The window, like most building products in our homes, has seen some pretty dramatic improvements over the last thousand years or so.  Our ancestors created windows using everything from translucent animal horns to paper.  Around 100 AD, in Roman-occupied Egypt, there was evidence of glass being used in windows.  Not surprisingly, glass windows back then weren’t very clear, but they were enough of an improvement compared to other materials that the future of glass was set in stone, so to speak.  

Over the years, several processes for making glass for windows have been used, from slab glass, cylinder glass, crown glass, cylinder sheet glass, cast glass, and drawn glass.  The drive to improve the clarity of glass while lowering the cost to produce glass yielded a technique invented by Alastair Pilkington in 1959 called float glass.  Float glass is the process of pouring molten glass onto a bed of molten tin.  The molten glass floats on the tin, spreading out and forming a level surface.  Float glass is the industry standard still in use today.

As the process for making glass was improving, window construction was getting better too.  Different framing and glazing methods were developed and adapted to specific applications and climates.  Framing materials used over the years include wood, metal, vinyl, fiberglass, and composite materials each with varying thermal and structural performance.  Some of the biggest energy improvements in glazing came from the movement to insulated glazing, both double and triple pane and gas-filled insulated glazing.  Other important energy efficiency glazing innovations include heat-absorbing tinted glass, reflective glass, low emissivity (Low-E), and spectrally sensitive glass. Heat gain and loss through windows account for 25-30% of home heating and cooling energy use so improving the efficiency of your windows can make a significant difference in your home’s heating and cooling efficiency, and monthly energy bills.

As more and more “smart” products are entering our lives, the window is gaining more intelligence too.  In this article, we are going to answer the question, what is a smart window and if you should consider a smart window when building a home or replacing the windows in your existing home.  We’ll explore the cost and benefits of smart windows and how smart windows work.

A smart window typically uses some form of smart glass or glazing. When voltage, light, or heat come in contact with the glass, the glass can change from transparent to translucent to completely opaque, which is why you’ll often see references to switchable glass, dimmable glass, or tintable glass when talking about smart glass.  This ability to block some or all wavelengths of light allows smart windows to provide additional energy efficiency gains compared to traditional windows.

How Do Smart Windows Work?

Smart glass is manufactured using a special film that is laminated onto the glass (typically acrylic or polycarbonate laminate).  You can actually purchase “smart film” and convert regular windows into smart windows.

There are several technologies used to create smart glass windows.

Electrochromic: electrochromic glass uses low voltage electricity to control the opacity of the glass, thereby controlling the amount of light and heat transmitted.  The change occurs slowly, but users have some control over the speed of the transition.  This technology may affect the durability of the glass.  Electrochromic glass is often found in museums when precise control is needed over the amount of light exposure an artwork or artifact receives.

Photochromic: photochromic glass reacts to light rather than electricity, most typically ultraviolet light.  You are likely familiar with photochromic glass if you’ve ever used or seen transition eyeglasses. Photochromic glass gets darker when exposed to light (from the sun for example) and clear again when the light source goes away (say when it gets cloudy).

Thermochromic: thermochromic glass changes when exposed to heat.  Changes to the translucency of the glass occur when the temperature exposed to the glass rises above the transition temperature activating the thermochromic compound, blocking light and heat.

Suspended-Particle (SPD): suspended-particle glass is constructed from nanoparticles suspended in a liquid that react to low voltage electrical current.  When electricity is applied, the particles line up and the glass becomes transparent and when no electrical current passes through the glass becomes opaque.

Micro-Blinds: smart glass that uses micro-blind technology also uses electrical currents to change the tint of the glass.  Micro-blinds are tiny sheets of metal (100 micrometers in size) that curl or flatten out when activated.  When there is no electrical current the blinds curl light passes through.  However, when an electrical current is applied, the blinds flatten out to block the light.  Like regular blinds, micro-blinds can be adjusted to user preference to control the amount of light that passes through.

Polymer Dispersed Liquid Crystal (PDLC): polymer dispersed liquid crystal glass is yet another type of smart film that reacts to electricity.  Similar to SPD, PDLC is liquid crystal droplets that are randomly arranged on the film when no electrical current is passing through the film.  However, when activated the liquid crystals align and the film becomes transparent allowing light to pass through.  Common applications for PDLC smart glass are found where privacy is needed due to the milky opaqueness such as conference rooms, hospital ICUs, bathroom and shower doors, and fitting rooms to name a few.

Some of the manufacturers of smart glass include Gauzy, Gentex, Halio, Heliotrope, Polytronix, Raven Window, Research Frontiers, SageGlass, View, and Vision Systems among others.  Most of these manufacturers are pursuing large-scale commercial projects, automotive and aerospace industries, but there are some that are venturing further into residential applications.  Marvin Windows signed an exclusive agreement with Halio to bring their smart glass technology to residential homeowners.

Benefits of Smart Glass

Despite the variety of technologies used to make smart glass, there are really only two primary benefits.

Convenience: smart glass can completely eliminate the need for window treatments, blinds or shades and inherently, your smart window system can automatically adapt to your climate allowing light in when you need it and blocking light when you don’t. 

Energy Efficiency: windows are one of the primary sources of heat loss in our homes and the promise of the smart window is to improve energy efficiency by controlling the amount of light that passes through the windows; some manufacturers claim 95% reduction in ultraviolet light and up to 30% reduction in your heating and cooling costs.

You may see manufacturers claim other key benefits like increased privacy and security.  While smart windows do offer some privacy benefits, they aren’t substantially better than regular windows that incorporate blinds or other window coverings.

Manufacturers of smart glass systems are continuing to innovate and experiment with intelligent glazing technologies that offer noise reduction, fire resistance, condensation protection, and self-cleaning.

How Much Do Smart Windows Cost?

When looking at the cost of smart windows compared to traditional windows you can expect to pay a hefty premium, anywhere from $50 to $100 per square foot for smart windows compared to $10 to $15 for regular windows.  This investment has the potential to produce a positive ROI over the lifetime of the product though due to the energy cost savings mentioned above.  Most manufacturers of smart glass are custom designing luxury home projects, making this a less accessible option for most of us.  However, we expect that to change over time as traditional window manufacturers look to incorporate smart glass technology into their product lineup.  

The smart film, on the other hand, is more widely available for both DIY projects and professionally installed applications, while the per square foot cost of the smart film can rival that of smart glass, the entry point to test out the product is much more accessible with starter kits offered at only a few hundred dollars.  The leading smart film manufacturers include InvisiShade, Rayno Film, Sonte. Smart Film and Smart Tint.