Glass is moving from a passive building material to an active building component.
For centuries, the window had a straightforward function: daylight, ventilation and views, while protecting the interior from weather. Modern glazing is being asked to do considerably more. A façade can now incorporate glass that changes its optical properties, responds to electrical signals, works with sensors and building controls, and integrates photovoltaic generation.
The International Energy Agency estimates that windows account for 5–10% of total building energy consumption. In highly glazed buildings, the contribution can be higher. For the glass industry, this makes the window an energy-performance issue, not simply a matter of transparency or aesthetics. The IEA identifies advanced windows and dynamic glazing among the technologies that can improve building performance.
Glass that changes its behaviour…
Electrochromic glazing is one of the clearest examples.
A conventional solar-control coating has a fixed optical performance. Electrochromic glazing can be electrically switched between different states, changing the amount of light and solar radiation transmitted through the glass. 
That difference matters on a large façade. The solar conditions at 9:00 in the morning are not the same as those at 14:00. A static glazing specification has to be selected to perform across those conditions. Dynamic glazing can be controlled according to the conditions at a particular moment.
Recent research is pushing this technology beyond simple darkening and clearing.
A 2025 study published in Nature Communications demonstrated a dual-band electrochromic window capable of independently controlling visible and near-infrared radiation across 320–2500 nanometres. The prototype recorded switching times of approximately 1.6 seconds for coloration and 0.8 seconds for bleaching. Outdoor testing produced temperature reductions of up to 25°C, while simulations indicated annual HVAC energy reductions exceeding 10% under the study’s conditions.
Those figures describe a research prototype, not the performance of every commercial smart window. They do, however, demonstrate the level of control researchers are now targeting.
The window enters the control system
Once glazing can change state, another question follows: who or what tells it when to change?
This is where glazing begins to overlap with building automation. Solar radiation, indoor temperature, exterior temperature, occupancy and glare can become inputs to a control strategy. The glazing becomes one element in a larger system:
environment → sensors → controls → glazing → indoor conditions
The technical challenge is no longer limited to producing an electrochromic material. Researchers also have to address switching speed, optical range, colour neutrality, durability, manufacturing scale and cost.
A review published in Nature Reviews Clean Technology in 2025 identifies durability, recyclability, scalability, cost and integration among the major challenges for electrochromic smart windows. That distinction is important for the construction industry. A laboratory device can demonstrate excellent switching performance. A façade component has to operate through years of temperature changes, solar exposure, moisture, maintenance cycles and mechanical stresses.
When the window becomes a power generator?
Photovoltaics add another function.
IEA PVPS research covers building-integrated photovoltaics in applications including windows and façade systems. The technology is designed to perform an architectural function while also generating electricity.
The 2025 IEA PVPS technical guidebook examines BIPV in façades, roofs and shading systems while considering electrical generation alongside thermal behaviour, daylighting, acoustics, safety and durability.
The conventional definition of a window therefore starts to break down:
- Traditional glazing — provides light and views.
- High-performance glazing — controls heat transfer and solar radiation.
- Dynamic glazing — changes optical properties.
- Connected glazing — interacts with sensors and building controls.
- BIPV glazing — adds electricity generation.
Each additional function brings another engineering discipline into the window: glass science, coatings, electrochemistry, electronics, controls, photovoltaics, façade engineering and energy modelling.
The question is not whether windows will disappear…
They will not. The more relevant question is whether the passive window remains the dominant model.
A building envelope that can regulate solar radiation, respond to environmental conditions and generate electricity is performing functions that once belonged to separate systems. For architects, façade engineers, glass processors and aluminium-system manufacturers, this changes the specification problem.
The question is no longer simply: “Which glass should we put in the window?”
It is becoming: “What should this part of the building do?”
That is a much bigger question. And if a future façade can regulate solar radiation, respond to environmental conditions and generate electricity, the distinction between window, façade and building technology becomes increasingly difficult to maintain.
Perhaps the traditional window is not dying. Perhaps its definition is.
Source: Nature Communications with additional information added by Glass Balkan
