A facade specification should be written as measurable performance criteria the specialist must prove, not just a shopping list of products. State what the envelope must achieve - thermal (U-value, SHGC, VLT), structural (design wind pressure, deflection), weather-tightness (air infiltration and water penetration), acoustic (Rw) and safety - and how each will be verified by test, then leave the specialist facade engineer room to size the sections, gaskets and anchorage. That is the single decision that separates a specification that ranks against tenders on a like-for-like basis from one that unravels on site.
The most common failures we see on drawings are silent gaps: no design wind pressure stated, no deflection limit, no water-test standard, and interfaces left to 'coordinate on site'. In Hyderabad's climate - 40°C-plus summers around Gachibowli and Kokapet, driving monsoon rain, and a fine construction dust that clogs drainage - those gaps become leaks, gasket roll-out and warranty disputes within two seasons. Over 25-plus years of service life, the cost of an ambiguous clause is paid many times over.
This guide walks the clauses that matter - performance, standards, tolerances, interfaces, submittals and testing - with realistic Telangana context and INR benchmarks, so your glazing specification is unambiguous, tenderable and enforceable once the system is installed.
What should a facade specification include?
A complete facade specification answers six questions in order: what performance must the envelope achieve, which codes govern it, what tolerances and movement must it absorb, how do the interfaces work, what must the specialist submit, and how will it be tested and warranted. Miss any one and the tender becomes non-comparable or the acceptance stage becomes a negotiation.
- Performance criteria: thermal, structural, air/water, acoustic and safety numbers, each with a governing standard and test method.
- Referenced standards: IS, NBC, ECBC and ASTM/AAMA methods, cited by number and edition.
- Tolerances and movement: deflection limits, thermal movement, and the building-frame tolerances the facade must absorb.
- Interfaces: every junction - head, sill, jamb, slab edge, parapet, roof and MEP penetration - detailed as a watertight condition.
- Submittals and testing: calculations, shop drawings, mock-ups, site water tests and warranties as contractual gates.
Whether you are specifying a structural glazing curtain wall for a Financial District tower or a modest office glass front, the skeleton is the same - only the numbers scale.
Performance criteria: the numbers that define the facade
State each criterion as a target with the governing standard and test method. Vague adjectives ('high performance', 'weatherproof') are unenforceable and cannot be priced.
- Thermal: glass U-value (W/m²K) and system U-value, plus SHGC and VLT. In Hyderabad's cooling-dominated climate a low SHGC (often 0.25-0.35) matters more than U-value; balance it against VLT for daylight and ECBC compliance. High-performance DGU facade units with a soft-coat low-E on surface 2 are the usual route to these numbers.
- Structural: design wind pressure derived per IS 875 (Part 3) for the specific site, height, terrain category and topography - state the value (kPa) or require the specialist to submit stamped calculations.
- Air infiltration and water penetration: reference ASTM E283 (air) and ASTM E331 / AAMA laboratory limits, and specify a site water-test acceptance clause.
- Acoustic: weighted sound reduction index Rw (dB) where the site fronts traffic or rail - relevant along the ORR service roads and near the Hitec City MMTS corridor. Specify the field or laboratory basis, and consider laminated acoustic glass build-ups to reach it.
- Safety glass: specify heat-strengthened or fully toughened glass, and laminated glass for overhead, balustrade and human-impact locations, per IS 2553 and NBC 2016.
How do I set SHGC and U-value for a Hyderabad project?
Start from the ECBC envelope thresholds for the warm-and-humid / composite zone that covers Telangana, then let cooling load drive the priority. Because air-conditioning dominates energy use here, solar heat gain through glass - not conductive U-value - is usually the decisive number.
- Target SHGC first: 0.25-0.35 for most commercial elevations in Madhapur, Kondapur and Kokapet; go lower on unshaded west and south faces.
- Retain VLT for daylight: aim for a SHGC-to-VLT ratio (light-to-solar-gain, LSG) above 1.25 so you get daylight without the heat - a reflective glass facade or high-selectivity low-E coating achieves this.
- Cross-check green credits: if the project targets IGBC, GRIHA or LEED, confirm the glass make-up satisfies the daylight and energy credits before locking the spec.
- Don't over-tint: a very low VLT saves cooling but forces daytime artificial lighting, which can erase the saving. Model it, don't guess.
As a rough benchmark, high-performance DGU curtain-wall glazing in Hyderabad runs roughly INR 1,600-3,000 per sq ft supplied-and-installed depending on coating, spacer and framing; premium unitised systems sit higher. State the performance and let that drive the glass, not the other way round.
Cite the standards by name
Anchor the specification in codes so tender responses are comparable and compliance is auditable. Naming the standard removes the argument later.
- IS 875 (Part 3) for wind loads; NBC 2016 for overall building and safety provisions, including glazing and fire.
- IS 2553 for safety glass, plus the relevant IS standards for aluminium alloys, sealants and hardware.
- ECBC for envelope energy performance (SHGC, U-value and VLT thresholds by climate zone).
- ASTM / AAMA test methods for air (E283), water (E331 lab, E1105 site) and structural performance where Indian equivalents are not specified.
- IGBC, GRIHA or LEED where the project targets green certification - align facade SHGC, VLT and daylight metrics with the rating's credits.
Only cite a code number you are certain of; where unsure, name the standard and require the specialist to confirm the current edition. A short pre-tender facade consultancy review can close these gaps before the document goes out.
Tolerances, deflection and movement
This is the clause most often missing - and the one that decides whether the facade cracks or leaks. Three different movements meet at the envelope, and each needs its own limit.
- Deflection: limit framing members to L/175 or 19 mm (whichever is less) under design wind load for glass-supporting mullions; state the limit explicitly rather than assuming the specialist will.
- Thermal movement: require the system to accommodate expansion of aluminium and glass across the site's temperature swing - a dark mullion on a west face in a Hyderabad May can exceed 70°C - without stress, gasket roll-out or seal failure.
- Building tolerances: state the permissible deviation of the primary RCC structure (slab edge, columns) that the facade must absorb, and require anchors with three-axis adjustment.
- Fabrication and installation tolerances: verticality, level and joint-width limits, so shop drawings and site work are held to the same numbers.
Where the design uses spider glazing or cable-net glazing, movement is even more critical because the glass itself carries load - specify the maximum panel movement and the fitting's rotational allowance.
Why do facades fail at the interfaces?
The facade rarely fails in the middle of a panel; it fails at junctions where two systems, two trades or two tolerances meet. Specify each interface as a detailed, watertight condition rather than a note to coordinate later.
- Head, sill, jamb and slab-edge conditions, including continuity of the air and water barrier and the thermal break across each junction.
- Facade-to-roof, parapet and coping junctions, and transitions to RCC, masonry or dry ACP cladding.
- Movement joints, fire-stopping and smoke-seals at each floor slab - the perimeter fire barrier is a code requirement under NBC 2016, not a detail to omit.
- Integration of shading fins, spandrel zones, aluminium louvers and any embedded MEP penetrations, each sealed and flashed.
- Waterproofing overlap sequence and the required flashing/membrane laps, drawn as a sequence so the site team installs them in the right order.
In Hyderabad's monsoon, wind-driven rain finds every unsealed lap; the interface clause is where a specification earns its keep. See how these details resolve in built work on our projects page.
Submittals, testing and quality gates
Define what the specialist must submit and what will be tested before and during installation - and make each a contractual gate, not a courtesy.
- Structural calculations, shop drawings and thermal/condensation analysis signed by a competent engineer.
- Samples and mock-ups: a visual mock-up for aesthetics and, on larger projects, a performance mock-up (PMU) tested for air, water and structural load before bulk fabrication.
- Site testing: field water test to ASTM E1105 or the AAMA 501.2 hose test on a defined percentage of installed area, with a stated pass criterion and remedial process.
- Warranties: separate warranties for glass (coating and edge integrity), sealants and the installed system, with clear durations.
- Maintenance and access: state cleaning, re-sealing and BMU/rope-access provisions, because a facade in dusty Telangana needs a realistic maintenance regime to hold its warranty.
A performance mock-up on a mid-size tower typically costs a few lakh INR - a small fraction of the cost of stripping and re-sealing a leaking elevation, which is why it belongs in the specification.
Common specification mistakes to avoid
Most disputes trace back to a handful of avoidable omissions. Check your draft against these before it goes to tender.
- Copy-pasting a wind pressure from another project instead of deriving it for the actual site, height and terrain.
- Specifying glass by tint or brand name only, with no SHGC, VLT or U-value target attached.
- Leaving deflection and thermal movement out entirely, or conflating the two into one line.
- Naming a test standard but no pass/fail criterion or tested area - an untestable clause is unenforceable.
- Detailing the typical panel beautifully but leaving corners, parapets and slab edges as 'to be coordinated'.
- Forgetting the maintenance and access clause, so the warranty lapses the first time the elevation is cleaned incorrectly.
If you want a second pair of eyes, Hakimi Aluminium and Glass offers design-assist, shop drawings, fabrication and installation for facade and glazing projects across Hyderabad, Secunderabad, Telangana and Andhra Pradesh - get a free quote and we will review a draft specification for buildability and testability before it goes out to tender.
