By Rick Duncan, Ph.D., P.E.,
Technical Director, Spray Polyurethane Foam Alliance (SPFA)
The sustainability focus in buildings has shifted lately to one on energy performance. Not only have building codes become more stringent, with a much greater emphasis on energy efficiency, but many incentives have been introduced and made available to owners, providing them with tax credits and savings for the integration of renewable energy sources such as solar onto their homes and projects.
Increasingly ambitious movements, including Passive House and Zero Net Energy (ZNE), are also gaining in popularity as immediate issues like climate change, and the catastrophic effects of it, are top-of-mind and ever present in the news.
Even though ZNE is a bigger energy goal than what is currently highlighted for many structures, architects, builders and owners are increasingly integrating sound energy practices in their buildings. As a key component of the building enclosure, roofing systems tend to transfer (i.e. lose) significant amounts of energy if not properly designed or well maintained. Thus, it is unsurprising that they are a key focus in buildings designed to minimize energy use.
Spray Polyurethane Foam (SPF) and Photovoltaic (PV) systems are now, more than ever, utilized together on the roof as a complete solution for energy savings. SPF reduces demand for the energy generated by photovoltaics, which can make a significant difference in ZNE buildings. When combined, they provide a joint solution for the generation of renewable energy, the conservation of heating and cooling energy, and, ultimately, the elimination of the structure’s dependence on fossil-fuel consuming electricity sources.
Regardless of whether ZNE is the end goal, SPF and PV integrated in roofing are an ideal combination for many structures, providing unparalleled return on investment through energy cost savings, as well as numerous additional benefits. However, contractors should be mindful of some important installation considerations when looking to join these two powerful systems on the roof of a building, to ensure highest possible performance and lifespan.
PV System Overview
PV cells are the basic unit used to convert light to 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.
The dominant type of PV panel used with SPF roofing is cSi, or crystalline silicon. cSi is a typically rigid panel with glass frame and metal frame and may be applied, unlike other dominant PV panel types, via rack installation methods.
A photovoltaic system includes many components in addition to the panels. Components include racks, rails, rooftop attachment devices, grounding systems, wiring and wiring harnesses, inverter(s), and connection to the main electrical panel. Components may also include control modules and storage batteries for off-grid PV system installations.
Photovoltaic panels must be handled and maintained with caution. Electricity is produced when a single panel is exposed to light, however, because a panel is not part of a circuit, that electricity will not flow until the circuit is complete. A worker may complete the circuit by connecting the two wires from the backside of a PV panel.
When maintaining a PV system, it may become necessary at some point to disconnect or remove an individual panel from a string or an array. The whole system must be shutdown properly as a precautionary measure to prevent shocks from occurring to workers and arcing between electrical connections. This “shutdown” procedure must be followed with precision 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.
Rooftop PV Installation Types for Use with SPF
Rooftop PV systems can vary significantly in size. Large footprint buildings can employ PV systems rated from 50 kW to 1000 kW or larger while residential rooftop PV systems are commonly 3 kW to 5 kW solutions.
Rooftop PV systems may be installed either on racks or adhered directly to the roof surface. When looking to combine PV with SPF, it is generally not advised to adhere or place the PV panels directly onto the roof surface. Solar heat as well as water can accumulate between the PV and roof coating and can negatively impact coating performance. Moreover, panels applied directly to a low-slope roof will, in nearly all cases, not optimally align with the sun, which will reduce energy production.
Non-penetrating rack systems may be placed directly on a rooftop and held in place with ballast. Racks may also be installed with penetrating supports that require flashings. Each type provides advantages and disadvantages. For example, ballasted racks may block water flow and affect drainage, while penetrations require leak and maintenance-prone flashings. SPF is unique in that it easily self-flashes around penetrating supports.
Rack-mounted arrays with penetrating attachments are fairly lightweight at two to three pounds per square foot, and ballasted arrays add four to six pounds per square foot. With the latter however, more ballast is utilized at the perimeters and corners of a PV array. Thus, localized loading from ballast may reach as high as 12-17 pounds per square foot, which must be considered. Most SPF roofing systems have a compressive strength of 40-60 psi.
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 if the additional weight of the PV system is acceptable.
Additionally, roofs are required by codes to provide “live load” capacity, a measurement, which includes people, snow, and other temporary weight-bearing scenarios that may occur. The weight of a PV system is typically 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 considerable weight to a roof, and must be factored in. When determining key considerations for wind load and fire safety, best practices require deferral to the PV supplier.
Drainage on rooftops is important for safety of the structure and longevity of the roof. 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.
Photovoltaic panels convert approximately 15-20 percent of light to electricity, leaving the remaining unconverted energy to be released as heat. Additionally, PV panels are more effective when their temperature drops. It is for each of these reasons that the majority of rooftop PV installations are designed to encourage airflow under panels, which reduces the temperature of the panels, improves conversion efficiency and releases heat effectively. Photovoltaic panels installed 4 to 5 inches above the roof will not change the temperature of the roof and, instead, provide shade to the surface of that roof. This additional shade may extend the life of SPF roof coatings.
Service Life and Maintenance
Ideally, a roof system, whether SPF or another material, and the PV system should have the same expected service life. Removal (decommissioning) and reinstallation (re-commissioning) of a PV system is costly, and the cost should be weighed relative to the residual service life of the existing roof and cost of roof replacement or recoating at the time of PV installation. Ballasted, rack-mounted PV systems are difficult, if not impossible, to reroof (or recoat) under and around. Elevated racks with adequate space beneath may be able to be left in place when reroofing. A PV system that covers, for example, 10% of the rooftop will be easier to relocate during reroofing than a PV system that covers 75% of the rooftop. Building owners should be advised of future reroofing and maintenance costs with roof-mounted PV systems.
The life expectancy of the SPF roof system should align with the service life of the PV system, and coatings factor in as they can extend the life and improve performance of SPF on the roof.
Roof systems used as platforms for PV systems must be tough and durable, and generally speaking, SPF has greater compressive strength as density increases. Higher density SPF systems may be preferred, especially when ballasted support systems are used.
An SPF system will be stressed during the installation of the PV system and coatings and granules will help protect the roof during this time, and during scheduled maintenance. Because a roof surface below PV panels will likely not dry as fast as non-covered portions, coatings that stand up better to standing water and biological growth should be selected. Installation of PV systems on SPF roofing will inevitably create additional foot traffic. It is important to protect heavily trafficked areas with additional coating and granules or walk pads. The cost to do so is low and will protect the service life of the roof.
All roof mounted PV systems should be inspected and maintained at least twice a year. Wiring, attachment points and flashings should be inspected and cleaning of the top surface of the PV panels may be required.
To maintain and service the roof and 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 without stepping on PV panels.
In closing, while there are many considerations to the application of PV systems in combination with SPF roofs, the complete energy generation and conservation solution provided by the two integrated systems performs notably well. The energy cost and earth saving benefits are both undisputable and hard to ignore.
About the Author
Rick Duncan, Ph.D., P.E is the Technical Director of the Spray Polyurethane Foam Alliance (SPFA), the industry’s leading organization representing contractors, material and equipment manufacturers, distributors and industry consultants. The SPFA promotes best practices in the installation of spray foam and offers a Professional Certification Program to all those involved in the installation of the product.
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