The heat inside the house often passes through a very precise point: the glass. It enters slowly, rests on the floor, warms the walls, transforms a normal room into a small domestic greenhouse. Then comes the air conditioner, with its hum, the bill, the electrical peak at the worst hours. The topic also concerns buildings, because the sector weighs heavily on global consumption: according to the International Energy Agency, buildings absorb approximately 30% of the world’s final energy and generate 26% of energy-related emissions. Cooling, meanwhile, is growing rapidly and putting pressure on electricity grids just when cities need to breathe most.
Within this scenario comes a smart window film developed by Hong Kong Polytechnic University, designed to regulate light, heat and humidity together. It’s called MRLR, an acronym for “Moisture-responsive and Light-regulating”: basically, a film that reacts to humidity and modulates light. The interesting thing lies in its apparent simplicity. It works by exploiting the properties of the materials and the day-night rhythm, without adding another device to power.
Glass does more
The structure consists of two layers. The first is made of polyacrylonitrile nanofibres, a hydrophilic material, therefore capable of attracting water. The second is a transparent polyacrylamide hydrogel, chosen to retain moisture. Together they build a small passive mechanism: during the day the accumulated water evaporates and helps reduce part of the solar radiation that passes through the glass; at night, when relative humidity tends to rise, the film absorbs vapor from the air and becomes less transparent.
In a room this means three effects at the same point: less heat during the sunny hours, less humidity during the night hours, more privacy when the lights come on outside and the windows begin to expose pieces of domestic life to those who pass by.
The results measured in a controlled environment are very clear. In a test chamber, the film reduced humidity from 91.73% to 53.76% in six hours during the night and lowered indoor air temperatures by up to 21.1°C during the day. That number should be taken for what it is: an experimental data, obtained under precise conditions, influenced by exposure, ventilation, climate, room size and type of glass. But it indicates a clear direction. The window, often treated as a weak point of the building envelope, can become an active surface in the control of internal comfort.
Out of humid climates
The technology was created with hot and humid areas in mind, such as Hong Kong and many tropical or subtropical cities. In those contexts, air conditioning works on two fronts together: high temperature and heavy humidity. The result can be seen in consumption, in bills, in summer electricity peaks.
The theme also speaks to the Mediterranean. In Italy, summers are longer, heat waves are harsher and many apartments continue to have poorly exposed glass, poor screening and weak insulation. A living room facing south or west, in an urban building without shade, can become unmanageable already in the early afternoon. Working on the glass means acting on one of the points where heat enters most easily.
According to the researchers’ global model, based on typical meteorological data from several large cities, the film could reduce average annual energy consumption by more than 20% and cut carbon emissions by more than 18 kg per square meter per year in the cities analyzed. The study, published on Advanced Energy Materialspresents this solution as a possible support for energy saving and privacy protection in buildings.
The duration account
A solution to be applied to windows must resist use, dust, repeated humidity, the real life of buildings. For this reason, the research group subjected the material to 300 humidity absorption and release cycles, verifying its stability and performance. Tests also indicated resistance to bacterial and fungal contamination, as well as dust-proof properties maintained after five days of continuous exposure to heavily polluted air.
Then there is the economic question. According to the researchers, the simple structure and low-cost materials could facilitate large-scale production, with an estimated payback time of one to two months. It is an estimate to be read with caution, because it depends on the price of energy, the local climate, the glass area, the type of building and the real installation costs. However, a concrete advantage remains: you work on a part already present in homes and offices, without redoing the entire building from scratch.
The next step will be outside the test chamber: real windows, entire seasons, different apartments, different facades, different habits. Only there will it be understood how much this thin film can take away work from the systems. For now it remains on the glass, where the heat enters. A fairly strategic place to start.
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