RoofViews

Building Science

Value Engineering Part 2: Retaining Performance

June 08, 2020

A wooden scale with balancing coins and a yellow hard hat

In the first Value Engineering post about the attributes of high performing roof designs, value engineering is defined as "a concept that states there are less expensive ways to get equivalent performance," and the post described the performance attributes that make for a long-lasting, high-performing roof system. These performance attributes include: images

Energy efficiency (reflectivity, thermal resistance, and air leakage)
Impact resistance
Wind resistance
Condensation prevention
High heat (and UV) resistance
Positive drainage
Wear resistance

These performance attributes manifest themselves in certain aspects of the overall design of the roof system. What type of membrane, membrane thickness, cover board or no cover board, adhered or mechanically attached cover board, insulation layout (including tapered), insulation attachment method(s), vapor retarder / air barrier or not, and attachment to the roof deck are some of the questions that drive the performance of a roof system.

And the roof design is…

After considering all of the above performance attributes, your design strategies for a long-lasting, high performing, and durable roof include:

Adhered reflective roof membrane with high heat-resistance
Adhered HD polyiso coverboard
2+ layers of adhered insulation, staggered and offset, with tapered insulation and crickets and saddles
Air barrier (over a substrate board) at the deck level
Third-party-tested edge metal details
Walkway pads

Energy Efficiency – reflectivity and thermal resistance

The attachment method is integral to a roof system's energy efficiency. Consider, for example, a roof that is designed to have the first layer of insulation fastened to the deck and the remainder of the components above are to be adhered. There is a cost for fasteners as well as adhesives. One way to "value engineer" this roof is to use fasteners through the entire system thereby removing the cost of the adhesive and labor to install the adhesive. While eliminating the adhesive is a cost reduction strategy, it is possible that longer fasteners will be needed. Sometimes something that is eliminated creates the need for a greater cost elsewhere. This is often not mentioned or discussed, so ask the question!

Value engineering the removal of the adhesive also, unfortunately, can result in a roof system with a 15 to 30% reduction in R-Value relative to the as-designed roof system. If the roof was designed to have an R-30, the actual R-value as installed would be about R-21 to R-25! It's been shown that over the life of a roof, roof systems that use adhered membranes and an adhered top layer of insulation can offset the cost of adhesives when the initial cost of 'lost' insulation performance and the additional annual cost for increased heating and cooling are factored into the analysis.

One of the key issues with value engineering of a roof system is that proponents of the reduction of costs are generally only looking at the initial cost to install a roof. However, the operational costs, the energy efficiency, the wind resistance, are commonly compromised. Operational costs, which are tied directly to a roof system's energy efficiency, are often much higher over the life of a roof. Also, roof systems with different wind-resistance ratings may need to be selected after value engineering revises the attachment method.

Also, is the mechanical system design and whole-building energy-use analysis predicated on a roof system R-value of 30? Determining the effect on mechanical unit sizing and cost, including differences in whole building energy use may show that the savings from value engineering the roof will have a negative effect on annualized energy use and the ability of the HVAC system to maintain occupant comfort. Whole building energy simulation modeling, such as EnergyPlus™ (EnergyPlus.net), can be used. EnergyPlus™ is funded by the U.S. Department of Energy's Building Technologies Office, and managed by the National Renewable Energy Laboratory.

Each "dot" is a fastener and plate; fasteners and plates are thermal bridges that reduce the R-value of the insulation layer.

Impact Resistance

Using any type of coverboard will improve the impact-resistance of a roof system because of the toughness of coverboards in general. However, a key issue with impact resistance is the location of the plate and fastener relative to the roof membrane. If a coverboard is mechanically attached (so the plates and fastener head are immediately beneath the membrane), impacts at fastener heads and plates have been shown to result in damage to the membrane. Adhered coverboards remove this concern, but do require the use of some type of adhesive. It's not just the use of a coverboard, but the installation method that matters.

ice.impact

Ice-ball impact above fasteners always punctures the membrane. Left to right: increasing damage from hairline crack to complete puncture.

Impacts come from hail as well as rooftop use by occupants and anyone performing work on rooftop mechanical units or to rising walls, for example. Reducing membrane thickness, changing from a fleece-back membrane to a smooth-back membrane, as well as changing to a more commodity-type membrane versus one that is shown to have a long service life are value engineering choices that can reduce cost, yet also reduce the impact resistance.

Wind Resistance

Designers make wind-design decisions that are intended to reduce risk (e.g., choosing Partially Enclosed versus Enclosed, selecting Exposure Category C instead of Exposure Category B). If a designer and owner have determined to reduce the potential risk of high-wind-event damage by increasing the design wind loads and subsequently the capacity of a roof system, that high capacity could be value engineered out. Coordinating wind design assumptions and documenting owner expectations and decisions can be critical pieces of information that alleviate the reduction of wind load capacity based on cost. Owners have their reasons; value engineering should not undermine owner expectations. By including design wind pressures for each roof zone as well as providing design loads for edge metal, and parapets and coping, minimum performance requirements are established in the construction documents.

For more on wind design, read our blog here.

Condensation prevention

Roofs that include air barriers above the deck and below insulation provide the best protection against the development of condensation in a roof system. Your roof design took into consideration the interior and exterior design temperatures and calculations were performed to determine potential dew point locations, which resulted in the correct placement of the air barrier.

This roof design is "above code." While the energy code allows the roof membrane to be the air barrier, air can still move up into the roof system. This is called air intrusion and can bring moisture into the roof system, potentially causing condensation issues.

Placing the air barrier layer on the top surface of a rigid board fastened to the roof deck is one option. However, with "first layer fastened and the upper layers (insulation, cover board, membrane) adhered" systems, there is an opportunity to include a vapor retarder / air barrier between the insulation layers without fasteners penetrating the vapor / air barrier layer. Reducing air intrusion and the potential for condensation is achieved using both designs; this is a real value engineering opportunity! The costs for materials and labor for each design can be determined, and the least expensive design that provides condensation control can be implemented, instead of eliminating the properly located air barrier.

While removing the air barrier and the associated adhesives and components reduces costs, changing to a roof system that uses only mechanical attachment also negatively affects the potential for condensation and thermal performance of the roof system. For example, consider a high-humidity building, such as a natatorium—the use of an air barrier at the deck level reduces air intrusion into the roof system from the interior and the moisture that air carries. This reduces the potential for condensation and damage to the roof system.

High Heat (and UV) Resistance

Heat and UV are two environmental factors that affect roof membrane service life. Understanding this and that storms are increasing in severity and frequency, in addition to recognizing temperatures are rising (ASHRAE's climate zone maps are moving northward), your roof design includes a membrane that has been shown to have a long service life. It's likely the specification includes GAF's TPO Everguard Extreme® membrane due to its ability to withstand high heat conditions.

It's possible that a high-performing roof membrane will be value engineered down to one of reduced initial cost. The rationale will be that it still keeps water out! But to what long-term cost? You designed a roof to meet your client's expectations for a long service life. By considering annualized costs, not simply initial cost, it may be shown that a longer service life roof is actually a better long-term value for the owner. A key strategy is to use annualized costs when pushing back against value engineering.

Positive Drainage

Most roof systems are not guaranteed or warranted against ponding water. The combination of UV and water is one mechanism that advances the deterioration of many roof membrane materials. And because water needs to be removed from a roof quickly and efficiently, your roof design includes properly sized drains and scuppers, a tapered insulation layout to efficiently move water to drains and scuppers, and crickets and saddles to prevent localized ponding.

Example of a tapered layout, courtesy GAF Tapered Design Group

When asked to reduce the cost of the tapered insulation, one possible answer is to install more drains that are closer together. This can reduce the build-up of tapered insulation, and the associated cost. Perhaps the use of lightweight insulating concrete is less costly? Perhaps the use of a TPO membrane, which does not have ponding water limitations in the guarantee/warranty, makes sense?

Wear Resistance

Thicker membranes, granulated surfaces, and walkway pads are helpful in preventing unwanted wear to roofing membranes. You've coordinated expectations with the owner, your client, that rooftop protection against wear and tear is important during the operation phase of the building. Reducing the thickness of a membrane reduces its potential wear resistance, as well as its impact resistance (as previously noted in this blog). As with many things, none of these issues are stand-alone issues.

Perform a cost comparison of a granule-surface modified bitumen membrane and a smooth surface system with a high-solids coating system to see if there are less expensive systems that still meet wear-resistance requirements. Don't just look at initial costs; consider maintenance costs over the life of the roof that affect annualized costs. A relatively small number of walkway pads, strategically placed around rooftop units and systems that will require regular maintenance (e.g., HVAC units and solar arrays), can be an effective and economic solution.

Product substitutions

Not all products with the same intended function (and described with the same terminology) are created equal. A vapor retarder is a good example. A self-adhered vapor retarder with a perm rating of 0.03 has a desired performance and was included for a specific reason – to effectively block the diffusion of moisture into the roof assembly. Substituting a less expensive vapor retarder with different properties (e.g., a higher perm rating) and physical characteristics (30 mil asphaltic vs. a 6 mil poly), won't be installed in the same fashion—even though they are both vapor retarders. The least expensive material may be the most costly to install properly.

Details

Details matter. Without a doubt, the ability for a roof to keep water out is most challenging at the details—penetrations, perimeters, and locations where there is an interruption, end, or change of direction. You designed a two-part counterflashing for ease of maintenance and future reroofing; a single-piece counterflashing is less expensive initially. When it's time to reroof, will the facade material (e.g., masonry, stucco, EIFS) need to be removed and repaired in order to install new counterflashing? Again, you've established with the owner that future efforts for maintenance and reroofing are important.

There are ways to value engineer roofing details, especially when it comes to air barrier and vapor retarder constructability. A good example is an overhang. It seems logical to design an air barrier to enclose the overhang (the blue lines in the graphic), but there are many potential dis-continuities (the red lines in the graphic). The air barrier can be inboard of the eave; the wall air barrier could be tied to the underside of the roof deck, and the above deck air barrier (whether at deck level or the roof membrane) is tied to the top of the roof deck making a continuous air barrier system. There likely are labor savings associated with this value-engineered design revision.

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Weather

A change of seasons is often the impetus for changing from an adhered system to one that strictly uses mechanical attachment. While this may not be value engineering, a change of seasons can mean daily temperatures are below manufacturer recommended minimums for adhesive materials. However, manufacturers have (and are developing) lower temperature adhesives. Regardless of why a roof system design eliminates adhesives in favor of fasteners, there are energy efficiency and an impact-resistance trade offs.

Owner expectations

Much of the prevention of value engineering is having a clear understanding of the owner's performance requirements and expectations. Getting 'buy-in' from an owner for high-performance roof systems is key. During the design phase, explaining the importance of design decisions and aligning them with the owner's performance requirements should be documented. Documented design decisions that are made in conjunction with the owner provide a defense when "it costs too much" is used as the basis for value engineering.

The Whole Building Design Guide, from the National Institute of Building Sciences, includes detailed information about value engineering during the design and construction phases of a project.

About the Author

James R Kirby

James R. Kirby, AIA, is a GAF building and roofing science architect. Jim has a Masters of Architectural Structures and is a licensed architect. He has over 25 years of experience in the roofing industry covering low-slope roof systems, steep-slope roof systems, metal panel roof systems, spray polyurethane foam roof systems, vegetative roof coverings, and rooftop photovoltaics. He understands the effects of heat, air, and moisture movement through a roof system. Jim presents building and roofing science information to architects, consultants and building owners, and writes articles and blogs for building owners and facility managers, and the roofing industry. Kirby is a member of AIA, ASTM, ICC, MRCA, NRCA, RCI, and the USGBC.

Have you ever thought about building products reducing the carbon dioxide emissions caused by your building? 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/

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. 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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