CoastalContractor ~ Winter 200522

Wind-borne debris carried by hur-ricane-force winds can crash

through window openings — themost vulnerable areas of a build-

ing envelope. Air rushing in theopenings can then over-pressurizethe envelope, just like blowing upa balloon until it bursts — a fate

that befell this home on NorthCarolina’s Outer Banks during

Hurricane Isabel in 2003.

SpeccingWindows in High-Wind

Zones

Winter 2005 ~ CoastalContractor 23

AAs Florida recovers from four devastating hurricanes in one sea-son, a clear lesson is emerging: The new codes worked. Betternailing, sturdier shear walls, more framing hardware, and beefed-up opening protection made a difference. Homes built to stringentnew standards fared far better than older houses approved underpre–Hurricane Andrew codes.

Florida’s lesson won’t be lost on builders or code officials in therest of the Atlantic and Gulf seaboards. But in states where thenew International Residential Code (IRC) and InternationalBuilding Code (IBC) are just coming into force, the beefed-updetails these codes require in high-wind zones pose an unfamiliarproblem for many contractors and inspectors. Window require-ments in the new codes are a particular source of confusion. Here,we take a look at what the codes say about windows near thecoast, and at what those requirements mean in practice.

EXPERIENCING TECHNICAL DIFFICULTIESBefore contractors in hurricane-prone areas can even start to tacklethe intricacies of actually building the house, they have to come toterms with the new codes and figure out what systems and prod-ucts their building is required to have.

In states north of Florida, windows need double-pane glass tomeet the energy code. That means a fatter sash and frame.Strengthening the glass, sash, and frame to stand up to higherwind pressures or flying objects has meant adding more thickness,which makes the windows too bulky and heavy for existing hard-ware to support. Working out the resulting technical issues hasbeen a real challenge for window suppliers, raising the cost ofcode-compliant windows and limiting the range of available prod-ucts from which coastal contractors can choose.

The good news is that the codes provide a “simplified proce-dure” that covers most residential work. Builders can usually findanswers by working their way through the look-up tables inChapter 3 of the IRC. The bad news is that using those tablesmay lock builders into some expensive choices when it’s time topick windows, as the prescriptive rules can seriously limit productand design options. But leaving the “simplified” path means get-ting help from an architect, an engineer, or both.

In regions vulnerable towind-borne debris, thecode offers more than oneway for coastal builders tocomply with new impact-resistance rules by Ted Cushman

COURTESY SOLUTIA

Main roof and wall areas receive moderate wind force

Hip and gable end corners experience the most extreme wind action

Roof edges and wall corners see increased pressure or suction

STRUCTURE VS. CLADDINGAndrew DiGiammo is an archi-tect and builder who specializesin custom homes in coastalMassachusetts and Rhode Island.Rhode Island adopted the IRC in2002, and Massachusetts is setto adopt it soon. So DiGiammohas begun to work the IRC’swind-related requirements intohis design process, starting withthe preliminary site analysis.

There are really two parts towind design, he explains. Thefirst is what the code calls theMain Wind Force ResistingSystem, or MWFRS. Thisincludes the structural compo-nents like wall framing, floor androof diaphragms, and shear walls— all the big elements that resistthe wind forces acting on thestructure of the building, keep-ing it from tipping over, sliding

off the foundation, or just gettingblown away.

Second, the wind designmust address Components andCladding, or C&C. That’s thesection of the code that appliesto elements such as roof sheath-ing, roof covering, exterior siding,windows, doors, soffits, fascias,and chimneys. This is where abuilder will consult design pressure tables and determinethe requirements for differentbuilding elements, including the windows.

Requirements for the mainstructure and for the compo-nents and cladding are foundtogether in the new codes. In theIRC, they’re located in Chapter3, “Building Planning,” and inthe IBC, they’re in Chapter 16under “Wind Loads.” Provisionsin each code ultimately go back

to the ASCE 7 engineering stan-dard, Minimum Design Loads forBuildings and Other Structures,published by the AmericanSociety of Civil Engineers. Theprescriptive tables in Chapter 3of the IRC are just ASCE 7’s“simplified method,” applied to alimited set of building sizes andshapes experiencing basic windspeeds lower than 110 mph. Ifyour needs go beyond thosetables, the IRC and IBC bothrefer you back to ASCE 7, or topublications based on it.

When designing to resist highwinds, the MWFRS and C&Cmust each be addressed sepa-rately. But DiGiammo doesn’t use the IRC tables in hisdesign process, because thatapproach limits his options forshear wall layout and windowplacement. Instead, he does a

wind-load analysis for eachbuilding that conforms to themore advanced method providedin the ASCE 7 standard.

“You have to start with shearpanels, so that you’ll know whereyou can put holes in your wall tobegin with,” DiGiammo pointsout. “That is all part of the MainWind Force Resisting System. Ifyou go with the prescriptivecode, the sheathing layout forshear panels is going to limitwhere you can and can’t put win-dows. But if the MWFRS has allbeen determined, and thebuilder just wants to know thedesign pressure rating requiredfor windows in a given plan, allyou really need to know is howto use the IRC.”

Once he has done hisMWFRS analysis, saysDiGiammo, finding out therequired window design pres-sures “is actually quite simple onmy end. I just go to theAndersen Windows website anduse the online Design PressureEstimator. That makes it easy.”Most other window manufactur-ers offering impact-resistantunits can provide some level ofdesign pressure analysis, as well.

Understanding how the IRCrules work is crucial, though. Themanufacturing and testing issuesmean that higher design pressureratings can’t be achieved in everywindow type or size; and whencompliant windows are available,the cost can be prohibitive. Also,different parts of a house mayexperience different wind pres-sures. Changing a window’s sizeor location can significantlyaffect how strong it has to be —so a simple thing like moving awindow or skylight may no longerbe so simple.

CoastalContractor ~ Winter 200524

n Zone 1: Main roof area (locations at least 4 feet from the eave, ridge, or hip)

n Zone 2: Roof edge locations within 4 feet of an eave, ridge, or hip, except for cor-ner areas

n Zone 3: Roof locations within 4 feet of the wall corner or gable-ridge intersection

n Zone 4: Main wall area (not within 4 feet of a corner)

n Zone 5: Wall corner area (within 4 feet of

Speccing Windows in High-Wind Zones

RICK VITULLO

FIGURE 1. WIND PRESSURE ZONES In a hurricane, the dynamics of swirling air currentsaround house corners create localized zones ofhigher positive or negative pressure. In an engi-neered design, pressures are calculated using for-mulas. But the “simplified method” used in theIRC just defines pressure zones on a basic house,and provides precalculated values for the expectedpressures. The builder can look up these requireddesign pressure ratings for each window in IRCTable R301.2(2).

“Calculating window designpressures might as well be left tothe window company’s engineer-ing staff,” says DiGiammo. Forthe designer or builder, the thingto do is to get familiar with thefigures and tables in Chapter 3of the IRC — especially thedrawings that show the windpressure zones on a house. Thatwill give you an intuitive grasp ofhow wind impacts your buildingand what that means for windowcharacteristics (Figure 1). Withthat clear, it’s easier to under-stand the design decisions orproduct choices.

NAVIGATING THE CODEThe figures and tables in theIRC amount to a road map fordetermining window require-ments. The first step is to figureout your site’s “basic windspeed,” as found in the newlyrevised wind speed zone mapfrom ASCE 7 (Figure 2). Thatgives you a starting value forwind pressures on your building.If you’re below the 110-mphthreshold, the whole buildingcan be designed with the IRCprescriptive rules. In the 110-mph and higher zones, thedesigner has to turn to the ASCE7 standard or to the AmericanForest & Paper Association(AF&PA) or SBCCI design manu-als that apply equivalent methods.

Your building departmentofficials should be able to tellyou the site’s wind speed. They’llalso make the determination of amodifying factor called“Exposure Classification.”Exposures range from ExposureA, a location sheltered by tallbuildings and rough terrain, upto Exposure D, a windsweptlocation facing a mile of open

ocean, mud flats, or ice.Sheltered locations let youreduce the required design pres-sures, but in exposed locations,you must increase them.

Once the shear-wall andanchoring details are decided,either by the prescriptive code orby engineering methods, thewindow design pressures can belooked up. These will varydepending on the size of thewindow, its placement in thewall, the height of the building,and the exposure classification ofthe site. Smaller windowsrequire higher design pressures,taller buildings have to use higher design pressures, andwindows near a corner, eave, hip,or ridge also need higher ratings.All of these factors are reflectedin the values in IRC TableR301.2(2). But for buildingstaller than 35 feet, and even forshorter buildings in exposureclassifications C or D (the relatively exposed sites), the val-ues from Table R301.2(2) haveto be increased by 21% to 87%,using an adjustment coefficientfrom Table R301.2(3).

Once the design pressure rat-ings have been established, thenext piece of the puzzle is todecide exactly how to protect theopenings. In 120-mph and higherwind speed zones, or within onemile of the shoreline in the 110-mph zone, the code requires allwindows to pass AmericanSociety for Testing and Materials(ASTM) testing for impactresistance or to be equipped withapproved storm shutters.Otherwise, the builder has toprovide plywood or OSB protec-tive panels cut to fit each win-dow opening, along with fasten-ers for attaching them. The

Winter 2005 ~ CoastalContractor 25

90

90

90

100 130120110

130

130

140 150140

140

120110

100

140150

FIGURE 2. WHERE THE WIND RULESThe basic wind speed map is the starting point for establish-ing window design pressure ratings. The 120-mph and higherzones and sites within a mile of shore in the 110-mph zoneare designated “wind-borne debris regions,” where impact-resistance rules apply. Regions inland of the 110-mph zonemay use prescriptive values defined in the code. Structures onthe coastal side of this line must be engineered.

While the map gives a rough idea of the regions affectedby the new rules, check with local jurisdictions. The actualboundaries between wind regions sometimes follow convenientmarkers, such as county lines or highways. Also, the actualdesign pressure designated within a specific wind zone willdepend on the degree of exposure or shelter at a specificbuilding site, and is higher for buildings that serve criticalpublic functions.

CoastalContractor ~ Winter 200526

impact glass adds yet more size,weight, and cost to the window— and so do the other beefed-upconstruction details needed forthe window’s frame, as well as itsglass, to survive the impact ofthe test missile (Figure 3).

Architect Bill Chaleff workson the east end of Long Island,N.Y., all of which falls into the

120-mph wind speed zone. Thewestern part of the island, closerto New York City, is in the 110-mph zone, so areas within a mileof the coast also require windowopening protection. “Windowmanufacturers have hustled toprovide us with compliant win-dows,” says Chaleff. “But they’redouble the cost. On our projects,

we’ve taken the other compliantpath — we supply the OSB pan-els and fasteners for every house.”

THE “PARTIALLY ENCLOSED”LOOPHOLE DEBATEChaleff points out that a closereading of the code shows thatimpact glazing, shutters, or pro-tective panels may not be neededat all. “The provision actuallyreads, ‘Windows in buildingslocated in wind-borne debrisregions shall have glazed open-ings protected from wind-bornedebris or the building shall bedesigned as a partially enclosedbuilding in accordance with theInternational Building Code,’ ”observes Chaleff.

According to New York Statebuilding code authorities, thatcode language lets builders anddesigners choose between twooptions: Either protect the win-dows, or make the buildingstrong enough to hold togethereven if the windows fail. Inshort, conclude the state engi-neers, “the code does not explic-itly require glazed openings to beequipped with protective woodstructural panels or costly spe-cialized windows. Instead, theseoptions are available if the build-ing is not designed as a partiallyenclosed structure.”

A letter from the New YorkCodes Division engineers indi-cates that a partially enclosedstructure could require strength-ening of framed exterior wallsand additional hardware for roofrafters. On a typical two-storyhome with a 30-degree roofpitch, the report specifies thatconnections would be affected,especially tiedowns for roofframing. As an example, if adesign would normally require

25-gauge tiedowns with four 8dnails to connect each rafter towall framing, it would nowrequire 20-gauge tiedowns withsix 8d nails. The engineers go onto say that rafter spacing mightneed to be reduced or sheathingthickness increased in order toprevent sheathing uplift in a“partially enclosed” design: Half-inch plywood on rafters spaced24 inches on-center should beacceptable. As for the framingmembers themselves, the engi-neers say roof framing that canhandle required snow loads isalready strong enough to handlethe allowable uplift loading.Conventional walls composed of2x4s spaced 16 inches on-centerand having a floor-to-ceilingheight of 8 feet would be capa-ble of resisting the increasedload. Walls composed of 2x6sspaced 24 inches on-center witha floor-to-ceiling height of 10feet would also be capable ofresisting the increased load.Exterior walls surrounding cathe-dral spaces, however, wouldrequire an engineered design.

In short, the constructionrequirements for a “partiallyenclosed structure” aren’t anymore rigorous than constructionstandards in most snow-loadregions. But there’s another sideto the problem. A structure thatis still standing after a hurricaneblows all its windows out couldstill be a total, or near-total, loss.Six to ten inches of rain drivenby hurricane-force winds is likelyto completely ruin the home’sinterior finishes, wreck all thefurniture and other belongings,and create a major risk to thehealth and safety of anyone try-ing to take shelter within thehouse.

FIGURE 3. PASSING THE IMPACT TEST Impact-resistant window units must pass the rigorous testprocedures defined by the American Standards for Testingand Materials in ASTM standard E1886. In the first part ofthe test (shown above), a “large missile” — a 2x4 — is shotfrom a compressed-air cannon at a velocity of 50 feet persecond (34 mph). Part two uses a “small missile,” or ballbearing, traveling at 80 feet per second (50 mph). Thesetests require multiple impacts at designated places on thewindow unit, and after each impact, the product — which isinstalled on a test wall or roof assembly — must undergo9,000 pressure cycles to simulate the damaging pressurefluctuations of turbulent winds.

These tests work in concert with ASTM standard E1996,which designates the wind zones where test-compliant prod-ucts are required and establishes pass/fail criteria for thetests. To pass after impact and pressure cycling, there canbe no hole in the assembly large enough to fit a 3-inchsphere, or no tear longer than 5 inches. By code, buildersworking within “wind-borne debris regions” (see “Where theWind Rules,” page 25) are required to install storm shuttersor laminated-glass windows and reinforced side-hinge doorsand garage doors that can pass the missile-impact and pres-sure-cycle tests.

Speccing Windows in High-Wind Zones

COURTESY SOLUTIA

CoastalContractor ~ Winter 2005

IRC and IBC wind provisions enacted by state

IRC and IBC wind provisions enacted by some localities

New Jersey

DelawareD.C.

Texas

Louisiana

Mississippi

South CarolinaGeorgia

Florida

Alabama

Virginia

NorthCarolina

NewYork

New Hampshire

Rhode IslandConnecticut

Maine

Maryland

Massachusetts

Considering these risks, engi-neers creating code referencedocuments clearly prefer theoption of tougher windows.Bradford K. Douglas, theDirector of Engineering for theAF&PA’s American WoodCouncil, phrased it this way in aletter to Florida code officials:“The consensus of the ANSI[American National StandardsInstitute] group overseeingdevelopment of our new WoodFrame Construction Manual forOne- and Two-Family Dwellings(WFCM) is that designing thestructural members of a buildingfor partially enclosed pressuresdoes not provide the same bene-fit as protecting the building

envelope from flying debris.Designing a building for partiallyenclosed pressures only protectsstructural members against acci-dental overloads. It does not pro-tect the more valuable interior fin-ishes, contents, and occupants.”

Based on their losses in paststorms, insurance companieshave a strong incentive to pushtougher windows. Insured dam-ages from Hurricane Andrewincluded many complete struc-tural failures; but after later andless powerful storms such asHurricane Floyd, insurers suf-fered the bulk of their multibil-lion-dollar losses in payouts forhomes that were still intact afterthe storm passed but sustained

so much interior water damagethat they were treated as a totalloss, and simply torn down andrebuilt from scratch. This sum-mer’s record crop of Floridastorms appears to follow a simi-lar pattern, with relatively fewstructural collapses but billionsof dollars worth of damagecaused by lost claddings andrain-saturated interiors.

Accordingly, insurance com-panies are moving to limit coverage for homeowners whoelect not to install storm-resistantwindows. Vin Andrews of A.W.Hastings & Co. is an architecturalrep for Marvin Windows servingcoastal areas of several Northeaststates. Andrews says that on onerecent project, an architect hadto go back to the drawing boardtwice — once to adjust to win-dows that would meet code-required design pressures, and asecond time when the homeownerrealized that her insurance com-pany would limit coverage if thewindows lacked impact protec-tion. “She wasn’t even in thezone where the code requiresimpact glazing,” says Andrews.

“But when she read her insur-ance policy carefully, it saidstorm damage to the home’s inte-rior was excluded unless thehouse conformed to that sectionof the code. Shutters didn’t fitthe style of the house she want-ed, and she didn’t want to messaround with plywood or OSBpanels. So it had to be impact-resistant windows.”

THE PRUDENT PATH Against this background,builders may be best advised totake on the window issue early intheir planning. If you let aestheticor other design factors drive win-dow choices without consideringthe required pressures or impactprotection, you may be forced torevisit earlier choices whenunexpected objections are raisedby the building department or aninsurance company — or both.With hurricane windows, as withhurricanes, it’s best to planahead. ~Ted Cushman reports on thebuilding industry from his homein Great Barrington, Mass.

TOUGH WINDOW CODES TAKE HOLDNew window requirements from the IBC and the IRC are now being enforced along the entireGulf of Mexico and Atlantic shorelines, either at the state level (dark blue states) or at least bysome municipalities (within light blue states).

In all states, contractors should always check with local authorities to see if and how theprovisions apply in their jurisdiction. Different local jurisdictions commonly interpret or applybuilding codes in different ways. Early in the planningprocess in any state, check with local code officials to deter-mine the requirements for each individual project. Keep inmind, too, that even where protective measures such aspressure-rated or impact-rated windows aren’t nec-essary to meet local requirements, they might still beneeded before the building can qualify for homeowner’sinsurance coverage.

Designing a building for “partially enclosed” pressures

does not protect the more valuable interior finishes,contents, and occupants.

ADAPTED FROM THE INTERNATIONAL CODE COUNCIL

28

Speccing Windows in High-Wind Zones