Transition Windows

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As you may know many of the LEED certified buildings have expansive windows which seems to beg the question “How energy efficient is this’? Whether it’s a high rise in Manhattan, multi-family dwelling or private home this is a great question to ask.

Windows are important to the efficiency of our homes and often account for a significant portion of our heating and cooling bills (although heating is probably more on your mind at the moment). New technologies are beginning to address common issues, such as solar heat gain and insulation, so that we can expect more from our windows in the future.

Windows allow sunlight into our homes. Sunlight is composed of short and long wavelengths. The short wavelengths represent the visible light that makes our homes bright and sunny. The long wavelengths produce the heat we feel from that sunlight. This heat is also known as solar radiation.

The amount of solar radiation transmitted through a window is represented by the Solar Heat Gain Coefficient (SHGC), a number between 0 and 1. A window with a high SHGC allows a lot of solar radiation into your home. The heat provided can be welcome in colder climates, but provides unnecessary heat in warmer climates. A window with a low SHGC blocks solar radiation, which reduces the need for air-conditioning in summer but is not ideal for cold climates.

The windows most commonly used today are double-paned with a special metal coating (low-E, E stands for emissivity). The double-pane increases these windows’ insulating abilities and the low-E coating helps block long-wave infrared radiation (heat) from entering the home while still permitting visible light. The drawback to the low-E coating is that in heating climates, it can prevent some passive (solar) heating in the winter.

The future may bring a new kind of window that optimizes its SHGC for all climates. They are called transition metal switchable windows and are good for any climate, especially climates that have four distinct seasons.

A small electrical current travels through the glass triggering a light sensor. As the current passes through the glass, it increases the reflectivity, preventing solar heat from entering the house. When the current is halted, the window becomes more transparent, which allows more light and heat to pass through. Since the current is triggered by light sensors and regulated by controls, it can automatically react to changing seasons and light conditions.

The upshot is that when it is hot and sunny outside, the glass automatically turns reflective to keep heat out of the house. When the temperature or light conditions drop, the glass automatically turns transparent to allow more heat and light into the house. This technology makes the glass optimal for any climate. Even a New York City winter!