Key takeaways<\/strong><\/p>\n You can name the system from a head detail in about ten seconds, and every cost and performance difference later in this post follows from that one fact. Both belong to the same family of aluminum facade systems<\/a>, which is exactly why they get confused in a tender package.<\/p>\n Curtain wall is a non-load-bearing facade hung from the structure. Dead load and wind load travel through anchors and brackets into the slab edge or the structural frame, and the facade runs continuously past the floor slabs. Window wall is infill: one unit fills the opening between two slabs, dead load bears on the sill or the slab below, the head anchors to the slab above, and one unit equals one floor height. Suppliers also sell it as slab-to-slab or infill glazing, so three names describe one product.<\/p>\n Two consequences fall out immediately. First, window wall inherits the as-built tolerances of the slabs, so dimensional error and movement both land in a head joint and a sill. Curtain wall is set out from a survey control line with adjustable anchors, which puts structural variation into the bracket rather than into a bead of sealant. Second, window wall is a series of framed openings, so the slab edge becomes a hard line in the elevation. Curtain wall puts its horizontals wherever the design wants them, independent of where the structure sits.<\/p>\n Then there is storefront. It is a third product, not a budget version of these two: a low-rise, punched-opening system, lighter in profile, with no inter-floor movement capacity. Buyers who quote storefront against a curtain wall number often wonder why the figure looks wrong. It looks wrong because the products are not comparable.<\/p>\n The conflict to watch for in a live tender is quieter. The drawings show a continuous plane, the priced alternate is a window wall dressed with cover caps, and the swap happens at submittal stage when nobody is comparing elevations any more.<\/p>\n Pick the elevation before you pick the system, because it is the one decision here you cannot reverse cheaply once the structural drawings are issued.<\/p>\n Curtain wall delivers a single plane. Spandrel glass or metal panels align to the design grid rather than to the slab, so the floor line can disappear completely. Window wall produces a horizontal break at every floor. In a banded residential block that break is an asset. On a tower sold to the client as one glass plane, it is a problem no amount of sealant solves.<\/p>\n You can dress a window wall with continuous vertical mullion covers, a shadow-box reveal, and a spandrel band wide enough to swallow the slab. What you have then is a disguise, and you have paid for it: extra profiles, extra sealed joints, extra site labor. That is the cost advantage you chose window wall for, spent on pretending to be the other system. If the elevation genuinely needs the continuous plane, specify curtain wall and price it honestly.<\/p>\n Two geometric factors decide how loud the break reads. The vision-to-spandrel ratio and the floor-to-floor height. A shallow spandrel puts the slab line at eye level from the street, which is the worst place for it. A deep spandrel reads as a deliberate band.<\/p>\n Program pushes the decision too. Balconies, punched windows and stepping floor plates suit window wall. A commercial tower with one vision band from lobby to roof suits curtain wall. Height on its own does not rule window wall out; it is the standard residential high-rise system in many markets. What rules it out is a movement or appearance requirement it cannot meet, not the storey count.<\/p>\n Three movements decide whether a facade leaks in year two, and usually only one of them is written down anywhere in the specification.<\/p>\n Take a tower where the structural spec states a drift limit but the facade spec states nothing. Three bidders price three different head joints, all technically compliant, and the one that wins on price is the one with the least movement capacity. That gap opens in service, not on the drawing.<\/p>\n Write drift and deflection values into the performance spec and require the fabricator to submit the joint detail that accommodates them. That single requirement prevents most of the argument at mockup stage. Name the tests as well: AAMA 501.4 for interstory drift and ASTM E330 for structural wind load, with test pressures derived from the project wind load rather than a catalog default. On system performance, APEX runs Structural Wind Load testing for strength, deformation limits and stability, which is the evidence a fabricator should be able to show at your project pressure, not at a generic tested size. If you want the input side of that calculation checked, see curtain wall wind load calculations<\/a>.<\/p>\n Water almost never enters through the glass. It enters at a horizontal joint, and on both systems that joint is at the slab.<\/p>\n Ask the supplier for the head and sill detail at the slab in writing, with the name of who seals it and who warrants it. A supplier who will not put that on paper has not solved it. The failure modes are set out one by one in curtain wall water leakage<\/a>, and they usually trace back to a joint like this rather than to a faulty unit.<\/p>\n Two tests separate a real system from a catalog claim: ASTM E331 for static water penetration and AAMA 501.1 for dynamic water penetration under wind-driven rain, both at a pressure derived from the project wind load. APEX runs Water Penetration testing for leakage resistance under wind-driven rain and Air Infiltration testing for air leakage control, which are the two numbers most likely to be missing from a bid package.<\/p>\n The U-value on the datasheet will not tell you how the building performs. The junction at the slab will.<\/p>\n Continuous mullions give curtain wall the chance to run a thermal break past the slab without breaking the frame. The interruption then moves to the anchor bracket, which has to be isolated deliberately or it becomes the thermal bridge you spent the budget on avoiding. Window wall is interrupted by the slab at every floor by definition. Each head and each sill is a linear bridge, and each one is also a flanking path for sound.<\/p>\n Both systems can hit the same target U-value on paper. The difference is how much of that performance depends on site-installed work that gets covered up, versus factory-assembled work that was tested before it shipped. Three places to look in the submittal: the insulated spandrel build-up, the thermal break inside the sill profile, and isolation pads at the brackets. If any of those is deferred to site, you are buying a drawing of a performance, not the performance.<\/p>\n On acoustics, window wall has two junctions per floor to seal instead of one continuous frame, so flanking risk concentrates at the slab edge rather than in the glazing. Take a hotel beside a traffic corridor: the detail to test is the head joint at the slab, not the glass specification, because a well-sealed window wall with ordinary glazing will usually beat a well-glazed curtain wall with a poorly sealed perimeter.<\/p>\n For hospitality and residential work, put the slab junction in the specification as a single owned and warranted item. Name the party responsible for the head and sill detail, require the facade contractor and the waterproofing trade to sign the same drawing before installation starts, and make the warranty cover the junction itself and not only the unit. If no name appears on that line, the risk stays with you once the ceiling closes.<\/p>\n Fire containment is the one item in this comparison that fails invisibly, and the geometry of the two systems decides how hard it is to inspect.<\/p>\n Curtain wall creates a continuous cavity between the spandrel and the slab edge, running the full height of the building. It needs a perimeter fire barrier at every floor, and that barrier is normally hidden behind the spandrel panel. Window wall sits on the slab, so the perimeter barrier sits at the slab edge and stays reachable and inspectable from inside the floor.<\/p>\n The reference test for the perimeter joint is ASTM E2307. The required rating and acceptance criteria come from the local code and vary by jurisdiction, building type and height, so the number has to come from the authority having jurisdiction, not from a supplier brochure. This is one of the areas where a generic international answer is worth less than a local one. Our own breakdown of fire safety ratings for aluminum curtain walls<\/a> covers how those requirements are usually structured.<\/p>\n Spandrel panels and the insulation behind them have to meet the combustibility limits the code sets for the wall assembly. Whether that means a mineral-core panel or an aluminium composite is a code question first and a price question second.<\/p>\n If the barrier ends up behind a spandrel nobody can see, add it to the pre-close inspection list with a photograph requirement per floor. Ask the fabricator how the system terminates at the slab and which tested assembly the barrier detail comes from. A tested perimeter joint detail is worth more than a fire rating quoted for a panel on its own.<\/p>\n A per-square-metre comparison between these two systems is only honest when both produce the same elevation. Four drivers do the real separating.<\/p>\n\n
Where the Weight Goes: Reading the Load Path Off a Drawing<\/h2>\n
![\"Adjustable](\"https:\/\/apexecobuilt.com\/wp-content\/uploads\/2026\/10\/curtain-wall-vs-window-wall-spec-decision-inline-1.jpg\")
What the Elevation Commits You To<\/h2>\n
Movement and Tolerance: The Numbers That Have to Be in the Spec<\/h2>\n
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![\"Interstory](\"https:\/\/apexecobuilt.com\/wp-content\/uploads\/2026\/10\/curtain-wall-vs-window-wall-spec-decision-inline-2.jpg\")
Water: The Slab Edge Is Where Both Systems Get Hurt<\/h2>\n
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![\"Window](\"https:\/\/apexecobuilt.com\/wp-content\/uploads\/2026\/10\/curtain-wall-vs-window-wall-spec-decision-inline-3.jpg\")
Thermal and Acoustic Performance: Which Junction Decides It<\/h2>\n
Fire Containment: Where the Concealed Cavity Sits<\/h2>\n
Cost and Lead Time: Four Drivers, Not One Price Per Square Metre<\/h2>\n
![\"Aluminium](\"https:\/\/apexecobuilt.com\/wp-content\/uploads\/2026\/10\/curtain-wall-vs-window-wall-spec-decision-inline-4.jpg\")