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The ABC of Low-E Windows and Doors

by Gates Dearen
A recent HomeRite project
If you’ve looked into getting replacement windows lately, you’ve probably run into some terminology you’re not familiar with. For example, one phrase that you’re almost certain to run into is “Low-e.” Just what does the “e” stand for? Emissivity. It’s not a word most people use or hear very often. Here’s a definition from the Merriam-Webster dictionary.
Emissivity:  the relative power of a surface to emit heat by radiation, the ratio of the radiant energy emitted by a surface to that emitted by a black body at the same temperature.
To illustrate emissivity in a way that most Floridians will understand, think of a trip to the beach in August. If you’ve ever walked barefoot in a paved parking to or from your car on a hot day, you know that the asphalt is very hot and the sand is much cooler. Both surfaces get the same amount of sunlight, but they have very different levels of emissivity. Asphalt has high emissivity.  90% of the heat directed at the asphalt is absorbed and emitted. Only 10% is reflected away. The sand has much lower emissivity than the asphalt, which is why it’s cooler to walk on. Most beachgoers would agree that lower emissivity is better.
Homes with windows from the last century frequently have plain, uncoated glass. Plain glass has a high level of emissivity, like asphalt. Old windows can get very hot, and the heat is emitted into the home. Low-emissivity windows and doors cause heat to be reflected off, rather than allowing it to be emitted into the home.
A is for Advances in Window Technology
Low-e coatings control heat transfer through windows with insulated glazing. A Low-e coating is a microscopically thin metallic oxide layer deposited directly on the surface of one or more of the panes of glass. The latest glazing options help to keep your home comfortable and energy efficient.
Windows manufactured with Low-e coatings typically reduce energy loss by as much as 30% to 50%. Other recent advances in windows also help. These include an insulating frame and sash, fusion-welded construction, and continuous weather stripping. The result is increased insulation value, increased warmth, and decreased condensation.
B is how it Beats the Heat (SHGC)
Windows by HomeRite of Jacksonville
SHGC stands for Solar Heat Gain Coefficient, another term that most people don’t use every day. SHGC measures cooling efficiency. This is a window’s ability to block the infrared heat from the sun while maintaining a cooler temperature in the room. The lower the number, the better. Homeowners in hot climates like we have in Jacksonville in the summer should look for windows and doors with low SHGC values, preferably under .27. This will help to keep air conditioning costs down.
The U-factor measures heating efficiency, the window’s ability to block the flow of heat through the glazing system, and to retain the heat in the room. The U-factor measures the insulating properties of windows. The U-factor incorporates the energy efficiency of the entire window. This includes the glazing, the frame, and the spacers. The lower the U-value, the better the efficiency. Many double-paned windows achieve a U-factor of .27 or below. This is desirable in warmer climates.
Argon gas
To improve the thermal performance of windows with insulated glazing, some manufacturers fill the space between two panes of glass with argon gas. Argon is an inert gas, and it has a higher resistance to heat flow than regular air. Triple pane windows have two layers of insulating argon gas, making them even more efficient.
C is for Can you stop the Sun?
Visible transmittance (VT) refers to the capacity of windows and door glass to allow outdoor light to pass through. In the recent past, desirable sunlight was accompanied by undesirable heat. Older types of glass couldn’t let the light in without also letting the heat in.
Modern glazing technologies, as well as the increased use of double-pane windows, can give glass a high VT while keeping the SHGC low. This is especially important for homeowners in sunny, hot climates, like we have in Northeast Florida during the summer. In addition to saving energy, modern windows reduce fading of carpets and furniture.
The National Fenestration Ratings Council (NFRC)
The National Fenestration Ratings Council (NFRC) is a nonprofit organization that tests windows and doors, along with other building materials, and assigns ratings to the products. The NFRC doesn’t make recommendations of what to buy. Their purpose is to provide information which informs consumers that a product performs as the manufacturer claims. This helps consumers to compare windows with confidence.
Energy Star
The ENERGY STAR® program defines thermal efficiency levels. ENERGY STAR® ratings are based on NFRC data. Replacing old windows with ENERGY STAR certified windows lowers household energy bills. Lower energy consumption also reduces greenhouse gas emissions from power plants and shrinks a house's carbon footprint. ENERGY STAR certified windows and doors do more than just lower energy bills. They deliver comfort that other products simply can’t match.
On cold winter nights, do you avoid seats near the window? The cold, inside surface of an inefficient window pulls heat away from your body, so you can feel chilly in a sweater with the thermostat at 70 degrees. With ENERGY STAR certified windows, the interior glass stays warmer, so you can enjoy your window seat even when the temperature outside dips below freezing.
Do some of your rooms feel like a sauna in the summer? With ENERGY STAR certified windows you can stay cool all summer long. Most ENERGY STAR certified windows reduce the “heat gain” into your home more than typical windows do, without reducing the visible light. You get the light you need without the uncomfortable heat.
The information in this blog will help you to understand the terminology you’ll hear as you look for new windows or doors. Talking to a professional from a reputable window and door company will help even more. He or she can guide you in your choice of the products that will work well for your home.
These days, customer reviews and testimonials are available online. Also, you may have friends and neighbors who’ve recently installed windows and who can tell you about the contractor they used. Consumers should take advantage of any opportunity to learn more about any vendors that may work on their homes.
When you do research on the available products and the vendors who can install them, it will help you to make a good decision which will improve the energy efficiency of your home and will also add beauty and value to your home.
In this article, I’ve explained what it means when windows are advertised as Low-e. I’ve also explained some of the other jargon associated with modern windows and I’ve written about the Energy Star designation.
For information about HomeRite’s energy efficient line of windows and doors, call 296-2515 or visit their showroom at 4801 Executive Park Court, Building 200, Suite 207, Jacksonville; FL 32216.
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HomeRite is a window and door dealer that specializes in energy efficient, quality windows and doors with warranties and service to match. The company has been in business since 2005. HomeRite has partnered with a manufacturer that has been producing high quality products and providing excellent customer service for over 60 years. Windows and doors from HomeRite are some of the highest quality, most thermally efficient windows and doors on the market.
HomeRite products add substantial value to their clients’ properties. HomeRite is a member of the United States Green Building Council, a non-profit organization comprised of leaders across the industry working to advance environmentally responsible buildings.
Gates Dearen is the co-owner of HomeRite Windows and Doors in Jacksonville, Florida. He and Richard Walden have been serving the building products industry in Florida for over 30 years. They strive to match homeowners with the right windows and doors for their homes and budgets. They make the home improvement process pleasant with first-rate, energy-efficient products, affordable pricing, and award-wining installers who employ the best practices and who always treat customers and their property with the utmost respect.


  1. Who knew that so much science went into today's windows? I always wondered what Low-E meant.

  2. Thanks for explaining the jargon. Prior to this I thought the E stood for energy. I've learned something new.

  3. Low-E and such has always been baffling to me...thanks so much for helping make it understandable.

  4. I've just downloaded iStripper, and now I enjoy having the hottest virtual strippers on my desktop.

  5. Styling your home with new windows and doors

  6. Thanks for sharing such a great blog... I am impressed with you taking time to post a nice info.
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