What Are Low-E Energy Efficient Windows?

What Low-E Glass Means and How It Makes for Energy-Efficient Windows

More homeowners are making the switch to energy efficient windows.

These windows reduce home energy consumption, heating and cooling costs, and carbon footprints.

If you’re looking to make your home more energy efficient with Low-E windows, look no further.

There are various options available for these windows. And to help reduce the costs of energy efficient windows, there’s an Ontario rebate program available through your local window manufacturer.

To help you find energy efficient windows for your home, consider the following information and look for top-rated windows in Canada using energy performance ratings.

What Are Energy Efficient Windows?

Energy-efficient windows reduce a building’s reliance on artificial heating and cooling systems by maintaining comfortable indoor air temperatures. These windows far surpass conventional windows when it comes to insulating homes against heat and cold.

While energy efficient windows will make an improvement in your home’s temperatures, you can make your home even more energy-efficient by choosing specific windows for the various sides of your home.

For example, east- and west-facing windows will get direct sunlight, so they should have strong solar protection. But you can also take advantage of passive solar heating from your north-facing windows.

You can choose your windows depending on the direction they face and how much sunlight they get during the day.

What Does Low-E Mean?

Glass naturally has high thermal emissivity. This means absorbing and allowing heat to transfer easily through to the other side.

High emissivity causes high energy consumption to maintain comfortable indoor temperatures. As a window absorbs heat from the sun during the day, it will also lose that heat just as easily at night.

In the summer, the heat from outside will come in, putting a strain on your air conditioning system. And in the winter, the heat from your home will escape through high thermal emissivity glass.

But Low-E windows will prevent heat from passing through. Low-E is short for low emissivity, referring to low thermal emissivity.

Low-E windows have a thin reflective coating that allows visible light in but reflects infrared light (heat energy) and ultraviolet light. This coating may be applied to the inside of the outer pane, the inner pane, or both panes of double-pane windows.

This means that heat from sunlight will be reflected off of windows instead of passing through. And the same goes for heat from inside your home, generated from your home heating system. Your heat will reflect back into your home instead of passing through the window’s glass when the outdoor air temperatures are colder.

Without Low-E coatings, ultraviolet light will fade furniture and infrared light (heat energy) will heat up a building.

Passive Low-E Coatings

Instead of reflecting all heat, these coatings maximize solar heat gain in a home or building. This provides passive heating instead of relying heavily on artificial heating.

Solar Control Low-E Coatings

These coatings limit the amount of solar heat that enters a home or building. They keep the internal temperatures cool and reduce energy consumption for air conditioning.

To maximize the energy-efficiency of your home, you can install windows with both passive and solar control low-e coatings. Use passive for north- and south-facing windows, and solar control for east- and west-facing windows.

Both types of low-E coatings are produced using the following methods:

Hard Coat – Pyrolytic Coatings

Pyrolytic coatings usually consist of tin dioxide. These coatings are fused to the glass at a very high temperature during the manufacturing process, creating a durable bond. The glass is then cut into sheets for window manufacturing.

Soft Coat – Magnetron Sputter Vacuum Deposition (MSVD)

The MSVD process applies thin layers of silver to pre-cut pieces of glass in a vacuum chamber.

How Energy Star® Ratings Work

Energy Star ratings use performance criteria certified by the National Fenestration Rating Council (NFRC) in the United States and the Canadian Standards Association in Canada.

These criteria vary by climate and building type.

Energy Star ratings for windows in the US are based on U-factor and SHGC. In Canada, the following values influence a window’s rated energy performance.


The rate of heat transfer from warm to cold areas—how much heat loss a window allows. The lower the U-factor value, the more energy-efficient the window is.

Solar Heat Gain Coefficient (SHGC)

The amount of solar heat that can pass through a window, represented as a ratio (fraction). The higher the number, the more solar heat gained through the window.


A value for the window’s resistance to heat transfer. The higher the number, the more energy-efficient the window is. (This isn’t part of energy performance standards, but is used by contractors and sales staff.)

Visible Transmittance (VT)

The amount of visible light that can pass through a window. The higher the number, the more visible light can pass through.

Centre-Of-Glass Rating

The centre-of-glass rating is a value for the energy-efficiency of the glass part of a window, not the entire window.

Energy Rating

A value representing the balance between U-factor, SHGC, and air leakage. The higher the number, the more energy-efficient the window.

What to Look for In Energy Efficient Windows

An energy-efficient window uses various components for optimal insulation. These include:

Window Frames

Window frames with a low U-factor value won’t allow for as much heat loss. These window frames are often made of uPVC, timber, or a composite aluminum/timber design.

Weather Seals

Gaps and cracks around window sashes and frames allow for plenty of hot and cold air to enter homes. This is where high-quality window seals come in to keep windows air-tight and well-insulated.

Low-E Glass

Low-e glass has a transparent coating that prevents solar heat from passing through or allows for passive solar heating.

You can have a single-pane window with low-E glass to block the radiant heat from the sun. But a single pane won’t insulate well enough to prevent conductive and convective heat transfer.

Double and Triple Glazed Windows

For high-performance, energy-efficient windows, look for double- and triple-glazed windows. A quality double-glazed window will have two window panes with a low-e coating on at least one pane, and argon gas between panes for added insulation.

Those with wider gaps between panes are even more energy-efficient. And triple-glazed windows (with three panes) are the ultimate design for insulation.

Now that you know what a low-E window is, you can look for the best types for your home to reap the many benefits of energy-efficient windows.

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