For decades, towers made entirely of glass symbolized transparency, corporate modernity, and spectacular city views. Today, however, this brilliance has faded. Overheated, energy-consuming interiors and dazzling light reflecting onto sidewalks reveal the disadvantages of these towers, while billions of birds lose their lives by colliding with invisible glass. With the rise of carbon regulations and extreme climate changes, massive glass buildings are being reevaluated.
Can a tower made entirely of glass exceed the 2030 carbon limits without extraordinary systems?
No, it’s almost impossible without a superhuman envelope. New regulations require much stricter facade performance. In New York, Local Law 97 (2019) limits annual CO₂ emissions for buildings larger than 25,000 ft². The initial compliance period begins in 2024 (penalties start in 2025), with much stricter limits introduced by 2030. The ultimate goal is net-zero emissions by 2050. Approximately 50,000 properties (about 60% of New York City’s total area) fall under this regulation. Therefore, leaky, glass-faced high-rises will require major HVAC retrofits or face mandatory fines.
U.S. baseline standards: ASHRAE 90.1-2022 (and IECC 2021) significantly tightened window rules. It has lowered the permitted U-factors and solar heat gain coefficients (SHGC) for all climate zones and now requires designers to document the glass U-factor, SHGC, and visible transmittance values for each facade orientation. In practice, this closes many “loopholes” that once led to the preference for inexpensive, high-gain glass.
The situation in the UK: The UK’s Part L (2021) regulations similarly reduce U-values, and the new Part O (overheating) rules require the dynamic assessment of solar energy gains (CIBSE TM52/TM59). CIBSE notes that many new or renovated homes have “high levels of glazing (excessive solar energy gain)” and that this leads to overheating. In short, UK regulations now require even commercial designs to demonstrate that they can expel heat (through shading, night ventilation, etc.), but a structure made entirely of glass struggles to cope with this.
Whole life cycle carbon: Glass walls also cause high carbon emissions. Research conducted by Arup reminds us that improving thermal performance generally increases the use of additional materials and therefore carbon emissions. For example, thickening walls or adding brise-soleil reduces energy consumption, but increases concrete/steel usage. Arup found that the carbon emissions of facades vary greatly depending on their type, and therefore bigger is not always better. Today, design teams (and guidelines such as LETI) use whole-life-cycle carbon accounting from day one: more walls and less glass is usually the most carbon-efficient approach.
Example application: These trade-offs are exemplified in Arup’s analysis of 55 Bishopsgate (London). The design team optimized the facade for carbon emissions throughout its entire life cycle by coordinating the glass area with solar control: they adjusted the glass size according to surface area and height, adding shading where necessary to balance daylight and heat gain. In short, the building envelope became a carbon decision rather than a render choice.
Lessons to be learned from the design: To achieve the 2030 targets without a massive cooling facility, tall buildings generally require a much lower window-to-wall ratio (WWR) on each facade, as well as external shading (brise-soleil, louvers, shutters). Use low SHGC spectrally selective glass (tuned for high visible VLT but providing low solar gain). Treat each facade as a carbon budget: add walls or insulated spandrel panels where necessary, and always calculate how the glazed area balances with the building’s energy.
2) How much glass is actually needed for daylight without glare and overheating?
The benefits of daylight do not require floor-to-ceiling windows everywhere. Today’s best practices use metrics (spatial daylight autonomy, DGP, etc.) instead of the old rule. Specifically, Daylight Glare Potential (DGP) measures the percentage of people who will be bothered by window glare. It classifies glare levels as follows: imperceptible (<35%), perceptible (35–40%), annoying (40–45%), intolerable (>45%). In practice, large, high-contrast windows easily push DGP into the “annoying” range.
What the metrics say: A window that looks great can disrupt focus if its DGP value exceeds ~0.40. Designers now also perform annual sDA (daylight autonomy) simulations alongside DGP to balance light and comfort. The goal is to provide sufficient daylight in workspaces without causing glare.
Envelope strategies: Limit WWR by orientation (use less glass on east/west facades where glare is highest). Choose glass with high visible light transmittance but low solar heat gain (spectral coatings). Always use fixed/dynamic external shading (blinds or awnings) to block the sun before it hits the glass. Interior blinds alone reduce glare, not heat.
Dynamic glass: Electrochromic/tintable glass is a useful tool for durable facades. Lawrence Berkeley Lab research shows that advanced “dual-band” EC glass can reduce annual heating, cooling, and lighting loads by 6 to 30 kWh/sq ft-window-year compared to standard glass. In hot climates, EC windows can significantly reduce peak and total cooling demand. (However, the energy and carbon emissions of the tinting system itself must also be considered, and controls must be carefully calibrated.)
Lessons to be learned from the design: Combine metrics: Aim for high daylight autonomy while keeping the DGP below ~0.40. In practice, this means a much lower WWR than a tower made entirely of glass – typically 30–50% rather than 80–100%. Always use glass in combination with shading (fixed louvers or automatic shades). Use electrochromic materials wisely in areas with the most intense sunlight, not as a cure-all. The key is to use good geometry first, then smart glass, not the other way around.
3) What effects do mirrored facades have on streets, neighbors, and wildlife?
Towers made entirely of glass tend to impose problems on the city. Glare and heat: When the sun hits a concave reflective surface, the results can be striking and dangerous. In 2013, London’s “Walkie-Talkie” (20 Fenchurch St) building created such an effect: its curved glass facade reflected a scorching beam onto the street. Pedestrians reported their shoes melting, and witnesses took a photo showing a Jaguar’s body panel warping under the reflected light. (The media even cooked eggs on the sidewalk.) Meanwhile, a few years ago, the Vdara Hotel in Las Vegas suffered a similar fate: its mirrored atrium showered intense sunlight onto the pool terrace, burning swimmers and literally melting plastic cups.
Bird collisions: Glass causes the deaths of hundreds of millions of birds. According to conventional estimates, the number of birds colliding with windows in the US ranges from 365 to 988 million per year, but a 2024 study revealed that even this figure is below the actual number. Researchers tracking birds brought to rehabilitation centers and their survival rates now say that over a billion birds die each year in the US alone from colliding with buildings. Birds cannot see transparent glass—they see reflections of the sky or trees and try to fly through them. This global crisis has spurred policy action: For example, New York City’s Local Law 15 (2020) now requires bird-friendly glass (fritted or patterned glass) on all new facades up to 75 feet above ground level (with only minor exceptions for areas under 10 square feet). Similar rules are spreading worldwide.
Design recommendations: For pedestrian safety: Avoid concave curves and ultra-reflective coatings that can focus sunlight. Simulate street-level sun reflections in the early stages of the massing phase. If bright glass is used in the building, consider using protective screens (such as brise-soleil) or changing the glass coating. For birds: Always use “bird-safe” glass (e.g., ceramic frit patterns, printed UV guides, or millimeter-scale dot screens). Comply with local regulations regarding pattern density (NYC requires a Threat Factor ≤25 up to 75 ft). Also, dim exterior lights at night to reduce attractiveness (especially during migration seasons). With today’s laws and tools, a bright tower can coexist with city life, but this requires designers to directly address these external factors.
4) If it’s not “all glass,” what will happen to high-performance long facades in 2025?
The new facade mantra: transparency, yes – everywhere, no. The trend is moving towards hybrid cladding that intelligently combines solid walls, shading, and glass. For example, many recent designs combine opaque walls with high R-values (insulated spandrels, brick, or composite panels) with perforated/strip windows and deep exterior wings or shutters. In cold climates, these reduce heat loss; in hot climates, they shade the underlying slab. Leading offices are now explicitly balancing carbon in the facade with energy use: adding mass and shading upfront to reduce operational emissions.
Hybrid structures: A facade analysis conducted in 2024 revealed that a typical curtain wall (composed entirely of glass) contains significantly more carbon than walls or insulated frames. For this reason, hybrid kits such as opaque insulated panels and narrow, high-performance viewing openings are increasingly preferred. Orientation is also important: more glass can be used on north walls, while more shade is needed on west/sun-exposed facades.
Double-skin facades (DSF): These “box window” systems with ventilated cavities yield mixed results. They provide bulk ventilation and solar control in some cool or noisy environments, but research shows their benefits are highly dependent on climate and control strategy. Furthermore, the extra glass and framing doubles maintenance costs and carbon emissions. Therefore, DSFs are not a panacea; they should be justified by real needs (e.g., pollution, acoustic, or natural ventilation targets).
Renovation (existing towers): Numerous studies have been conducted on how old glass towers can be improved without being completely demolished. Strategies range from low-emissivity films and better sealing materials (inexpensive, low-carbon solutions) to internal secondary glazing, partial cladding with additional external shading/shutters or insulated panels. A university study on a curtain wall building constructed in the 1950s found that all retrofit options (from film to a new curtain wall) yielded net carbon savings within a few years, while demolition never paid for itself. In short, renovation work (especially daylight films or secondary glazing) is almost always more advantageous than starting from scratch over a period of approximately 20-30 years.
Design steps: Start by creating geometry: deep glass lighting. Avoid challenging wide floor plates. Instead, plan for medium-depth floors and surrounding offices. Exterior shading/overhangs are important (these provide year-round energy savings). Use high-performance spandrels (thick, high R-value) below viewing windows. A typical high-performance facade sequence in temperate climates is: insulated spandrels + spectrally selective glass + fixed external brise-soleil + airtight construction + optional night ventilation – rather than using double-skin walls by default. In practice, today’s actual net-zero towers under construction carefully mix dense walls and shading with glass, rather than covering everything in glass.
5) What will happen to the glass towers built yesterday when the offices are vacated?
Following the pandemic, glass offices took the hardest hit. Converting old curtain wall offices into apartments is among the topics on the agenda in New York and London. (Even the US White House has launched a $100 million office-to-residential fund.) However, the feasibility of this varies. Design guidelines and case studies emphasize that simply changing the building’s intended use on paper is insufficient; for residential use, the facade typically needs to be redesigned as well.
Converting offices into residential units: Cities are revising regulations to facilitate conversions. In 2023, NYC’s Office Adaptive Reuse Task Force recommended changes to zoning plans to ease the conversion process. Currently, dozens of office buildings in Manhattan and Brooklyn are undergoing conversion. According to a report by the New York City Comptroller (mid-2025), there are 44 active conversion projects totaling 15.2 million square feet. This space is sufficient for approximately 17,400 apartments (mostly studio/one-bedroom). Similar programs (typically through Permitted Development Rights) are also being implemented in London and other markets, significantly increasing the housing supply.
Carbon logic: Studies (such as Arup’s “Carbon Story”) show that preserving and refurbishing a building generally yields better results in terms of lifetime carbon emissions than complete demolition. An analysis conducted in New York shows that expanding reuse could save approximately 5-11 million metric tons of CO₂ by 2050. However, curtain wall conversions still face fire, seismic, and acoustic standards. Facades often need to be renovated (or partially enclosed) to comply with new egress and insulation codes. The carbon balance depends on how much glass is replaced or renovated.
The UK perspective: In the UK, Permitted Development Rights have paved the way for many office-to-apartment conversions, but independent reviews highlight serious quality issues. Many conversions carried out without a full planning review result in deep internal corridors, small single-aspect units, and apartments lacking balconies or natural light. Critics note that the apartments often violate minimum space standards or lack adequate ventilation systems. This situation demonstrates that it is impossible to cheat the laws of physics – an office block built in the 1960s was not designed for residential use.
Design steps: Before drawing a flat layout plan, perform a daylight and ventilation analysis on the existing floor plan. Building codes typically require natural light in every living area – if direct window light is not possible, you will need to add light wells or skylights. Similarly, each unit requires legal ventilation (usually openable windows or mechanical ducts). In practice, many glass offices will require new exterior walls or perforated windows, extra insulation, and hybrid natural/mechanical ventilation to comply with residential regulations. Also, renovate with bird-safe glass and shading: you cannot expect a safe and comfortable residence by reusing a reflective glass shell.
Design Checklist
- Priority policy: Establish a legally compliant, definitive carbon budget (operational and concrete).
- Comfort criteria: Adhere to climate-based daylight/autonomy (sDA/DA) and glare (DGP) targets and full overheating criteria (CIBSE TM52/TM59 or equivalent).
- Glare and ecology: Model solar reflections along streets and sidewalks. Use bird-friendly glass (frit, films, or patterns) on lower floors to comply with regulations such as NYC LL15.
- Renovation hierarchy: Sealing leaks and replacing old gaskets should be done first, followed by low-emissivity films or insulated secondary glazing, then external shading devices, and finally selective window replacement or coating renewal. Carbon emissions should be tracked throughout the entire life cycle at each step. (Notably, renovation work often demonstrates that minimal interventions pay back carbon emissions more quickly than full curtain wall replacement.)
Sources and Further Reading:
NYC Local Law 97 (compliance 2024; penalties 2025; tougher 2030)
ASHRAE 90.1-2022 envelope/fenestration rules.
CIBSE TM52/TM59 (overheating) and UK Part O guidance.
LETI Climate Emergency Guide; Arup facade carbon studies.
Walkie-Talkie and Vdara glare incidents.
Bird collision science and NYC LL15 (bird-friendly glass).
NYC Office Reuse Task Force (2023) and carbon studies.
SF Adaptive Reuse code notes (daylight/vent).
McMillian (Univ. Penn) on curtainwall retrofit life-cycle.
These links provide detailed information on policies, criteria, and case studies for designers who wish to conduct more in-depth research.