As the sustainability conversation continues to focus on energy solutions within the built environment, movements such as net-zero energy are gaining traction. Net-zero energy buildings generate as much or more energy than they consume. Although net-zero energy is a bigger energy goal than what is identified for many structures, architects, builders and building owners continue to integrate stronger energy practices in their buildings.
As a key component of a building's enclosure, roof systems that are poorly designed can contribute significantly to the energy demands of buildings. Now, more than ever, spray polyurethane foam (SPF) and photovoltaic (PV) systems are being used together on roofs as a complete solution for energy savings. When joining these two powerful systems, be mindful there are important installation considerations to ensure the highest possible performance and lifespan.
How it works
SPF roof systems consist of three elements. The first is the substrate, which can be a metal, wood or concrete deck surface, or most types of structurally sound and dry, existing roof systems. The second element is a high-density (∼ 3.0 lb/ft3) two-part closed-cell SPF applied to the substrate. After application of the foam, one or more elastomeric coatings, the third element, are applied over the foam to protect the SPF against mechanical wear and ultraviolet light.
SPF reduces demand for heating and cooling energy, which, in turn, reduces the demand for power that is generated by PV systems. When combined, SPF and PV systems provide a joint solution for the generation of renewable energy, the conservation of heating and cooling energy, and, when used as part of an overall net-zero energy building design, the elimination of a structure's dependence on fossil-fuel consuming electricity sources.
PV system overview
PV cells are the basic unit used to convert light into electricity. Many PV cells are bundled together to make a PV panel or module. PV panels are grouped electrically to create a PV string. And depending on the system size, two or more strings are combined to create a PV array.
One dominant PV panel type used with SPF roofing is cSi, or crystalline silicon. Typically, cSi is a rigid panel with a glass and metal frame and, unlike other dominant PV panel types, it may be applied via rack installation methods.
In addition to panels, a PV system includes many components such as racks, rails, rooftop attachment devices, grounding systems, wiring and wiring harnesses, inverter(s), and connection to a main electrical panel. For off-grid PV system installations, components also may include control modules and storage batteries.
When maintaining a PV system, it may become necessary to disconnect or remove an individual panel from a string or array. Because electricity is produced when a single panel is exposed to light, PV panels must be handled and maintained with caution. As a precautionary measure, the whole system must be shut down to prevent shocks to workers and arcing between electrical connections. This shutdown procedure must be precisely followed as part of a lock-out/tag-out program and is provided by the inverter manufacturer. Under no circumstances should SPF contractors ever disconnect or decommission a PV panel or system unless they are trained and qualified to do so.
Use with SPF
Rooftop PV systems can vary significantly in size. Large footprint buildings can employ PV systems rated from 50 kilowatts (kW) to 1,000 kW or larger while residential rooftop PV systems commonly are 3 kW to 5 kW solutions.
Rooftop PV systems may be installed either on racks or adhered directly to a roof's surface. When looking to combine a PV component with SPF, it generally is not advised to adhere or place the film-type PV panels directly onto a roof surface because heat, as well as water, can accumulate between the PV panels and roof coating and negatively affect coating performance. Additionally, in nearly all cases, rackless panels applied with a small clearance above a low-slope roof will not optimally align with the sun, which will reduce energy production. Rackless PV panels also reduce access to the roof surface for regular inspection and maintenance.
There are two common approaches to rack-mounted PV panels: ballasted (nonpenetrating) racks and penetrating racks. Each type provides advantages and disadvantages.
Nonpenetrating rack systems are placed directly on top of a roof system and held in place with ballast. These provide a cost-effective means to install PV systems without interference with existing roof systems. Ballasted racks may not provide sufficient uplift resistance and are not recommended by NRCA as a best practice.
Rack systems with penetrating supports are tied to a roof's structural elements. Penetrating racks provide better wind-uplift resistance but generally are more costly to install because they require modifications to the existing roof. Additionally, rack systems with penetrating supports require leak and maintenance-prone flashings. An SPF roof system applied around these penetrating supports is self-flashing and provides an excellent flashing solution for penetrating support racks.
All rack systems may block water flow and affect rooftop drainage and must be properly integrated with rooftop drainage paths. Additionally, all rack systems should be evaluated by a qualified structural engineer to evaluate their effects on the structural performance of a building.
Design considerations
PV panels add weight to a rooftop, and this must be factored into the design and installation. Existing structures should be analyzed by a structural engineer to determine whether the additional weight from a PV system is acceptable.
Additionally, roof systems are required by building codes to provide "live load" capacity, a measurement that includes people, snow and other temporary weight-bearing scenarios that may occur. The weight of a PV system typically is below the live load capacity. However, in the absence of a structural analysis, the live load capacity will be reduced by the addition of the PV system.
A final consideration is whether a PV installation will create new locations for drifting snow, which may add significant weight to a roof. When determining key factors for wind load, best practices require deferral to a structural engineer to determine suitable loading and/or a PV supplier that can provide structural data/test reports for its systems.
Rooftop drainage is important for structural safety and roof system longevity. Improper drainage can accumulate water, adding to a roof's load. PV arrays often have many points of contact with a roof, and these are possible locations that will block or slow drainage. PV racking should be positioned to minimize ponding water and/or include methods such as notched pads to allow drainage under points of contact, especially for ballasted systems.
Typical PV panels convert about 15 to 22 percent of sunlight to electricity, according to SunPower Corp., San Jose, Calif., leaving unconverted energy to be released as heat. Because of this, a majority of rooftop PV installations are designed to encourage airflow under panels.
Service life and maintenance
Ideally, a roof system, whether SPF or another material, and a PV system should have the same expected service lives. Removal and reinstallation of a PV system is costly, and that cost should be weighed relative to the residual service life of the existing roof and cost of roof system replacement or recoating at the time of PV installation.
Reroofing (or recoating) under and around ballasted, rack-mounted PV systems is difficult if not impossible. Elevated racks with adequate space beneath may be able to be left in place during a reroofing project. In addition, a PV system that covers 10 percent of a rooftop will be easier to relocate during a reroofing installation than a PV system that covers 75 percent of a rooftop. Building owners should be advised of future reroofing and maintenance costs with roof-mounted PV systems.
Just as the life expectancy of an SPF roof system should align with the service life of a PV system, protective roof coatings also factor in as they can extend the life and improve performance of SPF on a roof. PV racking systems should be designed to facilitate periodic SPF recoating. Also, coatings should be selected to align with PV system service life so recoating can be done at the same time as PV panel replacement. Intermittent shading provided by PV panels should extend the life of most SPF roof coatings compared with coatings fully exposed to sunlight.
Roof systems used as platforms for PV systems must be tough and durable. Generally speaking, SPF has greater compressive strength as density increases. Higher density SPF systems may be preferred, up to 4.0 lb/ft3, especially when ballasted support systems are used.
An SPF roof system will be stressed during the installation of a PV system, and coatings and granules will help protect the roof during installation and scheduled maintenance. Because a roof surface below PV panels likely will not dry as fast as noncovered portions, coatings that stand up better against small areas of ponding water and biological growth should be selected. As a result of mild undulations in the surface, there can be small areas of water on an SPF roof system.
Installing PV systems on SPF will inevitably create additional foot traffic as a result of personnel on the roof during maintenance and inspections of the PV system. It is important to protect heavily trafficked areas with additional coating and granules or walkpads. The cost to do so is low and will protect the roof system's service life.
All roof-mounted PV systems should be inspected and maintained at least twice per year. Wiring, attachment points and flashings should be inspected, and cleaning the top surfaces of PV panels may be required to maintain efficient conversion of sunlight to electricity.
To maintain and service a roof system with a PV system, workers must be able to access both. PV systems should not block access to drains, penetrations, flashings, mechanical units or other rooftop equipment. Similarly, PV systems should be installed so maintenance workers can access wiring, inspect panel-to-racking connections and properly clean top surfaces of PV panels without stepping on adjacent PV panels.
A notable solution
SPF roof systems provide a continuous, monolithic layer of insulation without fasteners or extra adhesives; the monolithic layer also can be part of a continuous building envelope air-barrier system. The SPF can be coated with a variety of coatings depending on climate. Even cool roof coatings may be used to minimize solar heat gain. The unique self-flashing capability of SPF makes it an excellent roof system for roof-mounted PV systems.
Although there are many considerations regarding the application of combining PV and SPF roof systems, the complete energy generation and conservation solution provided is extremely notable.
To learn more of what can be accomplished when combining SPF and PV on a roof, see the example project case study below.
Rick Duncan, PH.D., P.E., is technical director for the Spray Polyurethane Foam Alliance.
Bee Sweet Citrus processing facility: As green as it gets
Founded in 1987 as an independent packer and shipper of California oranges, Bee Sweet Citrus, Fowler, Calif., has grown into a successful year-round operation, shipping throughout the U.S., Canada, Europe, Australia, New Zealand and several Pacific Rim countries. Now a "one-stop shop" for a number of citrus commodities (navel and Valencia oranges, lemons, grapefruit, mandarins, and exotic varietals such as minneolas and pummelos), the company is committed not only to excellence in the growing and distribution of citrus but also to sustainability.
The leadership behind Bee Sweet Citrus understands the environment is the most crucial driver of the company's success and recognizes that without the health of the land, the company would be nonexistent. In addition to tending regularly to the proper care of the soil and water resources used in irrigation and growing operations, the company recently shifted its focus to ensuring its headquarters and distribution facilities also reflect its corporate mission of environmental stewardship.
The 400,000-square-foot packing facility located in Fowler sits on 36 acres and operates five lines with enough power to run more than 3,500 bins of citrus per day and create 25,000 bags full of citrus per hour. The facility also features 31 degreening rooms and a 63,000-square-foot cold storage facility, enabling the safe holding of more than 280,000 cartons of citrus at any given time.
Because of its location adjacent to Highway 99 in the heart of California's central valley, an area known for its near year-round sunshine, an obvious means for aligning the facility with the company's environmental goals was to harness the power of a solar system on the roof in addition to making dramatic improvements to the roof structure's energy efficiency.
The plant consists of a series of conjoined metal buildings, and the buildings' waterproof elements needed to be improved. For this reason, as well as for the material's ability to protect the structure while acting as a high-performance insulator, Bee Sweet Citrus chose to add 88,600 square feet of spray-polyurethane foam (SPF) to the existing 133,720-square-foot SPF roof.
Engaging Madera, Calif.-based Central Coating Co. Inc. for the project, 3-pound Accella BaySeal® SPF was selected and effectively installed. Maximizing energy efficiency even further, Central Coating concurrently replaced and sealed 54 failing, existing fiberglass roofline skylight panels. New curb-mounted, double-lens skylights improved lighting indoors while increasing safety. Post-SPF installation, the roof was coated with BayBlock® II coating, a high-tensile acrylic coating option with granules.
In total, the SPF roof and coating installation required 10 installers during a seven-week period. The crew was challenged with meeting the project's deadlines while dealing with rain conditions for part of the project's duration. However, Central Coating was able to pull off the challenge. The company partnered with Fresno, Calif.-based Pickett Solar for the installation of the 8,034-panel, 2.49-megawatt photovoltaic (PV) system.
"One of the biggest challenges of this major roof project was that our combined teams had to complete all work without any disruptions to the Bee Sweet Citrus operations happening inside the facility," says Luke Nolan, president of Central Coating. "Pulling this off was a big accomplishment."
The massive rack-mounted solar system required a staggering 7,497 standoff brackets fastened into the roof structure with more than one-third of them sealed as the new SPF roof was installed. The remaining brackets had to be installed and sealed in the existing foam roof area.
"The sheer number of roof penetrations on this project had us concerned about roof leaks," says Mike Pickett, president of Pickett Solar. "The SPF roof system installed by Central Coating sealed over 7,000 roof penetrations with zero roof leaks."
The successfully installed SPF and PV systems act as a complete energy solution for the roof system, ensuring energy-efficiency performance and energy generation. The energy efficiency of the SPF roof significantly reduces the facility's energy consumption, while the solar component generates enough electricity to power 400 to 600 homes—more than enough to power the facility—and will save the company about $280,000 annually in electricity costs.
In addition to the vast energy and cost-saving benefits of the combined roofing solution, other financials of the new roof also proved attractive. Bee Sweet Citrus qualified for an attractive finance package, making the endeavor even more worthwhile. Still, reducing the company's impact to the environment was the main focus.
"Bee Sweet Citrus is a leader in the citrus industry, and reducing our carbon footprint is one of our main priorities," says Jim Marderosian, founder and owner of Bee Sweet Citrus. "It is our goal not only to provide the world with fresh, delicious citrus but to be an environmental steward, as well."
The Bee Sweet Citrus Processing Facility project and Central Coating were recognized by the Spray Polyurethane Foam Alliance as a First Place Winner in the organization's 2018 Annual Industry Excellence Awards in the Best SPF Roof Over 40,000 Square Feet category.
For an article related to this topic, see "A key player," November 2015 issue.
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