How to Specify a Curtain Wall System: A Complete Guide for Architects

Choose the curtain wall system type first

The stick-versus-unitised decision drives cost, programme, quality control and the tolerances you can realistically hold, so make it before you write a single performance clause. Getting this wrong forces expensive redesign once tenders are already in, because the section sizes, sealing strategy and crane logistics all flow from it.

• Stick systems: assembled mullion-by-mullion and glazed on site. They carry a lower tooling cost, suit low-rise, irregular grids and small quantities, and give the design team more flexibility late in the programme. The trade-off is more site labour and weather-dependent sealing quality, since gaskets and wet seals are applied in the open.
• Unitised systems: factory-assembled storey-height panels with interlocking split mullions that stack and interlock on site. They erect fast, carry factory-controlled weatherseals, and are ideal for high-rise towers and repetitive grids. The trade-off is higher tooling cost and a longer design and procurement lead time before the first panel arrives.
• Semi-unitised and structural glazing variants sit between these two poles. If you want a flush, gasket-free external face with silicone-bonded glass, specify our structural glazing approach explicitly in the tender rather than leaving the construction method open to the bidder's interpretation.

State the grid module, the floor-to-floor height, and whether spandrel zones are back-pan insulated or shadow-box, because these govern the section design and the visible sightline. As a rough Hyderabad benchmark, stick systems suit projects up to roughly G+4, while unitised construction earns its tooling premium above G+8, where crane time, repetition and reduced scaffold dominate the cost equation.

Set the structural performance criteria to IS 875

Curtain wall structural performance always starts with wind. Derive the design wind pressure from IS 875 Part 3 using the building's basic wind speed, terrain category, height and topography factors, then quote the resulting positive and negative design pressures in Pascals. A single nominal figure is not enough, because suction at corners and parapets is what pulls panels off buildings.

• Wind load: Hyderabad and Secunderabad fall in a basic wind speed zone of roughly 44 m/s, a moderate zone, but tall or exposed sites still generate significant negative (suction) pressures at corners, parapets and podium edges. State the design pressure and require the system to be both engineered and physically tested to it.
• Deflection: limit mullion deflection under design wind to L/175 or 19 mm (whichever is less) for spans up to 4.1 m; tighten to L/240 where the facade carries stone, terracotta or other fragile finishes that crack before the aluminium yields.
• Glass deflection and edge engagement: require a minimum edge bite and correctly positioned setting blocks per the glass processor's recommendation, typically 12-15 mm bite for insulated glass units, so the glass stays captured under full load and thermal movement.
• Movement budget: specify the differential slab movement, live-load deflection and thermal movement the system must absorb without transferring stress into the glass, gaskets or structural silicone. Aluminium moves noticeably across a Telangana summer-to-monsoon temperature swing.
• Seismic and inter-storey drift: state the drift the facade must accommodate where the structural engineer provides it, consistent with NBC 2016 and IS 1893, so anchors and stack joints are designed to slide rather than crush.

The load path ultimately runs from glass into transoms and mullions, then into brackets and back into the RCC slab. Every element in that chain must be rated for the same design pressure, and the anchor bracket is where you build in the adjustability to absorb real-world concrete tolerances.

Specify thermal, optical and acoustic values for Hyderabad's climate

Give the thermal and optical numbers as performance targets the vision and spandrel assemblies must meet, not a single named glass product. That way processors can offer compliant alternatives, you keep the pricing competitive, and you are not locked into one supplier's availability window.

• U-value: for Hyderabad's hot composite climate, target an overall vision U-value around 1.8-2.0 W/m2K with a thermally broken aluminium frame (a polyamide strip that separates inner and outer profiles) to meet the Energy Conservation Building Code (ECBC).
• SHGC: target roughly 0.25-0.27 for vision glazing to control the solar cooling load and support ECBC compliance and IGBC, GRIHA or LEED green-rating credits. Solar heat gain, not conducted heat, is the dominant load on a glazed facade in this climate.
• VLT: specify visible light transmission (commonly 0.40-0.60) balanced against SHGC through a good light-to-solar-gain ratio, and check glare on occupied floors facing east and west, where low sun angles cause the most complaints.
• Acoustic Rw: state a weighted sound reduction index (for example Rw 35-40 dB) where the site sits near arterial roads, the Outer Ring Road, or Rajiv Gandhi International Airport at Shamshabad; this drives the glass make-up, the interlayer choice and the interlock design.
• Reference glass to IS 2553 and require processor test certificates for thermal and optical performance before production release. High-performance double-silver or triple-silver coated units add roughly Rs 900-1,600 per sqm over a standard low-E insulated glass unit but repay it quickly through reduced HVAC capacity and running cost across a long Telangana cooling season.

Weather performance and the mandatory testing regime

Weathertightness is where curtain wall specifications most often go soft. Anchor it to named test methods with explicit numerical pass criteria, so the tender is enforceable rather than aspirational, and a bidder cannot claim compliance without evidence.

• Air infiltration: to ASTM E283, with a stated maximum leakage rate at a defined test pressure.
• Static water penetration: to ASTM E331, with no uncontrolled water at a test pressure that is a defined fraction (commonly 15-20 percent) of the design wind pressure.
• Dynamic water and structural: dynamic water to AAMA 501.1 where wind-driven monsoon rain is a real risk, and structural load to ASTM E330 at both design and 1.5x design pressure to prove the safety margin.
• Design philosophy: require a pressure-equalised, drained-and-vented rainscreen design rather than face-sealed reliance on a bead of external silicone, which is the first thing to fail under Hyderabad's combination of intense UV and daily thermal cycling.
• Mock-ups: call for a project-specific mock-up, both visual and performance, for facades above a threshold area, tested at an accredited laboratory before production is released. A full performance mock-up in India typically costs Rs 6-12 lakh, but it protects a facade package worth many crores and catches interface defects on the ground rather than at height.

Name these tests and their pass criteria in the schedule from day one. If you want testing scope and pricing benchmarks tailored to your specific building, get a free quote and we will map the regime to the facade area and exposure.

Select framing, glass and fixings that match the spec

Once performance is set, the framing and hardware must be specified to deliver it, not quietly value-engineered down after award. This is the stage where component knowledge matters as much as headline facade engineering, because the cheapest-looking substitution is usually the one that leaks or corrodes first.

• Aluminium: specify the alloy (typically 6063-T6), the thermal break type (polyamide strip), and the finish grade, whether powder-coat to a minimum film thickness or anodised to a stated micron class. Call up the framing system by a tested, named series rather than by a look-alike section, so the tested performance data actually applies.
• Glass: name the substrate, the coating surface position, the toughening or heat-strengthening spec and the interlayer for laminated units. Require heat-soak testing for all toughened glass to control the risk of spontaneous breakage from nickel-sulphide inclusions, which matters on any tall or hard-to-access facade.
• Fixings and accessories: specify the stainless-steel grade for external fasteners (typically 304 or 316 depending on exposure) and require compatible EPDM gaskets and structural silicone drawn from a single tested, warranted system rather than mixed sources.
• Doors and openings: where doors, vents or louvres are integrated into the curtain wall line, coordinate their hardware and framing interface early so the mullion accepts them without cutting or improvising on site.

Getting the specified components onto the delivery programme, rather than watching them get substituted at the last minute, is a large part of why single-source fabrication and installation reduces risk. Browse the full scope of what we fabricate and install through our services.

Detail the interfaces and tolerances explicitly

Curtain walls rarely fail in the middle of a panel; they fail at heads, sills, jambs, slab edges and the joints to other trades. Specify these explicitly or expect them to be improvised on site by whoever is holding the sealant gun that day.

• Slab edge and bracket: define the setting-out datum, the movement budget for slab deviation, and adjustable three-way brackets that let installers absorb the real construction tolerances of an RCC frame in all three axes.
• Perimeter fire and smoke: specify a continuous firestop and smoke seal at each floor slab edge to maintain compartmentation between storeys, tested and installed as a complete system rather than stuffed in as an afterthought.
• Trade interfaces: detail every junction to roof parapet, soffit, RCC and blockwork, doors, louvres and canopies with defined seals, backer rods and tolerances, so the boundary of the facade package is unambiguous.
• Tolerances: state the permissible deviation for line, level and plumb, and require the fabricator's tolerances to reconcile with the primary structure's construction tolerances. The two documents must add up, or the gap gets closed with silicone on site.
• Warranties: specify system, sealant and IGU warranties (commonly 10 years for IGU edge seal, and 5-10 years for structural silicone) and require compatible, tested sealant systems so a warranty claim is not defeated by a chemical incompatibility.

Reconciling facade tolerances against a real, as-built RCC frame is exactly the kind of buildability review that belongs in the design-assist phase. Doing it early, before the first bracket is cast in, is far cheaper than discovering a bust datum at the twentieth floor.

Understand curtain wall costs and how the spec affects price

A tight specification does not only improve quality; it makes competing tenders comparable and controls cost. A vague spec invites the lowest bidder to price the cheapest possible interpretation, which you then pay to correct through variations and remediation later.

• Baseline: a performance-tested curtain wall in Hyderabad and Secunderabad typically runs Rs 6,500-9,500 per sqm supplied and installed for a standard unitised or stick system with low-E double glazing and a mid-range finish.
• Upgrades: high-performance coated glass, wider sightlines, feature fins or acoustic laminated units can push this to Rs 11,000-16,000 per sqm, so name every upgrade in the tender rather than letting bidders assume the base case.
• Point-supported and spider glazing for lobbies, atria and double-height entrances is priced separately and depends heavily on the structural glazing detail and the fitting count, which can vary the rate substantially.
• Line items: testing, mock-ups and design-assist are worth naming explicitly in the bill of quantities, so they are not stripped out by the lowest bidder and then reintroduced as costly variations once the package is awarded.

Across a full facade package, a well-drafted specification usually pays for itself several times over by eliminating claims, rework and water-ingress remediation, which are the three biggest sources of facade cost overrun on Indian projects.

Why work with a local Hyderabad facade specialist

Specifying a curtain wall on paper is only half the job; delivering it needs a partner who fabricates, installs and stands behind the interfaces once the drawings become a building. Working with a local specialist shortens the loop between what is drawn and what is built, and keeps accountability in one place.

• Design-assist: early review of the grid, brackets and interfaces against your actual structural frame keeps the specification buildable and flags clashes while they are still cheap to fix on paper.
• Local supply chain and workmanship: serving Hyderabad, Secunderabad and the wider Telangana and Andhra Pradesh market means the specified system and hardware stay in the supply chain rather than being substituted on site under programme pressure.
• Single accountability: fabrication, glazing and installation under one specialist removes the finger-pointing that plagues split-package facades when water finds its way in and nobody owns the joint.

Hakimi Aluminium and Glass provides design-assist, shop drawings, fabrication and installation for architects and developers across Hyderabad, Secunderabad, Telangana and Andhra Pradesh, and can review your interface details early to keep a specification both buildable and enforceable. When you are ready to price a facade against a real specification, get a free quote and we will benchmark it for your project.