Photo courtesy GAF
The roof and its components, including the insulation, play an important role in a project’s sustainability.
A Roof
As simply defined by Oxford, a roof is the structure forming the upper covering of a building. Roofs are put on an enclosure to keep occupants warm and dry. However, they can also play a pivotal – and often underestimated – role in the sustainability story of a project.
The right roof can support efficiency and sustainability goals through proper insulation, solar reflectivity, and airtightness. Roofing materials and overburden technology can increase valuable usable space as well as providing enhanced stormwater management for the site. Through a combination of well-selected materials, roofs can be designed with long-term durability and product circularity in mind. This helps reduce the impacts of waste. Partnering with a reputable manufacturer allows design professionals to provide validation of critical attributes to use to certify their building design to sustainability goals.
There are many possibilities for making a roof that meets performance and sustainability goals. Designing and specifying the right roof for the project comes down to categorizing what attributes are most important to the client, the project, and the local environment.
The Roof and Energy Efficiency
As the top of any building enclosure, the roof has a direct impact on the overall energy efficiency of the structure. The main control layers for a roof assembly consist of air, water, thermal, and vapor control, provided by several different materials. Which materials provide the primary control layer and how the control layers are assembled depends on the needs of the building: the membrane and thermal insulation play a pivotal role in all four. The energy efficiency of the roof assembly is strongly influenced by air and thermal control properties of the roof “sandwich”.
Insulation
Insulation is a critical part of any roofing assembly as it contributes to the overall energy efficiency of the building. According to a study completed by ICF International in 2021 , roofing assemblies installed to current energy code requirements, as compared to those installed prior to 2015, resulted in whole building energy savings of 2-11%. The study also found that savings increase for colder climates and are highest for building types with large roof-to-floor ratios.
The basics
The effectiveness of roof insulation is determined by its R-value, which is a measure of thermal resistance. The higher the R-value when expressed per inch, the better the thermal performance of the insulation. The greater the total effective R-value of the assembly, the greater its effectiveness at maintaining interior temperatures.
The International Energy Conservation Code (IECC) specifies minimum insulation R-values. It is important to note that in each climate zone, next to at least part of each required R-value, ‘ci’ is denoted. This mark indicates continuous insulation. Continuous insulation must typically be outboard of the structural components. Similar to the continuous air barrier required by code, the insulation should also be continuous between assemblies: the insulation from the roof should continue, without interruption, to the exterior wall insulation. Continuous insulation will assist in maintaining interior temperatures but will also eliminate voids in the building envelope where warm air is likely to condense on uninsulated, cooler surfaces.
The most common roof insulation materials are polyisocyanurate (polyiso), extruded polystyrene (XPS), and expanded polystyrene (EPS), each with different chemical compositions and material properties. Polyiso has the highest R-value at 5.7 per inch, followed by XPS with an R-value of 4.7 per inch and EPS with an R-value of 3.6 per inch. Higher R-value per inch means less material thickness is required to achieve the desired insulating value. Selecting the right insulation material depends on factors like building use, climate, anticipated moisture exposure, and anticipated roof traffic. Vacuum insulated panels (VIP) with an R-value of 28 per inch and mineral wool (~R-4 per inch) round out the other common types of insulation.
Photo courtesy of Siplast
Applying lightweight insulating concrete (LWIC) on the roof.
LWIC
Lightweight insulating concrete (LWIC) is an alternate type of insulation where EPS boards are encapsulated in a slurry coat of concrete, upon which the membrane and any overburden would be placed. The R-value and overall thickness of the LWIC system are tailored to each unique project. LWIC can be advantageous for use on buildings with slope limitations because a desired slope can be achieved with the LWIC system itself. There are also no gaps within the installation, such as between standard insulation boards. Additionally, fasteners are not required to attach the LWIC, so there is limited thermal bridging and air leakage within the system.
The thickness of the overall installation has an additional impact on the design of the roof system. It is an industry standard for roof flashings to extend a minimum of 8-inches past the completed installation. Flashing heights are of particular importance at mechanical curbs and parapets.
The impact of roof insulation on energy usage
Insulation, and specifically continuous insulation on low-slope roofs, is especially relevant as insulation products occupy a unique position at the intersection of embodied and operational carbon emissions for a building. According to the Embodied Carbon 101: Envelope presentation by the Boston Society for Architecture, 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.
Just how much does insulation impact whole building energy use? As the R-value is increased above the roof deck, designers can increase energy savings by up to 12%. Design strategies can further increase insulation’s effectiveness. These include locating insulation above the deck, specifying insulation layers that are staggered and using offset joints to reduce air flow. Designs that reduce thermal transfer between the indoor and outdoor environment are of critical importance.
Strategies to make insulation work for your energy goals
Images courtesy GAF
Different attachment methods for insulation are shown here, including mechanically attached, induction welded, and an adhered assembly with only the first layer of insulation mechanically attached.
The installation method for the insulation impacts the heat and air flow through the roof assembly. Image 3 shows different attachment methods such as fully mechanically attached, induction welded and an adhered assembly with only the first layer of insulation mechanically attached. The 2021 IECC states that board insulation should be installed in a minimum of two layers, and the joints in the top layer should be offset from the joints in the underlying layer. The edges of adjacent boards should be in contact to limit gaps between boards. Air and heat that can travel between gaps between boards result in a thermal bypass and energy loss. Gaps between boards can increase condensation potential if there is air intrusion into the roof assembly. Air carries with it moisture vapor, which if allowed to condense adjacent to the insulation, can saturate the insulation boards. Wet insulation has a decreased R-value, even getting close to zero for some insulation types, which is like having no insulation at all.
The tighter and better insulated a building, the greater the difference in effective R-value from weak points in the thermal control layer. Roof fasteners have a measurable impact on the R-value of roof insulation. In recent years, changes in wind speeds, design wind pressures, and roof zones, as dictated by ASCE 7-16 and 7-22, mean that fastener patterns are, in many cases, becoming denser for roof assemblies. These changes require more metal on average per square foot of roof than ever before. More metal means that more heat can escape the building in winter and can enter the building in summer through thermal transfer.
By making buildings more robust against wind uplift, designers are also, in effect, making them less robust against the negative effects of hot and cold weather conditions. In a study presented at the RCI, Inc. Building Envelope Technology Symposium in 2018, researchers used a base case building with 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.
Image courtesy GAF
Insulative losses generated by location of fasteners within the roof assembly.
There are several design strategies and products that combat this loss of performance. Using an induction-welded system reduces the number of total roof fasteners compared to a mechanically attached system. The fasteners with special plates are installed to secure the insulation to the roof deck. The fastener plates are then welded to the underside of the roof membrane using an induction heat tool. This process eliminates the need for additional membrane fasteners. As a bonus, induction welded systems have less gap between the membrane and insulation or coverboard when the system is pressurized, resulting in less room for moist air and a decrease in the amount of potential condensation.
For additional reduction in thermal bridging impacts, designers should consider burying fasteners beneath one or more layers of insulation and adhering the remainder of the roof assembly. Multiple studies have shown that placing fastener heads and plates beneath a cover board such as high-density (HD) polyiso, or beneath one or two layers of staggered insulation, can lessen the thermal bridging effects of fasteners.
Using an insulating coverboard also prevents energy loss through thermal transfer and brings additional benefits. Coverboards provide enhanced longevity and system performance by protecting roof membranes and insulation from hail damage; they allow for improved aesthetics and offer additional R-value and mitigation of thermal bridging when adhered. If thermal bridging cannot be reduced enough to meet project needs, it may be necessary to increase the R-value of the roof insulation. If fasteners diminish the effective thermal performance of roof insulation, building owners are not getting the benefit of the designed R-value. Extra insulation beyond the code minimum can be specified to make up the difference.
Photo courtesy GAF
The roof and its components, including the insulation, play an important role in a project’s sustainability.
A Roof
As simply defined by Oxford, a roof is the structure forming the upper covering of a building. Roofs are put on an enclosure to keep occupants warm and dry. However, they can also play a pivotal – and often underestimated – role in the sustainability story of a project.
The right roof can support efficiency and sustainability goals through proper insulation, solar reflectivity, and airtightness. Roofing materials and overburden technology can increase valuable usable space as well as providing enhanced stormwater management for the site. Through a combination of well-selected materials, roofs can be designed with long-term durability and product circularity in mind. This helps reduce the impacts of waste. Partnering with a reputable manufacturer allows design professionals to provide validation of critical attributes to use to certify their building design to sustainability goals.
There are many possibilities for making a roof that meets performance and sustainability goals. Designing and specifying the right roof for the project comes down to categorizing what attributes are most important to the client, the project, and the local environment.
The Roof and Energy Efficiency
As the top of any building enclosure, the roof has a direct impact on the overall energy efficiency of the structure. The main control layers for a roof assembly consist of air, water, thermal, and vapor control, provided by several different materials. Which materials provide the primary control layer and how the control layers are assembled depends on the needs of the building: the membrane and thermal insulation play a pivotal role in all four. The energy efficiency of the roof assembly is strongly influenced by air and thermal control properties of the roof “sandwich”.
Insulation
Insulation is a critical part of any roofing assembly as it contributes to the overall energy efficiency of the building. According to a study completed by ICF International in 2021 , roofing assemblies installed to current energy code requirements, as compared to those installed prior to 2015, resulted in whole building energy savings of 2-11%. The study also found that savings increase for colder climates and are highest for building types with large roof-to-floor ratios.
The basics
The effectiveness of roof insulation is determined by its R-value, which is a measure of thermal resistance. The higher the R-value when expressed per inch, the better the thermal performance of the insulation. The greater the total effective R-value of the assembly, the greater its effectiveness at maintaining interior temperatures.
The International Energy Conservation Code (IECC) specifies minimum insulation R-values. It is important to note that in each climate zone, next to at least part of each required R-value, ‘ci’ is denoted. This mark indicates continuous insulation. Continuous insulation must typically be outboard of the structural components. Similar to the continuous air barrier required by code, the insulation should also be continuous between assemblies: the insulation from the roof should continue, without interruption, to the exterior wall insulation. Continuous insulation will assist in maintaining interior temperatures but will also eliminate voids in the building envelope where warm air is likely to condense on uninsulated, cooler surfaces.
The most common roof insulation materials are polyisocyanurate (polyiso), extruded polystyrene (XPS), and expanded polystyrene (EPS), each with different chemical compositions and material properties. Polyiso has the highest R-value at 5.7 per inch, followed by XPS with an R-value of 4.7 per inch and EPS with an R-value of 3.6 per inch. Higher R-value per inch means less material thickness is required to achieve the desired insulating value. Selecting the right insulation material depends on factors like building use, climate, anticipated moisture exposure, and anticipated roof traffic. Vacuum insulated panels (VIP) with an R-value of 28 per inch and mineral wool (~R-4 per inch) round out the other common types of insulation.
Photo courtesy of Siplast
Applying lightweight insulating concrete (LWIC) on the roof.
LWIC
Lightweight insulating concrete (LWIC) is an alternate type of insulation where EPS boards are encapsulated in a slurry coat of concrete, upon which the membrane and any overburden would be placed. The R-value and overall thickness of the LWIC system are tailored to each unique project. LWIC can be advantageous for use on buildings with slope limitations because a desired slope can be achieved with the LWIC system itself. There are also no gaps within the installation, such as between standard insulation boards. Additionally, fasteners are not required to attach the LWIC, so there is limited thermal bridging and air leakage within the system.
The thickness of the overall installation has an additional impact on the design of the roof system. It is an industry standard for roof flashings to extend a minimum of 8-inches past the completed installation. Flashing heights are of particular importance at mechanical curbs and parapets.
The impact of roof insulation on energy usage
Insulation, and specifically continuous insulation on low-slope roofs, is especially relevant as insulation products occupy a unique position at the intersection of embodied and operational carbon emissions for a building. According to the Embodied Carbon 101: Envelope presentation by the Boston Society for Architecture, 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.
Just how much does insulation impact whole building energy use? As the R-value is increased above the roof deck, designers can increase energy savings by up to 12%. Design strategies can further increase insulation’s effectiveness. These include locating insulation above the deck, specifying insulation layers that are staggered and using offset joints to reduce air flow. Designs that reduce thermal transfer between the indoor and outdoor environment are of critical importance.
Strategies to make insulation work for your energy goals
Images courtesy GAF
Different attachment methods for insulation are shown here, including mechanically attached, induction welded, and an adhered assembly with only the first layer of insulation mechanically attached.
The installation method for the insulation impacts the heat and air flow through the roof assembly. Image 3 shows different attachment methods such as fully mechanically attached, induction welded and an adhered assembly with only the first layer of insulation mechanically attached. The 2021 IECC states that board insulation should be installed in a minimum of two layers, and the joints in the top layer should be offset from the joints in the underlying layer. The edges of adjacent boards should be in contact to limit gaps between boards. Air and heat that can travel between gaps between boards result in a thermal bypass and energy loss. Gaps between boards can increase condensation potential if there is air intrusion into the roof assembly. Air carries with it moisture vapor, which if allowed to condense adjacent to the insulation, can saturate the insulation boards. Wet insulation has a decreased R-value, even getting close to zero for some insulation types, which is like having no insulation at all.
The tighter and better insulated a building, the greater the difference in effective R-value from weak points in the thermal control layer. Roof fasteners have a measurable impact on the R-value of roof insulation. In recent years, changes in wind speeds, design wind pressures, and roof zones, as dictated by ASCE 7-16 and 7-22, mean that fastener patterns are, in many cases, becoming denser for roof assemblies. These changes require more metal on average per square foot of roof than ever before. More metal means that more heat can escape the building in winter and can enter the building in summer through thermal transfer.
By making buildings more robust against wind uplift, designers are also, in effect, making them less robust against the negative effects of hot and cold weather conditions. In a study presented at the RCI, Inc. Building Envelope Technology Symposium in 2018, researchers used a base case building with 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.
Image courtesy GAF
Insulative losses generated by location of fasteners within the roof assembly.
There are several design strategies and products that combat this loss of performance. Using an induction-welded system reduces the number of total roof fasteners compared to a mechanically attached system. The fasteners with special plates are installed to secure the insulation to the roof deck. The fastener plates are then welded to the underside of the roof membrane using an induction heat tool. This process eliminates the need for additional membrane fasteners. As a bonus, induction welded systems have less gap between the membrane and insulation or coverboard when the system is pressurized, resulting in less room for moist air and a decrease in the amount of potential condensation.
For additional reduction in thermal bridging impacts, designers should consider burying fasteners beneath one or more layers of insulation and adhering the remainder of the roof assembly. Multiple studies have shown that placing fastener heads and plates beneath a cover board such as high-density (HD) polyiso, or beneath one or two layers of staggered insulation, can lessen the thermal bridging effects of fasteners.
Using an insulating coverboard also prevents energy loss through thermal transfer and brings additional benefits. Coverboards provide enhanced longevity and system performance by protecting roof membranes and insulation from hail damage; they allow for improved aesthetics and offer additional R-value and mitigation of thermal bridging when adhered. If thermal bridging cannot be reduced enough to meet project needs, it may be necessary to increase the R-value of the roof insulation. If fasteners diminish the effective thermal performance of roof insulation, building owners are not getting the benefit of the designed R-value. Extra insulation beyond the code minimum can be specified to make up the difference.
Air tightness
While perhaps the most pronounced criteria dictating a roof’s impact on a building’s energy efficiency is the insulation, mitigation of air infiltration is also extremely important.
Air moving through a building enclosure presents several kinds of risks. First, uncontrolled air flow reduces the efficiency of a building’s conditioning system (HVAC). Building owners pay a lot for conditioned air. Uncontrolled air infiltration and exfiltration can make up 25% to 40% of the total heat loss through a building enclosure in a cold climate. Air infiltration and exfiltration make up 10% to 15% of total heat gain in a hot climate. Losing one-third or more of conditioned air has a significant impact on the operational costs of a building. A 2005 study funded by the National Institute of Standards and Technology (NIST) simulated infiltration and exfiltration reductions in buildings and reported that “Predicted potential annual heating and energy cost savings…ranged from 2% to 36%, with the largest savings occurring in heating-dominated climates…”
Image courtesy GAF
Uncontrolled air movement can present serious concerns to a building’s overall performance. The air barrier is, critically, not usually just one material, but instead is an integrated system of many different materials and components. It controls air leakage, air intrusion and convective heat flow through the building enclosure.
Ensuring air tightness
In 2012, the IECC first published air barrier requirements, stating that a “Continuous air barrier shall be provided throughout the building envelope…” The purpose of an air barrier is straightforward: first, to minimize the uncontrolled movement of air through the building enclosure, and second, to reduce energy loss and increase building energy efficiency.
When it comes to the efficiency of the building enclosure, the goal of an air barrier is to prevent the loss of conditioned air from the interior to the exterior and the introduction of warm, humid air from the exterior to the interior. Just as thermal control is more than a layer of insulation as it needs to take into account thermal bridging, a single material that blocks air is insufficient in and of itself. An effective air barrier must be a part of a system: an interconnected series of materials, accessories and components spanning the entire enclosure. As such, the connection of air barriers at interfaces, such as the roof to exterior wall interface, is fundamental.
Image courtesy of Sustainable Building Partners
The detailing of the interface between the wall and roof is critical to get correct in order to prevent performance issues and increase building energy efficiency.
This starts by defining the air control layer within the roof assembly followed by detailing the air control layer at all penetrations, transitions and interfaces. This includes the transition from the roof assembly to the air control components in the wall assembly. Roof edges, or terminations of the roof assembly, strongly influence the amount of air leakage through a building enclosure as is highlighted in the infrared image in Image 6. This consideration for how the roof is connected with the surrounding walls, and the various control layers of the walls, must be evaluated during the design and documentation phases of a project to ensure success .
The air barrier is, critically, not usually just one material, but instead is an integrated system of many different materials and components. It controls air leakage, air intrusion and convective heat flow through the building enclosure
(Reference image 5).
Section 2: The Roof and Solar Reflectivity
Image courtesy of Siplast
Membrane color can play a significant role in a roof – and building’s – performance. This modified bitumen roof assembly has highly reflective granules that also remove greenhouse gas from the air.
For roof assemblies where the membrane is located at the top of the system, the membrane’s color can have a significant impact on both the performance of the system and on the roof surface temperatures. Reflective, or light- or white-colored roof membranes, can lower the ambient roof temperature. Two roof surface temperatures, differing only by color, can vary by as much as 60 degrees Fahrenheit in the summer heat. Dark colored roofs can reach higher than 150°F, whereas white or reflective roofing colors can have significantly lower surface temperatures.
Images courtesy GAF
A lighter colored roof membrane has a significantly lower surface temperature.
Using a lighter colored roof can decrease the urban heat island effect in cities and may also decrease the amount of heat that is able to radiate into a building’s interior. Conversely, the more heat that a roof assembly absorbs, the warmer the interior temperature will be. In the summer, the heat gain is offset by HVAC systems: the warmer the interior temperature is, the longer the cooling systems have to run, which can increase energy use, and potentially raise energy bills.
Understanding “cool roofs” and reflectivity
As defined by the Cool Roof Rating Council®, a cool roof is one “that strongly reflects sunlight (solar energy) and also cools itself by efficiently emitting any heat that was absorbed.” A cool roof does not need to be white. There are many “cool color” products which use darker-colored pigments that are highly reflective in the near infrared (non-visible) portion of the solar spectrum.
There is a set of terminology associated with cool roofing: these terms include albedo, emissivity, reflectivity, and SRI. For further information and definitions, visit the Cool Roof Rating Council (https://coolroofs.org/resources).
Albedo: Albedo is defined as the ratio of the diffuse reflection of solar radiation out of the total solar radiation hitting a surface. It is measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that reflects all incident radiation. It is commonly used in climate science as a measure of the reflectance of solar energy from the earth’s surface. Albedo is the potential for a surface to reflect sunlight.
Emissivity: Emissivity is defined as the ratio of the energy radiated from a material’s surface to the energy radiated from a perfect emitter, known as a blackbody, at the same temperature and wavelength and under the same viewing conditions. It is a dimensionless number between 0 (for a perfect reflector) and 1 (for a perfect emitter). Simply put, the emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation or heat. Dark surfaces absorb heat and have an emissivity value closer to 1. Emissivity is different from albedo in that emissivity involves the radiation emitted from a surface, whereas albedo involves the radiation reflected from a surface.
Reflectivity: Reflectivity is defined as the property of a material to reflect light or radiation. It is a measurement of reflectance irrespective of the thickness of a material. For most reasonably thick opaque materials, reflection occurs within the first few nanometers of the surface and any light transmitted more than about 100nm into the material is quickly absorbed. Reflectivity is measured on a scale from 0 to 1, where 1 is 100% reflective.
SRI: The Solar Reflectance Index (SRI) is an indicator of the ability of a roof surface to return solar energy to the atmosphere. Solar reflectance and thermal emittance measurements are combined to calculate the solar reflective index (SRI), which can range from -10 to 125 with most materials falling between 0 and 100. Roofing material surfaces with a higher SRI will be cooler than surfaces with a lower SRI under the same solar energy exposure, especially on a sunny day.
There are established standards governing and regulating cool roofs. The most common of these include California Energy Commission Title 24, ENERGY STAR, the U.S. Green Building Council’s Leadership in Energy and Environmental Design® (LEED), and ASHRAE 189.1. Additionally, the IECC sets minimum standards for solar reflectance, thermal emittance and SRI for Climate Zones 0-3: The 2021 IECC has a minimum 3-year aged value of solar reflectance of 0.55, minimum 3-year aged value of thermal emittance of 0.75 and minimum 3-year aged value for SRI of 64. LEED 5.0 has a requirement of a minimum initial SRI of 82 and 3-year aged SRI of 64 for low-sloped roofs.
The Cool Roof Rating Council® (CRRC) offers an online database to look up information on products and set desired project inputs. This tool is available at https://coolroofs.org/directory/roof.
Materials to meet project goals
The saying goes, “there is no bad roof, rather it is important to find the right roof for the project.” Reflective roof membranes, otherwise known as cool roofs, account for greater than 50% of roof surfaces installed onto low-slope commercial buildings each year. The percentage is even higher in the southern half of the United States. Such reflective roofs have shown the potential for decreasing cooling energy consumption by lowering roof temperatures (Freund et al. 2006; Ennis and Desjarlais 2009; Gaffin et al. 2010; Graveline 2013).
This research strongly suggests that, regardless of location and local climate, selecting high reflectivity roof materials can result in heating and cooling energy savings for a building. This is based on a building that is heated with natural gas and cooled by electric air conditioning. While a building designer should model energy costs versus roof reflectivity on a case-by-case basis, some level of savings can be expected. Even when aged reflectance and emittance values for cool roofs are factored in, SRI values remain at 85% of the initial installed values. This would suggest that energy cost savings achieved with cool roofs will remain, albeit slightly reduced, throughout the life expectancy of the roof.
Today, there are many product options that meet high reflectivity requirements. Most roof membrane technologies have a solution: thermoplastic membranes such as TPO and PVC are typically white or light in color; the same is true for liquid applied technologies. Modified bitumen roofs can integrate highly reflective granule cap sheets which meet or exceed current SRI standards, such as Title 24. Additionally, some modified bitumen cap sheets are available with depolluting granules. These granules (see Image 6) actively remove nitrous oxides, a greenhouse gas, from the air. Finally, while EPDM membranes are traditionally dark in color, higher reflectivity materials are also available.
Some types of roof membranes provide another property toward meeting sustainability goals: rainwater collection. Some single ply membranes, such as TPO, and some liquid applied membranes are certified to NSF International Protocol 151. This certifies that the membrane is able to be used in systems that harvest rainwater from the roof for capture and re-use.
Section 3: Understanding Overburden
Photo courtesy GAF
Installation of overburden. This roof combines vegetative space and a plaza deck for multiple sustainability benefits.
Current market trends and legislation are providing enhanced opportunities in sustainability certifications and code compliance for roofs. When building codes and certification bodies require high SRI roof membranes, there is typically an exception when a substantial portion of the roof is covered by overburden.
Roofing overburden is defined as “any manner of material, equipment or installation that is situated on top of, and covering all or a portion of, a roof or waterproofing membrane assembly.” There are many overburden options and roof assembly considerations to weigh in achieving sustainability goals. This can include vegetative roof assemblies, planters, plaza decks, stormwater management, and integration of equipment such as solar photovoltaic (PV) arrays.
Types of overburden
Vegetated roofs, or green roofs, can either be provided in multicourse, layered assemblies or in pre-vegetated trays or modules. Layered vegetated roof assemblies commonly include, root barriers, drainage and water retention layer(s), engineered growing media, and plant material. Layered assemblies can be easily tailored to build thinner extensive roofs or deeper, intensive vegetated roofs. Tray or module systems are generally available in shallower depths less than 8” and can include a range of extensive plants, which may include sedums and succulents, herbaceous perennials, or ornamental grasses..
Stormwater management on the roof seeks to achieve volume management through either retention or detention. Retention strategies are designed to hold on to water for use by the building, such as for irrigation or toilet flushing. Detention methods hold the water until it is allowed to slowly drain in a controlled-flow system. “Blue roof” is a term used to define a roof that helps to retain or detain stormwater. Blue roofs commonly incorporate a vessel or structural element to store the water. Blue roofs with the goal of detaining stormwater have drains fitted with flow control drains.
Paver and plaza deck assemblies are great ways to incorporate occupied space over low-slope roofs. They can extend the usable square footage of a building by adding occupiable space on the roof. The finished surface of the plaza deck can either be constructed utilizing traditional at-grade construction methods or pedestal set systems. Pedestal set systems allow for easy access to membrane inspection, drains, and other building mechanicals below while compensating for slope and providing a level finished surface and open joint drainage.
Municipalities benefit from detention roof assemblies as it reduces peak flow to the stormwater system by extending the amount of time over which the water is released. Rainfall onto the roof temporarily stored until its release is managed using orifices, weirs, or other outlet devices that control the discharge rate of rooftop runoff. A blue-green roof is a blue roof with a vegetative roof assembly installed over top. A purple roof is a patented “sponge” roof assembly that is designed to retain and detain rainwater. It incorporates a sponge-like layer made of hydrophilic mineral wool, a dense polyester fabric detention layer, and may or may not include an additional honeycomb layer to increase the volume of rainwater that can be managed.
Image courtesy of Siplast
Detailed cutaway of a blue-green roof assembly.
Rooftop Solar: Solar PV panels come in both single sided and bifacial (double-sided) modules. Modules are traditionally supported by racking systems on the rooftop that are either mechanically attached to the roof, mechanically attached to a structural canopy that is attached to the roof, or held in place with ballast. The emerging practice of agrivoltaics, as shown in Image 12, or agriculture combined with photovoltaics, is showing promise as a mutually beneficial, symbiotic solution to not only increase the efficiency of solar panels, but also increase plant size and crop yields by shading and limiting soil evaporation.
Photo courtesy GAF
A roof with agrivoltaics, the practice of combining green roofs with photovoltaics.
Membrane considerations
There are environmental, social, and economic benefits to designing overburden. These include the reduction of the urban heat island, increased habitat recreation, improved stormwater management and reduced risk of localized flooding, biophilic design, improved air quality, and increased usable space. Additionally, overburden enhances whole building energy efficiency, by saving dollars and increasing the membrane’s life expectancy, which is one of the biggest economic benefits to the building owner.
The overburden’s success depends on the membrane assembly beneath. The ability to manage stormwater and the increased water load means it is important to make sure the design incorporates the appropriate material for application. Waterproofing membranes like hot rubberized asphalt, SBS modified bitumen, and liquid applied PMMA are all robust, reinforced systems commonly employed in these assemblies. Quality control during the installation process, including detailing is essential. The compatibility of the membrane, flashing materials and accessory products must be verified.
For installations such as solar or vegetated, the plants and solar array will need to be accessed for service or removed for repairs or replacement. These anticipated actions require a more robust roof assembly for the long-term durability and serviceability of the complete roof. For fully covered roofs such as vegetative roofs or plaza decks, hot-rubberized asphalt waterproofing or a modified bitumen with a mineral cap sheet are durable, reinforced options. For solar arrays, the membrane must be assessed for its durability to heat, its adaptability to the mounting types being considered, and its ability to withstand the additional traffic from maintenance of the systems. High-performing TPO or modified bitumen are common solutions. Regardless of the specific system specified, a high level of durability is required for membranes to be buried under these assemblies.
Section 4: Durability of the Roof
Photo courtesy of Siplast.
The durability of a roof is an often overlooked part of its sustainability. Image of a roof assembly consisting of LWIC and modified bitumen.
When attendees of a recent webinar were asked what their number one desired characteristic of a sustainable roof was, the product’s durability comes first. Durability refers to a product or assembly’s capacity to withstand wear, pressure, or damage . At the roof, durability specifically refers to UV durability, weatherability and resistance to the loads acting upon it.
Defining durability and resiliency at the project level
Durability over the life expectancy of a roof assembly is a challenge, as the roof faces wear and tear not only from chronic stressors like moisture and temperature, but also from acute events like storms. Part of designing a durable roof is not just membrane selection, but designing the roof system for resilience. Further, technologies that offer an extended lifespan due to enhanced durability preserve the use of the building during storms and minimize waste from more frequent re-roof or replacement.
Depending on the building location, designing for impacts from hail can be one of the most important factors to implement durability into roof design. Impact resistance is a system discussion: it includes the membrane as well as the roof attachment method, coverboard, and insulation selection. Thicker membranes, including fleece-back membranes offer increased impact resistance for both hail and foot traffic. Attachment method also becomes crucial, as impact over a fastener can result in a laceration of the membrane. Adhered systems where fasteners are buried further in the assembly offer better impact resistance and higher hail resistance ratings. Finally, coverboards can bolster the roof system’s performance as they provide a more rigid substrate than standard insulation beneath the membrane. Higher density coverboards should be installed where there is greater risk of very severe hail and increased frequency of foot traffic.
Redundancy has extra importance for roofs in areas facing high storm damage risk. A hybrid roof approach can blend the best attributes of an asphaltic roof and single ply membrane together. The redundancy and toughness of an asphaltic membrane overlaid with the lighter colored single ply membrane utilizes the strengths of each roof technology. Asphaltic membranes, used as the first layer, provide redundancy and protection against punctures as they add 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 have generally darker color options as compared to single ply membranes. The addition of a single-ply membrane can 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. Single ply membranes also have a higher resistance to ponding water.
Section 5: Circularity and Sustainability
Photo courtesy GAF
Circularity aims to avoid waste and enhance sustainability.
“The circular economy is a system where materials never become waste and nature is regenerated. In a circular economy, products and materials are kept in circulation through processes like maintenance, reuse, refurbishment, remanufacture, recycling, and composting. The circular economy tackles climate change and other global challenges, like biodiversity loss, waste, and pollution, by decoupling economic activity from the consumption of finite resources.”
Defining circularity at the project level
On an individual project level, applying the principles of a circular economy (CE) helps to close the loop on material waste, preserves natural resources, and works seamlessly to address a project’s sustainability goals. Often, using products and materials that are recycled or reusable also improves the health, welfare, and safety of families in residential settings. Incorporation of pre- and post-consumer recycled content means consuming less raw materials and thereby generating a lower carbon impact for the site.
Circularity for the roof involves looking at what happens from initial manufacture, through useful life, and finally, what processes occur when the product is no longer useful as a roof. One of the main facets is recycling. Recycling or take back programs are already in place for asphalt shingles but are evolving for other materials, including tear-off recycling initiatives for single-ply membranes. Analyzing recycled content level is a valuable place for the designer to begin. One can start by assessing whether a product includes recycled content and at what percentage, and then verify if the recycled content claim is documented by a third party.
Designers can also review product Life Cycle Assessments (LCAs) to understand the environmental impact of each product’s production stages, from raw material extraction to end-of-life. In tandem, responsible manufacturers are using LCA information to identify areas of improvement and make informed decisions to further reduce a product’s environmental impact, resulting in a reduction in embodied carbon. The knowledge gained from LCAs creates the potential for product improvements and new innovations to help further sustainability goals.
Section 6: Validating Sustainability Through Documentation
Developers, designers, architects, and builders are rethinking development and design to improve the health and well-being of individuals and the community as a whole. The surge toward transparency is fostering collaboration in the broader building, construction, and design space to help reduce the built environment’s total carbon emissions . Roofing has long relied on industry-wide Environmental Product Declarations (EPDs) created from aggregate product data from multiple materials. As a result, architecture, engineering, and construction (AEC) community members have had fewer opportunities to make informed sustainability choices around roofing materials. While a step in the right direction, product-specific EPDs offer a more realistic picture. Leading roofing manufacturers are now issuing product-specific Environmental Product Declarations, allowing companies and customers to make more informed and accurate product sustainability decisions.
EPDs are critical to improving green building solutions. These standardized and third-party-verified documents outline the environmental impacts associated with a building product’s life cycle, from raw material extraction to end-of-life disposal or reuse. Independent, third party certification is especially valuable in bringing to market safer and more reliable products. Certified product claims are verified independently, rather than relying on a manufacturer’s word. Audits of a manufacturer’s facilities prior to certification occur under certain certification systems. This additional step allows consumers to have confidence in how a product or material is produced and provides a more transparent view into the product’s lifecycle. Through the EPD creation process, a manufacturer is better able to advance sustainability goals, demonstrate their commitment to the environment and customers, and increase product sustainability in the roofing industry.
Third party certifications offer the assurance of performance in today’s dynamic and innovative market sector. They allow for confident selection and installation of both cutting-edge and well-known materials and products, ensuring the best fit to improve the health and well-being of individuals, while also creating an attractive and high-investment return for developers and builders.
In addition to Environmental Product Declarations, other third-party product-specific disclosures and certifications such as product specific Health Product Declarations (HPDs), GREENGUARD Gold Certifications, Recycled Content Certifications, and Declare Labels are submittal requirements of several of the most popular green building rating systems such as LEED, The Living Building Challenge v4, and Well Building Certification (Well v2) systems.
A published Health Product Declaration discloses the ingredients in the building material or system, allowing the project team to specify products with confidence. The International Living Future Institute (ILFI) Declare certification creates a process for disclosing product ingredients in a transparent way, allowing manufacturers an opportunity to provide a clear list of materials in each building product. For design professionals, the Declare product database streamlines material specification and certification for sustainable design.
An active Declare label with a status of LBC Red List Free, LBC Red List Approved, or LBC Compliant at the time of specification is sufficient documentation of product compliance with I13 Red List. The Declare label also demonstrates alignment with other requirements within the Living Building Challenge and Core Green Building Certification, as well as LEED and WELL certifications. These requirements include ingredient disclosure thresholds, VOC content and emissions, embodied carbon, and responsible sourcing, including FSC Chain of Custody.
All of this product documentation proves that performance, safety, health, and sustainability goals for a project have been met. They demonstrate to the client how a project plans to meet the criteria of the rating system and other sustainability goals that the project is working towards.
Using roofing to meet project goals
The roof insulation and membrane system selected support net positive energy use and provide a durable substrate for the building’s solar arrays, a net positive water use through a rainwater catchment system incorporated into the roof deck, and materials that are safe for all species through time with membrane, insulation, and accessories that are Red List approved. The TPO membrane and non-halogenated insulation products helped meet the material specs of an LBC-compliant roof system. The high-performing TPO membrane is NSF P151 certified to support rainwater catchment and has higher UV durability to support the additional traffic and heat from the solar arrays.
The roof assembly specification for the Stanley Center included high-performing TPO membrane and non-halogenated polyiso insulation. “GAF already had a majority of their products approved for Red List Compliance, making them an ideal partner for this project”, explained Mike Kerker, Senior Estimator for Black Hawk. Integration of the manufacturer with the project team helped ensure that the production of the materials would meet the timeline. In addition to carrying a large solar array, the roof system is required to capture the rainwater that falls on it. “This requires a very specific material and [the TPO material selected] enables alignment and makes it an easy choice”, explained Brandon Fettes, Architectural Designer for Neumann Monson.
In Summary…
Roofs are put on a building to keep people warm and dry, but they can also be a key component of the sustainability story of a project. They support energy efficiency goals through insulation, solar reflectivity and airtightness. They can help with stormwater management, increase occupiable space or support on-site energy creation through the use of overburden. They can be designed with long-term durability and product circularity in mind to reduce the long-term impacts of the assembly at the end of the products’ life cycles. And, best of all, product documentation creates a transparent way to show clients exactly what is being accomplished through the building design.
References:
1. 79D Commercial Building Energy-Efficiency Tax Deduction.
2. “Embodied Carbon 101: Basic Literacy.” Boston Society for Architecture. Session hosted June 1, 2020. Accessed February 14, 2025.
3. Ray, Stephen and Leon Glicksman, PhD. “Potential Energy Savings of Various Roof Technologies.” 2010 ASHRAE. . Accessed March 28, 2025.
4. Molletti, Sudhakar PhD. “Mind the Gap.” Professional Roofing. September 2020. Accessed March 28, 2025.
5. Grant, Elizabeth. “Thermal Bridging Through Roof Fasteners: Why the Industry Should Take Note.” GAF Roof Views Blog. . Accessed February 14, 2025.
6. Ibid.
7. “Air Barriers.” Smart Energy Design Assistance Center. The University of Illinois Urbana-Champaign. Accessed February 14, 2025.
8. For further information on this topic, please see Meyer, Benjamin. “Parapets Part 1: Continuity of Control Layers.” GAF Roof Views Blog. September 27, 2019. Accessed March 28, 2025.
9. Oxford English Dictionary. Accessed March 31, 2025.
10. “What is a circular economy?” Ellen MacArthur Foundation. Accessed February 17, 2025.
11. Perez, Aly. “GAF Scaling Environmental Product Declarations – Publishes 21 EPDs.” August 13, 2024. Accessed February 19, 2025.
Aly Perez is the Sustainably Manager at GAF. With 6 years of experience in developing product sustainability documentation for building product manufacturers, Aly specializes in the creation of material health documentation. In her role at GAF, she manages the documentation development process, provides sustainability insight and education to internal teams, and collaborates closely with the integrated supply chain and research and development teams in searching for opportunities to embed sustainability across the product’s lifecycle.
Rick Kile is the Director of Green and Amenity Business for Siplast. Upon graduating from the University of Illinois, Rick practiced landscape architecture in both Illinois and Colorado, focusing on community development, commercial, institutional, and high end residential work. Rick has over ten years of experience in the vegetated roof industry, employed by leading roofing and waterproofing manufacturers where he has contributed to some of the most progressive and iconic vegetated roof and over structure landscape projects in the United States and abroad.
Andrea Wagner Watts is the Building Science Education Manager for GAF | Siplast Building & Roofing Science, engaging with industry professionals to provide guidance, technical support and education for roof and wall assemblies. With more than 15 years of experience in the industry, Andrea strives to improve the overall performance of the building enclosures through application innovation, product development and building science research. Andrea has published on building science, assembly interfaces, durability and resilience and holds multiple patents. She serves as an executive board member of ABAA, is the co-chair of their Technical Committee and chairs the ASTM E06 and D08 Task Groups on air barriers assemblies.
