Most in the roofing world are well acquainted with the idea of cool roof coatings, especially when it comes to metal roofing. But this isn’t the only technology available to help minimize surface temperatures on the roof and reduce cooling loads and energy needs.
ATAS International’s Jim Bush joined this year’s METALCON to discuss above-sheathing ventilation, how this cooling method differs from cool roof coatings, and how the combination of both systems can help your clients save energy and money.
What is above-sheathing ventilation?
Referred to by Bush as “the forgotten cool roof,” above-sheathing ventilation (ASV) is achieved by having continuous airflow between the roof sheathing and the metal panel system.
“Cool air enters at the eave through a vented eave assembly,” said Bush. “As cool air heats up, the buoyancy increases and air floats up and out.”
Ventilating the space between roof cladding material, the deck and the subassembly allows the temperature of the roof cladding to be closer to the ambient outdoor temperature, according to Bush, and reduces the heat gain passing through insulation during warm-weather months.
“This has a similar impact on internal cooling loads as a cool roof,” said Bush. He also noted that the heat island effect is a different phenomenon, though not discussed during his presentation.
Why ASV is important
Roofs experience much higher temperature fluctuations than any other surface, Bush said. On sunny days, roof temperatures can be higher than ambient temperatures by 50°F or more. On dry, cloudless nights, roof temperatures can be lower than ambient temperatures by 10° to 20°F.
“Ventilation mitigates this effect, whether it’s natural, forced or throttled,” Bush said. “It decouples the thermal connection between panel and insulation and works to push the roof temperature toward the ambient outdoor temperature.
Bush also noted that solar gain drives ASV—the higher the solar gain, the more air flows through the cavity. With direct-to-deck construction, the building order is deck/sheathing, insulation and roof panel. With ASV construction, however, it becomes more complex: deck/sheathing, insulation, perforated framing or shim (or another device to create an air cavity), low-E reflective insulation (if desired) and roof panel. (A screen can also be added on the top and bottom of the air cavity to keep out bugs, wildfire debris or other unwanted debris.)
How ASV differs from traditional cool roof methods
Cool roof coatings take solar reflectance and reflect it away from the system, explained Bush. As such, they tend to be lighter in color than panels that aren’t aimed at solar reflectance.
“Cool roof coatings and ASV are independent measures accomplishing the same thing: reduction of roof surface temps,” he said. “ASV works underneath the surface, and cool roof coatings work above. Because they work independently, they can be used separately or combined for maximum benefit.”
As an example, Bush used the winter or nighttime performance of a standard roof.
“Cool roofs with direct-to-deck attachments often have a winter heating penalty,” he said. “Rejected radiation could have been used to offset heating load, and the high emissivity of cool coatings maximizes nighttime cooling.”
With ASV, these effects are generally much less pronounced, Bush continued. The airspace created by ASV systems isolates the cooler roof surface at night, and the penalty can be further reduced using a damper or throttle.
Additionally, Bush said, many southern climates require cooling during the day and heating at night during the winter. In this scenario, a cool roof coating plus ASV and a low-E reflective insulation is ideal.
In the daytime, this system creates airflow up and out of the roof assembly, Bush said: Cool air enters, warmer air exits. At night, warm air enters at the top and cooler air exits at the bottom.
Other benefits of ASV
“Ventilated roofs have been used for hundreds of years and have been shown to have a longer service life,” he said. “They allow entrapped moisture to escape, preventing rust and rot, and temper thermal expansion and contraction. The ventilation also enhances fire performance in wildfire-prone areas and can minimize ice damming in cold climates.”
There are, however, numerous factors that play into the estimated performance of an ASV system, Bush said, including:
- Cavity air temperature.
- Heat flow direction (with or against gravity).
- Roof slope.
- Cavity dimension.
- Flow distance (or roof length).
- Choking effects of clips, framing, etc.
Additionally, where a dark roof is desired, ASV can make it perform like a cool roof, even with the lower solar reflectance. It is highly effective at reducing unconditioned attic temperatures, and its performance can be estimated using effective R-values (though Bush noted that modeling is needed for exact measurements).
How to implement ASV on your next project
Bush said there are a wide variety of ways to use ASV on your next roofing project, including:
- Offset clips.
- Integrated vent products.
- Perforated sub-framing.
- Battens/ridge vents.