benefit of the design R-value. 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. With easy-to-use resources quantifying thermal bridging through roof fasteners, roof designers will no longer be putting building owners at risk of wasting energy, or, even worse, of experiencing condensation problems due to under-insulated roof assemblies. Designers will have a much better picture of exactly what the building owner is getting when they specify a roof that includes fasteners, and which of the measures detailed above they might take into consideration to avoid any negative consequences.This research discussed in this blog was conducted with a grant from the RCI-IIBEC Foundation and was presented at IIBEC's 2023 Annual Trade Show and Convention in Houston on March 6. Contact IIBEC at https://iibec.org/ or GAF at BuildingScience@GAF.com for more information.

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