Read Building Envelope Design Using Metal and Glass Curtain Wall Systems text version

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D i v i s i o n of Euilding Research, National Research Council Canada

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September 1982




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Metal and g l a s s curtain wall systems have found growing favour in modern ar c h i t e c h r e . They are easily d i s t l n ~ i s h e d from other t y p e s of claddings by their thin rrmllions 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, e s p e c i a l l y w i t h respect to weather control performance. The early systems presented frequent rain penetration problems; large icicles would form on the o u t s i d e horizontal bars or condensation on the inside m u l l i o n surfaces; glazing seals were sometimes pumped out of the rabbet of sealed double glazing windw units. However, most of these d i f f i c u l t i e s were eventually overcome with improved detail design o f the system components. Today, most c u r t a i n w a l l manufacturers o f f e r a quality product l i n e of components which can be used to create one of the best overall exterior wall system.

.Somed.lfffculty remains, however, in obtaining equal or even .adequate performance from the j o i n t s a t the interface between t h e curtain wall system and the remainder Q£ the building, Numerous weather control prablam are encountered at parapets, s o f f i t s , corner

details a d grade connections, In addition to rainwater penetration -and the freeze-thaw action of concealed condensation, cold a i r seeps into soffits causing drain pipes, steam pipes, or sprinkler systems to freeze up. Large icicles which form along the roof edge are often a potential hazard t o pedestrians below.

Recent: field investigations and studies of construction d e t a i l design drawings have i n d i c a t e d some practices which continue to cause these problems. Design and construction requirements d i s c u s s e d in t h i s note can minimize undesirable conditions at the interface connection of curtain wall systems to other parts a£ the building.


A curtain wall system is a lightweight exterior cladding which is hung on the building structure, usually from floor t o floor. I can t provide a variety of exterior appearances but %s characterized by narrowly spaced vertical and horLzontd caps w i t h glass or metal infil panels. These systems provide a finished exterior appearance and most o f t e n a semi-finished interior as well. They are also -designed t o accomraodate structural deflections, control wind-driven r a i n and s i r leakage, minimize t h e effects of solar radiation and

provide for maintenance-free long term perfonmnce. Most of today's metal curtain w a l l system are constructed o f lightweight aluminum, although some may be of steel.

Since curtain w a l l systems are a good example of b u i l d i n g science p r i n c i p l e s applied to w a l l design, it may be useful to review s ' o ~ b a s i c p r i n c i p l e s through the details of a typical curtain wall system.

DESIGN PRINCIPLES FOR EXTERIOR WALLS A buflding enclosure may be broadly defined as a set of interconnecting elements which separate the outside from the inside. These elements would include exterior walls, a roof, other components such as windcws and doors, and sometimes e x p s e d floors. The function of a building enclosure is to control the penetration of s n w , wind, rain and sun t o the i n s i d e and t o contain the desired indoor c l i m t e . The enclosure m u s t m e t many individual requirements but for the purpose of this paper they are l i m i t e d t o the follarring six:


2 .

3, 4.



control of air flow, control of heat flaw, control over the entry of rain and snow, control of sunlight and other form of radiant energy, control o f water vapour diffusion, accommodation of building movemnts.

The requirement for alr tightness and consequently air leakage control is m e t by most curtain wall systerss because the a i r barrier of the wall is inherefit in the structural properties of glass and aluntlnm or steel tubes that comprise the system. The continuity of the air barrier (Figure 1) is achieved by the continuity of t h e glass panel through the a i r seal at the shoulder flanges of the tubular mullton, and through the aluminum section to the other flange surface. The air seal between the lmer shoulder flange of the curtain wall mullion and the metal pan of the spandrel panel provides continuity of air tightness to the air barrier metal pan and on to the next mullion connection. Such assemblies are regularly tested using a l r pressure to determine t h e structural propertiee of the glass, m e t a l , and seals and to d e t e r d n e the equivalent leakage area (RLA) that remains. Tn addition, the Architectural Aluminium Manufacturers Asgociation imposes upon its members many other requirements i n c l u d i n g a specification that the system must n m t leak more than -30 L/S per &

( - 0 6 c f m per f t 2 ) of wall a t a pressure difference equivalent t o a 40 kmlh (25 mph) wind.

THERMAL INSULATION (Control of heat f l t i w )

The control of heat flow is generally achieved through the use of i n s u l a t i o n . Although it i s not apparent from the exterior, the curtain wall system uses considerable insulation usually behind s p a n d r e l glass or any opaque panels. Because of the materials used In the structure, i e , glass and metal, which are highfy conductive, .. the system m u s t a l s o contend with potential condensation on t h e interior surfaces. To currafl this e f f e c t , most curtain w a l l systems incorporate t w o distinct features: f i r s t , a sealed double glazed window or an insulated metal pan and second, a thermally broken mullion, usually wfth a PVC p l a s t i c insert and mare recently, a

foamed-in-place polyurethane connection. A sealed double glazed windw unit can accommodate an indoor h d d i t y up to about 35I at an outdoor temperature of - 2 5 Y with little condewation appearing on the glass, Similarly, the thermal break in the aluminum at steel mullion ensures that the surface temperature of the structural mullion w i l l remain w e l l above t h e dew point temperature of the a i r f o r most b u i l d i n g types, except for high hursidity indoor environments such as i n s w i m m i n g pools or computer centers. The thermal break also ensures that the structural mullion is thermally s t a b l e , that is, not s n b j e c t t o extremes of expansion and contraction.

THE: "RAIN SCREEN" PRINCIPLE (Control of rafn and snow penetration)

To control rain pcmtration through exterior w a l l s the conventional apprsach is t o seal t h e exterior f a ~ a d eof the building. However, experience has shown that it is unreasonable to expect perfect scaling of a f*ade; most sealing strategies require continuous attention and maintenance.

Studies of the r a i n penetration problem have revealed a better solution than the faqade sealing approach. If the a i r that leaks i n and through racks and crevices of a f a ~ a d eduring a rain storm were l i m i t e d or stopped, most of the water impinging on the fwade would migrate stratght down the surface and little would penetrate the wall. This is the essence of t h e "Rain Screen" principle. If an a i r t tght element is poaitioned behind a faqade, the cavity formed between the exterior c l a d d i n g and t h e a i r t i g h t e l e m e n t may reach t h e same a i r pressure l e v e l as is exerted on the cladding surface, thus removing the force which causes air to flow through any fasade opening. The "Rain Screen Wall" f s therefore charactertzed by a c a v i t y behind the exterior surface that is connected t o the exterior but sealed t i g h t l y , or as tightly as reasonably p b s s i b l e , to the interior. The inner surface of the chamber is u s u a l l y referred to as the air barrier of the wall.

In most curtain wall systems the j o i n t between the i n f i l panel ( i . e . , window or spandrel panel) and the s t r u c t u r a l m u l l i o n is u s u a l l y designed to be part of a rain screen system (Figure 2). It comprises a p ressure-equalized cavity , connected t o the exterior by the d r a l n holes in the exterior caps, and a pressureequalized rain deflector s e a l between the outside surface of the glass and the mullian cap. The chamber portion o f the cav3ty is composed of the air seals connecting the i n s i d e f a c e of the window glass and the spandrel panel m e t a l pan, to the shoulder flanges of the structural mullion and other parts of the structural sectfon. Thus the set of elements cmprising the windm glass, the a i r seals, and the aluminum section and metal pan p e r f o r m the a i r barrier function for t h i s wall assembly. This design configuration for curtain w a l l s e c t i o n s has proven succeasfd and has become widely accepted.


RADIATION (Control of sunlight and other forms af

radiant energy)

Solar radiation falling on building surfaces may have t w o d i s t i n c t effects: the f i r s t is to cause a significant change in temperature of the faqade elements and t h e Gecond is the slow but destructive effect of u l t r a v i o l e t radtation impinging an all matetfals, particularly organic. On curtain wall systems the most important concerns w i t h s o l a r radiatcon have been the thermal expansion and contraction a£ curtain w a l l components, in p a r t i c u l a r those forming the outside cladding, and the effects of solar r a d i a t i o n on the glazing elements. A warping of glass occurs due t o differences in temperature between the Inner and outer panes, while pumping results from expansion and contraction of the a i r in the cavity of t h e sealed units. Daily and seasonal temperature differences can also cause t h i s effect. The action of the window (thermal pumping) is particularly stressing to the inner a i r s e a l ; however, serrated edges or recessed flanges keep the seals from pumptng out. Most of the ultraviolet-sensitive materials in curtain w a l l systems are located in the pocket and cavity areas of the joints and are partly shaded by metallic and glass components,

THE VAPOUR BARRIER (Control of water vapour diffusion)

Water in I t s gaseouq phase (water vapour ox humidity) always tries to migrate from a region of high water vapour p r e s s u r e t o a region of lower pressure. The migration o f water vapour through a

wall can be compared to heat flow; it moves through a l l materials at a rate that is dependent an both the resistance of the materials to water vapaur f l o w and the differeace in water vapour pressure on bat11 s i d e s of the material.

The migration of water vapour through an assembly of materials is not a serious problem in i t s e l f , provided it does not condense to l i q u i d form i n the material or wall. If water vapour is l i k e l y to condense in a wall, the p r i n c i p a l defense is to restrain L t s migration by using, a "vapour barrier" w i t h a high water vapour flow resistance, positioned on the warm s f d e of t h e insulation material or wall assembly.

The migration of water vapour through a curtain w a l l assembly is checked by the vapour barrier qualities of the glass and aluminum, as these materials have near perfect vapour flow resistance for a l l practical purposes. Thus the inner pane of the sealed double glazed unit and the aluminum or s t e e l inside surfaces of the m u l l i o n provide the necessary water vapmr diffusion control, Sealants a l s o contribute to the continuity of the vapour barrier.

JOINTS AND TOLERANCES (Accommodation of building mavements)

Movements of the structural elements of a building must be determined prior to the design of an exterior w a l l system. Movements may be grouped Into three types:


live load d e f l e c t i o n s duk t o occupancy loads or peak wind Loads on the b u i l d i n g f a ~ a d e ,and dead load d e f l e c t i o n s of the building structure,

expansion and contract&on of materials as a r e s u l t of temperature, radf a t i o n and sometims hygroscopic loading,



slow but inexorable movements due to gradual deformation, such as creep in concrete, foundation settlement, etc.

Although not a frequent cause of f a i l u r e , butlding movements are not adequately considered in the d e s i g n and construction a£ faqades. Masonry that has cracked or bulged, metal siding that has sheared its fasteners or buckled, o r caulking that is coqtletely squeezed out or broken are some of the effects of building movement.

With curtah wall systems, the b a s i c element which must


accommodated is the glass panel. Around it, the t y p i c a l curtain w a l l system of structural tubes, pressure plates and caps allows for a differential movement of about 4 to 5 mm (3116 to 114 in.) an a f l o o r

to floor basis and between each vertical riser.

This tolerance will

accommodate m o s t b u i l d i n g movement resulting in conpression, expans I o n and parallelograming of the frames. If t h e curtain wall system must

accommodate a p o t e n t i a l l y greater movement than above, i t is l i k e l y

that another system of mullion extrusions will be required. This inevitably l e a d s t o more soroplex detailing and usually a disproportionate Increase in the system cost.


A s s u m i n g that: most curtain w a l l systems can perform well or that the components are available to b u i l d a qualfty wall s y s t e m , we turn our attention t o the connections or Ehe interface j o i n r between the currain wall system and the other parts of t h e building,


When a curtain w a l l system is desfgned to extend upwards past the roof l i n e and t h u s t o g e t cold, several potential problems mst be considered. Without proper termination at the head of the curtain w a l l system, coadensation and frost m%y form in t h e tubes and eventually drain to the i n s i d e of the building, ar i c i n g may f a r c e p a r t s of the parapet cap off the b u i l d i n g , Also, if a l l o w e d t o fluctuate with the outdoor temperature, the structural p a r t of the curtain w a l l system m y expand and contract beyond its design limitations, thus straining all connections in and around the parapet elements.

Because the structural tubes of the curtain w a l l system are a l s o

miniature chimneys, particularly in high-rise h i l d i n g s , they may

conduct large volumes of (humid) air t o the outside if left open or unsealed at: the top. As there are many j o i n t s i n the structural system of the curtain wall, it is preferable that the a i r barrier component be connected from the t o p shoulder flange surface of the horizontal mullion and that it be made to bridge the gap from the c u r t a i n w a l l and parapet structure t o the a i r barrier component extending from the roof deck (Figure 3 ) - This may be d i f f i c u l t at times, particularly with a conventional b u i l t - u p roof in which the tnsulation is under rhe membrane and must cress over somewhere tn the interface detail. A l a o t h e a i r barrier between the curtain wall and the parapet must be insulated o n the o u t s i d e to prcvenr any condensatLon from forming on its i n s i d e

411rf ace.

Those parts of the structural mullion tubes from the thermal break and inward, must be kept on t h e w a r m side of the wall at all

times. Hence the structural mullions should face a warm cavity. 'It is not recommended that insulation be placed against t h e mullions or w i t h i n the mullion tubes. A parapet c a p can then be designed t o cover the parapet connection. Tt should b e s l o p e d to t h e roof s i d e and pressure equalized t o minimize rain e n t r y . This design approach a p p l i e s to a l l t y p e s of parapets, whether backed up by brick, block, precast or wood structure.

The material used to construct the p a r a p e t a i r barrier is of particular importance. While t h i s air seal may be a f l e x i b l e membrane, it must be a b l e t o carry air pressure loads as high as the combined wind load, stack effect, and ventilation pressure or it w i l l rupture and cause a major air exfiltration problem* It is better t o use a r i g i d material such as sheet metal with appropriate connection details because the insulation on its outboard face m s remain in ut place.


Buildings using curtain wall systems are often requfred to form an i n s i d e or outside corner, When t w o sections of curtafn w a l l meet, the interface d e t a i l must be designed to provide control over a l l a£ the aforwrentioned requirements. Because corner d e t a i l s vary consf derably f r m project to project, u suppliers do not have stock sections to draw upon to construct this intarf ace d e t a i l . However, cutrain wall s u p p l i e r s w i l l fabricate the necessary interface components provided that their participation is s o l i c i t e d early enough in the planning phase, preferably before tenders are closed.

This interface detail w f 11

requf re an a l r barrier, some insulation and an exterior c l a d d i n g (Figure 4 ) . The a i r barrier m u s t be structurally adequate t o carry the a i r pressure l o a d s on that corner. The material t o be used as an a i r barrier should be aluminum if t h e curta-ln wall system is aluminum, or at least a s h e e t of m e t a l , and at the very least a r i g i d element. G a l v a n i z e d sheet s t e e l m y be used w i t h aluminum; however, considerat i o n must be given t o the corrosive potential of dissimilar metals. Rond breakers such as paint or butyl tapes have proven satisfactory t o many of the curtain wall sysrem manufacturers, If t h e outside c l a d d i n g d m the corner of the b u i l d i n g is t o appear as a continuous s t r i p w i t h no mullion interruptions, then care must be taken to develop an a i r seal joint at the ends of the a i r barrier sheets.

When dealing w i t h an i n s i d e corrter, the same requirements apply { ~ i g u r e5 ) . However, If the mullibn caps are in near contact or overlap s l i g h t l y , it is not necessary to add a further "Rain Screen" b a f f l e over the insulation. The aXr barrier should be rigid and sealed against the shoulder flanges of the vertical mullions and h e l d in place by suitable pressure blocks, The f i n a l architectural s o l u t i o n may require a decorative finish inside t o follow the dotted l i n e profile. T h i s space should not he insulated, otherwise the structural a i r barrier a l s o becomes a vapour barrier on the w o n g s i d e QE the insulation, I n v i t i n g condensation problems.


Whether on a concrete curb, a block wall or a concrete floor s l a b edge, the grade connection interface between the curtain w a l l and the reminder of the b u i l d i n g is particularly s e n s i t i v e to r a i n penetration and air Infiltration. The most c - a 3 tnterface detail is am K n shown in Figure 6 . This design approach often r e s u l t s in t w o performance problems: f i r s t , coLd air Infiltrates through the ends of the vertical tubes, fncreasing t h e potential for condensation on the tube surfaces and f o r glass breakage, and second, rairwa ter accumulates in the c a v i t y between the curtain wall section and the floor, which w i l l prematurely deteriorate

the f l n o r t o l o u l l i o n a i t seal.

In keeping with the design requirements for cantinutty of the a i r barrier, it must be st.arted from the horizontal mullion lower shoulder flange and extended d i r e c t l y t o the floor or curb as in Figure 7. A sealant might be used to create a sloped edge or a water dam, and a small amount of Insulation w l l I c o n t r o l any condensation forming on the w a r m side of the a i r barrier

surface. Then pressure block water d r a i n i n g cavity instead

a flashing component should be i n s t a l l e d over a and under the a m p cap. This will ensure t h a t surface from the panel above is directed to t h e outside of the of i n t o I t .


In the recent p a s t there has been a trend in architectural practice to devfse new wags of abtatning the f l u s h facade. Specifically, window glass is often allgned or nearly aligned w i t h the exterior veneer or cladding to create a smooth unsculpturd exterior wall. Several general design weaknesses have been found in ehfs type of interface j o i n t , between the masonry and the curtain wall systems. Host often the detall shrrwn in Figure 8 results in condensation on the i n s i d e mullion surface and efflorescence on the outside surface of the brick veneer. The reasons are twofold: f i r s t , the wall insulatzon is out of l i n e with the thermal break o f the curtain wall mullion. This results In a discontinuity of the insulation plane and creates a thermal bridge that allows p a r t of the i n t e r i o r stractural or metallic components to become cooler than the i n s i d e dew point temperature. Condensation o f t e n shrrws up on the inside surfaces of t h e sill m l l i o n s . The second reason is a discontfnufty of the air barrler, because there was no provision for an air barrier element in t h e masonry w a l l ; if there is an a i r barrier, it may be attached to the wrong part of the curtain wall m l l f o n .

This configuration allows cold air which has entered the cavity from the weep holes in the brick veneer to i n f i l t r a t e past t h e n u l l i o n connections and i n t o the v e r t i c a l tubes, If this detail occurs i n the upper p a r t oE a b u i l d i n g there would be a tendency for a i r t a exfiltrate from rhe room. If m o i s t , warm air Elnds f ts way into the cavity between the brick and the concrete block, most of i t s m o i s t u r e would condense on the back of the brick veneer. This shows up as severe efflorescence o n the brickwork just below t h e window during the s p r i n g thaw; sometimes the brickwork w i l l spa11 and crack. This Interface detail must be designed so t h a t , regardless of its exterior appearance, an air barrier element is connected from the shoulder flange of the curtain w a l l rmllion to its complementary component on the masonry wall assembly. This may be the gypsum board on the room side face of the block w a l l or it may be a trowelled-on

mastic in between a layer of insulation and the backup block wall. Some element msr be chosen t o carry the f u l l wind o r pressure load that may occur across the building wall at t h a t Location.

The trowelled-an mastic has vapour barrier qualities, but it i s not considered an adequate air barrier under any circumstances. Even a pinhole in the, mastic will allow air to seep in between it and the insulation. If the insulatiors is not air-permable, the a i r w i l l exert pressure on it, and in some cases, force it off the wall.

14 the design of thSs interface detail, it is also desirable t h a t the wall insulation and the curtain wall m l l i o n thermal break he aligned. If t h i s is not practfcal because of a particular d e s i g n appearance requirement, then t h e interface d e t a t l mst provide sufficient space so that Ehe blanket of insulation may bridge the gap from the insulation on the masonry wall. t o the t h e m 1 break of the curtain wall section. The insulation should be installed tightly o n the cold s i d e of t h e a i r barrier component, which should have sonre vapaur diffusion control qualities as well.

When this represents a sill d e t a i l , the sill f l a s h i n g connected to the curtain wall rmlltan mst reach over the brickwork and have the appropriate drip profile. This flashing should not be connected on the i n n e r shoulder flange surface of the curtain wall s e c t i o n a s it w i l l cause a thermal bridge t o occur at the air barrier connection. Simtlarly the flashing should not go over the c a p or be f a s t e n e d to the cap surface, as more water than necessary will penetrate the cavity between the curtain w a l l and the masonry. The sill Elashfng will perform best i f it is installed over the pressure block, h e l d in place by the pressure p l a t e and cap. There are also other types of extrusion to s u i t t h i s application.


At times a curtain wall is used in a building fasabe system with conventional precast panels. Vertical s t r i p s of precast p a n e l s are interspaced with v e r t i c a l strips of windows or horizontal strfps of precast panels can be interspaced with horizontal strips of w i n d w s t o create a layered effect of glazing, precast, and glazing.

combination systems several questions should arise durtng t h e design phase: f i r s t , w i l l t h e curtain. wall system be connected t o a pressure-equalized wall or a precast panel wall using the face seal approach and second, does the sequence of construction a l l w or the successful assembly a£ the interface f o f n t ?

I n these

C o n s i d e r t h e following: a precast panel enclosure must be connected t o a curtain w a l l system assembly. The sequence of construction might be as follows: the precast panel is erected first o n the building and aligned; then t h e structural elerdehta composing

the curtain wall sysEem o r window system are mounted and installed; finally, an inner wall is built behind the conventional precast to complete the wall assembly. As simple as it might appear, it is likely that the interface components which connect the c u r t a i n wall system mlllon to the precast panel face will not be constructed as intended. This is because the interface couponents ( s e e Figure 9) which trust be i n s t a l l e d l a s t , an a i r barrier, i n s u l a t i o n and a cladding d e t a i l , require that t w o subcontractors, a drywall contractor and a c u r t a h wall

i n s t a l l e r , work on the i n t e r f a c e j o i n t at t h e same t i m e It invariably means that a swing stage will have to be used again, and p a r t of t h e c u r t a i n w a l l w i l l have to be dismantled.

In these cases it i s imperatlbe that the general contractor work closely with the designer to arrive at interface detail which is buildable within the sequence of construction and that this detail embody a l l the elements required of exterior w a l l design.


When connecting an aluminum curtain wall system t o a s t e e l s i d i n g , s o w thmght mst be given to the long term corrosion potential of the connecting elements. If a steel a i r barrier companent I s used to connect the curtain w a l l system mullion to the steel siding, the connection should not corrode appreciably. Thts is because the connection is on the w a r m side of the thermal break and s h w l d not come in contact with much moisture. Also it is l i k e l y to be separated from t h e aluminum by a t a p e or c a u l k i n g material. However, i f a stainless or galvanized steel element is used t o connect the steel siding to the alumfnum curtain wall mullion on the o u t s i d e of the packet pressure block, some measure of protection is required a t the interface between these two dissimilar metals, A tape, s p e c i d p a i n t , or a vinyl gasket, can prevent the s t a i n s which so o f t e n occur when rainwater washes off the oxidized mtals a t this interface connection. It is a l s o important to consider the type of material used for the pocket pressure block. This block s h w l d be of some plasric material to i n h i b i t any corrosive action of the materials in contact with i t or with the aluminum.


Thi.s part of the b u i l d i n g is perhaps one of the most troublesome and misunderstood parts of the buf l d i n g enclosure. F i r s t , it mst be

determlaed whether a soffit is t o he heated or unheated, as m o s t d e s tgn d e c l s i a n s mvt be based on one or the other a these preaf s e s .

I n the e v e n t t h a t t h e soff Lt 1s Lo be heated, then the enclosure area oE the s o f f i t , which includes the lower part of the w a l l fagade and the u n d e r s t d e surface of the s o f f i t , must be designed to m e ~ t l l a t h e requirements of an exterior w a l l .

Most often there is a discontinuity of the air barrier because the point of attachment for the a i r barrler component l s wrongly chosen. Also, the soffit portion that faces the w a r m cavity I s the s i t e of c o l d a i r tnfiltration because o f the inadequacy of the a i r h a r r i e r component to handle air pressure loads even under the mildest of wind conditions. A polyethylene film w i t h taped jaint aver a layer a£ insulation is inadequate for controlling air movement in and out of the s o f f i t .

In the design of a s o f f i t system, the ends of the curtain w a l l vertical tubes and the horizontal section must have a structural a i r barrier attached to the lower shoulder flange a£ the m l l i o n face; the barrier must return t o the underside of the s o f f i t t o link up w i t h i t s counterpart in the soffit construction (Figure LO). This should be followed by careful detailing aE a continuaus ~ n s u l a t i ~ n layer from the thermal break j o i n t at rhe curtain wall nullion, around the mullion section, t o j o i n eventually w i t h the so££it insulation. Depending on the d e s i g n humidity conditions, a vapour barrier may or may not be required, but i f it is, then care wst be exerclscd in the chalce of matertal and 10- tion,

In the event that a soffit is to be unheated, most of the building envelope d e s i g n requirements mist be achieved a t the t o o f / f l o o r element. In thts case, It would k wise to consider a cornpiete d i s c o n t i n u i t y of the structural components of the curtaln w a l l system composing the facade so that no v e r t i c a l tubes in the s o f f i t area are connected t o the corresponding vertical element starting at the floor above and gofng up the faqade of the building. An a l t e r n a t e approach is to construct a narrow v e r t i c a l enclosure around the curtain w a l l system f a c i n g the s o f f i t cavity so that the curtain w a l l system is maintained i n a w a r m environment at a l l ttmes and i s properly sealed and inaulared.

Soffit design and construction is somewhat complicated by the sequence of events that m i s t take place during constmction.

The sequence must be clearly d e l i n e a t e d by t h e d e s i g n e r and builder in order t o achieve q u a l i t y p e r f o r m n c e for this area. Icicle F o r m a t i o n under s o f f i t s , wet surface f i n i s h e s , frozen s p r i n k l e r p i p e s , and drafty lower f l o o r s are but a f e w of the p r o b l e m t h a t have h e e n reported with improperly d e t a i l e d or improperly constructed s o f f i t zones.

SLOPED WALLS (Glass ~ o o f s )

Architects and owners want t o see through their roofs. Skylights, and a t r i u m and mall spaces are to be bright, open and c l e a r and at the same t i m e provide a l l the functions of a normal roof. The f i r s t areempt at constmctiag sloping glass walls uslng conventional mullion sections revealed the major weakness almost immediately. Because of the angle at which the n u l l i o n s rested, the cavity could not drain and the window unit g l a z i n g seal c o u l d be c o n t i n u a l l y immersed in water. This caused t w o problems: it destroyed the inner air seal, which eventually allowed rain to penetrate, and the g l a z e d sealed window unit failed prematurely because the edges were frequently immersed in water. In addition, i t w a s discovered t h a t if s l o p e d walls were c o n s t r u c t e d in humid envtranments, severe condensatlon would form on the glass surfaces, accumulate at the lower edge, and d r i p from the horizontal bars- While t h i s was not a serious problem, sorrre measure would have to be taken to control the effects of condensation. These difficulties were corrected in most systems by t h e introduction of new mullion s e c t i o n s which p r o v i d e an interconnected rain gutter s y s t e m with drainage to the outside that is w e l l below t h e primary a i r s e a l o f the glass and aluminum fnterface ( F i g u r e 11). Also most of t h e s e new sections have condensation collector g u t t e r s a n d some manufacturers may soon p r o v i d e an i n s i d e drainage s y s t e m f a r the condensation c o l l e c t o r , e s p e c i a l l y for high humidity environments

where excess f v e condensat t o n may form. The i n t e r f a c e connection between a sloping glass wall and o t h e r bullding elements is somewhat complex. However, L E t h e sloping wall abuts a vertical wall that rises above the glass roof, t h e a i r seal requirement is the same as that f o r a l l of the interface d e t a i l s . An a l r barrier element must be joined from t h e flange surface of the h o r i z o n t a l m u l l i o n to the s i m i l a r f u n c t i o n element of t h e a d j o i n i n g w a l l , The air barrier must be insulated from the outside and a protective cover installed over the insulation. T h i s cover must also

serve another purpose. Recause of the possibilities of snow accumulation, the cladding cover should extend s u f f i c i e n t l y high up the vertical wall to guard against a water dam buildup at the upper This c l a d d i n g edge of the glass roof and over t h e cladding j o i n t s . should be o m piece where possible and may connect t o the normal m u l l i o n cap; however, it is probably better to extend it over t h e cap of the sloping glass curtain w a l l mullion and fasten it t o the mullion face cap.

Where two s l o p i n g glass walls meet as a R i p roof, the air barrier, ,insulation and cladding cover should be designed as a single element t o form the ridge.

Where a sloping glass wall is required to interface with a v e r t i c a l glass wall from underneath, consideration -st be given to melting snow and ice formation at the edge, and ta the condftion and continuity of drainage from the sloping roof mullion draf n gutters t o the wall mullion sectious below.


The Interface detail often referred to 'by othersf w i l l require a thorough well-developed design solution based on a l l the p r i n c i p l e s of wall performance. Consideration must also be gfven by the general contractor to h m he will implement the construction of the d e t a i l s developed by the design team. The seqGence of construction is p a r t t a l a r l y fmportant f o r a successfully performing interface d e t a l l .

The interface j o i n t , be it: a parapet, soffit; corner, or connection to another type of w a l l , w i l l requfre a s t r u c t u r a l l y adequate a i r barrier securely connected to the curtain wall at its proper location and joined t o a material of similar function at i t s other end. The air barrier component should be r i g i d so that insulation can be brought into intimate contact with its surface. I n m o s t cases the insulation d l 1 require a protective cover and this usually becomes the outside f i n i s h or cladding. The c a v i t y between the outer cladding and the i n s i d e air barrier should be pressureequalized and drafned t o the o ~ t s i d e . If the a i r barrier material has a sufficiently low permeability ta water vapour flow, it w i l l act as a v a p w r barrier; otherwide a separate vapour barrier may be required.

On very large projects where the designer intends to use much curtain w a l l , .Lt will be best t o obtain the services of a building science consultant durlrtg the development of working drawings and perhaps at the preliminary d e s i g n phase,

R L Quirouette, A Study of the Construction Process, Division of .. B u i l d i n g Research, National Research ~ o u h c i lof Canada, Building Practice Note 3 2 , 1982..


Building Envelope Design Using Metal and Glass Curtain Wall Systems

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