Agrivoltaics

Agrivoltaics means ‘“Agri-” relating to food production and “-voltaic” relating to electricity production."²³ The emerging practice of agrivoltaics, 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.

Janine Benyus, author and Co-Founder of the Biomimicry Institute has asked, “How do we make the act of asking nature’s advice a normal part of everyday inventing?”²⁴ Agrivoltaics does just that. With the solar canopy acting as a forest canopy, plants grow underneath like a forest floor and compete for sunlight. The shade from the solar canopy makes the plants grow bigger. A co-benefit to this approach is that it also conserves water by minimizing evaporation and transpiration. This microclimate created by the solar and vegetation keeps temperatures cooler in warmer climates and helps solar arrays perform more efficiently.

Similar to all other overburden strategies, Thomas Hickey, a research associate who develops agrivoltaic systems for Sandbox Solar notes “... the challenge is that you can’t just convert any roof to solar agrivoltaics. It has to be flat, for one thing. It might require a significant retrofit to support the extra weight of the soil, plants, and panels. And you’d need to be damn sure it’s waterproofed.”²⁵ This reinforces the benefits of engaging manufacturers and experts specializing in agrivoltaics to support holistic project goals.

Roof System Considerations

While design of the overburden system is important, selection of the roof membrane, the waterproofing layer that protects the building, is critical for the success of the overburden installation. Two main considerations are membrane performance (including color, thickness, and attachment) and roofing assembly configuration, including location of the membrane in the assembly. The improper selection of membrane can result in water infiltration into the building, costly repairs, or even replacement. Special considerations to the type of insulation and the presence of a cover board should be explored. Most importantly, the roof system should be designed to have an equivalent or longer lifespan than that of the overburden systems.

Membrane performance is critical since overburden would require removal in order to repair or replace the membrane. For solar installations, the panels would need to be disassembled, which in addition to the costly labor to remove, storage of panels, potential for panel damage, and the cost to reinstall, also results in lost electricity generation. The ability to easily remove vegetation from vegetative roofs is dependent on the type of roof installed; tray systems may be simpler to remove for replacement, however, storage location and the ability to keep those removed trays alive may prove difficult. Depending on the location of the repairs, the catchment systems for blue or purple roofs may need to be disassembled. Likewise, for a repair, the overburden would need to be removed in order to assess and access the repair location. However, membrane performance is not only dependent on the overburden and associated roofing assembly materials, the design and installation are equally important. The design begins with determining the appropriate roofing assembly including insulation and cover board selection, completing the design down to the last termination and penetration detail, and installing the roof in accordance with the contract documents.

Membrane Selection

Roofing, as with any project, is not a one-size fits all, however, roof system design that aligns with or exceeds the life expectancy of the overburden installations should be considered and is best practice. Project specific considerations should be taken into account to determine the best assembly for the project. The first decision is the orientation of the roofing assembly, including location of the membrane. The type of overburden will determine whether the assembly should be a traditional assembly, where the membrane is on top, or an IRMA (inverted roof membrane assembly) roof where the membrane is at the deck level. For overburden applications such as a vegetative roof, placement of the membrane at the deck level will protect it from possible damage from vegetation roots. When a blue roof is a consideration, placement of a membrane at the top of the roofing assembly is ideal, so that water can easily be collected from the rooftop.

Typical membrane options for roofing assemblies can be considered, including both single-ply and asphaltic based membranes. Comparison of the location, the intended purpose, and service life are fundamental considerations. Membranes that will be used at the deck level should be evaluated for durability and longevity since access to them for repairs will be limited. Typical membrane options at the deck level for vegetative roof systems can be both modified bitumen, and single-ply membranes. Another common waterproofing assembly used at the deck level are hot-fluid-applied membranes. While an excellent choice for vegetative roof installations, without the installation of a cap sheet for UV protection, these would not be considered for a traditional roofing assembly. For the purposes of this article, traditional single-ply and modified bitumen roofing membranes will be reviewed for overburden applications. Typical membranes placed at the top level for solar installations are modified bitumen and single-ply. For blue roofs and water catchment systems, single-ply membranes are commonly used due to performance during ponded water conditions and since they are less likely to leach contaminants into the water. For catchment systems, it is recommended that single-ply membranes meet NSF P151 water standards.

Single-ply membranes include EPDM, TPO, and PVC. The primary difference between the membranes is chemical composition, performance when exposed to chemicals at the roof surface, and treatment of the seams. Single-ply membranes are the most popular roofing membrane today due to their ease of installation and low cost compared to asphalt based materials. Single-ply membranes are, as their name implies, a single layer of membrane roof material and are produced in rolls. The chemical make-up of each of the membranes is different, and therefore, each of the membranes is preferable to different applications. EPDM and PVC perform well when exposed to chemicals, which is an important consideration depending on the building use and overburden type. For vegetative roofing assemblies where fertilizers may be used, all membranes should be checked for compatibility with the chemicals being deposited on the roof. For water collection systems, TPO may be a preferable membrane since it meets NSF P151 water standards. Another difference between the single-ply membranes is the treatment of the seams. TPO and PVC membranes are heat welded at the seams, which produces a monolithic roof membrane. EPDM seams are adhered, which creates a strong seam, but over time, the adhesives may become brittle or deteriorate, leaving open seams in the roofing membrane, which may be problematic for long term performance with overburden systems.

Modified bitumen is a modern day built-up roof that consists of the same asphalt plies, but instead of building the plies on the roof, they are produced in rolls in a factory. Modified bitumen roofs offer the same durable protection of built-up with a faster and more consistent installation since the rolls are pre-manufactured offsite. Modified bitumen roofs have a base ply and a granulated cap sheet (called a two ply system), where the cap sheet provides an excellent wearing surface. Modified bitumen roofs have a high tolerance for chemicals, which makes them suitable for vegetative roof assemblies. While durable, over time, the granules in the cap sheet may become loose. This is typical for granules in a cap sheet, but this type of roofing would not be ideal for blue or purple roofs where there are catchment systems for water as granules may dislodge and collect in the catchment systems.

Membrane selections can further be influenced by color, thickness and attachment.

Membrane Color

For roof assemblies where the membrane is at the top of the system, such as for solar panel installations and blue roof assemblies, the color can have a significant impact on the performance of the system and also on the roof surface temperatures; reflective roof membranes can lower the ambient roof temperature. EPDM membranes are traditionally dark in color and TPO and PVC are typically white or light in color. Modified bitumen roofs can have light colored granules which can increase reflectivity of a roof’s surface. 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 up to 150 degrees Fahrenheit, whereas white or reflective roofing colors can have significantly lower surface temperatures. Using a lighter colored roof can decrease the urban heat island effect in cities, and also may decrease the amount of heat that is able to radiate into a building’s interior. The more heat gain that a roof assembly absorbs, the warmer the interior temperature will be. In the summer, while the heat gain is offset by HVAC systems, the warmer the interior temperature, the longer the systems have to run, which can increase energy use, and potentially raise energy bills.

For solar panel installations, light colored or reflective roof membranes can lower the ambient roof temperature which allows the panels to function more efficiently. The temperature of a PV panel can significantly impact how much electricity the panel produces; as panels get hotter, they produce less power. According to an article published by GAF, “It is estimated that the efficiency of a PV panel can be up to 13 percent higher when installed over a highly reflective membrane compared to a dark membrane with low reflectance. Also, the use of bifacial PV panels over reflective roof membranes can increase the efficiency by 20-35 percent, as they take advantage of the reflected light.”²⁶ For overburden installations, such as blue or purple roofs where the membrane is exposed, it is advantageous to have reflective membranes. While the roof may not always be holding water, or presumably when water is present it is relatively translucent, a reflective membrane will contribute to lowering roof surface temperatures. Lower overall surface temperatures can contribute to mitigating the heat island effect and heat gain into the building interior. Additionally, vegetative roof systems can take advantage of reflective membranes where vegetation is not installed. Codes require that borders and paths are maintained on the roof for fire, access, and maintenance. Reflective membranes at these locations may lower roof temperatures which mitigates interior heat gain, and also decreases the strain of summer heat on vegetation.

Membrane Thickness

Roof assemblies should be installed to match or exceed the service life of the overburden systems. The risk of installing a less robust system, such as with a thinner single-ply membrane, could mean that the roof assembly would require replacement prior to the overburden reaching the end of its service life. The cost becomes significantly more than just the cost of a roof due to the cost of dismantling and reinstalling the overburden system, and loss of energy generation for PV systems and potential expiration of plants in a vegetative assembly.

Membrane thickness will increase resistance to puncture and foot traffic and extend the service life of the overall assembly. NRCA recommends a two-layer minimum for modified bitumen roof assemblies, minimum 60 mils reinforced for EPDM, minimum 60 mils reinforced for PVC, and minimum 72 mils reinforced for TPO in IRMA vegetative roof assemblies. Note that hybrid assemblies, which combine two membrane types, most often modified bitumen and single-ply TPO, will increase system robustness and additional protection from failure.

An unprotected roof membrane should offer enhanced protection against the effects of UV, high service temperatures, punctures, and added foot traffic to help ensure that the roof life expectancy will match that of the overburden. For single-ply membranes, additional membrane thickness can provide protection against punctures, which is especially important considering the extra foot traffic on the roof due to service and maintenance activities generally associated with overburden systems. According to a leading roof manufacturer, single-ply membrane thickness significantly improves impact resistance (such as by dropping a tool) by almost 80 percent from 45 mil to an 80 mil membrane. A thicker single-ply membrane also provides additional protection to both UV and high surface temperatures. For single-ply membranes, this is important since a thicker overall membrane means more thickness over the scrim, or the reinforcing layer. It is this portion of the membrane that provides the weather resistant properties, including UV resistance.

For modified bitumen membranes, two plies is considered typical and the addition of a third-ply is generally not required. Modified bitumen roofs are generally considered more durable as each ply thickness may be 120 mils or more.

A roof membrane in an IRMA assembly should have added protection to punctures and abrasion from roof elements above it, including root damage in a vegetative assembly. For installations where the membrane is on top, it is a best practice to install walkway pads around solar arrays or exposed areas of single-ply membranes to protect against the added foot traffic to service the installations.

Membrane Attachment

There are two broad categories of roof attachment; mechanically attached via use of fasteners, and adhered. The attachment method will vary depending on the membrane type, and project specific requirements such as energy efficiency, fume tolerance and fire hazards.

Selection of attachment methods should be reviewed for ease of installation in the short-term and energy efficiency over the long-term. Energy efficiency from the roof assembly can be directly related to thermal bridging, which occurs when components allow for heat transfer through the roof assembly. Loss of internal temperatures means that the mechanical equipment will have to work harder to maintain the desired set points. Thermal bridging has the potential to occur at gaps or discontinuities between materials, such as at fasteners in a mechanically attached system. Particularly where the fasteners penetrate the entire assembly from the membrane through the insulation and into the deck, the fasteners provide a direct thermal path from the exterior to the interior. Mechanically attached single-ply systems are also subject to billowing in high wind events. Billowing, or fluttering, of a membrane is when wind causes a negative pressure by pulling interior air into the roof assembly creating uplift force on the roof assembly. Although this is an acceptable behavior of single-ply membranes, over time, it can cause stress and fatigue on the mechanical attachments and membrane. Interior air that is pulled into the roof assembly equates to energy loss since often the temperature controlled air may warm or cool based on the temperature of the membrane.

Systems that do not use fasteners, such as with the use of adhesives, greatly reduce thermal bridging by eliminating the path from the interior of the roofing assembly to the exterior. Adhering also prevents billowing of the membrane, by mitigating the interior air that can be brought into the roof assembly.

Asphaltic based membranes use asphalt to adhere the roof assembly. Modified bitumen roofs can be installed several different ways, including using a torch to melt the asphalt plies or by using a cold applied adhesive. The multiple asphalt plies form a robust roofing system that is not penetrable to air or the effects of billowing. Asphalt, and particularly hot (torched-applied) asphalt, can have strong fumes. On an occupied building where overburden would be placed as a retrofit, or a building in close proximity to other buildings, the use of asphalt may not be preferable since HVAC intakes may transport asphalt fumes into the building. Many insurance companies and jurisdictions do not allow the use of torches on the roof due to the fire hazard of open flames. Cold applied modified bitumen applications are an alternative that use an asphaltic based adhesive that is rolled onto the substrate prior to installation of the membrane. Cold applied applications offer the same modified bitumen wearing surface, but with fewer fumes than a traditional torch applied application. Modified bitumen roofs are excellent for mitigating air movement as the multiple layers are impermeable to air, and therefore are not subject to billowing like mechanically fastened single-ply systems.

Single-ply membranes tolerate a wide variety of attachment methods and are able to be both adhered and mechanically attached. The types of adhesives can vary from melted asphalt (with fleece-backed membranes) to various types of commercial adhesives manufactured for specific single-ply membrane types. Although product specific, single-ply adhesives release fewer hazardous fumes during installation, in contrast to those by asphalt based products. These adhesives do not require heating or torching for application and generally come in pails or canisters and can be installed with a roller or a spray attachment. Single-ply adhesives are generally less messy and can be quicker to install than traditional asphaltic based products.

Mechanical attachment of single-ply systems uses fasteners to install both the insulation layers and the membrane. The fasteners must be continuous through each layer and into the structural roof deck for securement. The number of fasteners will depend on project specific requirements, but typically for a mechanically attached system, there is a minimum of six fasteners per 4-foot-by-8-foot insulation board, and additional fasteners for membrane attachment, which can lead to a substantial number of fasteners penetrating and creating thermal bridges within the roof assembly. It is important to note that most systems require mechanical attachment of the first layer of insulation to be attached with fasteners, even if the specified system is to be adhered. However, by burying the fasteners in the system, and adding adhered layers of insulation and membrane on top of the mechanically attached insulation layer, the interior air loss and thermal bridging is significantly reduced.

Overburden systems installed over mechanically attached systems will billow and flutter with the roof membrane during high wind events. As the membrane flutters and moves, the overburden will shift on the membrane. Intensive vegetative systems may act as ballast and may inhibit the billowing. But for extensive vegetative systems and solar arrays, the overburden will likely move with the fluttering of the membrane. Over time, this could create additional stress on the overburden systems and their interface with the membrane, including abrading the membrane surface. The overburden systems may also experience stress and fatigue due to movement, which could decrease their overall service lives. Additionally, for solar arrays, as the membrane flutters, the movement of their associated electrical connections may compromise the energy performance. Similarly, for blue and purple roofs, the water catchment systems may be impacted by the billowing of the membrane. The catchment systems, likely designed to be rigid, may also experience fatigue due to the movement.

Therefore, an adhered roof membrane, such as those installed with the use of adhesives for both single-ply and modified bitumen systems, can contribute to a roof system lifespan and is a recommended installation method for overburden assemblies.

Insulation

Insulation is a critical part of any roofing assembly as it contributes to the overall energy efficiency of the building. The effectiveness of roof insulation is determined by its R-value, which is a measure of thermal resistance. The higher the R-value, expressed per inch, the better the thermal performance of the insulation and its effectiveness at maintaining interior temperatures. The most common roof insulation materials are polyiso, XPS, and 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 is required to achieve the desired insulating value. In overburden systems, the thickness of the overall installation can have an impact on the overall design of the overburden system. It is an industry standard for roof flashings to extend a minimum of 8 inches past the completed installation. For vegetative systems, this means that the flashing must extend 8 inches past the vegetation. Flashing heights are of particular importance at mechanical curbs and parapets. For a new construction installation, it is possible to raise the heights of the curbs and parapet walls to the desired height, however, in an existing building, this can be problematic.