INTRO INTRODUC DUCTI TION ON Metal and glass curtain wall systems have found growing favour in modern architecture. They are easily dist distin ingu guis ishe hed d from from othe otherr type typess of clad claddi ding ngss by thei theirr thin thin mullions of horizontal and vertical metallic bars surrounding an all glass or metal panel. The curtain wall system has evolved rapidly over the last two decades, especially with respect to weat weathe herr cont contro roll perf perfor orma manc nce. e. The The earl early y syst system emss pres presen ente ted d frequent rain penetration problems; large icicles would form on the the outs outsid idee hori horizo zont ntal al bars bars or cond conden ensa sati tion on on the the insi inside de mullion surfaces; glazing seals were sometimes pumped out of the rabbet of sealed double glazing window units. However, most most of thes thesee dif difficul iculti ties es were were even eventu tual ally ly over overco com me with with improved detail design of the system components. Today, most curt curtai ain n wall wall manu manufa fact ctur urer erss off offer a qual qualit ity y prod produc uctt line line of components which can be used to create one of the best overall exterior wall systems.
WHAT IS A CURTAIN WALL?
curtain wall system is a lightweight exte exteri rior or clad claddi ding ng whic which h is hung hung on the the buil buildi ding ng stru struct ctur ure, e, usually from floor to floor. !t can provide a variety of exterior appearances but is characterized by narrowly spaced vertical and and hori horizo zont ntal al caps caps with with glas glasss or meta metall infi infill ll pane panels ls.. Thes Thesee systems provide a finished exterior appearance and most often a semi semi"f "fin inis ishe hed d inte interi rior or as well well.. They hey are are also also desi design gned ed to accommoda accommodate te structur structural al deflecti deflections, ons, control control wind"dri wind"driven ven rain and air lea#ag lea#age, e, minim minimize ize the effe effects cts of solar solar radia radiatio tion n and provide for maintenance"free maintenance"free long term performance. Most of today$s metal curtain wall syst ystems are constructed of lightweight aluminum, aluminum, although some may be of steel.
%ince curtain wall systems are a good example of building science principles applied to wall design, it may be useful to review some basic principles through the details of a typical curtain wall system.
THERMAL INSULATION (CONTROL OF HEAT FLOW)
The control of heat flow is generally achieved through the use of insulation. lthough it is not apparent from the exterior, the curtain wall system uses considerable insulation usually behind spandrel glass or any opaque panels. &ecause of the materials used in the structure, i.e., glass and metal, which are highly conductive, the system must also contend with potential condensation on the interior surfaces. To curtail this effect, most curtain wall systems incorporate two distinct features' first, a sealed double glazed window or an insulated metal pan and second, a thermally bro#en mullion, usually with a ()* plastic insert and more recently, a foamed"in"place polyurethane connection. sealed double glazed window unit can accommodate an indoor humidity up to about +- at an outdoor temperature of " / * with little condensation appearing on the glass. %imilarly, the thermal brea# in the aluminum or steel mullion ensures that the surface temperature of the structural
mullion will remain well above the dew point temperature of the air for most building types, except for high humidity indoor environments such as in swimming pools or computer centers. The thermal brea# also ensures that the structural mullion is thermally stable, that is, not sub0ect to extremes of expansion and contraction.
THE "RAIN SCREEN" PRINCIPLE (CONTROL OF RAIN AND SNOW PENETRATION) To control rain penetration through exterior walls the conventional approach is to seal the exterior fa1ade of the building. However, experience has shown that it is unreasonable to expect perfect sealing of a fa1ade; most sealing strategies require continuous attention and maintenance. %tudies of the rain penetration problem have revealed a better solution than the fa1ade sealing approach. !f the air that lea#s in and through crac#s and crevices of a fa1ade during a rain storm were limited or stopped, most of the water impinging on the fa1ade would migrate straight down the surface and little would penetrate the wall. This is the essence of the 23ain %creen2 principle. !f an airtight element is positioned behind a fa1ade, the cavity formed between the exterior cladding and the airtight element may reach the same air pressure level as is exerted on the cladding surface, thus removing the force which causes air to flow through any fa1ade opening. The 23ain %creen 4all2 is therefore characterized by a cavity behind the exterior surface that is connected to the exterior but sealed tightly, or as tightly as reasonably possible, to the interior. The inner surface of the chamber is usually referred to as the air barrier of the wall. !n most curtain wall systems the 0oint between the infil panel 5i.e., window or spandrel panel6 and the structural mullion is usually designed to be part of a rain screen system .!t comprises a pressure"equalized cavity, connected to the exterior by the drain holes in the exterior caps, and a pressure equalized rain deflector seal between the outside surface of the glass and the
mullion cap. The chamber portion of the cavity is composed of the air seals connecting the inside face of the window glass and the spandrel panel metal pan, to the shoulder flanges of the structural mullion and other parts of the structural section. Thus the set of elements comprising the window glass, the air seals, and the aluminum section and metal pan perform the air barrier function for this wall assembly. This design configuration for curtain wall sections has proven successful and has become widely accepted
SOLAR RADIATION (CONTROL OF SUNLIGHT AND OTHER FORMS OF RADIANT ENERGY) %olar radiation falling on building surfaces may have two distinct effects' the first is to cause a significant change in temperature of the fa1ade elements and the second is the slow but destructive effect of ultraviolet radiation impinging on all materials, particularly organic. 7n curtain wall systems the most important concerns with solar radiation have been the thermal expansion and
contraction of curtain wall components, in particular those forming the outside cladding, and the effects of solar radiation
on the glazing elements. warping of glass occurs due to differences in temperature between the inner and outer panes, while pumping results from expansion and contraction of the air in the cavity of the sealed units. 8aily and seasonal temperature differences can also cause this effect. The action of the window 5thermal pumping6 is particularly stressing to the inner air seal; however, serrated edges or recessed flanges #eep the seals from pumping out. Most of the ultraviolet"sensitive materials in curtain wall systems are located in the poc#et and cavity areas of the 0oints and are partly shaded by metallic and glass components.
THE VAPOUR BARRIER (CONTROL OF WATER VAPOUR DIFFUSION) 4ater in its gaseous phase 5water vapour or humidity6 always tries to migrate from a region of high water vapour pressure to a region of lower pressure. The migration of water vapour through a wall can be compared to heat flow; it moves through all materials at a rate that is dependent on both the resistance of the materials to water vapour flow and the difference in water vapour pressure on both sides of the material. The migration of water vapour through an assembly of materials is not a serious problem in itself, provided it does not condense to liquid form in the material or wall. !f water vapour is li#ely to condense in a wall, the principal defense is to restrain its migration by using, a 2vapour barrier2 with a high water vapour flow resistance, positioned on the warm side of the insulation material or wall assembly.
The migration of water vapour through a curtain wall assembly is chec#ed by the vapour barrier qualities of the glass and aluminum, as these materials have near perfect vapour flow
resistance for all practical purposes. Thus the inner pane of the sealed double glazed unit and the aluminum or steel inside surfaces of the mullion provides the necessary water vapour diffusion control. %ealants also contribute to the continuity of the vapour barrier.
JOINTS AND TOLERANCES (ACCOMMODATION OF BUILDING MOVEMENTS) Movements of the structural elements of a building must be determined prior to the design of an exterior wall system. Movements may be grouped into three types' 9. :ive load deflections due to occupancy loads or pea# wind loads on the building fa1ade, and dead load deflections of the building structure, . xpansion and contraction of materials as a result of temperature, radiation and sometimes hygroscopic loading, +. %low but inexorable movements due to gradual deformation, such as creep in concrete, foundation settlement, etc.
THE PARAPET 4hen a curtain wall system is designed to extend upwards past the roof line and thus to get cold, several potential problems must be considered. 4ithout proper termination at the head of the curtain wall system, condensation and frost may form in the tubes and eventually drain to the inside of the building, or icing may force parts of the parapet cap off the building. lso, if allowed to fluctuate with the outdoor temperature, the structural part of the curtain wall system may expand and contract beyond its design
limitations, thus straining all connections in and around the parapet elements. &ecause the structural tubes of the curtain wall system are also miniature chimneys, particularly in high"rise buildings, they may
conduct large volumes of 5humid6 air to the outside if left open or unsealed at the top. s there are many 0oints in the structural system of the curtain wall, it is preferable that the air barrier component be connected from the top shoulder flange surface of the horizontal mullion and that it be made to bridge the gap from the curtain wall and parapet structure to the air barrier component extending from the roof dec#. This may be difficult at times, particularly with a conventional built"up roof in which the insulation is under the membrane and must cross over somewhere in the interface detail. lso the air barrier between the curtain wall and the parapet must be insulated on the outside to prevent any condensation from forming on its inside surface.
THE CORNER INTERFACE
&uildings using curtain wall systems are often required to form an inside or outside corner. 4hen two sections of curtain wall meet, the interface detail must be designed to provide control over all of the aforementioned requirements. &ecause corner details vary considerably from pro0ect to pro0ect, suppliers do not have stoc# sections to draw upon to construct this interface detail. However, curtain wall suppliers will fabricate the necessary interface components provided that their participation is solicited early enough in the planning phase, preferably before tenders are closed. This interface detail will require an air barrier, some insulation and an exterior cladding. The air barrier must be structurally adequate to carry the air pressure loads on that corner. The material to be used as an air barrier should be aluminum if the curtain wall system is aluminum, or at least a sheet of metal, and at the very least a rigid element.
building is to appear as a continuous strip with no mullion interruptions, then care must be ta#en to develop an air seal 0oint at the ends of the air barrier sheets.
4hen dealing with an inside corner, the same requirements apply. However, if the mullion caps are in near contact or overlap slightly, it is not necessary to add a further 23ain %creen2 baffle over the insulation. The air barrier should be rigid and sealed against the shoulder flanges of the vertical mullions and held in place by suitable pressure bloc#s. The final architectural solution may require a decorative finish inside to follow the dotted line profile. This space should not be insulated; otherwise the structural air barrier becomes a vapour barrier on the wrong side of the insulation, inviting condensation problems.
THE MASONRY INTERFACE !n the recent past there has been a trend in architectural practice to devise new ways of obtaining the flush fa1ade. %pecifically, window glass is often aligned or nearly aligned with the exterior veneer or cladding to create a smooth unsculptured exterior wall. %everal general design wea#nesses have been found in this type of interface 0oint, between the masonry and the curtain wall systems. Most often the detail shown in =igure > results in condensation on the inside mullion surface and efflorescence on the outside surface of the bric# veneer. The reasons are twofold' first, the wall insulation is out of line with the thermal brea# of the curtain wall mullion. This results in a discontinuity of the insulation plane and creates a thermal bridge that allows part of the interior structural or metallic components to become cooler than the inside dew point temperature. *ondensation often shows up on the inside surfaces of the sill mullions. The second reason is a discontinuity of the air barrier, because there was no provision for an air barrier element in
the masonry wall; if there is an air barrier; it may be attached to the wrong part of the curtain wall mullion.
THE PRECAST PANEL INTERFACE t times a curtain wall is used in a building fa1ade system with conventional precast panels. )ertical strips of precast panels are interspaced with vertical strips of windows or horizontal strips of precast panels can be interspaced with horizontal strips of windows to create a layered effect of glazing, precast, and glazing. !n these combination systems several questions should arise during the design phase' first, will the curtain wall system be connected to a pressure"equalized wall or a precast panel wall using the face seal approach and second, does the sequence of construction allow for the successful assembly of the interface 0oint?
*onsider the following' a precast panel enclosure must be connected to a curtain wall system assembly. The sequence of construction might be as follows' the precast panel is erected first on the building and aligned; then the structural elements composing the curtain wall system or window system are mounted and installed; finally, an inner wall is built behind the conventional precast to complete the wall assembly. s simple as it might appear, it is li#ely that the interface components which connect the curtain wall system mullion to the precast panel face will not be constructed as intended. This is because the interface components which must be installed last, an air barrier, insulation and a cladding detail, require that two subcontractors, a drywall contractor and a curtain wall installer, wor# on the interface 0oint at the same time. !t invariably means that a swing stage will have to be used again, and part of the curtain wall will have to be dismantled.
ASSIGNMENT NO. 1 BUILDING CONSTRUCTION & MATERIALS DETAILS OF CURTAIN WALLS
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