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Measuring Energy Efficiency in Windows

What are Energy-Efficient Windows?

Energy efficiency can be equated with the worth of windows since one of their functions is to save you money on your home’s energy costs. Low-E coated glass is one of the features for windows, skylights, and doors that improves a product’s energy efficiency.

Having Low-E coating on windows improves their function as barriers by preventing or hindering hot and cold air transfer. Without this inhibition, it is common for undesirable energy loss to occur. 

Low-E windows come with a variety of additions that increase its insulative powers. Consequently, each of these unique additions has a measurement associated with it. When combined, these equations create numbers that determine the overall efficiency and worth of a window.

 

Do Energy-Efficient Windows Save Money?

Energy-efficient windows prevent unwanted energy transfer between hot and cold air. In plain terms, this means they prevent unwanted heat loss during the winter, unwanted cold air loss during the summer, and unwanted heat gain through external sources. 

According to a Canadian Government publication, “Windows and doors can account for up to 25 percent of total house heat loss.”

Similarly, a study shows that Canadians spend a significant amount on energy at home. “Ontario also had a comparatively high incidence of energy poverty, with a 2013 measurement of 7.5% of households.”

Investing in energy-efficient windows for your home typically costs more upfront. Still, it is worth it to increase your home’s property value and save you money each month on your energy bills.

 

What Makes a Window Energy-Efficient? 

To estimate the total efficiency of a window, we need to measure and compare four variables. Altogether, they represent how well the glass can insulate and how well it performs in lighting. These variables are the U-value, SHGC, VLT, and LTG ratio. 

U-value

This measures how much heat loss a window incurs. The problem that insulation tries to solve is to prevent hot and cold air from exchanging with one another. These fluctuations create un-ideal conditions for living and may incur extra energy costs as temperature regulation is needed. 

For instance, heated homes suffer during the winter if heat can escape through a window’s glass. Once again, U-value measures how much heat can escape through a window unit. 


Solar Heat Gain Coefficient (SHGC)

While the U-factor measures heat going from inside to outside, the SHGC measures the amount of solar radiation going from outside to inside. Lower values of SHGC indicate less heat transmission occurring. 

 

Is a High SHGC Better, or a Low SHGC? 

A high SHGC allows a lot of heat to transfer through a window. This would be useful for homes located in a colder climate like Canada since it absorbs as much sunlight as possible. It is energy efficient since it increases internal heating. Therefore, a high SHGC is less optimal for a home located in a dry, hot climate. 

Low SGHC then is logically suited for hot climates, since it prevents external heat from warming your home’s interior. 

 

Visible Light Transmittance

VT is much simpler to define. While SGHC measures solar radiation, visual transmittance measures the amount of visible light that filters through a window. 

Glass with high VT lets more light in, and lower VT lets less light through. Factors influencing preference of these values are personal taste. 

If you want to decrease glare, then lower VT would be better suited. Consequently, high VT glass helps brighten up rooms. 

Light to Solar Gain

A ratio representing visible light compared to the solar gain coefficient. It takes the two previous factors and uses them to determine the lighting efficiency of the glass.

For example, a higher LSG means that the window is efficient at improving ambient lighting, while not absorbing solar radiation. An application of this is a window that lights a room very well but reduces the solar radiation that comes with it. 

 

How Do You Measure All of This?


You don’t need to worry about buying fancy scientific equipment tuned for measuring delicate things like light and heat transfer. Luckily, all of these measurements have been calculated beforehand and are available on the windows’ product label. 

All you need to know are the significance of the numbers and what each variable means. 

 

Factors that Reduce Energy Efficiency

It’s essential for you also to understand some factors that can reduce energy efficiency for a window. Damages due to ageing can reduce the energy efficiency of a window, so you should keep an eye on them as they get older.

Some other factors that can reduce a window’s energy efficiency are:

 

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