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Building Science

3 Types of Moisture in Low Slope Roofing Systems

By Thomas J Taylor

December 14, 2015

Low-slope roofing system

Roofing systems must be designed and installed to address the many sources of moisture that can affect a building. Roofing manufacturers can provide the materials with a range of properties and options for the roofing system designer, but because each building is unique the final responsibility for ensuring that moisture and condensation don't cause problems lies with the architect, system designer, project engineer, and/or roofing system professional.

Several years ago, SPRI issued an advisory bulletin on moisture and its effect on roofing systems. Their classification of the types of situations leading to moisture build-up is a good way to think about the main scenarios. I've listed them here, along with some general roofing system guidelines that may be useful. Of course, these guidelines should be superseded by those of a professional roof system designer.

1. Buildings with Small Amounts of Occupancy-Generated Moisture

These are the most common situations, covering office, retail, and warehouse spaces, for example. Warm air migrates upward and carries moisture with it. In northern climates, regardless of the membrane type, condensation can occur between the membrane and the insulation. Here is what to do in this situation.

  • Try to always use two layers of insulation with staggered joints. This helps prevent migration of warm, moist air up to the underside of the roof membrane. Warm air and moisture generally move upward, so drying out the roof system during warmer/sunnier days is a slow and difficult process. The goal should be to minimize or prevent condensation in the first place.
  • If a single layer of insulation is used, install cover boards, especially a high-density polyiso board, and use staggered joints.
  • If the membrane is to be fully adhered with a water-based adhesive, special care is needed to select the right one. Some have been shown to re-liquify in the presence of moisture. Our GAF water-based adhesive was formulated to limit this effect.

2. Buildings with Large Amounts of Occupancy-Generated Moisture

This category includes paper mills, laundries, buildings with indoor swimming pools, and the like. The building's air handling and ventilation systems should be carefully specified to take into account the moisture loading. Also, the entire building envelope needs to be designed and constructed in such a way that damaging condensation and moisture build-up doesn't occur. It is critical that a building science professional experienced in these types of building occupancy and designs be involved.

  • Some approaches used by building design professionals for these buildings involve the use of air and vapor barriers. Care is needed to make sure these are applied properly and without any damage to their integrity. Pay particular attention to how they are terminated around the perimeters and at penetrations.
  • Remember that roof membranes are essentially impermeable. An air and vapor barrier placed lower in the roof system can prevent any water that has managed to leak in from going elsewhere.

3. Construction–Related Moisture

Most construction practices release some amount of moisture into the building space. These can be as straightforward as drywall installation and painting although these are typically relatively short term. However, some practices can release large amounts of water over a considerable time frame into the building. Concrete slab floors have been a challenge to use with some flooring materials, and building design professionals have methods to cope with the moisture release.

Concrete roof decks can present a challenge for roof system designers especially in new construction. Regardless of the type of concrete, significant amounts of water remain after curing is completed.

  • There is very little correlation between cure time and the amount of water remaining. Guidelines such as not closing up the system until a minimum of 30 days after pouring and forming are not particularly effective at reducing or eliminating issues.
  • Concrete in new construction that appears to be dry rarely is. Water takes a long time to diffuse out of a four inch-thick slab. In fact, the NRCA no longer considers the "plastic sheet test" to be a viable indicator of concrete's surface dryness. That test involved taping a plastic sheet to the surface and monitoring moisture under it.
  • Many roof system designers require concrete decks be covered with a vapor barrier. As with buildings that have occupancy-generated moisture, care must be taken to ensure that these are specified and installed correctly. Roof membranes are essentially impermeable and any moisture that does get into the roof system will not have a path out.
  • Adopt a "belt and suspenders" approach to the roofing system and make sure any vapor barrier is terminated around the perimeter and penetrations properly. To do this, use two layers of insulation with staggered joints. When installing a fully adhered membrane with a water-based adhesive, make sure the adhesive will not re-liquify in the presence of moisture.
  • Make sure professionals who have experience with this type of construction are involved. These could include structural engineers, building science experts, and concrete suppliers.

The vast majority of roof systems give trouble-free performance for many years. When problems do occur within a few years of installation, it could be due to installation defects or moisture build-up. Roofing manufacturers try to provide a wide range of material choices so that the design professional can configure the optimum system for each building. The guidance offered here might help you to ask the right questions. However, to lower the risk of issues, make sure design professionals experienced with moisture and humidity in your region are involved.

About the Author

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

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

What is going on here?No, this roof does not have measles, it has a problem with thermal bridging through the roof fasteners holding its components in place, and this problem is not one to be ignored.As building construction evolves, you'd think these tiny breaches through the insulating layers of the assembly, known as point thermal bridges, would matter less and less. But, as it happens, the reverse is true! The tighter and better-insulated a building, the bigger the difference all of the weak points, in its thermal enclosure, make. 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Likewise, The National Energy Code of Canada for Buildings: 2020 addresses thermal bridging of a number of building components, but also explicitly excludes fasteners: "in calculating the overall thermal transmittance of assemblies…fasteners need not be taken into account" (Section 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. 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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. 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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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