QA-spotlightheader image
0 Helpful?

Rebuilding in Tornado Country

Devastating storms in Oklahoma spark a discussion about the most appropriate type of construction for the region

Posted on Jun 17 2013 by Scott Gibson

Tornadoes have struck the Midwest with a vengeance this year, killing dozens of people and causing widespread destruction of property. In the city of Moore, Oklahoma, a tornado with winds topping 200 miles per hour struck on May 20, reducing whole neighborhoods to rubble.

Many homeowners will rebuild, so what should their new houses look like? In a post at GreenBuildingAdvisor's Q&A forum, David Gregory raises that question.

“The likelihood of another direct hit is tiny,” Gregory writes “The biggest opportunity is improvements in comfort and reduced energy consumption. But durability, I'm sure, is on people's minds, while costs are, of course, an issue.”

Questions that Gregory finds important concern the design and construction of tornado safe rooms, construction details for roof overhangs, the costs and benefits of roll-down metal shutters to protect windows, and whether attics should be vented or unvented.

"I wanted to tap the collective intelligence to work through how to build back better (or retrofit surviving homes for resilience), both in terms of ‘what’ and ‘how’ — especially, to discuss what is locally appropriate and cost-effective, specific technical resources, etc., given the unique and extreme conditions in Oklahoma.”

Tapping online resources

GBAGreenBuildingAdvisor.com senior editor Martin Holladay, Lucy Foxworth, and Trevor Trainor point to a number of Web resources that can help people decide the specifics of rebuilding their homes.

Among them:

  • Information published by the Federal Emergency Management Agency (FEMA) .
  • "Tornados of the South: Structural Performance of Newly Constructed Homes in North Carolina, Alabama, and Georgia," at the web site of APAAPA-The Engineered Wood Association. Nonprofit trade association for manufacturers of engineered wood products, including glue-laminated timber (glulams), composite panels, wood I-joists, and laminated veneer lumber (lvls). APA and APA EWS (Engineered Wood Systems) trademarks identify products that meet the organization's manufacturing and performance guidelines. Formerly known as the American Plywood Association. — The Engineered Wood Association. The download is free, Foxworth says, after completion of a .
  • An article from The Journal of Light Construction (to download the article, it looks like you'll need to subscribe).
  • A free article from JLC about .
  • Recommendations from .

Reinforced concrete structures resist tornado damage

Dana Dorsett suggests concrete walls and roofs reinforced with rebar would probably be able to resist just about any tornado structurally. "But," he adds, "you'd still have to replace all the blown windows and doors, and some or all of the siding and exterior insulation, depending on the severity of the storm."

Code minimums for a concrete wall assembly in Oklahoma would require continuous exterior insulation rated at R-5, he adds, would easily be met with just 1 in. of foil-faced polyisocyanurate. Installing 1 1/2 in. of the insulation would bring the performance of the wall assembly up substantially "for not a huge uptick in cost," Dorsett says.

"How building with reinforced concrete compares to timber-framing costs depends on the costs of concrete and the cost of labor in that area," he writes, "but for most homes in most markets it doesn't rise to a double-digit cost adder to the project as a whole."

Mike Collignon, like Holladay, thinks walls built of insulated concrete forms (ICFs) would be a good way of guarding against tornado damage, although Collignon concedes that some homeowners will be scared off by the increased costs of building that way.

Taking a lesson from Greensburg

One model for post-tornado reconstruction comes from Greensburg, Kansas, a community of 1,400 people devastated by an extremely powerful tornado in May 2007.

Thanks to emergency warnings about the approaching storm, casualties were kept to a minimum. Even so, 11 people were killed and 90% of the town's buildings were destroyed by the 1.7-mile-wide tornado.

The people of Greensburg decided to rebuild their town as a model green community, and their efforts were later described in a . The rebuilding effort also is described in a report by the , which was hired to help design replacement houses and train builders.

Foxworth says some of the BSC recommendations for rebuilding are "a little surprising."

"They recommended building with 2x6 advanced framingHouse-framing techniques in which lumber use is optimized, saving material and improving the energy performance of the building envelope. with OSB shear panels in the corners and insulated foam sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. elsewhere," she says. "I thought foam sheathing was not a good barrier to windblown objects in tornado-prone areas."

No, Collignon says, foam sheathing is not much of a barrier to windblown objects unless "it is backed with something of significance, like Kevlar or concrete.

"I've personally witnessed 2x4s fly right through brick veneer/batt insulationInsulation, usually of fiberglass or mineral wool and often faced with paper, typically installed between studs in walls and between joists in ceiling cavities. Correct installation is crucial to performance. /frame construction during wind-cannon testing, and the speed of the stud was only simulating an EF-1 tornado (110 mph wind speed, 60 mph stud speed," Collignon writes. "Foam insulation in that instance wouldn't have fared too much better, in my opinion."

He adds, however, that he's seen a video clip in which an ICF box built to simulate a closet was subjected to a TNT blast from only 6 ft. away. "There were areas of compression and minor burn marks on the foam," he says, "but that was the limit of the damage."

Reinforcing wood-framed buildings

Patrick Walshe says that if homeowners decide to rebuilt with wood because concrete is too expensive, there are a number of ways to reinforce the structure.

“Our house has the Simpson tie-down metal straps on the rafters and between the stories,” he writes. “There are 3-in. metal plate washers on the anchor bolts fastening the house to the slab as well as some heavy-duty metal brackets connecting studs to some anchor bolts. The house was built with pre-made stud panels sheathed with plywood. This plywood overlaps adjacent panels and the second floor in a jigsaw fashion. The design limited expanses of windows to allow enough shear strength in each wall.”

In addition, he adds, the architect insisted that nail heads be flush with the plywood, not set below the surface. Double-stud walls are set on 2x10 plates and have 2 inches of Roxul insulation on the outside to make them stronger and to increase resistance to penetration by windblown objects.

Hurricane ties between the rafters and wall framing, adds Dana Dorsett, “keep the roof from flying off in one piece.

“Construction adhesive between the rafters and roof sheathing and tighter fastener spacing keep the sheathing from popping off a sheet at a time,” he adds. “If the wind strips the deck bare of roofing and foam, that's a much cheaper repair than the whole roof (or whole house).”

Our experts' opinions

Here's what GBA technical director Peter Yost had to say:

Martin and others have done a great job covering the issues and providing resources. I just have two perspectives to add:

With support from the Lawrence Berkeley National Lab and the Department of Energy, BuildingGreen has been developing a . One of the coverings included in the resource is roller shutters. While not inexpensive, these shutters have a significant impact on energy efficiency and storm protection of windows. Check out the of the site for qualifying roller shutters.

Alex Wilson, founder of BuildingGreen, Inc. recently created the Resilient Design Institute and adds his perspective:

The recommends that safe rooms be installed in all new homes as well as existing homes undergoing major renovations in the parts of the country most vulnerable to tornados (red area in the ). Guidance on building safe rooms is available from . In most of the country (all shaded and cross-hatched areas of the FEMA map), wind resistance should also be provided in home construction (for example, hurricane straps); in these areas, RDI recommends building to the or its equivalent.


Tags: , , , , , , , , ,

Image Credits:

  1. Greg Henshall/FEMA

1.
Jun 17, 2013 8:09 PM ET

Disaster Resistant Buildings - Superior Walls
by Thomas Dugan

I will continue to comment on all of these articles touted as having solutions to tornadoes, wild-fires, hurricanes, etc. The simple and readily available solution is from a company called Superior Walls of America. I am a builder in coastal North Carolina and only build hurricane resistant homes using their wall system. They are made of 5000 mix strength concrete panels that are rebar reinforced and come pre-insulated with Dow foam. The walls are inherently waterproof and missile proof. I use Viwinco brand impact rated windows and closed cell foam in the rafters. It is the only way to build, in my opinion.


2.
Jun 18, 2013 2:15 PM ET

Closed cell & Dow & Superior Wall (response to Thomas Dugan)
by Dana Dorsett

At code-min R the most typical blowing agent used (HFC245fa) in most closed cell foam at the rafters will far outstrip the global warming potential (GWP) of the heating/cooling energy saved over the life of the building. It's a net-negative for the planet. At 3" it's already structural, but still sub-code for R, and still probably lifecycle net-damaging from a global warming perspective. At 2" it's pretty-structural, but may someday break-even, depending on the actual energy sources. HFC245fa is on the order of 1000 x CO2 as a greenhouse gas- it doesn't take a whole lot to do a whole lot of damage- the less you can get away with, the better.

Similarly, Dow Styrofoam XPS, is blown with combination of HFC134a (about 1400 x CO2 by itself) and other HFCs, with a combined GWP of over 800 x CO2, and will never break-even using the code-min Xi version of Superior Wall. The R5 version might when used in combination with other insulation to reach code min, depending on the actual heating/cooling energy sources. But it's hard to turn Superior Wall R5 into a truly high thermal performance wall due to the excessive thermal bridging of it's structural stud-like members.

Until that changes use of these materials as the primary insulation is the antithesis of "green building". There are newer blowing agents out there for closed cell polyurethane that with sub-10 x CO2 GWP,- notably DuPont FEA-1100®, & Honeywell Solstice®, but I'm not sure which (if any) of the major players are using them. I don't know of any vendor in north America using a truly low GWP blowing agent for XPS, but in Europe Dow used CO2 as the blowing agent for XPS (with, strangely, 1x CO2 GWP :-) ).

Polyiso & EPS are both blown with pentane (at about 7x CO2 GWP), and where they CAN be substituted for XPS or closed cell polyurethane, they arguably SHOULD. A bare-bones minimalist R16 ICF (yes, they're still available) is a far greener solution than any version of Superior Wall, with substantially higher thermal performance than the Xi (R12.5) version of Superior Wall .

Open cell foam is not at all structural, but is blown with water, and uses 1/4-1/3 the total polymer of most closed cell foams, and a much greener (though not the greenest) for going high-R, or even code-min. In the climate-zone 3 parts of NC an all open cell solution can be used on the underside of the roof deck without damaging the roof deck with excess moisture, but not in the cooler zone 4 parts. But a flash-inch of closed cell in combination of blown fiber works everywhere in your area, except possibly under "cool roof" finishes of metal roofing. See table 3 on this document:

There are at least two vendors with a 2lb spray polyurethane that is blown with water (with very low GWP). Aloha Energy (a regional player in NY), and Icynene's MD-R-200 product (not to be confused with MD-C-200, which is blown with HFC245fa). The 1.8lb goods from Aloha are fairly similar in most respect to HFC blown foam. The Icynene goods run about R5.1/inch, and are about 3-4x more vapor permeable (which is in some ways better, for roof deck applications, since even at code-min it's still a class-II vapor retarder, with at least some drying potential.) If you're going to stay stuck on the all-closed-cell solution to the roof insulation, PLEASE seek out one of these, or find a product/installer that stipulates that they use only the very-low/no GWP blowing agents- anything under 10x CO2 . The Enovate® 3000 blowing agent has sometimes been touted by Demilec as a low GWP, but it's just a trade name for HFC245fa, and thus on the order of 1000 x CO2.

It matters that the materials used to insulate a house aren't more damaging to the environment the energy use they're offsetting, and I hope you can make accommodations & adjust to your "It is the only way to build, in my opinion" statement. The solutions proposed may be hurricane proof, but are net-damaging to the climate. There are better, lower impact ways to get there.


3.
Jun 19, 2013 9:24 PM ET

Edited Jun 19, 2013 9:27 PM ET.

let's keep the conversation going
by David Gregory

Thanks for all the feedback and great resources so far. Hope the conversation can continue; esp. on the 'pretty good house for OK (re: energy use)' theme, regardless of where along the concrete - to - wood framed continuum people fall.

One topic to go deeper on: Garage doors. There have been discussions here on how leaky they are, and the misleading 'R-value' claims of manufacturers; but in addition, garage doors are often the weak point that initiates the collapse of the structure. Even so, I heard that Midwest City recently started requiring garages as part of all new construction (still tracking down details)!

Could a better-sealed garage door system also be more high-wind resistant? I prefer single-car garages (if any) for other reasons, but if a multi-car garage is unavoidable, breaking it into multiple doors would be preferable from a durability standpoint, though at the cost of more perimeter for leakage. I will review the literature on garage door failures for clues to improvements; welcome advice from anyone with first-hand knowledge.


4.
Jun 19, 2013 9:28 PM ET

Edited Jun 22, 2013 11:17 PM ET.

type-o
by David Gregory

last par., 2nd sentence: safe rooms, not roofs


5.
Jun 20, 2013 3:31 AM ET

Response to David Gregory
by Martin Holladay

David,
Thanks for your help. I've corrected the error.


6.
Jun 23, 2013 12:13 AM ET

'Superior Walls'
by David Gregory

Thomas - thanks for the ref to 'Superior Walls'; good to know what builders are using. How do construction costs compare? Any other builder insights? Looks like they were originally foundations, now moving into walls...

I have concerns about pre-fabricated systems in general, though, especially in disaster situations. Though fast to build once on-site, needs lead time for translation of designs, then time/money/environmental costs to shipping, and specialized (heavy) equipment for installation. Looks like the nearest manufacturing facility for SW is in Tennessee; not bad, but not great... Given that the majority of the components of concrete can usually be sourced more locally (water, stone and sand, even portland cement; and in OK, probably fly ash as a supplement; and demo concrete as new aggregate), it's hard for me to imagine that the net benefits of pre-fab (accounting for risks of things going wrong and harder to fix in the field) outweigh the flexibility of on-site construction with a similar final result (ICF).

I second Dana's concerns about the foam; also, some of their products use concrete studs, sometimes with metal facings, which means thermal bridging. And I'm skeptical about their claims that no water proofing is needed (for foundations...); at least would need to see a convincing detail for sealing the joints between units. Perhaps better for higher-end homes with adequate lead time and attention to details, than rapidly built replacements for more modest incomes?

Finally, my soapbox: If we're going to the trouble of building with reinforced concrete, does it still make sense to build single-story detached homes? (some multi-story on SW's website, admittedly).


Register for a free account and join the conversation


Get a free account and join the conversation!
Become a GBA PRO!