How the two differ in kind
The two systems are separated by where they meet the building, and every other difference is downstream of that. A storefront sits inside an opening the structure has already made. Its dead load goes straight down into the floor through the sill, the jambs are fixed to whatever forms the sides of the opening, and the head is anchored to the beam or slab above with a deliberately slotted connection — because the one thing a storefront absolutely must survive is the structure over it deflecting under live load and trying to stand on the glass. That deflection head is the single most important detail in the system, it is the most commonly botched one, and a storefront that has been screwed tight at the head is a cracked lite waiting for the first fully occupied floor above it. A curtain wall does the opposite. It passes in front of the slab edge and hangs off it, continuous past the floor line, carrying no floor load at all and delivering its own weight and the wind on its face back to anchors set at each level. Nothing about that arrangement is available to a storefront, which is why the choice is genuinely binary the moment an elevation is more than one storey tall in one uninterrupted run.
Crossing that floor line is not free, and it is the reason curtain wall carries a cost structure a storefront never sees. A wall anchored at every level has to tolerate everything the structure does between levels: slabs deflecting as the building is fitted out and occupied, columns shortening as concrete creeps, floors racking relative to each other in wind and in a seismic event, and the aluminium itself growing and shrinking over a run that may be many storeys long. That is what the horizontal stack joint at each floor is for, and what the paired anchors are for — one bracket taking gravity plus wind, its partner slotted so the frame can move vertically without prying its own fixings out of the concrete. The gap behind the wall at the slab edge then has to be closed as a rated perimeter joint in its own right, tested as a system with the wall it belongs to. None of that exists on a storefront, which is one storey tall, bears on the floor, and needs its head to move and nothing else.
Where estimators get burnt is the middle ground, because the trade's shorthand for all of this is a height in feet and that shorthand is not what governs. What governs is a mullion's ability to span between its two supports at the design wind pressure for that height and that part of the elevation — and corner zones carry higher pressures than the field of the wall, which is why the same system can be fine across a facade and short at the return. A tall single-storey lobby is one span and by definition not a curtain wall condition, and it can still be well past what an ordinary storefront mullion holds. The usual answer there is a reinforced storefront, with steel tube pushed inside the aluminium until the section works, and that is a legitimate and common solution. It is also the point at which the honest question changes: the reinforcement buys back the span, but it does not buy the drained and pressure-equalised water management, the tested assembly or the anchor engineering that came free with the other system. Somewhere on that path the reader stops saving money and starts paying curtain wall prices for storefront performance, and knowing where that point sits on a given job is worth more than any rule of thumb about storeys.
The factors that actually differ
Show
| Curtain wall | Storefront system | |
|---|---|---|
| How it meets the structure | Passes in front of the slab edge and is anchored back to it, continuous across the floor line. It carries nothing but itself and the wind, and it hands both to brackets at each level — typically a pair per mullion, one taking gravity and wind, the other slotted so it takes wind only. The gap left behind it at every slab edge becomes a rated perimeter fire joint that has to be detailed, installed and inspected as its own scope. | Sits inside an opening the structure already made. Dead load goes down through the sill into the floor, the jambs fix to the sides of the opening, and nothing passes a floor line — so there is no slab edge condition, no perimeter joint at the back of the wall, and no anchor engineering beyond fixing into what is there. |
| The movement it has to absorb | All of it. Slab deflection as floors are loaded, column shortening as the frame creeps, storey-to-storey drift in wind and in an earthquake, and thermal growth accumulated over a multi-storey run. Horizontal stack joints at each floor and slotted anchors are how the system survives a building that never stops moving, and racking performance is something the assembly can be tested for rather than assumed to have. | One joint, and it is the whole game: the deflection head. The beam or slab above will drop under live load, and the head anchorage has to be slotted to take the movement the structural engineer states for that member, with the sealant joint sized to match. Get it right and storefront is untroubled by movement; get it wrong and the frame becomes a very expensive prop. |
| Span, mullion depth and wind | Deep sections designed to span floor to floor at the design pressure for their height, with the pressure rising as the building gets taller and rising again in the corner zones of the elevation. The depth is the point of the system, and the span chart is where the engineering starts rather than where it stops. | Shallow sections, and the span limit arrives fast. Beyond it the standard move is to reinforce — steel tube inside the aluminium until the section works — which is common, legitimate, and worth watching, because a heavily reinforced storefront costs real money and still has a storefront's water management and a storefront's test data behind it. |
| Water and air management | Designed as a drained system: glazing pockets drained and weeped at every horizontal, internal gutters routing water back out, joinery detailed on the assumption that some water WILL get past the outer line, and pressure equalisation used to stop it being driven inward in the first place. Assemblies are tested to the standard air, water and structural methods, and on larger jobs a performance mock-up is built and tested before production starts. | A simpler, sealant-dependent system, generally weeped at the sill and reliant on the face seal doing its job. Ratings are lower and the performance target is usually taken from the manufacturer's published figures rather than a project-specific mock-up. That is entirely adequate over a lobby with a hard floor and a drain in it, and it is the wrong system in front of a finished, occupied, carpeted space. |
| Where it gets assembled | Two routes with different economics. Stick systems are built up on site, mullion by mullion, and glazed in place — usually needing exterior access. Unitised systems arrive as complete factory-glazed panels that are landed on their anchors from inside the floor plate, which removes the swing stages, moves the labour and the quality control into a shop, and makes the facade go weathertight fast enough to release the interior trades. | Always stick-built on site by a glazing crew, cut and screw-splined and glazed in the opening. Many storefront systems can be glazed from the inside, which at grade means no scaffold, no lift and no exterior access at all — a genuine programme advantage on a tight pavement. |
| When the decision becomes irreversible | Early, and much earlier than the glass. Anchors want cast-in embeds or channels set with the concrete, which means the facade geometry is committed to the structural package months before a panel arrives. Delegated engineering, shop drawings and — on a unitised job — the factory run all sit ahead of that too. | Late. Storefront is field-measured into an opening that already exists, so built tolerances are absorbed by the survey rather than designed around, and a change to a division or a lite size is a re-measure and a re-order rather than a structural conversation. |
| Entrance doors | Not its native job. Curtain wall systems have door options, but the standard commercial entrance package — stile-and-rail doors, thresholds, closers, panic hardware, all with a long service life of daily abuse behind it — belongs to the storefront product line, so entrances in a curtain wall are usually a storefront or door frame set into an opening in it. | The system entrances are built from. Door jambs, thresholds and hardware are catalogue parts of the same line as the framing, which is a substantial part of why ground-floor retail and lobby elevations are storefront in the first place. |
| What the cost scales with | Glazed area, plus a fixed package that a small elevation cannot dilute: delegated design, calculations, shop drawings, embeds coordinated into someone else's concrete pour, and often a tested mock-up. Unitising shifts labour off the site and into a factory, which pays back across enough repeating modules to absorb the shop setup and nowhere near as well across a handful. | Framing length and the number of lites — perimeter and divisions rather than area — and it is dominated by site labour. Every intermediate mullion the layout needs is more cut framing, more sealed joints and more crew hours, which is why tightening a maximum lite width costs noticeably more and making a lite larger within the system's limits costs very little. |
| Cost of changing it later | High, and it starts accruing before anything is glazed. Once embeds are cast the elevation is largely fixed, and a change on a unitised job means changed panels coming out of a production run rather than a different cut on site. | Low. The system is site-measured, stick-built and made of catalogue lengths, so an altered opening, a moved door or a revised division is re-measured and re-ordered without touching the structure. |
Which one, and when
Choose curtain wall when…
- One continuous run of framing has to cross a floor line. This is not a preference — a storefront has no way to be anchored at a slab edge, no stack joint and no path for its own dead load past that floor, so above one storey there is no storefront answer at any budget.
- The span and the design wind pressure have already beaten the storefront span chart, including at the corner zones where the pressure is higher than across the field of the wall, and reinforcing the section is no longer a cheap fix.
- What sits behind the glass cannot tolerate water. A drained, pressure-equalised, tested assembly is the system that expects to be wet inside and is built to get rid of it — the sensible answer in front of finished, occupied space.
- The structure will move enough to matter: real storey drift, seismic demand, a frame that will creep and deflect over the height of the elevation.
- There are enough repeating modules to make unitising worth it, and the programme needs the building weathertight quickly so interior trades can start behind the facade.
Choose storefront system when…
- A single-storey opening within the structure — the framing bears on the floor, is restrained at the head, and never passes a slab edge.
- Entrances are the point of the elevation. Doors, thresholds and hardware are native parts of the system rather than something set into a hole cut in a different one.
- The opening will exist before the glazing is ordered, and field measurement is the sane way to absorb whatever the concrete and blockwork actually gave you.
- Behind the glass is a lobby, vestibule or shell retail unit with a hard floor and drainage — a place where the storefront's simpler water management is genuinely appropriate rather than merely cheaper.
- The job cannot carry delegated engineering, cast-in embeds and a mock-up programme for one elevation, and does not need to.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Where exactly is the line between them?
- Ask three questions in this order and the answer usually falls out. First: does one continuous run of framing have to cross a floor line? If yes, stop — it is a curtain wall, because only a curtain wall passes the slab edge, hangs off it, joints at each floor and firestops behind itself. That question is binary and no budget changes it. If no, ask the second: does the mullion hold the span between its supports at the design wind pressure for that height and that zone of the elevation, remembering that corners see more pressure than the middle of a facade? That is a span chart question, not a rule-of-thumb question, and it is where a tall lobby usually gets interesting. Third: what gets ruined if water gets past the outer seal? A drained and pressure-equalised assembly is designed on the assumption that water will get in and must be routed back out; a face-sealed storefront is not, and the honest test is what is on the other side of the glass. Height in feet, the way the trade normally states this, is a rough proxy for the second question and tells you nothing about the first or the third.
- Can a storefront system go up a tall two-storey lobby?
- Often, and it is a fair question rather than a cheat, because a two-storey lobby with no floor plate crossing it is a single span — the floor-line test simply does not apply. What applies is the span chart, and an ordinary storefront mullion will usually be short. The standard answer is reinforcement: steel tube inserted inside the aluminium, sized until the section carries the design pressure over the full height, with the anchorage at head and sill designed to match the larger reactions that come with it. Manufacturers publish this and it is done constantly. The thing worth checking before committing is what you have and have not bought. The reinforcement restores the span and nothing else — the water management, the drainage detailing and the test data are still the storefront's, and the reinforced framing, the heavier anchors and the engineering to justify them are no longer cheap. Some manufacturers sell heavier front-set systems specifically for this condition that sit between the two product families. Price the reinforced storefront and the single-storey curtain wall against each other rather than assuming the storefront wins because it started cheaper.
- Which one costs less?
- They have different cost shapes, which is more useful than a figure and is the only honest thing to give you. Storefront is site-labour-heavy and its cost follows framing length and lite count — perimeter and divisions, not glazed area. Every extra intermediate mullion is more cut and sealed framing and more crew hours, which is why tightening a maximum lite width shows up in the price and enlarging a lite inside the system's limits barely does. Curtain wall has two parts that behave differently: one scales with glazed area in a way a small job cannot avoid, and the other is a fixed package that a small job cannot dilute either — delegated engineering, calculations, shop drawings, embeds coordinated into the concrete package and, on larger work, a mock-up built and tested before production. Unitising is not a discount; it moves labour into a factory, and that shop setup has to be amortised across enough repeating units to be worth it. Then there is the ledger nobody prices at tender: a storefront is measured after the opening exists and changed by re-ordering catalogue lengths, while a curtain wall is committed the day its embeds are cast. The calculators here will each take off one quantity — a unitised panel count on one side, an interior mullion count on the other — and neither is a bill of materials, so price each one against your own supply chain rather than reading the two outputs as comparable totals.
- What is window wall, and is it a third option?
- Yes, and on residential and hotel work it is frequently the actual answer rather than either system on this page. Window wall spans slab to slab but does NOT pass in front of the slab edge — each storey's frame bears on the floor below and is restrained under the floor above, so it is installed floor by floor without exterior access and without the anchor engineering a hung wall needs. What it gives up is the continuous outer plane: the slab edge is left exposed and needs its own cover and insulation, every floor line is a horizontal joint on the outside of the building, and acoustic and thermal performance at that edge takes work to recover. It is cheaper and simpler than curtain wall and it goes higher than storefront, which is exactly why it dominates residential towers and is rare on commercial offices where the facade is doing an architectural job.
- Can both appear on the same elevation?
- Constantly, and it is the normal answer for a commercial building rather than a compromise: storefront at grade where the entrances, the daily abuse and the field measurement are, curtain wall above where the wall has to cross floor lines. What deserves attention is the joint between them. The two systems move differently, are anchored to different things, drain differently and — critically — are very often supplied by different subcontractors on different packages. That transition is where water finds its way in and where responsibility is easiest to lose, so it wants a detail drawn by someone, a named party owning it, and the drainage from the system above given somewhere deliberate to go rather than being allowed to arrive on the head of the system below.
- What do these two calculators actually tell me?
- Quantities for a layout you have already decided, and nothing more — worth saying plainly, because neither is a design tool. The panel count is a unitised curtain wall tool: it divides total facade area by the area of one typical unit and rounds up to a whole panel, so it answers nothing on a stick-built wall, which is bought as framing lengths and glass rather than as panels. It also says itself that corner units, custom-sized panels at edges and setbacks, and openings for louvres and doors are not included; those come off the elevations and get counted separately. The mullion count takes one straight opening width, divides it by your maximum lite width, rounds up to a whole number of equal lites and subtracts one, because the two ends of the opening are bounded by jambs rather than by interior mullions — and it excludes corner mullions and door jambs for the same reason. Neither output says anything about whether the mullion spans, what the anchors carry, or whether the assembly will pass a water test. Use them for takeoff and pricing, and take the span, the anchorage and the performance from the manufacturer's data and the project engineer.