You may have heard of the term “U-value,” used to describe energy-efficient windows.

U-value (also referred to as “U-factor”) is the rating that determines the insulation properties of a window, door or skylight.

Put simply, a low U-value indicates a slower transfer of heat from a warm area to a cold area, and higher levels of insulation. Conversely, the higher the U-value of a material, the worse it performs in relation to retaining heat.

In other words, the lower the U-value number, the more efficient the product.

Understanding R-values

To understand how U-value is measured, you must first know what a “R-value” is. That’s because the R-value goes into determining the U-value.

R-value is a rating for thermal resistance. The higher the R-value, the better the product is at preventing heat from flowing out of your home in winter; and keeping cool and conditioned air inside over the summer months.

While U-values are usually attributed to doors and windows, the R-value measures the insulation value of other construction materials.

Calculating a U-value

As you can see here, calculating a U-value can be incredibly complicated.

The challenge with U-value numbers is that not everybody applies the same units of measure.

R-values are measured using the metric system, in square feet per hour in degrees Fahrenheit per British thermal unit (sq. ft. x h x °F/Btu).

Meanwhile, U-values can be measured in one of two ways:

  • In imperial units—more specifically, watts per square metre Kelvin (W/m2 x K), or
  • In metric units—specifically, British thermal units (BTUs) per hour per square foot Fahrenheit (Btu/h x sq. ft. x °F)

This sample table from Natural Resources Canada shows U-values in both imperial (in the left-hand column) and metric units (in the middle column), and their corresponding R-values:

Generally speaking, if you want to compare apples to apples using the metric formula (which is more commonly used across North America), a U-value can be easily measured as 1 divided by R-value.

Where to find a U-value

In Canada, all windows with the ENERGY STAR® symbol provide the U-value on the label. In fact, every ENERGY STAR certified window, door and skylight must feature a removable label including not only the U-value but its climate zone, description of the product, and certification information.

All ENERGY STAR products are certified as meeting or exceeding energy efficiency specifications through guidelines set by the Canadian government. This means that you are guaranteed a low U-value.

example of energy star window label with u-value

Check out these sample labels created by Natural Resources Canada.

Double-glazed and triple-glazed windows

Today, the window industry manufactures double- and triple-glazed windows with features above and beyond what single panes ever did.

The terms “double-glazed” and “triple-glazed” refer to the number of panes, or glass layers, that make up your window. Double refers to two panes, while triple refers to three. (Some people may also call these double-glazing or double-pane windows, or triple-glazing or triple-pane windows.) A spacer separates the panes, and those “pockets” of air are filled with argon or other gasses to slow the transfer of heat through the window. Meanwhile, an airtight seal prevents any leakage. Put simply, double- and triple-glazing have become the standard of energy-efficient windows.

Moreover, the Efficient Windows Collaborative says glazing makes for better insulation performance: double-pane windows can have U-values of 0.3 or even lower, while some triple-pane windows can be as low as 0.15.

image of triple glazed window section

Remember: the lower the U-value, the better

While U-value isn’t the only metric that makes up a good window, it is certainly one worth paying attention to before you make your purchase. The lower the U-value, the better the insulation, and that means cost savings on your energy bills.

According to ENERGY STAR, you can save as much as USD$538 per year by installing a double-pane clear glass window!

Do you need new windows?

At Weaver Exterior Remodeling, we take great pride in the work of our skilled, certified professional installers. Over 20 years, we have earned our track record of success. That’s because we’ve taken it upon ourselves to elevate industry standards of professionalism, customer service, and attention to detail to where we think they should be.

Investing in your home’s exterior is a big decision, and we can help make that decision easier with our proven eight-step process, as well as a Lifetime Warranty on our windows.

Thinking of replacing your windows? Let’s talk about how we can transform your house into your energy-efficient dream home. Try out our interactive quote tool and set up a free estimate with us.

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