Russell M. Sanders, AIA is Executive Vice President and Director, Technical Services with HoffmannArchitects, Inc. He is specifically experienced in the rehabilitation of curtain wall structures.

Russell M. Sanders, AIA

Journal of architectural

technology published by

Hoffmann Architects, Inc.,

specialists in the rehabilitation

of building exteriors.

I S S U E 1 / 2 0 0 6 V O L U M E 23 N U M B E R 1

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Curtain Walls:Not Just Another Pretty Façade

hey’re unique, they’re aestheticallypleasing and they serve as a barrier tothe elements. And with today’s demandfor energy efficiency and alternativepower sources, a curtain wall provides asolution. During the last two decades,curtain wall systems have not onlyevolved, they’ve grown incrementallypopular in building construction. In fact,the curtain wall is considered one of themost influential developments in thehistory of modern architecture.

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The curtain wall is one of the mostrecognizable components of today’sbuilding. Modern structures featurecreative and extremely efficient curtainwall systems comprising lightweight glass,stone, aluminum, marble, metal orcomposite materials. These systemsminimize air and water infiltration byequalizing the substantial wind pressureson high-rise buildings. When designedproperly, they are beautiful and highlyfunctional in keeping the elements outand the temperate environment in.

In a nutshell, a curtain wall literally hangsfrom a structure like curtains hang in ahome. A curtain wall system is anyexterior wall that is attached to the

building structure, but is not load-bearing.A true curtain wall is the façade elementthat forms the weather barrier for thebuilding, but it doesn’t support thestructure. It can have many differentappearances, but its characteristicmakeup features narrowly spacedvertical and horizontal mullions withglass, stone, metal or composite panels.

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The development of curtain walls hasadded another dimension to the buildingindustry. Today, buildings faced withcurtain walls are associated with modern,commercial architecture. Their stream-lined look is a result of the marriage ofmetal framing with gleaming glass.

Curtain walls first appeared on thescene in 1918. While designing theHallidie Building in San Francisco,Architect Willis Polk came up with thenotion that a contiguous, non-load-bearing, exterior glass wall could beconstructed at the face of the entirebuilding.

By the mid-1930s, newly created tech-nology in sheet metal and aestheticsassociated with the mass-production ofairplanes and automobiles paved the

Curtain walls may be rehabilitated for aes-thetic reasons or to improve energy efficiency, inaddition to the need to resolve deterioration as-sociated with weathering, age and materials orinstallation failures.

Figure 1b. Illustration demonstrates the characteristics in a typical stick curtainwall system, shown in Figure 1a to the left.

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Figure 1a.

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way for new developments in buildingconstruction, most notably in metalcurtain wall panels. After World War II,metal and glass curtain wall systemsstarted appearing on commercial andinstitutional buildings. Large areas of glassbecame possible in the 1950s with thenewly invented float process. Insulatedglazing, air conditioning and insulationtechnology helped solve heating andcooling issues that accompanied largeareas of vision glass. The United Nationsheadquarters, built in 1949-1950, featuredthe first complete glass curtain wall.

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A wide variety of materials and designsare available to meet different require-ments in curtain wall construction. Butit’s not just what’s in your curtain wall;curtain walls are classified by how theyare built.

While there are many variations ofcurtain wall construction, there are fivesystems implemented most frequently:stick system; unit panel system; unitmullion system; column cover andspandrel systems; and point-loadedstructural glazing systems.

The oldest type of curtain wall is thestick system. (Figures 1a and 1b below)It features a cladding and exterior wallsystem that is hung on the buildingstructure from floor to floor. The sticksystem is assembled in the field usingvarious components, including anchors,mullions, rails, vision glass, spandrel glass,insulation and metal back pans. Inaddition, there are various hardwarecomponents, including connectors, settingblocks, corner blocks, pressure plates,caps, gaskets and sealants. Not only isthe stick system airtight and resistant towater penetration, it’s cheap, and quick

and easy to install, making it one of themost commonly used types of curtain walls.

The unit panel (unitized) system is acurtain wall fabricated and installed as apanel system. It contains the sameingredients as a stick-built curtain wallsystem, but most of the system compo-nents are factory-assembled undercontrolled work conditions, instead of inthe field. The unitized system offersmany advantages, including higher quality.It allows for fabrication lead-time andrapid closure of the building.

The unit mullion system is a compromisebetween the stick and unitized systems.Pre-assembled units are installed behindone- or two-story individual mullions.The advantage of this particular systemis that it provides some of the factoryquality control of the unitized system,costs a little less, and offers a shorter

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V O L U M E 23 N U M B E R 1

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Figure 2

manufacturing lead time because there isless customization than with a fullyunitized system. Field labor time andbuilding costs are more comparable withthe stick system.

Column cover and spandrel systems arerelatively new but are becomingincreasingly popular because they offeraesthetic choices. Units can be pre-assembled or completed onsite andfeature infill vision glass and spandrelpanels between columns. The beauty ofthis system is that it allows for thestructural framework of the building tobe expressed in the façade and visuallyintegrated with the fenestration. Sincethese systems are customized, manufac-turing lead times are longer and framingconstruction tolerances more important,as the units must fit precisely withincolumn bays.

Point-loaded structural glazing systemsconsist of laminated and tempered glasssupported by proprietary hardwareembedded in the glass. Unlike thealternatives, this system eliminates anyvisible metal framework. The variety ofavailable support systems, includingtension cables, trusses and glass mullions,offers considerable freedom andaesthetic options within varying degreesof transparency, stiffness and cost. Glassmullions are the most transparent andare the usual choice for very large,monumental spaces in spite of havingvery long lead times. Product andinstallation costs are higher thanconventional frame-supported curtainwall systems.

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Glazed curtain walls are multiple-storyexterior building walls constructed ofvertical and horizontal metal framingmembers (mullions) that contain andsupport fixed glass vision panels, opaque

glass panels or operable windows.

When designing and selecting glass for aglazed curtain wall, there are manyfactors to consider: safety, stability,impact-resistance, durability and cost arethe most important aspects to look at inglazed curtain walls.

Float glass is ordinary glass, and can bereferred to as sheet or plate glass. Unlikethe other types of glass, it undergoes nospecial treatment to increase its strength.Float glass breaks into large, sharp shardsthat can constitute a safety hazard.Annealed float glass is becoming popular;annealing (heating and cooling) helpsreduce breakage during handling and inservice. The advantages of float glassare availability, workability and low cost.

Tempered glass is typically used whenbetter impact resistance, increasedbending strength and shatter control arerequired. Tempered glass shatters intovery tiny pieces without sharp edgesinstead of large shards, providing agreater degree of safety in case of glassbreakage. It has been factory-treated bya heat-tempering process that makes itresistant to breakage. Any cutting, drillingor edging must be done before the

tempering process or the glass willshatter. Tempered glass also is relativelycostly.

Heat-strengthened glass is stronger thanstandard annealed float glass, but not asstrong as tempered glass. Whenshattered, heat-strengthened glass shardsare larger than those of tempered glass,though not as sharp as annealed glassshards. Heat-strengthened glassundergoes factory treatment to increaseits breakage resistance. It exhibits lessdistortion and is less costly thantempered glass, making it suitable forwindows with limited access.

Chemically strengthened glass is similar totempered glass in its qualities and uses. Thestrengthening process involves chemicaltempering rather than heat tempering.

Laminated glass is made of two or morelayers of any type of glass with a plasticinterlayer, usually polyvinyl butyral (PVB).In the manufacturing process, a vinyl,polycarbonate or cured resin interlayer isbonded applying heat and pressure tothe glass layers (Figure 2). This interlayerholds the pieces together when the glassis broken, keeping the glass fromshattering. Like tempered and chemicallystrengthened glass, it can meet safetyglazing requirements. This glass can beassembled in combination with any othertype of glass. For example, combininglaminated and tempered glass in a singlepane produces a very strong, secureproduct. This combination provides anunsurpassed level of security frombreakage, and is typically specified foroverhead applications and areasvulnerable to impact damage, such asstorm debris, bullets or a bomb blast.Laminated glass also deadens soundimpact, as in airports or near highways,but is very costly due to the additionalsteps required in manufacturing.

Tinted/reflective glass blocks portions of

Laminated Glass

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solar light transmittance and are typicallyused for aesthetic reasons or to controlthe amount of light and heat entering abuilding.

Spandrel glass is tinted, reflective-coatedor film-coated and is typically used foraesthetic reasons in the fenestration.Solid insulated metal panels are oftenused in place of glass.

Insulating glass units (IGUs) are manufac-tured with a powdered gas or air-filledspace between two or more panes ofglass to provide energy efficiency.

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Many factors affect the performance ofcurtain walls, and can lead to deteriora-tion and failure if not addressed in aproper and timely fashion. Weather,especially wind and rain, is a leadingsource of deterioration to the exteriorcomponents of a building. Gasket andsealant material selection is critical inpreventing air and water infiltration;inferior quality can lead to early disinte-gration and failure. And proper installa-tion of the components is key.

A curtain wall is a combination of manyelements with two common interfacesbetween the parts: gaskets and seals.Gaskets and seals provide watertight,flexible connections between the panelsand framework. Generally, curtain wallframework and panel materials rarelydeteriorate; failure of those componentsis a result of damage. But gaskets andseals are made of compounded elasticmaterials that can dry, shrink and crackand lose adhesion (Figure 3).

As this happens, tiny openings occur,allowing water or air to intrude. Moisturecan enter a curtain wall system and rearits ugly head in a multitude of ways.

Dripping water at window heads or wetflooring are the most obvious indicationsof moisture problems. If the existingcurtain wall is a thermal pane system,condensation can form between thepanes of glass. Insulated panels that nolonger provide insulating qualities canmean that moisture is entering thesystem. Deformation of the frameworkcan occur if water that has intruded thesystem collects and expands duringfreeze-thaw cycling (Figure 4).

A compromised thermal seal in thecurtain wall can have several effects. Themost obvious is water infiltration into aninterior space. Fogged or etched thermalwindowpanes can be one result of aless-than-tight seal (Figure 5). Air leaksmay not be quite as obvious, but canshow up as drafts, hot and cold spots atpanel edges or increased heating andcooling costs.

Leaks in a curtain wall, in the forms ofboth air and water, can contribute toindoor air quality problems by supplyingmoisture for mold growth. Water orcondensation can often remain hiddenwithin the wall system and not becomeevident until concealed wall componentsexperience significant deterioration andmold growth, requiring costly repairs orreplacement.

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Once your building’s curtain wall hasgone awry, it’s time for a difficult decision.Are repairs sufficient or is a completereplacement in order? Weighing thebenefits with the long-range costs oftenhelps guide this determination. Rehabilita-tion of an old curtain wall system mayrefurbish the existing system, but notresolve all of the issues. And dependingon the extent of the rehabilitation, thework might need to be repeated withinthe next decade as a maintenance task.

Figure 4. The metal support system behindthe mullion cover has rusted, causing it to extrude.

Figure 3. Failed gasket at the base of the glazedcurtain wall will permit water and air penetration.

Figure 5. Loose gaskets permit water infiltra-tion; winds can loosen and eventually blow outthe glass panel. Staining is the result of contami-nants that have washed into the glass panel andmay lead to permanent etching.

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V O L U M E 23 N U M B E R 1

Most renewable energy comes fromthe sun. The sun’s energy can becollected and stored using electrical ormechanical devices called solar systems.Sunlight, or solar energy, is used forheating and lighting buildings, generatingelectricity and hot water heating andsolar cooling.

Solar cells, or photovoltaics, convert sun-light directly into electricity (Figure 6).Solar cells are made of semi-conductingmaterials similar to those used incomputer chips. When these materialsabsorb sunlight, the solar energy knockselectrons loose from their atoms,allowing the electrons to flow throughthe material to produce electricity.

Thin film technology has made itpossible for solar cells to be used inglazed curtain walls. Thin film solar cellsuse layers of the semiconductormaterials only a few micrometers thick.The solar cells are laminated and makeup the solar modules, which are thefundamental building blocks ofphotovoltaic systems. Solar panelsinclude one or more photovoltaic

modules assembled as a pre-wired,field-installable unit. A photovoltaicarray is the complete power-generatingunit, consisting of any number ofphoto-voltaic modules and panels.

As technology has progressed, thethermal performance of newersystems has improved greatly.Replacement with new panels oraugmentation with additional panels canhelp achieve greater thermal efficiency.The thermal performance of insulatingglazing depends mainly on the solarenergy transmittance through theglazing and the reflectance of theglazing. Reflective coatings help tominimize interior solar heat gain byreflecting solar energy.

Glass and glazing choices areinstrumental to the curtain wall’sthermal performance. Single glazinghas poor thermal performance and issuitable only where interior andexterior temperatures do not varysubstantially. If the curtain wall systemcomprises single-pane, non-insulatingglass panels, a fair amount of heat

loss/gain may take place through thecurtain wall.

Double-pane and triple-pane glasspanels can effectively enhance thermalperformance in a curtain wall. Adouble-glazed panel combines twoglass panels, creating an air spacebetween the panels, while a triple-glazed panel has two separate airspaces. Use of these panels providesimproved insulation and condensationcontrol, and allows for between-glassshading options, such as blinds orpleated shades.

Double-skin systems, which use aventilated space between the innerand outer walls, are becomingincreasingly popular in the U.S. Theventilated space helps conserve energyby balancing the temperaturessurrounding the curtain wall. Duringthe heating season, the space acts asa buffer between the exterior andinterior and can be used to temper thecooler outdoor air. During the coolingseason, warm interior air is exhaustedinto the ventilated space.

Renewable Energy and Thermal Performance

Figure 6

>

Figure 7

>

Below, Figure 6 shows a factory builtwith spandrel glass incorporating photovol-taic cells.

At right, Figure 7 shows a sports sta-dium with solar cells incorporated into thecurtain wall.

Photos courtesy of Open Energy, a manufacturer of photovoltaic glazing systems for curtain walls, skylights and canopy systems. (openenergycorp.com)

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There are a variety of assessments tomeasure how well your curtain wallsystem is performing. These tests includemeasuring air leakage; water resistance;water drainage; wind resistance; ability ofthe curtain wall to support its ownweight; safety; and thermal performance.

If a curtain wall is intact but its gasketsand seals are deficient, rehabilitationmight be limited to replacement ofgaskets and seals, depending on theavailability of the correct gaskets. Whenreplacement gaskets cannot be obtained,wet sealing the assembly might offer theonly option. Replacing troubled panelscan resolve moisture infiltration andachieve greater thermal efficiency.

Extensive repairs can turn into expensiverepairs, and in some cases it might bemore economically practical to replacethat failing curtain wall.

A building owner might opt to overhaula curtain wall for simple aestheticreasons. The curtain wall might functionproperly, but its appearance cries for anupdate, which can be accomplished bycurtain wall replacement.

Design ConsiderationsDesign ConsiderationsDesign ConsiderationsDesign ConsiderationsDesign ConsiderationsFor ReplacementFor ReplacementFor ReplacementFor ReplacementFor Replacement

During the design and constructionprocess of a replacement curtain wall,there are several important points totake into consideration. Cost is a factor;therefore, a choice must be madebetween a custom curtain wall systemand a standard curtain wall system.Custom curtain wall systems aredesigned and built specifically for oneparticular project. Standard systems

utilize manufacturers’ stock componentsand details, and are generally lessexpensive than custom systems.

Preventing water penetration is aprimary focus when designing andconstructing a curtain wall. Steps toprevent moisture migration are critical.Many curtain wall systems includecondensation drainage provisions, suchas condensate gutters, that collect andweep condensate from spandrel areasto the exterior. Water penetrationresistance is a function of glazing details,frame connection and drainage details,weather stripping and frame gaskets, andperimeter flashings and seals.

Building code requirements govern manyaspects of curtain wall design, such asthickness and type of glass required for acertain application, maximum area ofglass permitted, design wind loads andfirestopping of the cavity behind the wallat each floor.

In high-rise curtain walls, the spread offire from floor to floor is prevented byfirestops, which are fire-resistant materialsplaced in concealed hollow spaces of thebuilding frame. Close cooperationbetween the architect and the builder inthe early stages of design is essential toensure that proper firestops areprovided and potentially hazardousconditions are eliminated.

Fire safing and smoke seal at gapsbetween the floors and the back of thecurtain wall are essential to slow thepassage of fire and combustion gasesbetween floors. Spandrel areas musthave non-combustible insulation at theinterior face of the curtain wall. Fireman

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From top to bottom, figures illustrate how a curtain wall is reclad. Figure 8 shows the curtain wall priorto replacement. Figure 9 shows the horizontal rail attached to the structural mullion. Figure 10 shows newveneer rails attached to existing curtain wall mullions. Figure 11 shows the installation of new vision, spandreland insulation panels. Figure 12 shows the completely reclad curtain wall.

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

7

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These representative projects includedcurtain wall rehabilitation:

Chase Manhattan CentreChase Manhattan CentreChase Manhattan CentreChase Manhattan CentreChase Manhattan CentreWilmington, DelawareConcrete panel curtain wall

780 780 780 780 780 Third Third Third Third Third AAAAAvvvvvenenenenenueueueueueNew York, New YorkGranite panel curtain wall with metal-framed windows

AltrAltrAltrAltrAltria Coria Coria Coria Coria Corporporporporporate Headquarate Headquarate Headquarate Headquarate HeadquartertertertertersssssNew York, New YorkGranite panel curtain wall with metal-framed windows

MetLife BuildingMetLife BuildingMetLife BuildingMetLife BuildingMetLife BuildingNew York, New YorkConcrete panel curtain wall with metal-framed windows

101 101 101 101 101 AAAAAvvvvvenenenenenue of the ue of the ue of the ue of the ue of the AmerAmerAmerAmerAmericasicasicasicasicasNew York, New YorkBrick masonry curtain wall with metal-framed windows

Hoffmann-La RocheHoffmann-La RocheHoffmann-La RocheHoffmann-La RocheHoffmann-La RocheBuilding 76Building 76Building 76Building 76Building 76Nutley, New JerseyGlass and marble panel curtain wall

PfPfPfPfPfizizizizizer Incer Incer Incer Incer Inc..... WWWWWorororororld Headquarld Headquarld Headquarld Headquarld Headquar tertertertertersssssNew York, New YorkAluminum-framed glass curtain wall

1251 1251 1251 1251 1251 AAAAAvvvvvenenenenenue of the ue of the ue of the ue of the ue of the AmerAmerAmerAmerAmericasicasicasicasicasNew York, New YorkMetal and glass curtain wall

BrBrBrBrBristol-Myistol-Myistol-Myistol-Myistol-Myererererers Squibs Squibs Squibs Squibs SquibbbbbbPharmaceutical Research FacilityPharmaceutical Research FacilityPharmaceutical Research FacilityPharmaceutical Research FacilityPharmaceutical Research FacilityWallingford, ConnecticutSteel structure supports curtain wallwith granite or concrete spandrelpanels and metal-framed ribbonwindows

MontgomerMontgomerMontgomerMontgomerMontgomery Collegey Collegey Collegey Collegey CollegeCampus Campus Campus Campus Campus TTTTTooooowwwwwer Buildinger Buildinger Buildinger Buildinger BuildingRockville, MarylandAluminum-framed curtain wall with glassand metal spandrel panels

VVVVVerererererizizizizizononononon240 E. 38th St240 E. 38th St240 E. 38th St240 E. 38th St240 E. 38th StNew York, New YorkLimestone, granite and aluminum-framed insulated glass curtain wall

The Ford Foundation HeadquarThe Ford Foundation HeadquarThe Ford Foundation HeadquarThe Ford Foundation HeadquarThe Ford Foundation HeadquartertertertertersssssNew York, New YorkGranite panel curtain wall with metal-framed windows

V O L U M E 23 N U M B E R 1

AmerAmerAmerAmerAmerican Express ican Express ican Express ican Express ican Express TTTTTooooowwwwwerererererNew York, New YorkPolished granite curtain wall withaluminum-framed windows

633 633 633 633 633 Third Third Third Third Third AAAAAvvvvvenenenenenueueueueueNew York, New YorkMasonry curtain wall with aluminum-framed insulated glass windows

One Beacon StreetOne Beacon StreetOne Beacon StreetOne Beacon StreetOne Beacon StreetBoston, MassachusettsPre-cast concrete panel curtain wallwith aluminum-framed glass windows

One Magnificent MileOne Magnificent MileOne Magnificent MileOne Magnificent MileOne Magnificent MileChicago, IllinoisAluminum-framed polished granite andglass curtain wall system

GMH Management, Inc.GMH Management, Inc.GMH Management, Inc.GMH Management, Inc.GMH Management, Inc.King of Prussia, PennsylvaniaCurtain wall system with aluminum-framed, double thermal pane windowsand pre-cast concrete panels

VVVVVerererererizizizizizon Headquaron Headquaron Headquaron Headquaron Headquartertertertertersssss1095 1095 1095 1095 1095 AAAAAvvvvvenenenenenue of the ue of the ue of the ue of the ue of the AmerAmerAmerAmerAmericasicasicasicasicasNew York, New YorkAluminum-framed insulating glass andmarble panel curtain wall

25 Sigourney Street25 Sigourney Street25 Sigourney Street25 Sigourney Street25 Sigourney StreetHartford, ConnecticutMetal-framed glass and brick masonryveneer curtain wall

1211 1211 1211 1211 1211 AAAAAvvvvvenenenenenue of the ue of the ue of the ue of the ue of the AmerAmerAmerAmerAmericasicasicasicasicasNew York, New YorkMetal and glass curtain wall

AmerAmerAmerAmerAmerican Expressican Expressican Expressican Expressican ExpressTTTTTooooowwwwwerererererNew York, New York

> 1425 New 1425 New 1425 New 1425 New 1425 New YYYYYorororororkkkkkAvenueAvenueAvenueAvenueAvenueWashington D.C.

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JOURNAL is a publication of HoffmannArchitects, Inc., specialists in the rehabilita-tion of building exteriors. The firm’s workincludes investigative and rehabilitativearchitecture and engineering services forthe analysis and resolution of problemswithin roofs, façades, glazing, andstructural systems of existing buildings,plazas, terraces and parking garages.

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knock-out glazing panels are oftenrequired for venting and emergencyaccess from the exterior. Generally,knock-out panels are made of fullytempered glass, which allows completefracturing of the panel into small piecesand relatively safe removal from theopening.

Evaluation and MaintenanceEvaluation and MaintenanceEvaluation and MaintenanceEvaluation and MaintenanceEvaluation and Maintenance

Once you’ve invested in a new curtainwall, you’ll want to optimize the benefitsfrom that investment. Routine inspectionsand evaluations will help identify anyissues that can crop up and compromiseyour curtain wall’s efficiency.

Regular checks of the system areparamount to resolving problems. Athorough evaluation will include surveyingthe condition of the various gaskets andseals, inspecting the system joints toascertain if the framing components areadmitting water into the curtain wallsystem, and evaluating the thermalinsulation capabilities of the vision andinsulating panels of the system.

Facility managers must routinely examinegaskets or sealants for splits, breaks oropenings and replace any that have failed.

The three keys to a maintenanceprogram are:

• Follow the preventive maintenanceprogram recommended by themanufacturer of the system or productinstalled.

• Schedule regular inspection,cleaning and prompt repair of minorproblems. It is essential that inspectionreports of problems be referred tomanagement for long-range planningand action. Any repairs or replacementof hardware should be performed byprofessionals.

• Maintain written records of themaintenance procedures to documentproblems and their solutions. Thisfacilitates long-term tracking of themaintenance program, improving theability of maintenance personnel tomake effective decisions.

ConclusionConclusionConclusionConclusionConclusion

Curtain wall performance is essential toany façade, in terms of function andaesthetics. As with any buildingenvelope component, curtain walls needrepairs and sometimes replacement tocombat the effects of deterioration andallow maximum performance of abuilding. With routine inspections anda consistent maintenance schedule, anyrehabilitation can be kept to aminimum.