Practical solutions for sustainable building with Steve Easley
The Real Deal
This test apparatus is used to evaluate the wind uplift capacities of OSB sheathing attached to 2x4 framing at 24” O.C. at the University of Florida. In these tests, the closed cell spray foam and sheathing assembly was subjected to pressure cycles that increased by 15 psf every 10 seconds until failure. For the fully foamed assemblies (with 3” thick layer), the average failure occurred around 240 psf. The 3” fillet assemblies failed at 160 psf. The traditional nailing schedule OSB assembly having 6d (2” long) nails at a 6”/12” fastener schedule failed at about 75 psf.
and through joints in the sheathing. Wouldn’t it be great if the house also had a secondary water barrier to pre- vent water leaks? Existing older homes are particularly
Uplift Update
New approaches offer better options for building and retrofitting storm- resistant homes. By Steve Easley
As summer progresses, residents in the South- east and Gulf Coast states prepare themselves for another hurricane season. We are all well aware that hurricanes can cost residents their lives and cause billions in property damage. Most of the property damage sustained by residential construction is caused by water intrusion into the home after hurricane forces failed one or more components of the building envelope. When it comes to hurricane-prone areas,
it is prudent to build and to retrofi t homes so they are more storm resistant. This entails making sure the home is structurally sound
and resistant to wind-driven rain. One of the weakest areas in a home is the
garage door. In a wind event, the garage door can easily blow inward (as wheels are pulled off their track). This fast-moving air entering the garage would positively pressurize the house, increasing the uplift pressure on the roof sheathing and trusses. Wouldn’t it be great if the roof framing and sheathing could remain intact? Further, even in cases where the roof remains in place but the house loses a few shingles and underlayment, there can be extensive water damage to the interior as strong winds can drive the rainfall uphill
vulnerable to wind damage because the code minimum roof nailing schedules that were in place at the time those homes were constructed specifi ed far fewer and smaller nails than what’s currently accepted by the code today. Interesting new research out of the University of Florida, Gainesville, shows that builders can provide signifi cant uplift resistance for customers without tearing off existing roof coverings to re-nail the roof sheathing.
New Research Dr. David O. Prevatt, PE (Massachusetts), from the Civil and Coastal Engineering Department at UF, has been researching the use of closed cell spray polyurethane foam (ccSPF) insulation as a structural retrofi t applied to the underside of the roof sheathing as a means to glue it to rafters or trusses and reduce roof blow off . Closed cell foam has tenacious adhesive properties in addition to being a high-performance insulator. The research has shown that using
According to test results, the 3” x 3” triangular fillet (left) doubled the uplift resistance for a 1/2” OSB sheathing fastened to 2x4 framing at 24” O.C. The full 3” foam layer (right) provides nearly a 300% increase in uplift resistance compared to traditional framing methods.
www.greenbuildermag.com
3” of spray foam applied to the under- side of roof sheathing between framing members will glue these components together providing nearly 300% increase in uplift resistance compared to tradi- tional roof installation methods. Based on U.S. Census data, Dr. Prevatt
estimates that about 80%–90% of the existing roof decks may be fastened in
July > August 2010 GreenBuilder 47
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