Concrete
Curtain wall, storefront, and glazing installation field guide
What a curtain wall is and why a good one drains water instead of sealing it out, the systems, the glass, the thermal break, the air and water barrier tie-in, anchorage and movement, and the performance mock-up.
Direct answer
A curtain wall is a non-structural aluminum-and-glass skin hung off the structure, carrying only its own weight and wind. The best systems do not face-seal against water. They are pressure-equalized rainscreens that let some water into a drained glazing pocket and weep it back out. Anchorage, movement, the thermal break, and a tested mock-up decide whether it holds.
Key takeaways
- A curtain wall is a non-structural aluminum-and-glass skin hung off the structure, carrying only its own weight and wind load, not the floors.
- High-performance curtain walls drain water using a pressure-equalized rainscreen and weep it out, rather than face-sealing, because a single face seal always fails eventually.
- Performance is proven by lab tests: ASTM E283 air, ASTM E331 water, ASTM E330 structural wind load, plus the ASTM E1105 field water test on the installed wall.
- Aluminum frames need a thermal break (polyamide or poured-and-debridged resin) to cut heat flow and stop the interior face from sweating in cold weather.
- An IGU fails by edge-seal failure: moisture saturates the desiccant and fogs the cavity, and argon leaks out, so the glazing pocket must drain and never pond water.
What a curtain wall is, and what holds it up
A curtain wall is a non-structural building skin, usually aluminum framing and glass, hung off the structure to keep the weather out. It carries its own weight and the wind that hits it, then passes those loads back to the floors through anchors. It does not hold the building up. The slabs and columns do that, and the curtain wall hangs on the outside of them like a curtain, which is where the name comes from.
The detail that decides whether a curtain wall leaks is the same one that decides whether a masonry cavity wall leaks. The good systems do not try to seal water out at a single face. They let a controlled amount of water in, then drain it back out through the glazing pockets and weeps. A face seal is one line of defense, and one line always fails somewhere over thirty years of sun, movement, and dirty gaskets.
The work is five decisions, and missing any one of them is how these walls leak. Pick the right system for the building. Manage the water by draining it, not damming it. Tie the curtain wall air and water barrier into the wall barrier so the line is continuous. Anchor it for load while letting it move. Break the frame thermally so it does not sweat. Each of those is proven, not assumed, by performance testing and a mock-up before the real wall goes up. The envelope air barrier and the building movement joint sit right next to this work and are covered in their own guides; this one is the glass-and-aluminum skin.
Why does a curtain wall drain water instead of sealing it out?
Because a single face seal always fails, and a drained wall survives the failure. Water gets through an exterior seal eventually, driven by wind pressure through the smallest gap at a gasket lap, a corner, or a tooling void in the sealant. A face-sealed wall has nowhere to send that water but inside. A drained wall expects it and has a way out.
The high-performance approach is the pressure-equalized rainscreen, the same principle the masonry trade uses on a cavity wall. The outer gaskets and caps shed most of the water. The glazing pocket behind them is vented to the outside so the air pressure in the pocket equals the pressure on the face, which removes the pressure difference that drives water in. The little that still gets past the outer seal collects in the pocket and drains down internal gutters and out the weep holes at the horizontals.
There is a middle option, the water-managed or drained-and-back-ventilated system, which drains the pocket without full pressure equalization. In a water-managed system the weeps mostly drain; in a true pressure-equalized system the same openings also equalize air. The trade ranks them the way every reference does: pressure-equalized resists air and water best, water-managed next, and face-sealed last. Which class a project gets is the system manufacturer's design, proven by the test, so confirm the system class against the spec and the test report rather than the brochure.
The four systems: stick, unitized, storefront, window wall
Four aluminum-and-glass systems show up on commercial work, and they are not interchangeable. Two are curtain wall, separated by how they are assembled. The other two are separated by where they sit relative to the slab. Get the category wrong on a bid or a detail and the water, fire, and structural assumptions all move with it.
| System | How it is built or where it sits | Typical use |
|---|---|---|
| Stick-built curtain wall | Mullions and glass assembled and glazed piece by piece in the field | Low to mid-rise, complex shapes, smaller jobs |
| Unitized curtain wall | Factory-glazed panels craned in and locked together | High-rise, repetitive elevations, fast schedules |
| Storefront | Lighter framing at the ground floor, often center-set, carrying the doors | Ground-floor and low-rise glazing, retail fronts |
| Window wall | Floor-to-floor units that sit on the slab and tuck under the slab above | Mid-rise residential and condos, where cost rules |
What is the difference between curtain wall and window wall?
A curtain wall hangs past the slab edge as one continuous skin. A window wall sits between the floors, resting on each slab and dying into the slab above. That single difference, where the system meets the floor, changes the structure, the water path, and the fire detail.
Curtain wall runs continuous and outboard of the slab, so it spans the floor line uninterrupted and the slab edge is hidden behind it. That is the cleaner facade and the higher performer, and it forces a perimeter fire-safing detail at every floor because a non-rated wall is passing a rated floor. Window wall sits on the slab, so each floor's units are independent, the slab edge is exposed or clad separately, and the floor-to-floor break contains fire more readily without the same continuous safing run.
The tradeoff is cost and water. Window wall is cheaper and simpler to install one floor at a time, which is why it dominates mid-rise residential. It also stacks a horizontal joint on the slab at every floor, sitting in the worst spot for water, so the head and sill detail and the slab-edge flashing carry the whole risk. The structural and fire implications belong to the engineer and the code official, so confirm the system against the project documents rather than assuming one behaves like the other.
Stick-built vs unitized curtain wall: which fits the job?
Stick-built is cheaper to buy and slower to install, with most of the sealing happening in the field. Unitized costs more up front and goes up fast with factory quality control, which is why it is the high-rise standard. The choice is schedule, height, and how much you trust field labor over shop labor.
Stick systems ship as loose mullions and glass that the crew assembles and glazes in place, joint by joint, hung off the structure with anchors at each floor. That means a lot of field-applied seals, a lot of labor up on the wall, and a lot of weather exposure during the work, which is where the quality risk lives. Stick suits low to mid-rise, odd geometry, and smaller jobs where craning panels makes no sense.
Unitized systems arrive as complete factory-glazed panels, often a floor tall and a module wide, that crane into place and interlock at the vertical and horizontal stack joints. The glazing and the critical seals happen in a clean shop under controlled conditions, the field work is mostly setting and locking panels, and the wall closes in fast. The split joints between units are engineered, gasketed, pressure-equalized interlocks that also absorb movement. Unitized costs more and needs the design locked early because the panels are built before they ship, but on a tall, repetitive building it wins on speed and consistency. The crossover point is project-specific, so let the manufacturer and the schedule drive it.
Storefront: the ground-floor system
Storefront is the lighter aluminum-and-glass system that fills the ground floor and low-rise openings, and it is where most entrance doors live. The framing is shallower and lighter than curtain wall, the glass is often center-set in the frame rather than hung on the face, and the spans are short because the floor-to-floor height at grade is modest.
Two things separate storefront from curtain wall in the field. It is not built to the same wind and water performance as a tall curtain wall, because it does not see the same pressures down at grade behind canopies and recesses. And it carries the doors, which means it carries the most-used, most-abused, most-leak-prone openings on the building. The threshold and the door perimeter are where storefront leaks, so the sill pan and the door head flashing deserve more attention than the glass.
A common mistake is treating storefront as a smaller curtain wall and skipping the drainage. Better storefront framing still drains and weeps. Center-set glass with no path for the water that gets behind it is the cheap version, and it leaks at the sill the same way a face-sealed curtain wall leaks, just lower down where everyone sees it.
The glass: IGU, coatings, safety glass, and spandrel
The glass is not one product. A curtain wall lite is usually an insulated glass unit with a low-E coating, made of tempered or laminated safety glass where the code requires it, and the opaque bands at the floor lines are spandrel. The makeup is specified for vision area, energy, safety, and what has to be hidden, so the glass order is a schedule, not a single part number.
| Element | What it is | Why it is there |
|---|---|---|
| IGU (insulated glass unit) | Two or more panes with a perimeter spacer and a sealed, gas-filled cavity | Cuts conductive heat flow across the glass |
| Low-E coating | A microscopic metallic coating on an interior surface of the IGU | Controls heat gain and loss; the main energy lever |
| Tempered glass | Heat-treated glass that breaks into small dice | Safety glazing where impact risk is high |
| Laminated glass | Two lites bonded to an interlayer that holds the fragments | Safety, security, sound, and fall protection |
| Spandrel | Opaque glass (ceramic frit or shadow box) at the floor bands | Hides the slab edge, the safing, and the plenum |
The insulated glass unit and why it fogs
An insulated glass unit is two or more glass panes separated by a spacer around the perimeter, with the cavity between them sealed and usually filled with argon. The spacer holds a desiccant that absorbs the trace moisture sealed inside at fabrication. A primary seal blocks vapor and gas, and a secondary seal around the edge holds the unit together structurally. The whole thing lives or dies at that edge seal.
The failure mode is fogging between the panes, and it is always the edge seal. Moisture works through the seal over years, the desiccant absorbs it until it saturates, and after that the moisture condenses inside the cavity where you cannot wipe it off. The argon leaks out the same path it came in, and the U-factor you paid for degrades as the gas fill drops. Low-E coatings can corrode at the perimeter when the seal lets moisture reach them, showing as haze creeping in from the edge.
The edge is everything, so two field rules matter. Keep the IGU drained and never let it sit in standing water, because a unit in a wet glazing pocket fails its seal early, which is one more reason the pocket has to drain. And honor the manufacturer's setting blocks and edge clearances, because pinching the edge seal or blocking it wrong shortens the life of the unit. IGU performance and warranty belong to the glass fabricator, tested to the ASTM durability methods, so the spec and the warranty control the makeup.
When does code require safety glazing?
Code requires safety glazing, tempered or laminated, in the locations the building code calls hazardous: in and next to doors, in large panels close to the floor, in glazing near walking surfaces and stairs, and in wet areas like tub and shower enclosures. The International Building Code addresses this in its safety-glazing provisions, and the adopted edition and local amendments control the exact triggers.
The triggers a glazier carries in their head are size, height above the floor, and proximity to a door or a walking surface. A big lite with its bottom edge low to the floor, a sidelite next to an entrance door, glazing in the door itself, and railings or guards that double as glass all land in the hazardous category and need safety glass. Annealed glass in those spots is a code violation and a cut-someone hazard.
Tempered breaks into small dice that are less likely to lacerate. Laminated holds its fragments on the interlayer, which is why it is used where the glass also has to stay in the opening after it breaks, like overhead glazing and fall-protection guards. Which one a given location needs depends on the application and the code, so confirm the safety-glazing requirement against the adopted IBC edition and the project documents instead of defaulting to tempered everywhere.
The framing: thermally broken aluminum mullions
The framing is extruded aluminum, shaped into vertical mullions and horizontal members that grid the wall and hold the glass. Aluminum is light, strong, and easy to extrude into the complex shapes a glazing pocket needs, and it is a fast conductor of heat, which is the problem the rest of the frame design has to solve. Left as solid metal, an aluminum mullion runs cold on the inside in winter and sweats.
The glass is held into the frame one of two ways. A captured system clamps the glass with an exterior pressure plate screwed to the mullion, with gaskets making the seal, then a snap-on cap covers the plate. That is the framed, gridded look, and the pressure plate is a mechanical backup that holds the glass no matter what the sealant does. The other way is structural silicone glazing, where the glass is bonded to the frame with structural silicone and there is no exterior cap, giving an uninterrupted glass face.
The mullion is also the drainage channel and the structure. It spans floor to floor carrying wind load, it houses the glazing pocket that drains the water, and it carries the anchors back to the slab. So the frame is doing structure, water management, and thermal control at once, which is why the thermal break inside it matters as much as the aluminum around it.
The thermal break and the sweating frame
A thermal break is a low-conductivity barrier built into the aluminum frame that stops heat from conducting straight through the metal. It does two jobs: it cuts the energy lost through the frame, and it keeps the interior face of the frame warm enough that room air does not condense on it. A frame without a break runs cold inside and sweats, and that condensation rots sills, stains finishes, and grows mold.
Two methods dominate. Polyamide strips, a glass-reinforced nylon, are crimped into channels in the inner and outer aluminum extrusions, joining two separate metal pieces with the plastic carrying the gap between them. Poured-and-debridged breaks fill a channel in a single extrusion with a polyurethane resin, then cut away the aluminum bridge underneath, so only the resin connects the two halves. Polyamide generally gives the better thermal number on high-performance frames, but both are real thermal breaks and both get specified depending on the system.
The break is not optional on a conditioned building in a cold climate. The energy code drives the U-factor, and the condensation resistance is measured by the condensation resistance factor, a higher number meaning a warmer interior surface. The continuous exterior-insulation logic and the thermal-bridging story for the rest of the building live in the envelope guide; on the curtain wall, the frame break is the equivalent move.
Water management: the drained glazing pocket and the weeps
The glazing pocket is the channel in the mullion that the glass edge sits in, and in a good system it is a drainage cavity, not a sealed box. Water that gets past the outer gaskets collects in the pocket. From there it runs along internal gutters at the horizontals, steps down through the verticals, and exits at the weep holes cut in the bottom of each horizontal member. The water that gets in goes out. That is the whole idea.
The weeps have to actually weep. Blocked weeps, weeps sealed shut by a well-meaning caulker, or a pocket that does not slope to drain will pond water against the IGU edge seal and the frame, which is how a drained system gets defeated and starts behaving like a face-sealed one. On the mock-up and in the field, confirming the weeps drain is a real test step, not a formality.
The pocket also has to be continuous with itself. Water draining down a vertical has to find its way to a horizontal gutter and then to a weep without hitting a dead end at an intersection. The mullion-to-transom joints are where that path breaks, and they are where a drained wall leaks if the gaskets and the internal seals at the intersection are not built right. This is the same drain-the-water logic the envelope guide applies to the wall cavity, carried into the glazing pocket.
Tying the air and water barrier into the wall
The single biggest leak point on a curtain wall is not the wall itself. It is where the curtain wall meets the rest of the building. The curtain wall has its own air and water barrier built into the framing and the glazing seals. The adjacent wall has its own air barrier and water-resistive barrier. If those two lines do not connect into one continuous plane, you have a hole in the envelope exactly at the transition, and air and water find it.
The transition is a designed detail, not a bead of caulk at the end. The curtain wall perimeter has to seal to the wall's air barrier and to its water barrier, usually with a transition membrane and a backer, lapped so water sheds outward and the air seal stays continuous. At the head, the jambs, and the sill, the curtain wall ties into the wall above, beside, and below it, and each of those is a place the continuity can break.
This is where the curtain wall trade and the wall trade point at each other, and the gap between their scopes is the gap in the barrier. The perimeter seal and the membrane transition are the detail that decides whether the whole assembly is continuous. The continuous-air-barrier principle and how to detail the transitions through the rest of the envelope are covered in the envelope air-sealing guide; on the curtain wall, the perimeter tie-in is the work.
How is curtain wall performance tested?
Curtain wall performance is proven by testing for three things: air infiltration, water penetration, and structural load under wind, run in the lab on a mock-up and again in the field on the real wall. The standards come from ASTM and from AAMA, now part of the Fenestration and Glazing Industry Alliance (FGIA), with the North American Fenestration Standard (NAFS) covering rated fenestration products.
The lab tests on the mock-up follow a sequence. Air infiltration is measured under ASTM E283, the rate of air leakage at a specified pressure across the specimen. Water penetration is checked under ASTM E331, a uniform static air pressure with a calibrated water spray, and often a dynamic water test as well. Structural performance is verified under ASTM E330, loading the wall to its design wind pressure and beyond, commonly to 1.5 times design, checking for permanent set and damage. AAMA 501 covers the overall test methods for the mock-up program.
The field tests confirm the building got what the lab approved. AAMA 502 covers field testing of newly installed fenestration, and AAMA 501.2 is the diagnostic hose test for fixed, non-operable curtain wall and storefront seals. The numbers, the test pressures, and the pass criteria are set by the project specification and the engineer, so the spec and the test reports control whether the wall passed, not a general rule of thumb.
The performance mock-up and the field water test
A performance mock-up is a full-size section of the actual curtain wall, built on a test frame before the building wall goes up and run through the air, water, and structural tests, so the system gets proven before it is committed to thousands of square feet. It is the cheapest place to find out the design leaks, because fixing it on the mock-up costs a detail change and fixing it on the building costs a facade.
The mock-up catches what drawings hide. It exposes the corner conditions, the mullion-to-transom intersections, the transitions to adjacent construction, and the drainage path, which are exactly the spots where curtain walls leak and exactly the spots a flat detail makes look fine. A mock-up that passes air, then fails the dynamic water test at the intersections, has just earned its cost by finding the leak before the building did.
On the installed wall, the field water test under ASTM E1105 applies a calibrated spray and a static pressure difference to a section of the real curtain wall to confirm it performs as the mock-up did. Skipping the mock-up or the field test is the gamble that the drawing and the installed reality are the same, and on a curtain wall they rarely are. Whether a project requires a mock-up and how much field testing it gets is set by the spec and the engineer, so confirm the testing scope in the project documents.
The anchors: carrying the weight and the wind
A curtain wall hangs on two kinds of anchors, and they do different jobs. The dead-load anchors carry the weight of the wall down to the structure. The wind anchors take the lateral push and pull of wind pressure and transfer it back to the slab. Most designs use a dead-load anchor that carries gravity and restrains wind, plus separate wind anchors that take lateral load only and let the wall move vertically.
The anchors land at the slab edge, usually on an embed cast into the concrete or a bolted connection to the structural steel or the slab edge. The connection has to be adjustable in three directions, because the structure is never built to the tolerance the glass needs. The anchor has slots and shims to take up the difference between where the slab actually landed and where the curtain wall has to hang plumb and true. That adjustability is what lets a precise aluminum-and-glass wall hang off a rough concrete frame.
Undersized or wrong anchorage is a structural failure, not a leak, so this is the engineer's calculation and not a field judgment. The anchors, the embeds, and the connection back to the structure are designed for the dead load and the project wind pressure by the structural engineer and the system manufacturer. Confirm the anchor against the approved shop drawings and the embed against what was actually cast, because a missing or mislocated embed at the slab edge is the anchorage problem that shows up after the concrete is poured.
Allowing the wall to move
A curtain wall has to move, and the system is designed to let it. Aluminum expands and contracts a lot with temperature, and a long mullion in direct sun grows measurably over a hot day. The building moves too: it sways and drifts under wind, the floors deflect as live load comes and goes, and in a seismic zone the structure displaces between floors. If the wall is locked rigid against all of that, the aluminum buckles, the anchors shear, or the glass breaks.
The movement is absorbed at designed joints. The stack joint between units in a unitized wall is a slip joint that lets each panel move relative to the one above and below. The anchors are slotted so the wall can move vertically as the slab deflects without dragging on the structure. The glazing pocket gives the glass room to move within the frame on its setting blocks and edge clearances, so building movement does not transfer into the glass as a point load and crack it.
Seismic and inter-story drift are their own design case. A wall that has to survive the floor above moving sideways relative to the floor below needs joints sized for that drift, and that is engineering, not field improvisation. The general logic of sizing a gap to real movement and running it continuously through the assembly is the subject of the movement-joint guide; on the curtain wall, the stack joints and slotted anchors are how that logic is built into the skin. Confirm the movement allowance against the engineer's calculation.
Glazing methods: wet, dry, captured, and structural silicone
How the glass seals to the frame comes down to two pairs of choices: wet versus dry, and captured versus structural silicone. Wet glazing uses a gunnable sealant to seal the glass to the frame. Dry glazing uses preformed gaskets that compress against the glass. Many systems use both, a dry gasket on one side and a wet seal on the other, and the spec calls which.
Captured glazing holds the glass mechanically with a pressure plate and cap, with the gaskets or sealant doing the weather seal while the metal holds the glass. It is the more forgiving method, because if a seal fails the glass is still mechanically retained. Structural silicone glazing (SSG) bonds the glass to the frame with structural silicone and no exterior cap, so the silicone is carrying the wind load on the glass back to the frame, not just sealing it. That is a structural bond, designed and sized as structure.
Four-side SSG, with all four edges bonded and no caps anywhere, gives the uninterrupted glass face architects want and relies entirely on the silicone bond to hold the glass. That puts the whole burden on the sealant adhesion, the joint design, and the shop conditions the bond was made under, which is why four-side SSG is usually shop-glazed under controlled conditions rather than field-bonded. The structural silicone design, the bite, and the joint sizing belong to the sealant manufacturer and the engineer, so confirm the SSG design against their documents.
Sealants: structural, weather, and adhesion
Two different sealants do two different jobs on a curtain wall, and confusing them is a real mistake. Structural silicone carries load, bonding the glass to the frame and transferring wind pressure into the structure. Weather-seal sealant keeps water and air out at the perimeter and the joints and carries no structural load. They are formulated differently and they are not interchangeable, no matter how similar the tubes look.
Adhesion is the whole game with sealant, and it is where field failures concentrate. A sealant only works if it bonds to a clean, sound, compatible surface, which means the right cleaning, the right primer where the manufacturer calls for one, and a substrate the sealant is tested to stick to. Silicone will not bond to a dirty or contaminated surface, and it will not bond reliably to some plastics, coatings, and gaskets without a compatibility check first. Putting a structural sealant against an incompatible setting block or gasket can quietly poison the bond.
Joint design matters as much as the chemistry. A sealant joint needs the right width-to-depth ratio and a backer rod or bond breaker so the sealant stretches across the joint instead of being glued on three sides and tearing. The structural bite on an SSG joint is sized by calculation for the wind load and the glass size. Sealant selection, primer, compatibility, and joint design are the sealant manufacturer's call, proven by adhesion and compatibility testing, so follow their data and the project spec.
Thermal performance, U-factor, and condensation
The thermal performance of a curtain wall is rated by its U-factor, the rate of heat flow through the whole assembly, and its solar heat gain coefficient (SHGC), the fraction of solar heat the glass lets through. The energy code sets the maximum U-factor and the SHGC limit for the climate zone and the building type, and the whole-assembly numbers, frame included, are what has to comply, not just the center-of-glass number that looks better on a cut sheet.
The energy lever is the glass and the frame together. The IGU with its low-E coating and gas fill controls most of the glass performance, and the thermal break controls the frame. A great IGU in a frame with no thermal break still performs poorly at the edges and sweats, because the frame is the weak path. The two have to be specified as a system to hit the assembly U-factor the code wants.
Condensation resistance is the other half, measured by the condensation resistance factor (CRF), with a higher number meaning a warmer interior surface that is less likely to condense. It is determined in the AAMA 1503 test, which sets a warm side and a cold side and measures the surface temperatures and the heat flow. In a cold climate, a low CRF means the frame sweats even when the U-factor looks fine on paper. The required U-factor, SHGC, and condensation performance are set by the adopted energy code and the project, so confirm the targets against the code edition and the spec.
Installation: survey, layout, tolerances, and protection
Curtain wall installation starts with a survey of the structure, because the wall is precise and the building is not. The crew shoots the slab edges and the embeds against the control lines to find where the structure actually landed versus where the drawings put it, then sets the anchors to bring the wall back to plumb and true within the system's adjustment range. Skip the survey and you find the out-of-tolerance slab when the panels will not line up, which is the worst time to find it.
Out-of-plumb kills a curtain wall. The whole system depends on the glass hanging in a flat, plumb plane so the gaskets seat, the pockets drain, the panels align at the joints, and the glass is not racked into a parallelogram that loads it in the corner. The adjustable anchors absorb the normal slab tolerance, but they cannot absorb a structure that is out past their range, and forcing it shows up as failed seals and broken glass later. Layout and plumb are the work, not a setup step you rush through.
The sequence and the protection matter through the whole job. Set and anchor, then glaze or set the unitized panels in order, weep and seal as you go, and protect the finished aluminum and glass from the rest of the trades. Aluminum scratches, glass breaks, and weld splatter from the ironworkers etches glass permanently, so the protection is part of the install, not an afterthought. The installation tolerances, the sequence, and the acceptable adjustment range are the system manufacturer's, so build to the approved shop drawings and the manufacturer's instructions.
Perimeter fire-safing at the slab edge
Where a curtain wall passes a fire-rated floor, there is a gap between the back of the wall and the edge of the slab, and that gap is a path for fire and smoke to spread floor to floor unless it is sealed with a perimeter fire-containment system. Because the curtain wall itself is not fire-rated and the floor is, the code requires the joint between them to maintain the floor's rating. The International Building Code addresses this in its perimeter fire-barrier provisions.
The system is safing insulation, usually mineral wool packed into the slab-edge gap and held with steel clips, topped with a firestop smoke seal, and it has to be detailed to move. The point of the test method, ASTM E2307, is that the perimeter fire barrier keeps its seal while the curtain wall in front of it deflects and deforms in a fire, so the safing and the firestop are an engineered, tested assembly, not loose insulation stuffed in the gap.
This is the detail people get wrong because it is hidden behind the spandrel and nobody sees it until there is a fire or an inspection. The mineral wool has to be the right density and depth, compressed the right amount, and continuous along the slab edge, and the smoke seal has to be continuous over it. The perimeter fire-containment system is a tested, listed assembly, so confirm the detail against the tested system and the adopted code edition.
What to record
A curtain wall is a system of decisions that have to be defended years later when a unit fogs, a joint leaks, or a panel has to be replaced. The record is what tells the next person which system went in, what glass is in that opening, how the anchors were set, and whether the wall passed its tests. Tie it to a field tool like FieldOS so the survey, the anchor sign-offs, the glass schedule, the mock-up result, and the field water-test reports live in one place against the actual openings instead of in a binder nobody can find. Capture the anchor sign-off as the work happens, because the anchor you cannot see after the spandrel goes on is the one someone will ask about.
| Element | Requirement | Note to record |
|---|---|---|
| System type and class | Per spec and approved submittal | Stick or unitized; pressure-equalized, water-managed, or face-sealed |
| Glass makeup | Per glass schedule | IGU buildup, coating, tempered or laminated, spandrel locations |
| IGU warranty | Per fabricator | Warranty term and fabricator, recorded by opening |
| Anchors and embeds | Per engineer's drawings | Dead-load and wind anchors set and signed off; embed locations |
| Thermal break and energy | Per energy code and spec | Assembly U-factor, SHGC, and CRF as specified |
| Mock-up result | Per test program | Air, water, structural pass; corrections made |
| Field water test | ASTM E1105 or AAMA 502 | Sections tested, pressures, pass or fail |
| Perimeter fire-safing | Tested ASTM E2307 system | Listed system used; continuous at the slab edge |
Common mistakes
- Relying on a face seal instead of a drained, pressure-equalized system, so the one seal that fails has nowhere to send the water.
- Not tying the curtain wall air and water barrier into the wall barrier, leaving a hole in the envelope at the transition.
- No thermal break in the frame, so the interior of the aluminum sweats and rots the sill in cold weather.
- A wrong or failed IGU: blocked drainage at the edge seal, the wrong makeup for the energy code, or a unit set in standing water.
- No movement allowance, so thermal growth, slab deflection, or seismic drift buckles the frame or breaks the glass.
- Skipping the mock-up and the field water test, then finding the leak on the finished building instead of the test frame.
- Undersized or mislocated anchorage, including a missing slab-edge embed nobody caught until the panels would not hang.
- Blocked or sealed-over weeps that defeat the drainage and turn a drained wall back into a face-sealed one.
Field checklist
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Standards and references
The curtain wall standards come from a few bodies, each for its own piece. AAMA, now part of the Fenestration and Glazing Industry Alliance (FGIA), publishes the curtain wall test methods and the North American Fenestration Standard (NAFS) for rated fenestration products. ASTM publishes the performance test methods. The structural engineer and the system manufacturer own the design that the standards verify.
For performance, the ones that come up are ASTM E283 for air infiltration, ASTM E331 for water penetration under static pressure, ASTM E330 for structural performance under wind load, and ASTM E1105 for the field water test on the installed wall. AAMA 501 covers the mock-up test methods, AAMA 502 the field testing of newly installed fenestration, and AAMA 501.2 the diagnostic hose test for fixed seals. Thermal transmittance and the condensation resistance factor come from AAMA 1503, and IGU durability from the ASTM E2190 family.
For the design itself, the system type, the performance class, and the anchorage are the structural engineer's and the system manufacturer's, proven by the mock-up and the field test, so hedge to their calculations and the test reports rather than a rule of thumb. The safety-glazing locations and the perimeter fire-containment requirement come from the adopted International Building Code edition, and the U-factor, SHGC, and condensation targets from the adopted energy code, both with local amendments. The point that holds across all of it: drain the wall with a pressure-equalized system instead of face-sealing it, tie the air and water barrier into the wall and break the frame thermally, and anchor for load while allowing movement, all proven on a mock-up before the building wears the system. Confirm every number against the project documents and the adopted code.
Units and terms
Curtain wall work mixes structural, glazing, and energy terms, and the same part can read differently across a shop drawing, a glass schedule, and an energy report. These are the ones that carry the meaning.
- Curtain wall
- A non-structural aluminum-and-glass building skin hung off the structure, carrying only its own weight and wind.
- Stick-built vs unitized
- Stick is field-assembled mullion by mullion; unitized is factory-glazed panels craned in and interlocked.
- Storefront vs window wall
- Storefront is the lighter ground-floor system with doors; window wall sits between floors on the slab, while curtain wall hangs past the slab.
- IGU (insulated glass unit)
- Two or more panes with a perimeter spacer and a sealed, usually argon-filled cavity; fails by edge-seal failure and fogging.
- Pressure-equalized rainscreen
- A drained, vented glazing pocket that equalizes air pressure so water is not driven in, then weeps out what gets past the outer seal.
- Thermal break
- A low-conductivity barrier (polyamide or poured-and-debridged resin) in the aluminum frame that cuts heat flow and stops the frame sweating.
- Structural silicone glazing (SSG)
- Glass bonded to the frame with structural silicone that carries the wind load, with no exterior cap; four-side SSG bonds all edges.
- Mock-up and field water test
- A full-size test wall proven for air, water, and structural load before the building, then confirmed on the installed wall by an ASTM E1105 water test.
FAQ
What is a curtain wall?
A curtain wall is a non-structural aluminum-and-glass skin hung off the building structure to keep the weather out. It carries only its own weight and the wind load, not the floors, and passes those loads to the slabs through anchors. The structure holds the building up; the curtain wall just hangs on the outside.
Stick-built vs unitized curtain wall: which is better?
Neither is better outright. Stick-built is cheaper and slower, assembled and glazed in the field with more on-site seals, and suits low-rise and complex shapes. Unitized costs more but goes up fast with factory-glazed panels and better quality control, which makes it the high-rise standard. Schedule, height, and quality risk drive the choice.
What is the difference between curtain wall and window wall?
A curtain wall hangs past the slab edge as one continuous skin, outboard of the floors. A window wall sits between the floors, resting on each slab and dying into the slab above. Curtain wall performs better and needs perimeter fire-safing at every floor; window wall is cheaper and stacks a joint on each slab.
What is structural silicone glazing?
Structural silicone glazing (SSG) bonds the glass to the aluminum frame with structural silicone that carries the wind load back to the frame, with no exterior pressure plate or cap. It gives an uninterrupted glass face. Four-side SSG bonds all four edges and relies entirely on the silicone, so it is usually shop-glazed under controlled conditions.
Why does a curtain wall have weep holes?
Weep holes drain water out of the glazing pockets. A good curtain wall is a pressure-equalized rainscreen, not a face seal, so it lets some water past the outer gaskets into a drained pocket and weeps it back out at the horizontals. Blocked or caulked-over weeps defeat the drainage, and the wall starts to leak.
How is a curtain wall tested for water leaks?
In the lab on a mock-up, water penetration is tested under ASTM E331 with a static pressure and a calibrated spray, often with a dynamic test too. On the installed wall, the field water test runs under ASTM E1105, and AAMA 501.2 is the diagnostic hose test for fixed seals. The spec sets the pressures and pass criteria.
Why is my curtain wall frame sweating?
Condensation on the inside of an aluminum frame means the frame has no thermal break, or a weak one, so the interior face runs cold enough for room air to condense on it. The fix is a frame with a real thermal break, polyamide or poured-and-debridged, sized for the climate. Condensation resistance is rated by the CRF in AAMA 1503.
When does code require tempered or laminated safety glass?
Code requires safety glazing in hazardous locations: in and next to doors, in large panels low to the floor, near walking surfaces and stairs, and in wet areas. The International Building Code sets the triggers and the adopted edition controls. Tempered breaks into small dice; laminated holds its fragments, used where the glass must stay in the opening.
What is a curtain wall mock-up?
A mock-up is a full-size section of the actual curtain wall built on a test frame before the building, run through air, water, and structural tests so the system is proven before thousands of square feet go up. It exposes the corners, intersections, and transitions where curtain walls leak. Fixing a leak there is far cheaper than on the facade.
Why does an insulated glass unit fog up?
An IGU fogs between the panes when its edge seal fails. Moisture works through the seal over years, the desiccant in the spacer saturates, and after that moisture condenses inside the sealed cavity where you cannot wipe it off. The argon fill leaks out the same path, degrading the U-factor. A unit sitting in a wet pocket fails early.
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Codes cited in this guide
This guide is written and reviewed against the published standards below. Always confirm the current adopted edition with the authority having jurisdiction.