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Proceedings of the RCI 26thInternational Convention & Trade Show

APRIL 7-12, 2011PEPPERMILL RESORT SPA CASINO

RENO, NV

MASTERING THE DESIGN ISSUES OF INSTALLING PHOTOVOLTAICS ON EXISTING ROOFS............................................................................1Karim P. Allana

DYNAMIC WATER VAPOR PERMEANCE OF BUILDING MATERIALS AND THE BENEFITS TO BUILDINGS..........................................................13Marysusan Couturier and Craig Boucher

SUSTAINABLE INSULATION: THE HARD CHOICE ..................................................................................................................................21Thomas Hutchinson

THE CORRELATION BETWEEN WIND RESISTANCE AND THE PHYSICAL PROPERTIES OF FIBERGLASS SHINGLES ........................................27Jim D. Koontz

THE DEVELOPMENT OF FM APPROVAL STANDARDS FOR CLASS-ONE STEEP-ROOF COVERS, FLEXIBLE PHOTOVOLTAIC MODULES,AND VEGETATIVE ROOF SYSTEMS ....................................................................................................................................................37

David L. Alves, Michael C. Burke, and Jill E. Norcott

DEVELOPMENT OF A ROOF SAVINGS CALCULATOR ..............................................................................................................................47Joshua New, William Miller, André Desjarlais, Yu Joe Huang, Ender Erdem

DYNAMIC BUILDING ENVELOPE CLOSURE SYSTEMS ............................................................................................................................57Douglas Pearmain and David G. Dixon

CASE STUDY: RESTORATION OF A HISTORIC CLAY-TILE ROOF SYSTEM ................................................................................................67Christopher W. Giffin and James M. Brown

FULL OF HOT AIR: HOW THE IRC FAILS TO ADDRESS EFFECTIVE ATTIC VENTILATION............................................................................79Jeremiah M. Edwards and Ashley Aiken McDuffee

THE GREEN ROOF: COMMON-SENSE ADVICE ON DURABILITY..............................................................................................................89Daniel A. Delisle and Jeffrey D. Kerr

INVESTIGATION AND LITIGATION OF A ROOF FAILURE: WHO WILL PAY? ................................................................................................99Edward O. Betker and Michael G. Taylor

ROOFS, ENERGY EFFICIENCY, CODES, AND SUSTAINABILITY: COMPLEXITY AND COMPROMISE ON THE ROAD TO NET ZERO ....................107R. Christopher Mathis

CASE STUDIES OF ROOFING AND CLADDING FAILURES INVOLVING INTERNAL PRESSURIZATION AND TOPOGRAPHIC EFFECTS ....................115Warren R. French

CLADDING ATTACHMENT SCHEMES FOR THREE-COAT STUCCO VENEER ON THICK CONTINUOUS INSULATION (CI), BASEDON EXPERIMENTALLY VALIDATED FINITE-ELEMENT MODELING ............................................................................................................125

Gary Parsons and Jeff Hansbro

HOW BIG IS TOO BIG? THE EFFECT OF TABLE SIZE ON WIND UPLIFT RESISTANCE EVALUATION..........................................................135A. “Bas” Baskaran, S. Molleti, W. Li, S. Ko

REHABILITATION OF HISTORIC LIMESTONE FAÇADES..........................................................................................................................145Jonathan E. Lewis, Mark K. Schmidt, and Blake M. Andrews

TABLE OF CONTENTS

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26 T H RC I I N T E R N AT I O N A L C O N V E N T I O N A N D T R A D E S H O W • A P R I L 7 - 1 2 , 2 0 1 1 E D W A R D S A N D M C D U F F E E • 7 9

FULL OF HOT AIR: HOW THE IRC FAILSTO ADDRESS EFFECTIVE ATTIC VENTILATION

BY JEREMIAH M. EDWARDS, RRC, PE,AND ASHLEY AIKEN MCDUFFEE, RRO, EIT, LEED AP BD+C

STAFFORD CONSULTING ENGINEERS130 Edinburgh South Dr., Suite 202, Cary, NC 27511

P: 919-461-8125 • F: 919-461-8127 E-mail: [email protected]

ABSTRACT

The intent of the International Residential Code (IRC) regarding enclosed attic spaces isto provide an effective system that is balanced and cross-ventilated to minimize moistureand heat buildup in unconditioned attic spaces and to reduce energy consumption and cool-ing costs for the residential building. However, the objective is compromised by ambiguouscode language that leaves interpretation to local code officials, yields ventilation decisionsto contractors and not designers, and ultimately contradicts sound engineering judgment.The presentation will address discrepancies between the IRC and IBC and illustrate currentIRC implications on attic ventilation design and installation with a case study.

SPEAKERS

JEREMIAH M. EDWARDS, RRC, PE — STAFFORD CONSULTING ENGINEERS

JEREMIAH M. EDWARDS has over 13 years of experience in investigating and design-ing roofing, waterproofing, and exterior wall assemblies on multiple commercial, institu-tional, government, and residential building projects. Mr. Edwards graduated with a B.S.and graduate-level professional degree in civil engineering from North Carolina StateUniversity. He specializes in consulting with architects, contractors, and building owners toevaluate, diagnose, and repair moisture-intrusion problems and construction defects. Mr.Edwards is a licensed professional engineer in North Carolina and South Carolina and anRCI, Inc. Registered Roof Consultant.

ASHLEY AIKEN MCDUFFEE, RRO, EIT, LEED AP BD+C — STAFFORD CONSULTING ENGINEERS

ASHLEY AIKEN MCDUFFEE graduated with a B.S. in civil engineering from BucknellUniversity in 2005 and an M.S. in historic Preservation from the University of Pennsylvaniain 2007. McDuffee joined Stafford Consulting Engineers in 2008 and has experience sur-veying, designing, and monitoring various building envelope assemblies and investigatingroof and wall failures. McDuffee specializes in building science and preservation engineer-ing and is certified as an RCI, Inc. Registered Roof Observer, USGBC LEED-AccreditedProfessional, and Engineer in Training.

80 • E D W A R D S A N D M C D U F F E E 2 6 T H RC I I N T E R N AT I O N A L C O N V E N T I O N A N D T R A D E S H O W • A P R I L 7 - 1 2 , 2 0 1 1


INTRODUCTIONAttic ventilation is common practice and

has been incorporated into residentialbuilding design for over 60 years. Whileboth vented and unvented roof assembliesare allowed by model building codes, ventedroof designs have the advantage of a long,proven, successful performance history.However, with the increasing complexity ofroof designs, vented roof assemblies havebecome much more difficult to construct incomparison to unvented roof assemblies,resulting in mounting problems in venteddesigns and increased popularity of unvent-ed designs. For the purposes of this paper,the focus is exclusively on the performanceof vented attic designs.

When designed and installed correctly,vented roof designs can be used successful-ly in all North American hygrothermal cli-mate zones. The rationale for ventilatingunconditioned attic spaces varies, depend-ing on the climate zone, but in all casesaims to limit premature deterioration ofbuilding components. The primary purposeof attic ventilation in cold climates is toremove moisture originating from the condi-tioned spaces in order to prevent condensa-tion and decay of roof framing and deckingand minimize ice dams. The prime motiva-tion for attic ventilation in hot climates is tominimize moisture and solar heat buildupin the attic spaces, thereby reducing heatgain, energy consumption, and coolingcosts for the building.

The International Residential Code (IRC)and International Building Code (IBC)include requirements for attic ventilation,which intend to provide an effective ventila-tion system that is balanced and cross-ven-tilated. However, the intent is compromisedby insufficient and ambiguous code lan-guage that leaves interpretation, jurisdic-tion, and enforcement to local code officials.Moreover, language in the IRC deviates frombalanced system requirements found in theIBC, further impairing effective ventilationdesign and ultimately contradicting soundengineering judgment. Industry profession-als, International Code Council (ICC) offi-cials, and building owners should be madeaware of these flaws in the IRC and IBC and

how they, along with complex roof designs,can negatively influence attic ventilationdesign and installation.

HISTORY AND BACKGROUNDThe basis of attic ventilation require-

ments found in the IRC and IBC can betraced back to research and code literaturepublished in the 1930s and 1940s.

Larry V. Teesdale, in 1937,1 was one ofthe first researchers to present the case forattic ventilation to help reduce indoorhumidity and condensation in cold cli-mates. Recognizing the need for a designedattic ventilation system, the FederalHousing Administration first publishedattic ventilation requirements in 1942.2 Thefirst requirements were to provide 1 sq ft ofventilation for every 300 sq ft of attic floorspace (1/300). The genesis for this ratioappears to be loosely based upon theresearch by Tyler S. Rogers (1938)3 andFrank B. Rowley et al. (1939)4 for condensa-tion control within the attic during coldweather. The research was performed dueto the observation of frost on the undersideof the roof deck sheathing during the winteron facilities in Minnesota.

The Building Officials and CodeAdministrators International, Inc. (BOCA)adopted language in 19485 that reads, “Allattic spaces and unoccupied spacesbetween roofs and top floor ceilings shall beventilated by not less than two (2) oppositelouvers or vents with a total clear area ofopening not less than one-third (1/3) of one(1) percent of the horizontally projected roofarea.” BOCA promulgated the building codethrough the mid- to late 1990s, until theInternational Code Council issued the 2000IRC. The BOCA National Building Codethrough 19946 read, in part:

The net free-ventilating area shall benot less than 1 to 150 of the area ofthe space ventilated except that thearea may be 1 to 300, provided atleast 50 percent of the required ven-tilating area is provided by ventila-tors located in the upper portion ofthe space to be ventilated.

Code Change No. R82-94 revised thewording to read “the area is permitted to be1 to 300, provided at least 50 percent, andnot more than 80 percent, of the requiredventilating area is provided by ventilators.”The reasoning was that the wording “wouldallow 100 percent of the required ventilationarea to be in the upper portion of the attic.The purpose of the reduction in ventilatingarea [at the upper portion], however, is toaddress cross ventilation between the highand low portions of the attic.”

The International Code Council adoptedthe language of the 1995 edition of theBOCA building code into the 2000 Editionof the International Residential Code and itis incorporated in the 2009 IRC.7 The 2009IBC,8 Section 1203.2, does not have thisparticular exception and instead reads,“The net free ventilating area shall not beless than 1/300 of the area of the spaceventilated, with 50 percent of the requiredventilating area provided by ventilators.”

Multiple studies have also been per-formed on the effects of ventilation on attictemperature by monitored comparison ofunvented and properly vented attic spaces.Several researchers have indicated that theaverage difference in temperature betweenthe ambient temperature and a properlyvented attic is in the range of 25ºF. Forexample, in 1992, William B. Rose present-ed findings9 from a study he performedshowing a maximum difference in air tem-perature of 28ºF between vented andunvented attic spaces. It should be notedthat there remains some debate on thedegree of impact that ventilation has onattic temperature due to the existence ofsimilar studies with less-extreme results.

Studies have also been performed onthe effects of attic ventilation on interiorcomfort levels. Inadequate attic ventilationcan be detrimental to interior comfort levelsbecause heat gain in the ceiling and duct-work increases the supply air temperature.Increases in the supply air temperature canresult in inadequate mixing of supply airand room air and significant variation ofroom air temperatures at various levels.This lowers the elevation of the stagnant airlayer in the living spaces and ultimately

FULL OF HOT AIR: HOW THE IRC FAILSTO ADDRESS EFFECTIVE ATTIC VENTILATION

affects occupant comfort.In the mid 1950s, research was per-

formed by J. R. Wright et al10 on the perfor-mance of air conditioning systems on anoccupied house in Illinois. Many differentaspects of the subject house were evaluat-ed, including temperature gain through theceiling, temperature gain of the duct work,natural ventilation with gable and soffitvents, and forced attic ventilation at variousventilation rates. The total net free ventilat-ing area (NFVA) was 3.59 sq ft, which repre-sented a ratio of 1/290, while the totalNFVA of the soffit was 3.13 sq ft. Theresearch found that increasing the atticventilation rate (cfm/sf) resulted in a signif-icant decrease in the attic temperature, asshown on Figure 1. Additionally, thedecrease in attic temperature resulted in adecrease in the heat gain and temperaturein the ceiling and ductwork, increasingcomfort levels in the living space.

2009 IRC: IMPLICATIONS ANDSHORTCOMINGS

For a fair analysis and critique of theIRC and IBC, it should be determined whatthe current code states, what it means, andwhy it is a problem. Excerpts from the 2009IRC and 2009 IBC are included in order tointerpret and recognize potential problemswith the IRC and IBC and identify devia-tions between the two codes. While mostjurisdictions have promulgated code based

on the 2006 IRC, there is little differencewith regard to attic ventilation requirementsfrom the 2009 IRC. Hence, the code lan-guage from the most-current available ver-sions of the International Residential Codewas used for examination of the IRC’s andIBC’s potential implications and shortcom-ings in attic ventilation.

DefinitionsSection R102.4 of the 2009 IRC reads as

follows:

R102.4 Referenced codes andstandards. The codes and stan-dards referenced in this code shallbe considered part of the require-ments of this code to the prescribedextent of each such reference.Where differences occur betweenprovisions of this code and refer-enced codes and standards, the pro-visions of this code shall apply.

Exception: Where enforcement of acode provision would violate theconditions of the listing of the equip-ment or appliance, the conditions ofthe listing and manufacturer’sinstructions shall apply.

Chapter 2, Definitions of the 2009 IRC,defines equipment as follows:

Equipment. All piping, ducts,vents, control devices, andother components of systemsother than appliances that arepermanently installed and inte-grated to provide control ofenvironmental conditions forbuildings. This definition shallalso include other systemsspecifically regulated in thiscode.

Under Section R102.4’s excep-tion, manufacturers’ installationrequirements appear to overrulecode provisions in instances wherethey conflict with one another.Based upon the definition of“equipment,” the soffit vents andridge vents should be classified asequipment because they do providecontrol of the environmental condi-tions of the attic and, therefore, theenvironmental conditions of the liv-ing space. Hence, R102.4’s excep-tion indicates manufacturers’

requirements for roofing products and atticvent equipment would have authority overthe installation and design of an attic venti-lation system.

Cross VentilationSection R806.1 of the 2009 IRC reads as

follows:

R806.1 Ventilation required.Enclosed attics and enclosed rafterspaces formed where ceilings areapplied directly to the underside ofroof rafters shall have cross ventila-tion for each separate space by ven-tilating openings protected againstthe entrance of rain or snow.Ventilation openings shall have aleast dimension of 1/16 in (1.6 mm)minimum and ¼ in (6.4 mm) maxi-mum.

The term “cross ventilation” is notdefined within the Code in Section R806.1.Chris Holland of the Chicago office of ICChas stated, “What is meant by the term isthat there should be a free flow of airthrough the attic space. This can beachieved when outside air is drawn into theattic through the eave or cornice vents andexits through the ridge or gable vents.”

While this section of the code addressessize requirements for ventilation openingsand specifies cross ventilation, it does notaddress several key design components thatcan substantially impact their effectiveness,including soffit vent placement and spacingand the ventilation of contiguous spaces.Soffit vents should be uniformly spaced atthe eaves to ventilate the entire roof deck inorder to provide balanced cross ventilation.All contiguous spaces, separate areas creat-ed by unique roof design features, shouldbe ventilated and not isolated from the freeflow of air.

Vent ClearanceSection R806.3 of the 2009 IRC reads as

follows:

R806.3 Vent and insulation clear-ance. Where eave or cornice ventsare installed, insulation shall notblock the free flow of air. A minimumof a 1-inch (25-mm) space shall beprovided between the insulation andthe roof sheathing and at the loca-tion of the vent.

Although this section of the code speci-

Figure 1 – Relation between maximum attic airtemperature and maximum outdoor airtemperature at various attic ventilation rates.Original chart from J.R. Wright, D.R.Bahnfleth, and E.J. Brown, “ComparativePerformance of Year-Around Systems Used inAir Conditioning Research Residence No. 2,”University of Illinois Engineering ExperimentStation Bulletin No. 465, 1963, Figure 28.


fies the clearance of ventilation openingsand holding back insulation, it does notaddress proper baffle installation, which iscritical in providing a free flow of air fromthe eave to the upper ventilators. Bafflesshould be properly installed in betweenrafters near the eave area to maintain apath for the free flow of air and should notbe obstructed by insulation.

CONTRAST OF THE 2009 IRC AND2009 IBC

Section R806.2 of the 2009 IRC reads asfollows:

R806.2 Minimum Area. The totalnet-free ventilating area shall not beless than 1/150 of the area of thespace ventilated, except that reduc-tion of the total area to 1/300 is per-mitted provided at least 50 percentand not more than 80 percent of therequired ventilating area is providedby ventilators located in the upperportion of the space to be ventilatedat least 3 ft (914 mm) above the eaveor cornice vents, with the balance ofthe required ventilation provided byeave or cornice vents. As an alterna-tive, the net free cross-ventilationarea may be reduced to 1/300 whena Class I or II vapor barrier isinstalled on the warm-in-winter sideof the ceiling.

Section 1203.2 of the 2009 IBC reads asfollows:

R1203.2 Attic Spaces. Enclosedattics and enclosed rafter spacesformed where ceilings are applieddirectly to the underside of roofframing members shall have crossventilation for each separate spaceby ventilating openings protectedagainst the entrance of rain andsnow. Blocking and bridging shallbe arranged so as not to interferewith the movement of air. A mini-mum of 1 in (25 mm) of airspaceshall be provided between the insu-lation and the roof sheathing. Thenet-free ventilating area shall not beless than 1/300 of the area of thespace ventilated, with 50 percent ofthe required ventilating area provid-ed by ventilators located in theupper portion of the space to be ven-tilated at least 3 ft (914 mm) aboveeave or cornice vents with the bal-

ance of the required ventilation pro-vided by eave or cornice vents.

The 2009 IBC differs from the 2009 IRCin the following ways:

• IBC does not require a total net-freeventilating area (NFVA) of 1/150,

• IBC does not have a reduction forvapor barriers, and

• IBC does not have the exception of“at least 50 percent and not morethan 80 percent of the required ven-tilating area is provided by ventila-tors located in the upper portion tobe ventilated.”

Although the 2009 editions of the IRCand IBC appear to be very similar, the keydifferences between them are the NFVArequirements and the percentage of ventila-tion that is allowed to be in the upper partof the roof system, which significantlyimpacts the amount of ventilation by natur-al convection in the attic space.

NET FREE VENTILATING AREASound engineering judgment would

suggest that you can only exhaust as muchair as the amount of air intake.

Assuming the code maximum and airmovement solely by natural convection, if80% of the ventilating area is provided by aridge vent, and 20% is provided by soffitvents, then the effective ventilating areawould be limited by the soffit vents andwould result in 20% intake plus 20%exhaust, totaling 40% of the total NFVA.The only way to use 100% of the NFVA is to

balance the system and provide 50% of theventilating area at both the ridge vents andsoffit vents. While it is acknowledged thatwind forces may increase the effective ven-tilating area through the system, they can-not be assumed to be present in all climatezones at all times and, therefore, cannot beincluded in the calculation of the NFVA. Theamount of effective NFVA should be suffi-cient to provide ventilation by air movementthrough natural convection alone.

Figure 2 illustrates how increasing thepercentage of ventilation at the upper roofareas, resulting in an unbalanced ventila-tion system, can reduce the effective NFVAof the attic ventilation system, assuming aninitial effective NFVA of 1885 s.f. at a 1/150ventilation rate. The graph compares theeffective NFVA of ventilation rates 1/150and 1/300 as the ventilation systembecomes more unbalanced in favor of theupper roof areas.

For further clarification on the code lan-guage, Chris Holland, with the ICC, wasconsulted. Holland stated, “The eave or cor-nice vents must be greater than or equal tothe area of the ridge or gable vents.”However, the ICC’s response brings up thefollowing question: If the ICC recognizes theneed for a greater amount of ventilation atthe eave and cornice vents, then why doesthe code only allow for a greater percentageof ventilating area at the ridge or gablevents?

By allowing a greater percentage of ven-tilating area at the upper ventilators, onesignificantly limits the amount of effectiveventilating area of the roof design. Hence,

Figure 2 – Impact of unbalanced ventilation rates on effective NFVA.


the limiting factor in the effective ventilatingarea would be the lesser of the soffit, ridge,or baffle ventilating area.

Additionally, between the 2006 and the2009 IBC, the total NFVA was changed from1/150 to 1/300 of the area of the space tobe ventilated. However, both the NationalRoofing Contractors Association (NRCA)11

and Asphalt Roofing ManufacturersAssociation (ARMA)12 have issued industrystandards for attic ventilation and roofingthat require a greater amount of NFVA.These organizations have stated that theminimum free-flow ventilation area shouldbe equal to 1 sq ft per 150 sq ft of attic floorarea (1/150) and must be designed andproperly installed to provide proper ventila-tion. Both organizations suggest that theamount of attic ventilation be balancedbetween the eave and ridge.

Manufacturers’ RequirementsSection R905.1 of the 2009 IRC reads as

follows:

R905.1 Roof covering application.Roof coverings shall be applied inaccordance with the applicable pro-visions of this section and the man-ufacturers’ installation instructions.

This section of the 2009 IRC states thatroof materials must be installed per manu-facturers’ requirements, and most asphaltshingle manufacturers state that the atticventilation must be balanced. CertainTeedstates in its Ventilation Standards andSystems in its Applicator’s Manual, “It’s crit-ical that this airflow is distributed uniform-ly. That means intake and exhaust ventsmust be balanced – for both position andairflow capacities. Otherwise, ‘hot spots’can develop under roof sheathing, drastical-ly reducing the efficiency and effectivenessof whatever ventilation is installed.”

CertainTeed Shingle Applicator’s Manualstates in Chapter 7, “When in-and-out ven-tilation cannot be equally balanced,research indicates that it is better to havesomewhat more ventilation at the lower partof the roof.”

Furthermore, GAF, manufacturer ofCobra® Exhaust Vent rolled ridge vent,clearly states in its product’s literature, “Forproper ventilation, the amount of undereaveventilation must equal the amount of venti-lation at the ridge. Note: In no case shouldthe amount of exhaust ventilation exceedthe amount of intake ventilation.”

Additionally, most shingle manufactur-

ers and some attic ventilation system man-ufacturers recommend that the NFVAshould be 1/150.\, since the 1/300 ratioequates to less than ½ inch of vent area persq ft of attic space, which is barely enoughto create airflow, and that “this standardassumes a proper balance of exhaust andintake venting. Unfortunately, it’s probablysafer to assume that assumption rarelyholds true.” Several attic ventilation sys-tems manufacturers state, “In no caseshould the amount of exhaust ventilationexceed the amount of intake ventilation.”

Code Adaptations Based on ClimateZone

In Section R806.4 of the 2009 IRC,requirements for unvented attic assembliesare outlined. Section R806.4 offers specificrequirements based on the air permeabilityof the selected insulation and the climatezone. A vapor retarder is only required onthe underside of the insulation for ClimateZones 5 through 8. Unlike unvented atticassemblies, the vented attic assembliescode language does not tailor its require-ments for different climate zones. In fact,the vented attic design commentary foundin 2009 IRC Code and Commentary editionsolely addresses cold-climate issues. Thecommentary only recognizes interiorsources of water vapor and vapor drives tothe exterior, of which the opposite condi-tions are present for hot and humid cli-mates.

Also, the commentary on the Code onlyreferences condensation of vapor on build-ing components. While this is applicable toareas with an average daily winter tempera-ture below 25ºF (approximately Zones 5-8),the Code doesallow for thereduction ofthe total NFVAto 1 to 300, ifan interiorvapor retarderis installed atthe winter-warm side ofthe ceiling. Theinstallation of avapor retarderon the interioris generally notrecommendedfor hot andhumid cli-mates, due tothe potential

for condensation from the vapor drive intothe interior. Hence, the 1 to 300 ratio seemsto be applicable to condensation and thereduction should only be considered if avapor retarder is installed.

Code InterpretationWhile ICC officials write the code lan-

guage, it is up to the local code official toread, interpret, and enforce how the code isimplemented. In situations where the lan-guage or intent of the code is unclear, localcode officials will have the jurisdiction toprovide clarification and decide on thecode’s rationale and enforcement. Inessence, local code officials may be forced tomake decisions on subject matter on whichthey may or may not have been properlyinformed. This can potentially have detri-mental effects on building performancebecause local code officials may not fullyunderstand the situation and the ramifica-tions of their actions.

CASE STUDY: TOWNHOUSES, UNITS#7 AND #25Background

The following case study centers on aninvestigation of two townhouses, units #7(Figure 3) and #25 located in Bluffton,South Carolina. The case study is present-ed to illustrate how the IRC and its inter-pretation can negatively affect attic ventila-tion design, installation, and operation.

The townhouses were designed and con-structed in 2004 under the 2000 IRC, whichwas the current building code at the time ofconstruction. The residences are one-story,wood-framed, attic-ventilated townhouseswith stucco and/or exterior insulation fin-

Figure 3 – Townhouse unit #7 in Bluffton, South Carolina.


ish system (EIFS). The main roof is com-posed of 2x4 wood trusses with approxi-mately 5/12 pitch and “overbuild” rooftrusses that are 7/12 pitch. The roof systemconsists of CertainTeed Landmark 30 archi-tectural shingles, GAF MaterialsCorporation Cobra® Exhaust Vent rolledridge vent, and Lomanco, Inc., Model C616soffit vents.

One resident, hereafter referred to as

the homeowner,owns the resi-dences at units #7and #25. Prior toDecember 5, 2005,the homeownerraised concernsregarding elevatedtemperatures expe-rienced within theunits, garages, andin the attics. Theseconcerns werebased upon havingto significantlyreduce the thermo-stat set-point inorder to reach acomfortable tem-perature within

bedroom #2 in unit #7 and bedroom #2 andmaster bedroom in unit #25.

An investigation was conducted on theunits to determine the condition of theexisting roof/attic assembly; the probablecause or causes of failures experienced, ifany; compliance of the existing assemblywith appropriate contract documents; andthe corrective actions required to make theexisting system effective or to provide an

effective system.

ObservationsDuring the

inspection, severalobservations of theunits indicatedineffective atticventilation:• The greatest

observed temperature differentialbetween the ambient temperatureand attic spaces in units #7 and #25was 42ºF when the ambient temper-atures were 96ºF and the attic tem-peratures were 138ºF (Figure 4). Perthe homeowner, attic temperaturesof 138ºF were a common occurrenceduring the summer months andwere exceeded on many days.

• Insulation was missing over thefront and rear porches, allowingheat transfer between the attic andexterior.

• The overbuild roof trusses for thedecorative gable roofs were installedon top of the OSB roof decking(Figure 5), which was typicallyinstalled on top of the main rooftrusses from eave to ridge, andtherefore were effectively isolatedand unventilated.

• Ridge vents were located at multipleridges and within 3 ft below thehigher ridge vent, allowing the venti-lation system to potentially short-circuit and reduce its effectiveness.

• Soffit vents were not installed con-sistently around the perimeter of thebuilding, including the rear porches(Figure 6), master bedroom bay win-dows, and garage eaves, therebyindicating an unbalanced system.

• Polystyrene insulation baffles(Figure 7) were either not present,obstructed by insulation, notsecured to the bottom of the roofdeck, and/or butted against theback of the fascia board, forcing hotair to build up within the eave area

Figure 4 – Attic temperature recorded in unit #7 duringinvestigation.

Figure 5 – Decorative gable roofs were installed overthe main OSB roof decking in unit #7 attic andisolated from ventilation.

Figure 6 – No vents were present at thesoffit at the rear porch of unit #7.


and spill into the baffle at a lowerelevation, with minimal assistancefrom the wind or gravity. Further-more, all polystyrene insulation baf-fles were less than half the width ofthe roof truss spacing. All of theseinstallation deficiencies significantlyreduce the NFVA and effectivenessof the ventilation system.

Ventilation CalculationsThe manufacturer’s published values

for the NFVA for the ridge vent, soffit vent,and baffle vents were utilized in calculatingthe existing ventilation. Ridge vent manu-facturer-published values for the ridge ven-tilation are 14.1 sq in per linear foot forgun-nailed installation and 16.9 sq in perlinear foot of ridge vent for hand-nailedinstallation. Since the installation was notobserved, the most conservative ap-proachwas used and the gun-nailed installation

was assumed. (SeeTables 1 and 2.)

Case StudyConclusions

After thoroughreview of designand constructiondocuments, existingconditions, and per-formance of ventila-tion calculations,the following coreconclusions weremade regarding theattic ventilationdesign and installa-tion in units #7 and#25:• The extremeattic temperatures

observed increased heat gain in theceiling and HVAC ductwork, whichtypically ran through the atticspaces, leading to higher latent loadsand higher supply air temperatures.The heat transferred from the atticspace to the living spaces belowincreased occupant discomfort andenergy costs for the homeowner.

• Based on the ventilation calcula-tions, the installed ventilation sys-tem in unit #7 does not meet thecode requirements and violatesSection R806.2 with over 80% of therequired ventilating area providedby ventilators located at or near theridge when compared to the limitingeffects of the baffle vents.

• Per the ventilation calculations, theinstalled ventilation system in unit#25 narrowly meets the coderequirements outlined in Section

R806.2 with just under 80% of therequired ventilating area providedby ventilators located at or near theridge.

• When the limiting effects of the baf-fle vents are excluded, both unit #7and unit #25 appear to be well with-in the ventilation requirements inaccordance with R806.2 withapproximately 64-66% of NFVA atthe ridge.

• Although unit #25 meets the IRCrequirements, the elevated tempera-tures in the unit, garage, and atticspace indicate that the attic ventila-tion system is not operating effec-tively.

• Per the vent clearance requirementsin Section R806.1, installation of theoverbuild trusses on top of the decksheathing is a code violation andhas further reduced the effective-ness of the attic ventilation system.

• There were several underlyingdesign and application issues withthe observed attic ventilation systemthat contributed to the attic ventila-tion system’s poor performance andviolated other aspects of the code.However, deficient code languageand provisions permitted by the IRChave primarily contributed to thecreation of an unbalanced, ineffec-tive ventilation system that in noway meets the original intent of thecode. The IRC allows an unbalancedattic ventilation system, as observedin unit #25, which significantlyreduces the effective NFVA. The IRCdoes not address common applica-tion issues such as soffit vent place-ment and spacing, ventilation ofcontiguous spaces, and baffle design

Figure 7 – Installed insulation baffles were less than halfthe width of the roof truss spacing and not secured to thebottom of the roof deck in unit #25.

Table 1 Table 2


and installation, which are critical tothe performance of the attic ventila-tion system. Furthermore, the IRCdoes not require attic ventilation tobe a designed system.

• Several roofing and attic ventilationproducts were not installed per theiroriginal manufacturer’s installationrequirements, including the poly-styrene insulation baffle vents,asphalt shingles, and the ridgevents. Since “attic ventilation” is asystem that provides control of envi-ronmental conditions, and its com-ponents are equipment, a violationof manufacturer’s installationrequirements can also be considereda code violation per R905.1 and thedefinition of “Equipment” in Chapter2. However, the IRC does not requirethe recognition of attic ventilationcomponents as equipment.

RECOMMENDATIONSBased on close examination of the IRC

and IBC attic ventilation language and pro-ject experience observing the effects of thecurrent IRC on attic ventilation design andperformance, the following revisions, clarifi-cations, and amendments to the IRC andIBC are recommended:

Revisions• The area requirements for ventila-

tors in the IRC should be modified toemulate language in the IBC requir-ing equally balanced (50%) ventila-tion at the lower intake vents andthe upper exhaust vents.Specifically, the exception permit-ting a reduction of the total NFVA to1/300 on the provision that “at least50 percent and not more than 80percent of the required ventilatingarea is provided by ventilators locat-ed in the upper portion of thespace,” which encourages unbal-anced attic ventilation, should beremoved.

• The minimum ventilation arearequirements in the IRC, as well asthe IBC (due to recent changes),should be revised to accommodatethe needs of different climate zones.The IRC and IBC should be revisedto require a minimum total net freeventilating area of 1/150 for hot andhumid climates and allow a 1/300ventilation rate for cold climateswith the inclusion of a vapor

retarder at the warm-in-winter sideof the ceiling.

Clarifications• The ventilation area requirements in

the IRC and IBC should more clear-ly specify that intake air at the eaveand cornice vents should always beequal or greater than exhaust air atridge and gable vents.

• IRC and IBC definitions of“Equipment” should be clarified toprevent misinterpretations by localcode officials, contractors, anddesigners. The IRC and IBC shouldspecifically identify attic ventilationcomponents, such as soffit vents,ridge vents, and insulation baffles,as equipment because they do pro-vide control of the environmentalconditions of the attic and thereforethe environmental conditions of theliving space.

Amendments• The IRC and IBC should include lan-

guage that specifies that attic venti-lation should be a designed system.Ventilation calculations should beperformed in conjunction with thedesign of other building systems andincorporated into the design docu-ments, since providing a properlyfunctioning attic ventilation systemmay impact the aesthetics and otherassemblies of the building. The IRCand IBC should require that theattic ventilation system be designedby someone knowledgeable aboutthe principles of attic ventilation,such as a Registered RoofConsultant.

• The NFVA requirements in the IRCand IBC should be expanded toinclude the following: The NFVA ofthe soffit vents should be equal to orgreater than the ridge NFVA; thenumber of baffles should be suffi-cient to have a NFVA greater than orequal to the soffit vent area.

• The IRC and IBC ventilation require-ments should address soffit ventplacement and spacing and the ven-tilation of contiguous spaces.

CONCLUSIONVented attic designs can be used suc-

cessfully in all hygrothermal climate zones;however, they must be designed andinstalled correctly. A building’s attic ventila-

tion system should be treated as an essen-tial building component designed by knowl-edgeable industry experts and profession-als. Accordingly, model building codes mustbe written to promote effective attic ventila-tion. Attic ventilation requirements in theIRC and IBC should be revised to realizetheir original intent of providing effectiveventilation systems that are balanced andcross-ventilated. Attic ventilation require-ments should also be modified to addressthe specific needs of hot and cold climates.An attic ventilation system designed andinstalled per its climate requirements offersimmediate benefits of both reducing tem-perature differentials between the attic andthe outside air temperatures and accumu-lation of moisture in attic spaces and holdslong-term environmental benefits of pro-moting durability and reducing energy con-sumption and costs in buildings.

REFERENCES1. L.V. Teesdale, Condensation in Walls

and Attics, U.S. Department ofAgriculture, Forest Service, MadisonWI, 197, 12 pp.

2. FHA, Property Standards and Mini-mum Construction Requirements forDwellings, Washington, D.C.: Fed-eral Housing Administration, 1942.

3. T.S. Rogers, “Preventing Conden-sation in Insulated Structures,”Architectural Record, March 1983,pp. 109-119.

4. F. Rowley, A. Algren, and C. Lund,“Condensation of Moisture and ItsRelation to Building Constructionand Operation,” ASHVE Transac-tions, 45 No. 1115, 1939.

5. BOCA Abridged Building Code.Building Officials and Code Ad-ministrators International, Inc.,adopted September 16, 1948.

6. BOCA National Building Code,Building Officials and Code Ad-ministrators International, CountryClub Hills, IL, 1994.

7. 2009 International Residential Code:Code and Commentary-Volume 1, In-ternational Code Council, Inc.Country Club Hills, IL, 2010.

8. 2009 International Building Code:International Code Council, Inc.,Country Club Hills, IL, 2009.

9. W. Rose, “Measured Values ofTemperature and Sheathing Mois-ture Content in Residential AtticAssemblies,” Thermal Performance ofthe Exterior Envelopes of Buildings V,


1992, pp. 379-390.10. J.R. Wright, D.R. Bahnfleth, and

E.J. Brown, “Comparative Perfor-mance of Year-Around SystemsUsed in Air Conditioning ResearchResidence No. 2,” University ofIllinois Engineering ExperimentStation Bulletin No. 465, 1963, p.62.

11. The NRCA Roofing and Water-proofing Manual, Fifth Edition,Volume 3, National Roofing Con-tractors Association, Rosemont, IL,2001, pp. 834-35.

12. “Ventilation and Moisture Controlfor Residential Roofing,” AsphaltRoofing Manufacturers Association,Washington, DC, 2007, ARMA Form

No. 209-RR-86, pp.1-2.13. CertainTeed Shingle Applicator’s

Manual, 9th Edition, CertainTeed,2009, pp.70-81.


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