Keeping it Clear: A Guide to Window Glass

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With electricity costs rising, more and more homeowners are becoming interested in improving energy efficiency in their homes, starting with exploring how replacing their windows can help. The only problem is, choosing a replacement window can get tricky because you have all these terms being thrown around. I’m here to address that, covering the basics to help you understand window glass better and assist you in choosing the right window for you.

Getting Started: A Few Important Terms

Window glass come with labels specifying ratings in certain criteria. Generally, these criteria include:

  • R-Value. The measure of thermal resistance, the R-value refers to a window’s ability to insulate against the flow of heat from outside your home to the inside, and vice versa. The higher the R-value, the better a window’s thermal resistance is.
  • U-Factor. The measure of heat lost over time, the U-factor indicates how much heat escapes through the window per hour. For energy efficiency, you want a lower U-factor for your windows.

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  • Solar Heat Gain Coefficient. The measure of how much heat enters a building while the sun is shining, ideal SHGC will depend on the kind of climate you live in. In warmer climates, where cooling needs are high, you’ll want a low SHGC, to minimize the amount of heat entering your home. On the other hand, in colder climates, you’ll want a higher SHGC, so you can take advantage of heating from the sun to help meet your heating requirements.
  • Visible Transmittance. The measure of how much sunlight a window lets in, VT determines how helpful a window will be when it comes to daylighting. The higher the VT, the higher the amount of natural light that will be let in.

About the Glass

You will also encounter different window glass options, like:

  • Double-Pane Glass. Also known as double-glazed glass, it’s the standard glazing option today, made up of two panes of glass separated by a layer of air or inert gas like argon or krypton. The more panes a window has, the higher the level of insulation it provides so you can expect double-paned windows to perform better than their single-paned counterparts.

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  • Low-emissivity Glass. Also called low-E glass, this glass option comes with a very thin metallic oxide coating that limits the entry of radiant heat and ultraviolet light, reducing heat that can affect indoor comfort and fading in furniture and other items close to the window without affecting visible transmittance.
  • Tinted glass. As it reduces heat and glare from sunlight and also blocks UV light, tinted glass functions similarly to low-E glass. However, they differ in that tinted glass cuts back on a window’s visible transmittance, meaning it lets in less natural light compared to low-E glass. Just how much a window blocks will depend on the kind of tint it uses. The most common gray and bronze tints reduce heat and light more or less equally, while blue and green tints let in more light than heat, and black tints block more light than heat.

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Choosing the Right Window Glass

The right window glass for is one that addresses specific requirements put forth by the climate you live in. For instance, those in hotter climates, you’ll want a window that lets in less heat so look for windows that offer the lowest U-factor and SHGC. Those in colder areas, on the other hand, will benefit from windows with high SHGC that will help heat your home during the day, but with low U-factor, so that the heat that does enter your house doesn’t dissipate too fast.

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Author Bio:

Scott Barr currently heads Southwest Exteriors and has been with the company since 1990.  He’s committed to delivering the “Ultimate Client Experience” so he finds great enjoyment in hearing customers talk (even rave) about how satisfied they are with their home improvement project. For updates from Scott, you can check out the company blog.

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