About Rick Duncan

Rick Duncan is the technical director of the Fairfax, Va.-based Spray Polyurethane Foam Alliance, an organization representing contractors, material and equipment manufacturers, distributors and industry consultants involved in spray foam.

Spray Polyurethane Foam and Photovoltaic Roofing Systems

Spray polyurethane foam and photovoltaic systems are increasingly utilized together as
a joint solution for energy savings. With the continued push toward sustainability and growing
movements, like net-zero-energy construction, SPF and PV systems are a logical combined solution for the generation of renewable energy, the conservation of heating and cooling energy, and the elimination of the structure’s dependence on fossil-fuel-consuming electricity sources. Regardless of whether net-zero energy is the end goal, SPF and PV combined in roofing can be quite effective for many structures. Here are some considerations when looking to join these two powerful systems on the roof of a building.

ROOFTOP PV INSTALLATION TYPES FOR USE WITH SPF

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.

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.


Rooftop PV systems can vary significantly in size. Large-footprint buildings can employ PV systems rated from 50 kilowatts to 1,000 kW or larger while residential rooftop PV systems are commonly 3 kW to 5 kW.

Rooftop PV systems may be installed 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 and water can accumulate between the PV and roof coating which could negatively impact coating performance. Moreover, panels applied directly to a low-slope roof will not be properly aligned with the sun to achieve optimal performance.

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.

PV EXPLANATION

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. 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 a glass and metal frame and may be applied, unlike other dominant PV panel types, via rack installation methods.

A PV system includes many components in addition to the panels. Components include racks, rails, rooftop attachment devices, grounding systems, wiring and wiring harnesses, combiner boxes, inverter(s) and connection to the main electrical panel. Components may also include control modules and storage batteries for off-grid PV system installations.

ELECTRICAL SAFETY

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. This procedure 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.

HEAT BUILDUP

Photovoltaic panels convert approximately 15 to 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 these reasons that the majority of rooftop PV systems are installed 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.

LOAD

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.

Rack-mounted arrays with penetrating attachments are fairly lightweight at 2 to 3 pounds per square foot, and ballasted arrays add 4 to 6 pounds per square foot. However, with the latter, more ballast is utilized at the perimeters and corners of a PV array. Thus, localized loading from ballast may reach as high as 12 to 17 pounds per square foot, which must be considered. Most SPF roofing systems have a compressive strength of 40 to 60 psi.

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Spray Polyurethane Foam: A Key Component to Any Net Zero Solution

SPF has the ability to insulate, air and water seal, as well as control moisture throughout the structure, acting as a single-source solution, reducing the need for multiple products.

SPF has the ability to insulate, air and water seal, as well as control moisture throughout the structure, acting as a single-source solution, reducing the need for multiple products.

In July 2014, California initiated the revision process to the 2016 version of Title 24, California’s building energy efficiency codes, which are designed to move the state’s residential and commercial buildings toward zero net energy (ZNE). All new residential construction is to be ZNE by 2020, and all new commercial buildings are to achieve ZNE by 2030. While aggressive, these goals are achievable with the right design implementation and accessibility to proper building materials.

As one of the world’s most influential economies, the state of California has demonstrated its power in leading the other 49 states in the implementation of progressive initiatives. California traditionally takes an environmental stance with a history of enforcing regulations designed to protect the physical environment and health of the state’s residents. These efforts often result in national trending with other states and municipalities following suit with similar regulations. It is widely anticipated a similar phenomenon will occur with ZNE goals.

The design of a ZNE building focuses on the reduction of energy consumption and on the generation of the structure’s own renewable energy (such as via solar panel solutions). Long-term ZNE begins with a quality building enclosure. High-performance attics and wall systems are a key focus of 2016 Title 24 as they make a significant impact in the reduction of peak cooling demand in structures.

SPF may be installed in a continuous layer, eliminating thermal bypasses, and boasts one of the highest R-values of all insulation options.

SPF may be installed in a continuous layer, eliminating thermal bypasses, and boasts one of the highest R-values of all insulation options.

Because of spray polyurethane foam’s unique attributes, the material is widely recognized as an optimal solution for unvented attics, as well as for roofing, walls and ceilings. SPF has the ability to insulate, air and water seal, as well as control moisture throughout the structure, acting as a single-source solution, reducing the need for multiple products.

Energy loss may occur at various points throughout the roof, walls and ceiling via air leakage. Thus the air-sealing ability of SPF is extremely beneficial when trying to improve energy efficiency.

In roofing, SPF acts as a protective roofing solution and as an insulator.

In roofing, SPF acts as a protective roofing solution and as an insulator.

As a thermal insulator, SPF forms in place and fully adheres, almost completely eliminating the cracks and gaps that allow escape of conditioned air. It may be installed in a continuous layer, eliminating thermal bypasses typically found with cavity insulations and boasts one of the highest R-values of all insulation options.

In roofing, SPF acts as a protective roofing solution and as an insulator. The effectiveness of insulation is measured through moisture control, air leakage, health, safety, durability, comfort and energy efficiency factors, and SPF scores exceptional marks in all.

These combined characteristics are integral to SPF’s ability to contribute to total ZNE solutions—solutions, which will become increasingly necessary as the net zero revolution takes hold across the U.S.

Spray Polyurethane Foam Has Structure-strengthening and Energy-efficiency Capabilities

A high-performance building material, spray polyurethane foam (SPF) is widely used as an effective, lasting roofing solution. With positive benefits, including versatility, thermal insulation, resistance to inclement weather cycling and storms, strengthening of the building envelope, long life span and durability, spray foam has enjoyed increased use among builders and roofing contractors alike.

A roof’s primary purpose is to protect the structure underneath it. As a roofing material, closed-cell SPF acts as a protective roofing mechanism and a thermal insulator. The lightweight material is ideal as a roofing solution when:

 As a roofing material, closed-cell SPF acts as a protective roofing mechanism and a thermal insulator.

As a roofing material, closed-cell SPF acts as a protective roofing mechanism and a thermal insulator.

  • the roof substrate has many penetrations.
  • the roof deck is an unusual shape or configuration.
  • the roof is being applied to a structure located in a severe-weather environment.
  • a lightweight option is needed.
  • a slope application is preferred to provide extra drainage capabilities.
  • keeping the existing roof cover is desired.

STRENGTH AND DURABILITY

SPF is considered a highly durable building material. The physical properties of the foam change little with time, accounting for a life span up to 30 years with regular care and maintenance. SPF roofing systems also strengthen the roof in multiple ways. Roofing spray foams possess a compressive strength of 40 to more than 60 pounds per inch. Spray foam’s adhesion strengthening capabilities are key, especially in locations where severe weather cycling, storms, wind, hail and other conditions are prevalent and commonly cause structure damage. Coastal and hurricane-prone regions are prime examples.

When applied to the interior side of a roof, closed-cell SPF can increase a building’s resistance to wind uplift during severe storms. When SPF is applied to built-up roofing and metal substrates, it increases resistance to wind uplift even further. A study conducted by the University of Florida, Gainesville, in 2007 found that applying closed-cell spray foam under a roof deck provides up to three times the resistance to wind uplift for wood roof sheathing panels when compared to a conventionally fastened roof.

Spray foam is a good solution for unusual configurations and areas with many penetrations.

Spray foam is a good solution for unusual configurations and areas with many penetrations.

Spray foam also is resistant to progressive peeling failure. Caused by wind, peeling happens at the roof’s edges when wind pulls flashings and copings away from their installed positions. Peeling looks like a tin can after it has been cut around the perimeter. When this happens, a chain reaction may occur and lead to catastrophic building failure. After the roof membrane, panels or tiles pull away, the board-stock insulation is exposed, often with less resistance to the lateral and uplift wind forces. Then the sheathing below and the substructure are subject to movement and wind or water damage, potentially leaving the entire building interior underneath open and vulnerable. SPF roofing is continuous, so it provides a water-resistant layer that is well adhered to the substrate.

When the Gaithersburg, Md.-based National Institute of Standards and Technology examined roofs following Hurricane Katrina, it found buildings with spray-foam roofs performed rather well without blow-off of the SPF or damage to flashings. The 2006 “Performance of Physical Structures in Hurricane Katrina and Hurricane Rita: A Reconnaissance Report” found that only one of the examined SPF roofs incurred notable damage, and that damage was confined to only 1 percent of the total roof system. The report concluded spray foam kept the roofs intact, prevented moisture from entering the buildings, and protected the structures from hail and debris.

Hurricane Katrina played a significant role in one of the largest reroofing projects ever on one of the largest metal-framed domed structures in the world: the Superdome in New Orleans. Katrina destroyed the dome’s second roof; the structure’s original roof was constructed with polyisocyanurate foam covered with a fluid-applied elastomeric coating but was replaced in 1989 with a single-ply EPDM roofing system. After the damages suffered during Katrina, the EPDM roof system was replaced with a spray foam roof system.

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