What are Low-E windows, and why buy them?
Updated: Oct 22, 2022
The question we always get about Low-E windows is whether or not they’re really energy efficient windows. In short, Low-E means low emissivity, or more precisely, low thermal emissivity. So yes, Low-E windows are energy efficient because any material that reflects, absorbs, and emits radiant energy is going to save energy.
In terms of window glass, it is by nature highly thermal emissive and clear glass absorbs a ton of energy without reflecting much. This means if you have clear glass in your windows, you are absorbing all of the heat energy from the sun as well as radiant heat from inside, but none of it is being put to use in creating energy efficient windows. Any heat taken in through the glass during the day is being released right back out through the window at night. This is why in some houses a room can be very hot during the day and freezing when the sun goes down. The net result is that we end up using our heating and cooling appliances more than we need to.
To improve insulation or thermal efficiency in a window, a thin film coating is applied to the raw glass. The process is done at the glass manufacturing plant where specially designed coatings are applied to one or more surfaces of an insulated glass unit. For example: usually the glass is coated on the inside of the outer pane for double-pane windows. This film coating reflects radiant infrared energy, which is meant to keep the energy on the same side it originates from while still letting in visible light. So when the sun shines, a Low-E coated window reflects a lot of that unwanted energy back towards the sun instead of letting the heat pass through the glass. It works the same way if you are running your heater. The energy from your heater comes up against the Low-E window and is reflected back toward the inside of the house so it isn’t lost.
There are a couple different methods used for coating Low-E windows. Pyrolytic coatings are applied at very high temperatures at the plant when the glass is manufactured. Pyrolytic coatings are usually tin dioxide and are also called ‘hard-coat’. Hard coat Low-E glass surfaces are considered to be medium grade energy efficient windows and perform much better than plain clear glass. The second method is called Magnetron Sputtering. This process takes place when the glass is placed in a vacuum chamber and has several thin layers of silver with antireflective properties applied to it. This is considered ‘soft-coat’ and must be enclosed in double-pane window units to protect it. Soft coat Low-E glass is the most efficient and highest performing of the two energy efficient window types. Soft coat Low-E windows are the type we sell at Wood Windows and Doors.
Most companies will offer both hard and soft coat Low-E windows. Hard coat Low-E windows are relatively durable, can be handled easily, and are something most often used in single-pane applications; however, hard coat surfaces have a higher solar gain than soft coat and have the possibility of developing a slight haze over time. On the other hand, soft coat Low-E windows must be in a double-pane unit and are nearly invisible coatings which provide more visible light. Soft coat windows have much better U-value than hard coat surfaces and allow less UV transmittance by almost 75% over clear glass.
No matter what type of Low-E windows you get, they will perform better than windows that have only clear glass. Low-E windows are definitely the way to go and will soon be the standard that all builders use. There have been some concerns about the effects of Low-E glass on house plants, but the quick answer is that Low-E glass is designed to block heat; not light. The visible spectrum is what’s necessary for plant growth. In fact, some studies show that Low-E windows can actually improve plant growth over clear uncoated glass.
Wood Windows and Doors offers the option of standard with soft coat Low-E glass. We use Solarban60 and Solarban70 solar control glass. This glass has a very low U-value, which means a higher performance. Our products also help with furnace heat loss, protect fabrics and carpet from fading, provide glare control, and of course reduce heating and cooling energy costs. When considering ways to make your home more energy efficient.
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