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Optimizing Thermal Resistance and Inertia in Low-Slope Roof Design

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Optimizing Thermal Resistance and Inertia in Low-Slope Roof Design

January 24, 2020

Can Energy Efficiency be Further Improved?

The energy efficiency of a building's enclosure is generally analyzed in terms of thermal resistance. This is a static property and doesn't take into account the time of day and the effects of thermal mass. This article shows how adding thermal mass to a roof assembly might offer a way to improve energy efficiency, especially for buildings only occupied during daylight hours such as offices and schools. In addition, thermal mass could help reduce electric grid demand swings.

Introduction

Building codes continue to improve energy efficiency requirements with a long term goal of realizing net-zero energy buildings. Standard practice in designing buildings generally only considers the thermal resistance of the building enclosure because of its simplicity. Conceptually, thermal resistance is readily understood, measured, and its effects easily calculated. However, thermal resistance is a steady-state property yet the thermal performance of building enclosures is also impacted by thermal mass and other related properties. Thermal mass affects the dynamic flow of heat into or out of buildings. While important, thermal mass has not received as much attention as thermal resistance in roofing.

In the first part of this series the use of thermal mass to slow down heat flux through a roof assembly was discussed. In buildings where air conditioning costs dominate and heating use is relatively low, higher thermal mass assemblies can potentially improve energy efficiency. This is particularly the case of buildings such as offices that are only occupied during daylight hours. Thermal mass could delay the transmission of heat into a building towards the end of the day, increasing thermal comfort and allowing facility managers to reduce cooling demand during working hours.

In the second part of this series the physical properties that contribute to thermal mass were described. The key property values for common roofing materials were listed.

This article shows how thermal mass can significantly affect heat flow through some example roof assemblies.

Background

In any roof assembly, the amount of thermal energy entering a building is reduced and delayed as shown below:

Conventional building enclosure design normally considers only thermal resistance and not the potential delaying effect of thermal mass. There are several key characteristics of thermal mass; time delay and the decrement factor.

The time delay is calculated as:

calculation

Where t[Tout(max)] and t[Tin(max)] are the times of day when the inside and outside surface temperatures reach maximum. Time delay could be important for buildings that are only occupied during the day, such as offices. The time delay caused by a building's enclosure could be why some offices become uncomfortably hot during the late afternoon, as the HVAC system fails to compensate for the heat flow into the interior space.

The decrement factor, DF, is calculated as:

calculation

The DF is a measure of the dampening effect of the building enclosure on external temperature swings.

Finally, in any real-world situation, there is the dynamic effect of exterior temperature swings which lead to a "periodic thermal transmittance". If the periodic thermal transmittance is low, there will be a reduction in the impact of outside thermal load.

The key physical properties for some common roof assembly components are shown below (derivation of these values was explained in part two of this series):

Calculating Thermal Mass Effects in Roof Assemblies

Calculation of the effects of thermal mass has been described in the international standard ISO 13786. A validated tool from HT Flux was been used to perform the calculations for four roof assemblies:

system and layers

System 1 is a well-insulated assembly often found over steel decks (R-value of approximately 34).

System 2 is included for comparison, having no additional thermal resistance above that of the concrete deck.

System 3 is for a combination of polyiso insulation over a lightweight structural concrete deck.

System 4 represents an example of a high performance roof assembly. The gypsum board is used as a possible base for a vapor retarder. The HD polyiso would provide for improved impact resistance.

Peak Temperature Time Delay

The table shows the time delay between the peak external temperature and the internal peak temperature. System 1, based on polyiso over a steel deck, has a time delay of 1.64 hours. This potentially explains why the top floors of buildings become warmer during mid-afternoon periods in the summer. The external temperature will peak between noon and 1pm when the sun is overhead. However, the heat flux into the building is delayed until 2 to 4 pm.

A structural lightweight concrete roof with no added insulation, System 2, is projected to delay the peak internal temperature by 4.24 hours. However, a combination of lightweight structural concrete deck with polyiso delays that peak temperature by over 8 hours (System 3). This would mean that, for an office building, the peak heat flux into the building would be delayed until after normal working hours.

A high performance roof assembly over a steel deck, System 4, delays the peak temperature by 2.60 hours. Such an assembly is optimized for impact resistance and includes a gypsum board as a substrate for a vapor retarder. This time delay would still result in the peak heat flux into the building being during working hours for a building occupied only during the day.

Periodic Thermal Transmittance

System 3, consisting of a structural lightweight concrete deck, polyiso, and TPO membrane has the lowest periodic transmittance of 0.048 W/m2K. This means that System 3 not only has a large time delay in peak heat flux, but that the amount of energy entering the building through the roof would be lower. The polyiso insulation helps resist the heat flux and the lightweight structural concrete has a high density so that it absorbs what heat does come through the polyiso. This would have the effect of significantly dampening the effect of exterior temperature swings and increasing occupant comfort.

For buildings such as offices, schools, and any other building occupied only during daytimes, the combination of high thermal resistance and thermal mass could have a significant advantage. The combination of a large time delay and low periodic thermal transmittance for System 3 would lower the thermal demand on air conditioning units and also reduce temperature swings within the building. This would improve comfort for occupants as well as reducing HVAC cycling. (Of course, energy cost savings are not guaranteed and the amount of savings may vary based on climate zone, utility rates, radiative properties of roofing products, insulation levels, HVAC equipment efficiency and other factors.)

A further benefit of adding thermal mass to roof assemblies could be that, by delaying heat transmission into a building, peak demands on the electric grid might be reduced. Typically, air conditioning demand results in large grid loads during afternoon hours during summer periods. By delaying heat flux into buildings it might be possible to not only smooth out interior temperature swings but also swings in electrical demand on the grid.

Conclusions

The thermal property data shown here could be used in modeling exercises to better understand how to design and optimize energy-efficient buildings.
Clearly, lightweight structural concrete has a far higher thermal mass than other materials commonly used in roof assemblies.
Conventional roof assembly design normally only considers thermal resistance. However, for further improvements in energy efficiency, it could be worthwhile to consider thermal mass, particularly for buildings only occupied during the daytime. (Of course, whether a particular roof design is suitable for a given project depends upon a number of factors that must be evaluated and prioritized by the design professional.)
While this article suggests that including thermal mass into a building enclosure could bring benefits, it should be noted that there are other factors that were not taken into account here. These include realistic weather conditions, occupant's interactions, and a building's overall geometry.

Thanks to Martha VanGeem (www.vangeemconsulting.com) for commenting on an earlier draft of this article. Martha is a notable authority on the topic of using thermal mass in construction to both improve energy efficiency and dampen electrical grid demand.

Sources Used

Perino, M. and Serra, V., Switching from static to adaptable building envelopes; a paradigm shift for the energy efficiency in buildings. J. Façade Des. Eng., 2015, 3(2), pp. 143-163.
Stazi, F., Bonfigli, E., Tomassoni, E., Di Perna, C., and Munafò, P. The effect of high thermal insulation on high thermal mass: is the dynamic behavior of traditional envelopes in Mediterranean climates still permissible? Energy Build. 2015, 88, pp. 367-383.
VanGeem, M. G., Holm, T. A., and Ries, J. P., Optimal thermal mass and R-value in concrete, Proc. First Int. Conf. on Concrete Sustainability, Tokyo, May 2013, pp. 411-418.
Kuczynski, T. and Staszczuk, Experimental study of the influence of thermal mass on thermal comfort and cooling energy demand in residential buildings. Energy, 2020, 195.
International Organization for Standardization, ISO 13786, Thermal Performance of Building Components – Dynamic Thermal Characteristics – Calculation Methods, 2007.
Diana K. Fisler, Manfred Kehrer, and Francis Babineau, Sensitivity Analysis of Moisture Dependent Properties of Roofing Materials. ASTM International STP1599 Advances in Hygrothermal Performance of Building Envelopes: Materials, Systems and Simulations eds. Phalguni M. and Fisler D. K., 2017 (pp.166-185)
Verbeke, S., Thermal inertia in dwellings. Quantifying the relative effects of building thermal mass on energy use and overheating risk in a temperate climate. PhD Thesis, University of Antwerp, 2017.
Balaji, N. C., Mani, M., and Venkatarama, R. B. V., Thermal performance of building walls. 1st IBPSA Italy Conference, Building Simulation, 2013, pp. 151 – 159.
Roberz, F., Loonen, R. C. G. M., Hoes, P., and Hensen, J. L. M., Ultra-lightweight concrete: energy and comfort performance evaluation in relation to buildings with low and high thermal mass. Energy and Buildings, 2017, 138, pp. 432-442.
Heat flux calculations performed using a tool provided courtesy of Daniel Radisser, HT Flux, www.htflux.com

About the Author

Thomas J Taylor

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.

Extreme heat has been the greatest weather-related cause of death in the US for the last 30 years. In fact, between 2010 and 2020, roughly 12,000 Americans died from extreme heat, with Native American and Black communities disproportionately affected. Around the globe, excess heat collects in urban areas and can contribute to increased heat-related death and illness, diminished quality of life, and reduced economic opportunities.One example of this urban heat is Los Angeles's Pacoima neighborhood. During days of extreme temperatures, Pacoima had seven times the number of excess heat emergency room visits (19,000 between 2009 and 2018) compared with nearby Santa Monica, an affluent community of similar size, according to Bloomberg.Economic sectors, such as tourism and local businesses, also feel the effects of urban heat. Pacoima food truck vendor Jennifer Ramirez told Bloomberg she couldn't start work until outdoor and street temperatures cooled. Otherwise, her food truck generator could blow out.Fortunately, cooling strategies are now available and being used in initiatives like the GAF Cool Community Project to help mitigate urban heat.Understanding Heat IslandsYou can envision heat islands as heat-saturated sponges spread out across vast, shimmering urban spaces. These "islands" tend to have minimal shady vegetation, so their blacktop and hard surfaces—such as pavements, parking lots, concrete recreation areas, and roofs—absorb and retain the sun's heat well into the evening. Temperatures in urban heat islands can often be 15°F to 20°F hotter than in equivalent vegetated areas.In technical terms, darker, tarmac-covered surfaces have a "low albedo" effect, which describes the ability to reflect and diffuse the sun's rays back to the upper atmosphere. Meanwhile, reflective, snow-covered regions such as the Arctic have "high albedo." Areas with low albedo are at greater risk of experiencing increased heat retention.Densely packed buildings compound a low albedo effect by preventing cooling breezes from lowering temperatures. To top it off, heat islands can trap waste heat from cars, air conditioners, and other heat-emitting devices—exacerbating the effects. In terms of detrimental environmental impacts, this is the perfect, overheated, storm.Shifting from Heat Islands to Cool CommunitiesThe GAF Cool Community Project addresses the complex issue of urban heat with a simple solution: Take a hot, sun-exposed community with an abundance of heat-retaining surfaces. Then, apply colorful, solar-reflective coating to its streets and public hardscaping—such as GAF StreetBond® pavement coatings with Invisible Shade™, which Time magazine named one of the Best Inventions of 2022.The result? Cooler surfaces and ambient air temperatures.Making Progress on Cooling PacoimaLA's Pacoima neighborhood is one example of a community hit hard by heat. A true urban heat island, it has consistently been one of the city's hottest areas. On one summer afternoon in 2022, Bloomberg reported a pavement temperature of 127°F at noon. Just one hour later, it had risen to 141.8°F.Many of Pacoima's residents are in lower-income brackets and live in spaces that haven't been fully optimized to mitigate heat island effects. So, combatting the heat became a priority for local officials and organizations. Melanie Torres, community organizer with Pacoima Beautiful, said that "the cooling solutions that were brought here to Pacoima were actually very impactful to our community." Torres shared, "it definitely cooled things down, but we definitely did need the educational aspect —beginning the discussion of climate exchange and what extreme heat is, and how we can create solutions to combat it."With resources and support from GAF and non-profit partners, more than 700,000 square feet worth of streets, parking lots, and other hardscapes were coated with StreetBond® over a contiguous 10-block area. Attractive, colorful solar-reflective materials cover the park and playground areas, and blue and white coatings brighten the public basketball court. The formerly dark streets are now a cool gray-blue.According to Miguel Angel Luna, president of Urban Semillas, Pacoima residents are right "on the front line of climate change." Urban Semillas is an LA-based social and environmental justice consultancy working with GAF on the Cool Community Project.Achieving Positive Results in PacoimaOver the past year, Cool Community Project members monitored pavement and air temperatures using sensors, satellite technology, and drones. Findings from the past 12 months of observation have shown a positive impact on the Pacoima community:Ambient air temperature (Surface temperatures have been reduced by an average of 10°F during the daytime on sunny days.The second phase of the project includes applying pavement coatings to an additional 500,000 square feet of streets and introducing a new cooling technology on select residences—Timberline® CS Shingles. These shingles are made with a highly reflective material that can help reduce attic temperatures and air conditioning costs.In addition to heat reduction and urban beautification, GAF StreetBond® can help preserve pavement. Longer-lasting pavement can save cities time, money, and resources while giving them another tool to help mitigate the impacts of climate change. This is just another way GAF is looking to strengthen communities from the ground up.Through this project, GAF and its partners aim to improve the lives of the roughly 2,000 people living within the project area of Pacoima. They're accomplishing this by taking what Jeff Terry, GAF vice president of sustainability, describes as a "complete community approach," which involves residents and local organizations coming together to create a model for cities around the world.Reducing Temperatures at HomeWhether urban or rural, all communities can use technology to help reduce indoor and outdoor temperatures. Some cooling strategies that can be effective across cities or in your own home include:Cool SurfacesSurfaces treated with solar-reflective coatings can help reduce heat absorption and lower street and ambient air temperatures. Plus, products such as StreetBond® come in a range of colors to highlight urban areas and preserve pavement.Cool RoofsSolar reflective shingles like Timberline® CS Shingles may save homeowners an average of 7% to 15% on their total cooling costs.* This cooling technology can minimize a building's solar heat gain by reflecting incoming sun rays and reemitting the absorbed energy.Ready to get started in your neighborhood? Check out the inspiring community improvement projects at streetbond.com. (A division of GAF, StreetBond® offers the same quality and reliability customers have come to rely on from North America's largest roofing and waterproofing manufacturer.)*Potential for savings depends on various factors including, but not limited to, climate zone, utility rates, location, and HVAC equipment efficiency.

Companies, organizations, and firms working in the building, construction, and design space have a unique opportunity and responsibility. Collectively, we are contributing to nearly 40% of energy-related carbon emissions worldwide. While the goals, commitments, pledges, and promises around these challenges are a step in the right direction, no one entity alone will make major improvements to this daunting issue.We need to come together, demonstrate courageous change leadership, and take collective approaches to address the built environment's impacts on climate. Collectively, we have a unique opportunity to improve people's lives and make positive, measurable changes to impact:Buildings, homes, and hardscapesCommunity planningConsumer, commercial, and public sector behaviorOur Collective Challenge to Reduce our Carbon FootprintAccording to many sources, including the U.S. Green Building Council (USGBC), the built environment accounts for 39% of global energy-related carbon emissions worldwide. Operational emissions from buildings make up 28% and the remaining 11% comes from materials and construction.By definition, embodied carbon is emitted by the manufacture, transport, and installation of construction materials, and operational carbon typically results from heating, cooling, electrical use, and waste disposal of a building. Embodied carbon emissions are set during construction. This 11% of carbon attributed to the building materials and construction sector is something each company could impact individually based on manufacturing processes and material selection.The more significant 28% of carbon emissions from the built environment is produced through the daily operations of buildings. This is a dynamic that no company can influence alone. Improving the energy performance of existing and new buildings is a must, as it accounts for between 60–80% of greenhouse gas emissions from the building and construction sector. Improving energy sources for buildings, and increasing energy efficiency in the buildings' envelope and operating systems are all necessary for future carbon and economic performance.Why It Is Imperative to Reduce our Carbon Emissions TodayThere are numerous collectives that are driving awareness, understanding, and action at the governmental and organizational levels, largely inspired by the Paris Agreement enacted at the United Nations Climate Change Conference of Parties (COP21) in 2015. The Architecture 2030 Challenge was inspired by the Paris Agreement and seeks to reduce climate impacts from carbon in the built environment.Since the enactment of the Paris Agreement and Architecture 2030 Challenge, myopic approaches to addressing carbon have prevailed, including the rampant net-zero carbon goals for individual companies, firms, and building projects. Though these efforts are admirable, many lack real roadmaps to achieve these goals. In light of this, the US Security and Exchange Commission has issued requirements for companies, firms, and others to divulge plans to meet these lofty goals and ultimately report to the government on progress in reaching targets. These individual actions will only take us so far.Additionally, the regulatory environment continues to evolve and drive change. If we consider the legislative activity in Europe, which frequently leads the way for the rest of the world, we can all expect carbon taxes to become the standard. There are currently 15 proposed bills that would implement a price on carbon dioxide emissions. Several states have introduced carbon pricing schemes that cover emissions within their territory, including California, Oregon, Washington, Hawaii, Pennsylvania, and Massachusetts. Currently, these schemes primarily rely on cap and trade programs within the power sector. It is not a matter of if but when carbon taxes will become a reality in the US.Theory of ChangeClimate issues are immediate and immense. Our industry is so interdependent that we can't have one sector delivering amazing results while another is idle. Making changes and improvements requires an effort bigger than any one organization could manage. Working together, we can share resources and ideas in new ways. We can create advantages and efficiencies in shared R&D, supply chain, manufacturing, transportation, design, installation, and more.Collaboration will bring measurable near-term positive change that would enable buildings and homes to become net-positive beacons for their surrounding communities. We can create a network where each building/home has a positive multiplier effect. The network is then compounded by linking to other elements that contribute to a community's overall carbon footprint.Proof of Concept: GAF Cool Community ProjectAn estimated 85% of Americans, around 280 million people, live in metropolitan areas. As the climate continues to change, many urban areas are experiencing extreme heat or a "heat island effect." Not only is excess heat uncomfortable, but heat islands are public health and economic concerns, especially for vulnerable communities that are often most impacted.Pacoima, a neighborhood in Los Angeles, was selected by a consortium of partners as a key community to develop a first-of-its-kind community-wide research initiative to understand the impacts various cooling solutions have on urban heat and livability. Pacoima is a lower income community in one of the hottest areas in the greater Los Angeles area. The neighborhood represents other communities that are disproportionately impacted by climate change and often underinvested in.Implementation:Phase 1: This included the application of GAF StreetBond® DuraShield cool, solar-reflective pavement coatings on all ground-level hard surfaces, including neighborhood streets, crosswalks, basketball courts, parking lots, and playgrounds. The project also includes a robust community engagement process to support local involvement in the project, measure qualitative and quantitative impact on how cooling improves living conditions, and ensure the success of the project.Phase 2: After 12 months of monitoring and research, GAF and partners will evaluate the impact of the cool pavements with the intent to scale the plan to include reflective roofing and solar solutions.This ongoing project will allow us to evaluate for proof of concept and assess a variety of solutions as well as how different interventions can work together effectively (i.e., increasing tree canopies, greenspacing, cool pavements, cool roofs, etc.). Through community-wide approaches such as this, it's possible that we could get ahead of the legislation and make significant innovative contributions to communities locally, nationally, and globally.GAF Is Taking Action to Create Community-wide Climate SolutionsWith collaboration from leaders across the building space and adjacent sectors, we believe it is possible to drive a priority shift from net neutral to net positive. Addressing both embodied and operational carbon can help build real-world, net-positive communities.We invite all who are able and interested in working together in the following ways:Join a consortium of individuals, organizations, and companies to identify and develop opportunities and solutions for collective action in the built environment. The group will answer questions about how to improve the carbon impacts of the existing and future built environment through scalable, practical, and nimble approaches. Solutions could range from unique design concepts to materials, applications, testing, and measurement so we can operationalize solutions across the built environment.Help to scale the Cool Community project that was started in Pacoima. This can be done by joining in with a collaborative and collective approach to climate adaptation for Phase 2 in Pacoima and other cities around the country where similar work is beginning.Collaborate in designing and building scientific approaches to determine effective carbon avoidance—or reduction—efforts that are scalable to create net-positive carbon communities. Explore efforts to use climate adaptation and community cooling approaches (i.e., design solutions, roofing and pavement solutions, improved building envelope technologies, green spacing, tree coverage, and shading opportunities) to increase albedo of hard surfaces. Improve energy efficiency to existing buildings and homes and ultimately reduce carbon at the community level.To learn more and to engage in any of these efforts, please reach out to us at sustainability@gaf.com.

Exactly how hot does a roof get in the summer? According to the US Department of Energy, traditional darker asphalt shingle roofs can get up to 150°F on a sunny summer day. And prolonged exposure to high heat like this can damage your roof.Four Potential Effects of Summer Weather on Your RoofHere are four common ways hot summer weather can adversely affect your roof system:1. High HeatOver time, high heat can damage roofing materials.2. Ultraviolet LightUV light shines down on your roof on both sunny and cloudy days during the summer. Over time it can cause materials to deteriorate and diminish the amount of protection your roof shingles ultimately provide, leaving your roof vulnerable to leaks.3. HumiditySummer's high humidity can also damage your roof by creating condensation that collects under the shingles. If left unaddressed, this moisture can cause water damage, leaks, or mold growth.4. Thermal ShockThermal shock occurs when high temperatures during the day are followed by quickly cooling temperatures at night. This temperature swing can cause roofs to expand and contract rapidly, weakening the roof's integrity.How to Reduce the Risk of Heat Damage to Your RoofHere are a few strategies to help reduce the risk of heat damage:Ventilate the AtticA properly balanced attic ventilation system helps damaging heat and moisture to escape. Make sure you have the proper amount of ventilation needed to keep attic temperatures down.Maintain and Clean ShinglesRegularly removing debris and replacing damaged shingles can help ensure your roofing system is operating correctly. If you don't take care of your roof, its integrity could be compromised. It's best to leave this work to the professionals, so consider scheduling regular maintenance.Have the Roof InspectedHave your roof inspected at least twice a year for damage. Have a professional perform these inspections, as they know what to look for and can do so safely. Damage can often be seen inside your home before it shows up on your roof, so be sure the inspection looks both inside and outside for potential issues.Choose a Cool RoofInstalling a highly reflective roof (often known as a "cool roof") can help prevent early damage from the sun and may even help reduce cooling costs.1 How hot does a roof get in the summer when it's designed with "cooler" materials? A highly reflective roof can keep the surface 50°F cooler compared to traditional roofs, notes the US Department of Energy.2Benefits of Cool RoofsTwo factors affect the coolness of a roof: thermal emittance and solar reflectance. Thermal emittance is a roof's ability to shed heat by giving off thermal infrared radiation. Solar reflectance is its ability to reflect or bounce sunlight off the roof before heat is created.In general, light-colored roofs stay cooler than darker ones. However, new advances have made it possible to get cool roofs in a variety of architecturally pleasing colors. The benefits of a cool roof may include:Lowering cooling costs1Reducing the load on air conditionersEligibility for rebates and other incentives from utilities or government programs4Decreasing roof temperature, which may help extend the life of the roofCurious to learn more? Contact a local roofing professional today to find out how you can reduce the temperature of your roof, either through increased ventilation or by installing a cool roof system.1 Energy cost savings are not guaranteed and the amount of savings may vary based on climate zone, utility rates, radiative properties of roofing products, insulation levels, HVAC equipment efficiency, and other factors.2,3: From energy.gov/energysaver/cool-roofs.4 Rebates and Incentives, and their eligibility requirements vary and availability is not guaranteed.

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Optimizing Thermal Resistance and Inertia in Low-Slope Roof Design

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