Tom Shafer

How low-e glass can save you money

February 06, 2014

From frame style to the number of panes of glass, you'll have a lot of choices to make when you pick replacement windows. One of the questions you're bound to hear is, "Do you want low-e glass?" If you've never shopped for windows before, this term could leave you scratching your head. However, it's actually a simple concept - not to mention one that could save you money on your utility bills.

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Low-e glass means that a metallic, reflective coating has been added to the window to increase its insulation value. It does this by bouncing heat back to its source rather than allowing the heat to move through the glass as quickly as it would with clear, uncoated glass. In the summer, the heat is reflected back outside, and in the winter, the heat is reflected back into the home.

The efficiency of low-e glass is measured as a u-value. If you've ever purchased insulation, you may remember the R-value used to measure it's efficiency. Similarly, a u-value is used to measure heat transference through windows. The lower the u-value, the less heat transfer.

All glass manufacturers have their own methods for creating low-e glass. Generally speaking though, there are two approaches. In the first, the glass sheet is heated to glowing orange and the metallic (a combination of silver and other reflective metals) is sprayed onto the glass surface, which allows the coating to migrate into the glass and become part of it. This method is referred to as a hard coat. Hard coat low-e is more versatile, but it has slightly lower insulating values.

In the other method, the metallic coating is sprayed onto the glass and sits on top of it instead of becoming part of the glass itself. This is called soft coat. It has a shorter shelf life than hard coat and is more difficult to use when making insulated double pane glass. Since the sealants used to seal the insulated unit will now adhere to the coating, the edges must be ground off before the unit is made. Soft coat gives a slightly higher u-value than hard coat.

Low-e glass, when sealed into double pane insulated glass, usually cannot be touched. It's on the inside of the double pane unit.

To achieve better u-values, glass manufacturers have begun making low-e glasses with multiple coatings - low-e 2 has two coatings and low-e3 has three. There are also special low-e glasses that filter more sunlight, which can be especially useful in sunny climes like Florida.

Because of Energy Star requirements, low-e glass is now the required standard in many parts of the country. While this has increased the cost of windows, it lowers utility bills in return. Eventually, the windows pay for themselves in savings - and low-e glass is a big part of the reason why.

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