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Blog | Energy Efficiency, Windows | The Magic of Low-E Glass

The Magic of Low-E Glass

July 29th, 2009

If you step in any window dealer or manufacturer showroom, including Primary Seal,  you would hear about high efficiency windows with glass units that have Low-E + Argon. However, if you begin asking about the details of how this technology works, you may hear a variety of explanations followed by pictographic attempts at illustrating how this technology works. In my experience, both salespersons and clients alike use these terms often but understand little about the inner workings of Low-E technology.

So, let us shed some light on this magical Low-E glass.

As usual, something that appears magical either involves a trick or can be eventually described using physics. Since Low-E glass is no trick, we move on to the physics. Now, I will keep the detail to a minimum but try and stick with me for a moment. Low-E stands for low emissivity or emittance. Low Emittance glass radiates or emits low levels of radiant energy. Any object that has been heated either by the sun or another heat source will radiate heat. Radiated heat is also known as long-wave radiation, and, it is this type of radiation that the Low-E glass is designed to repel. Direct sunlight provides a different type of energy known as short-wave radiation which the Low-E glass allows to pass through in a certain proportion depending on the climate for which it was designed. When short-wave radiation hits an object, that object becomes heated and then proceeds to radiate heat in the form of long-wave radiation.

Keeping the above in mind, let’s look at a couple of examples:

The Winter – It is cold outside and hot inside. Your entire home is heated by your furnace and all the objects in your home are radiating heat. Heat hits the pane of Low-E glass as it tries to escape from your home, but only a part of it escapes while a part of it is reflected back into your home. Along with this, during the day, the sun’s radiation will pass through the windows in some amount and heat the objects in your house which then radiate heat … and this becomes a repetitive cycle.

The Summer – It is hot outside and cold inside. Your air conditioner is working overtime to cool off your home but the heat from the outside keeps wanting to come in. As pavement, sidewalks, and the earth itself radiate the heat of the sun, the Low-E glass reflects a significant portion of this radiation back outside. The sun does penetrate the windows somewhat, and it will slightly heat the objects in your home. However, less short-wave solar radiation enters your home when compared with clear glass and the winter heat gains are more important in our climate.

A bonus feature of Low-E glass is that it repels harmful UV radiation which damages furniture, hardwood floors, and carpet.

Low-E glass is described generally by two variables: solar heat gain (SHG in percent) and light transmission. For example, the glass that Weather Seal Windows uses is called ClimaGuard 75/68, meaning it has 75% light transmission and 68% solar heat gain. A high solar heat gain is ideal for our climate as we spend more money on heating and much less on cooling as we have a short summer. To give you a comparison, a hot climate like New Mexico or Arizona would use ClimaGuard 55/27. It is also important to see that windows designed for cooler climates allow more light to pass through, making for a brighter home and an acceptable decrease in lighting when compared to clear glass.

So how is Low-E glass made you ask?

Well that part is mostly due to high-tech manufacturing and sadly not very magical. Low-E glass is made by applying a thin, almost invisible layer of a metallic compound (usually tin or silver) to the surface of the glass. There are two ways to apply this coat. The first is called a “hard” coat and is a process which is performed while the glass sheet is still hot. While this process is very resilient because the metal is virtually fused with the glass – hence, the term “hard” –  it is not as energy efficient as the “soft” coat, a process which is applied after the glass is cooled. The way in which they apply the “soft” coat involves splattering little metallic droplets in a vacuum on a sheet of glass in an electrically charged chamber. The “soft” coat process is very delicate however, and Low-E glass made with this process must be sealed in a glass unit, as the metal particles cannot be exposed to moisture, abrasion, or air. Weather Seal Windows only uses “soft” coat Low-E glass as the energy performance is far superior to the “hard” coat Low-E.

What about the Argon?

The argon gas inside most insulated glass units today is there for two reasons: to increase energy efficiency and protect the Low-E coating from oxidation. Argon gas does increase the energy efficiency of the glass unit by a small percentage, however its main purpose is to protect the Low-E coating. Oxidation or rusting of any metal will occur if air and moisture are present. It is for that reason that an inert (unreactive) gas like argon is pumped into insulated glass units in order to prevent unwanted oxidation.

Low-E glass is becoming an industry standard as more and more companies – including Weather Seal Windows – are certified as part of the Energy Star program. This program tests the energy efficiency of windows to meet certain baseline standards in order that consumers are able to identify a window as highly efficient when it bears the Energy Star logo sticker.

If you have any questions or comments we would love to hear from you.

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