Hurricane Resistance Ensured By SPF
How thorough testing showed that SPF can provide substantial wind uplift resistance
By Jason Hoerter, Senior Product Manager at NCFI Polyurethanes
At the turn of the century, homeowners in Florida were under attack from hurricanes and the ever increasing cost of home owners insurance. Several hurricanes crushed the coast of Florida in the 90s and mid-2000s that devastated the insurance companies in Florida. To this day, the cost of homeowners insurance in Florida is twice that of the United States’ average. Close behind are Louisiana and Mississippi, two states massively attacked by hurricane Katrina in 2005. As
a result, programs started popping up in Florida to encourage homeowners to retrofit their homes with many types of hurricane mitigation products. Some of these included making stronger windows or shutters, stronger walls, and stronger roof attachments.
When I started working for NCFI Polyurethanes in August of 2007, one of my first projects was to look at how spray polyurethane foam (SPF) insulation installed to the roof deck of residential homes affected the wind uplift of the wooden roof deck attachment to the rafters. SPF was already being installed in the roof decks all over the United States, so this was a study to see if this application had any effect on the roof deck to rafter connection. Earlier that year, we started
a project with Huntsman and Honeywell under the direction of Dr. David Prevatt at the University of Florida that had some promising initial results.
The team first had to devise a way to study the effect of SPF on the roof deck. They laid out four-by-eight sheets of plywood and attached two-by-fours spaced 24 inches on center. Ten of these panels were made with the two-by-fours attached using 6d ring-shank nails, thirteen were made using SPF fillets along each side of the rafters, and five specimens had SPF completely filling the cavity between each rafter.
Results from this initial study showed real promise that SPF increased the wind uplift resistance. The no foam samples failed at a pressure of 78 psf. The foam fillet and full foam samples failed at averages of 153 and 244 psf, respectively. With statistics being as they are, more testing needed to be done, but this test confirmed the hypothesis.
According to research collected by Dr. Prevatt at the time, 80 percent of homes in Florida consisted of wood-framed roofing construction that were built before tougher code requirements surrounding hurricane resistance were introduced in Florida’s building codes around 2000. Field studies also showed that many homes were increasingly vulnerable because of faulty construction such as missing fastners or where the fastner didn’t make substantial contact with the roof rafter. With these concerns, a retrofit fix like SPF was very appealing, but the industry had to prove that it was a viable option.
The team created a second experiment to repeat the results with some minor modifications. To accurately determine the final design uplift resistance after SPF is installed in an existing home, the existing construction roof deck attachment strength had to be ignored. There is too much variability in attachment methods over the years and no way to tell how many fastners were used or what type. To come up with usable data, only the strength of the SPF could be calculated.
For the second experiment, the fasteners used to create the test assembly were removed after the SPF was applied and before the decks were tested. In this study, three sets of 10 panels were made. The first set was manufactured using a three-inch bead of foam applied to the angle at each side of the rafter members (see photos above). For the second set, the same three-inch bead of foam was applied. Then, the space between each rafter was sprayed with half-an-inch of SPF. For the third set, three inches of foam was applied between each rafter. Before the uplift resistance was measured, the fasteners holding the two-by-four to the OSB were removed. The only thing keeping the rafters attached to the OSB sheathing was the SPF.
The results of these test showed wind uplift resistance numbers between 178-209 psf. This is around twice the uplift resistance of roof panels with nails alone. This second research project proved SPF provided significant wind uplift strength to the roofdeck assembly. Also, the results demonstrated there was no statistical difference between the three spray foam configurations. So whether you are only adding spray foam to provide wind uplift resistance with the three-inch beads or filling the cavity for insulation purposes too, you should get similar wind uplift results.
At this point we knew we were onto something, but we had to find a way to convince others, especially insurance agencies, building code inspectors, and wind mitigation inspectors. The best way to do this was to obtain a Florida Product Approval (FPA). These reports are similar to engineering evaluation reports that many SPF manufactures obtain to show how their SPF systems meet code. In Florida, to claim a structural benefit to a product, you must go through the Florida Product Approval process and receive a report. For select counties including Dade and Broward, they require the Miami-Dade Notice of Acceptance (NOA) report. There are many unique differences between the two reports, but for this article, the important difference is the safety factor and the amount of testing involved. Since the NOA’s cover parts of Florida that are on the coast, an area designated as High-Velocity Hurricane Zones (HVHZ), they require additional structural testing and the safety factor is 2 instead of 1.5 for the FPA.
NCFI was referred to CBUCK Engineering to lead us through the process of obtaining an FPA and later an NOA. CBUCK’s Jimmy Buckner, a licensed engineer in the state of Florida whose area of expertise is all things structural and the Florida Building Code, helped us out. He and his team were able to design a test method, select a test laboratory, witness the testing, write the engineering evaluation report, and follow that through obtaining a complete and approved FPA and NOA. CBUCK determined that TAS 202-94 was the correct test to determine the wind uplift rating for SPF attached to a wooden roof deck. The test set up was essentially the same except the test assembly had 2×6 boxed frame around the outside.
Getting the right laboratory equipment and set up was a challenge that involved many iterations. Being involved in a new product application is exciting because you are developing the test method with the testing laboratories. SPF is such a good adhesive, the test laboratories had to change how they were testing. They were used to testing fasteners, but SPF proved to have failure pressures measuring over three times that of common fasteners of the time. The laboratories had to use multiple or larger vacuum pumps and had to design the test frame that would isolate the failure of the spray foam adhesive over the failure of the test frame itself.
During some of the first test runs using third party laboratories, I was able to demonstrate to the laboratories that failures were occurring due to the test assembly, not the spray foam connection. Looking at videos frame by frame showed that the two-by-fours were separating from the test frame before separating from the plywood. Also by analyzing the test assemblies after a failure, we could see that the test frame had to be modified. It was failing before the foam adhesive connection was failing. After a couple witnessed tests, the test laboratory was able to provide us with a test frame and vacuum equipment strong enough to test SPF.
The final tests were able to demonstrate that SPF was able to hold the roof deck down to uplift pressures two to three times stronger than standard fastening systems. With this data, NCFI became the first spray foam insulation company to obtain an FPA recognizing the structural benefit of their InsulStar product. Soon, other manufacturers recognized how valuable this product application was. Now there are ten manufacturers that have FPAs. Two manufacturers, including NCFI, have obtained an NOA for this application.
Now that manufacturers were able to obtain FPA’s, they were able to demonstrate that foam is a viable wind mitigation product. Homeowners in Florida could now use SPF to qualify for significant insurance discounts and decrease their energy use through adding insulation.
In addition to wind uplift resistance, spay polyurethane foam has other benefits that can help add to the mitigation program. Insurance agencies are also looking for ways to decrease the amount of water entering through the roof deck during a storm where the primary roof material is damaged or completely removed. The primary roof material is typically roof shingles or tiles which are susceptible to damage by flying debris or high wind. This should bring up images of blue tarp neighborhoods in Florida after devastating hurricanes. Spray polyurethane foam can provide a secondary water barrier in case the primary water barrier is damaged. During the time the primary
barrier is damaged until the time it can be repaired, closed-cell SPF can prevent water from entering the building saving the homeowner from having to replace their ceiling, furniture and carpeting that would be otherwise damaged.
Other states have programs similar to Florida in recognizing wind mitigation products but none are as extensive. Hopefully, other states will begin to incentivize homeowners to retrofit their homes to make them safer in hurricane prone areas around the Eastern and Gulf Coast areas of the United States. There are many options, but none have as many multiple benefits as SPF.
1. Yes, it’s 2017, we can start saying that now. I just love that I’m old enough to say I’ve been out of college since the turn of the century. Now, everyone else say it. Doesn’t it feel great?
2. The internet led me to a 2016 report by the National Association of Insurance Commissioners. I’m not an insurance expert, so I did my best guess understanding the results. See for yourself at http://www.naic.org/prod_serv/HMR-ZU- 16.pdf and make your own conclusions. No arguments that Florida’s insurance premiums are as high or higher than the rest and very likely twice as high at the United States average, I just couldn’t reliably give you hard numbers.
3. “Wind Uplift Behavior of Wood Roof Sheathing Panels Retrofitted with Spray-applied Polyurethane Foam,” Report No. 03-07, August 31, 2007, Department of Civil and Coastal Engineering, University of Florida. Can be found here: http://www.davidoprevatt.com/wp-content/uploads/2010/01/ccspf-test-report-8- 31-07.pdf
4. Thanks to my cousin Andrew Kester, P.E. for the simple definition of a safety factor. He’s
a Civil Engineer. I knew he’d come in handy one day.
For more information about this test, contact Jason Hoerter (email@example.com) or visit www.ncfi.com.
Photos courtesy of NCFI Polyurethanes
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