Energy-Efficient Roofs with Above Sheathing Ventilation

ASV Affect

ASV has a similar impact on a building’s cooling loads as does a cool roof, where the surface of the roofing material has a high solar reflectance index (SRI) value, so that the sun’s energy is reflected and not drawn into the building. Cool roof coatings and ASV are measures that are working to accomplish the same thing, to reduce the roof surface temperature. The difference is that the cool roof coating works at the roof surface while ASV works underneath the surface. They work independently from each other, however, and by combining the two, it can maximize the benefit of a cool roof.

Roofs experience much higher temperature fluctuations than any other surface on a building. On sunny days, the roof’s temperature can be higher than the ambient outdoor temperature by 50 F or more, and lower than ambient by 10 to 20 F on dry, cloudless nights. The use of ASV on a roof mitigates this effect. By incorporating that air space, it always works to push the roof temperature toward ambient outdoor temperature (either naturally or forced). The rate of air flow is driven by solar gain; the higher the solar gain, the more air flows through the cavity.

When cool roofing is attached directly to the roof deck, there is often what is known as a winter heating penalty. However, with the addition of ASV, those effects are not as pronounced since the air space created by the ASV isolates the cooler roof surface at night.

Longevity and Performance

Ventilated roofs have been used for hundreds of years and have been shown to have a longer service life than unventilated roofs. The ventilation allows for moisture to escape a roof assembly, which helps to prevent corrosion and roof deck deterioration. It also assists in tempering expansion and contraction of the roofing materials and can minimize ice damming in cold climates. Another benefit is in wildfire prone areas, where the airspace created with ASV enhances fire performance of roofing materials that can burn.

In a series of studies from 2008 to 2015, the Oak Ridge National Labs (ORNL), supported by the Metal Construction Association (MCA), evaluated ASV in conjunction with other energy-saving technologies. As it pertains to metal roofing, ASV outperformed a direct-to-deck cool metal roofing application. Resulting from their studies, ORNL stated, “A painted metal roof on a 4:12 roof slope with a 3/4-inch ASV cavity needed only a 0.1 solar reflectance (SR) to have the same annual cooling load as a 0.25 direct-to-deck metal roof.” By combining cool pigments in the painted metal roof with ASV, it provided a 45% reduction in the cooling load. Even non-cool, dark colors achieved a 30% reduction with ASV.

Many may be familiar with the perfect wall concept, developed by Joseph Lstiburek of Building Science Corp., Westford, Mass. Lstiburek states, “In concept, ‘The Perfect Wall’ has the rainwater control layer, the air control layer, the vapor control layer and the thermal control layer on the exterior of a structure. The cladding’s function is principally to function as an ultra-violet screen. Oh, and architects might consider the aesthetics of the cladding to be important.”

If you take that perfect wall concept and flip it up, you end up with a perfect roof assembly. That perfect metal roof assembly begins with the roof decking, followed by a fire-retardant board, an air and vapor barrier, insulation, a vapor permeable secondary underlayment, ASV shims and, finally, the metal roof panels.

There are many ways to implement ASV in a roof assembly:

• Battens and subpurlins
• Offset clips
• Integrated vent products
• Perforated sub-framing
• Battens/ridge vents
• Shims/lifts

ATAS International offers patented ASV spacer shims, which are 3/8-inch thick and can be stackable to achieve a 3/4-inch airspace. They are manufactured of a highly engineered polyoxymethylene that aids in thermal transfer reductions and have a very high heat resistance. The use of these ASV spacer shims in a roof assembly can reduce cooling costs and negate heating penalties associated with cool roof technologies. The spacer shims can be used in both roof and wall applications.

ASV Case Study: Erving Public Library, Erving, Mass.

A new public library, designed by Somerville, Mass.-based Johnson Roberts Associates, was built in Erving, Mass., with typical wall and roof construction. The roof deck construction consisted of structural insulated panels (SIPs), which are all-in-one structural panels consisting of insulation encapsulated by two layers of sheathing. The challenge facing the architect, installer and distributor was that the project required a 20-year weathertight warranty. Therefore, they needed an approved roof assembly using a weather-resistant barrier (WRB) that is vapor permeable. Working with the manufacturer’s technical team, a solution incorporating a breathable vapor barrier (VaproShield’s SlopeShield Plus), ASV spacer shims, a drainage mat with continuous nylon filaments used at perimeter areas to support flashing assemblies (Enka Solutions’ EnkaMat), and vented trims was developed and implemented.

With the vapor permeable WRB, it allowed for long-term drying of a roofing assembly and stands up to six months of UV and climate exposure on the roof. By using this WRB on the Erving Public Library project, it allowed any vapor or moisture created from inside the building to vent through the assembly. This resulted in positive air flow between the roof panel and the deck. The metal roofing manufacturer engineered and approved the assembly for the specified 20-year weathertight warranty.

Video: Will Ragano

The architectural firm for this project was Johnson Roberts Associates Inc., Somerville, Mass. They stated, “The new Erving Public Library … replaces the current one-room library with a state-of-the-art new building, featuring a dedicated teen room, children’s room (with an activity room), meeting rooms and adult study areas. The new building has been designed to respect the architectural heritage of the community. This is a green, sustainable design that reduced energy usage by 81%—it is an all-electric building, using no fossil fuels, with a large photovoltaic array to provide energy. The project is seeking LEED Silver certification and construction was completed in 2021.”

LEED certification is still pending. The use of SIPs, along with a weather-resistant barrier, ASV and metal roofing can contribute toward LEED certification in the following categories:

• Integrative Process Category and Credit: The energy efficiency attributes for cool roofs, above sheathing ventilation and SIPs can be factored into the energy modeling analysis during the integrative process.
• Sustainable Sites Category
  • Heat Island Reduction Credit: Cool pigment paint reflects infrared radiation, keeping the material cooler and reducing the urban heat island effect. When combined with an above sheathing ventilation strategy, it can further reduce cooling loads and air conditioning appliance usage (a secondary contributor to urban heat island effect).
• Energy and Atmosphere Category
  • Minimum Energy Performance Prerequisite: A number of ATAS products can be used to help maximize energy efficiency, including cool metal roof systems and ASV.
  • Optimize Energy Performance Credit: Energy efficient cool metal roof systems can help realize many of the design elements that are key to reducing energy demand. Energy-efficient building envelope systems, including cool roofing and ASV, help to reduce heat gain (and loss in the winter), reduce peak energy demand, and improve energy performance of the building.

ATAS’ Dutch Seam standing seam metal roofing was chosen for the Erving Public Library. The 10,520 square feet of Dove-Grey panels were manufactured in 0.040 aluminum. They were 19 1/4 inches in width with stiffening ribs. Dove Grey is a lighter grey color with an SRI value of 55. With ASV shims incorporated into the roof assembly, it further contributes toward keeping the roof cool and decreasing the cooling load on the building.

This project included a ground-mounted solar array. However, solar panels could have mounted on the roof. Standing seam metal roofing panels are a perfect platform for solar panels due to their long service life. When installing a crystalline system (typically warranted for 25 years of power generation), a building owner benefits from a roofing substrate that has a greater life expectancy than the solar panels. Standing seam metal panels also provide a natural platform for attaching crystalline systems without any roof penetration. Additionally, highly reflective roofs with cooler rooftop temperatures (with or without the addition of ASV to the roof assembly) result in better performance of solar panels.

Incorporating ASV into a roofing system is one of several components that can contribute to a better performing roof and a more energy efficient building, resulting in energy cost savings and a means to reduce carbon impact on the environment.

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Jim Bush, CSI, is the vice president of sales and marketing for ATAS International Inc., Allentown, Pa. He has over 40 years of experience in the metal wall panel and metal roofing industry. Bush is the immediate past chair of the Metal Construction Association (MCA). He is also the past chairperson of the MCA retrofit council, MCA roofing council, MCA roofing certification committee, and is the past organizer and leader of the demonstration area at METALCON, the industry’s annual trade show.

Lee Ann Slattery, FCSI, CDT, CCPR, LEED AP BD+C, is the sales support manager for ATAS. She is the chairperson of the MCA’s market development committee, a member of the National Women in Roofing’s education committee and recently served for four years as the Construction Specifications Institute director for the Middle Atlantic Region. She has over 16 years of experience in the metal wall panel and metal roofing industry.