Measuring Energy Efficiency in Windows

What Makes a Window Energy-Efficient? To estimate the total efficiency of a window, we need to measure and compare four variables; U-value, Solar Heat Gain Coefficient (SHGC), Visible Light Transmittance(VLT), and Light to Solar Gain (LTG) ratio. Together, they tell how well the glass insulates and how well light passes through the glass.


What are Energy-Efficient Windows?

Energy efficiency can be equated with a window’s value since one of its functions is to save you money on your home’s energy costs. Low-E coated glass, for instance, is a film that improves a product’s energy efficiency. Read more about insulative qualities  in our  article ‘Insulating your Doors and Windows – Before or After’

A Low-E coating on windows improves their function of blocking hot and cold air transference. Low-E windows have add-ons that increase their insulative powers and likewise their R-value. The overall efficiency and worth of a window increase with its R-value.


Learn more about the windows Forest City Window & Door Ltd. here.


Do Energy-Efficient Windows Save Money?

Energy-efficient windows prevent unwanted energy transfer between hot and cold air. 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 but also increases your home’s property value and saves you money each month on your energy bills.

Read more in our article ‘Are Replacement Windows a Good Investment?’

U-value measures how much heat loss a window incurs.

U-value measures how much heat can escape through a window unit. The problem that insulation tries to solve is to prevent hot and cold air from exchanging. These fluctuations create poor living conditions, and you 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.

On top of preventing heat loss, the benefits of energy-efficient new windows are innumerable, and you’ll wonder how you lived without them.

Read the benefits of new windows in our article ’10 Reasons You Should Consider Replacing all Your Windows at Once’

Is a high Solar Heat Gain Coefficient (SHGC) better than having a low SHGC? 

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. 

A high SHGC allows more heat transference through a window. This would be useful for homes 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 is logically suited for hot climates since it prevents external heat from warming your home’s interior. 


What is Visible Light Transmittance (VLT)?

Visible Light Transmittance (VLT) is much simpler to define. While SGHC measures solar radiation, VLT measures the amount of visible light that filters through a window. 

Glass with a high VLT number lets more light in, and a lower VT number allows less light to pass through. Factors influencing preference of these values are personal taste. If you want to decrease glare, then a lower VLT would be better suited. Consequently, high VLT glass helps brighten room interiors.



What is a Light to Solar Gain (LSG) ratio?

Light to Solar Gain (LSG) represents 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 number means the window efficiently improves ambient lighting without 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. 


But how do I 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, these measurements have been calculated beforehand and are available on the windows’ product label. 

It’s helpful to know what the numbers refer to and what each variable means. 

Visit our window page on our website to get started.


Factors that Reduce Energy Efficiency

It’s essential for you also to understand some factors that can reduce energy efficiency for a window. Damage due to ageing reduces energy efficiency, so you should watch your windows as they get older.

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


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