July 21, 2010

American roofing innovation limits wind damage

JEAN SORENSEN

correspondent

VANCOUVER

Build a better roof vent and the world will beat a path to your rooftop. It is the quest underway in the U.S., where they are developing a roof vent that looks a little like two halves of a ball with the curved ends butted together on stilts.

It’s known as the omnidirectional roof vent, with one version already in use by a roofing company.

It uses destructive high-force winds in a way that prevents the roof or roof membrane from lifting off.

The importance of the vent today is not just in preventing conventional roof membranes from lifting off or tearing, but in preserving new forms of roof membranes, such as reflective ones that are now becoming popular.

“They perform similar to a green roof, but they are a white roof,” said Elizabeth J. Grant, a research team member, who wrote her doctoral thesis on the omnidirectional vent development.

She spoke of how it can be used at Vancouver’s recent International Conference on Building Envelope Systems and Technologies (ICBEST) 2010 conference.

The new vent for low-sloped membrane roofing systems has been a collaborative effort by U.S. roofing company Acrylife principals Chuck and Pat Johnson (who had the idea for the vent in 2002) and took it to a research team from Virginia Tech College of Architecture and Urban Studies and College of Engineering.

The Johnson brothers and researchers saw its potential to mitigate billions in wind damage ranging from storms through to the destructive forces of hurricanes.

“Wind actually creates a suction on the roof,” said Grant. “And, the wind is worse at the corners and edges of the roof.”

The omnidirectional vent captures wind, from all directions, and creates a Venturi effect, which then produces the suction needed to keep the roof or membrane in place.

Essentially, the wind is constricted (making it more forceful) as it enters the lower half-globe and as it is directed upward, this powerful constricted wind stream pulls out air below in the roofing system creating a strong suction effect.

Virginia Tech College’s Jim Jones, an associate professor of architecture and principal investigator, described the vent, in a patent application in 2004, as a “360 degree membrane roof pressure equalizer” with no moving parts.

While Acrylife has been incorporating a version of the vent it calls the Venturi Vent Technology (V2T) into its roofing systems, the role of the research team has been to develop information on its use, collect test results and further development of the vent, with the intent of bringing a low-cost version that can be commercially manufactured.

Grant, who attended ICBEST 2010, reported on a full-scale prototype model of the vent that was mounted on a wind tunnel’s floor and subjected to wind speeds that were up 232 kilometres per hour.

The speeds are typical of what is seen on the U.S. East Coast.

The testing showed the prototype was stable in such wind forces, but it also created significant wind draw for the ambient air under the roof.

The test results, said Grant, showed that pressure inside the vent base was 2,447 Pa below the ambient pressure and capable of drawing the membrane down onto the roof substrate.

She said that on-going research has been looking at issues that surround the use of the vent and will aid installers and manufacturers of roofing materials with the installation and use, as well as warranty provisions.

Issues being looked at include what is the correct spacing of the vents on the roof, how are they impacted by infiltration, how do they react to different roof membranes and also what is the suction impact on the various kinds of insulation used today.

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