Introduction to Energy-Saving Glass

Energy-saving glass, also known as insulating glass, refers to a type of glass that improves the energy efficiency of buildings. It is constructed of two or more panes of glass separated by a spacer and sealed together at the edges. The air space between the panes acts as an insulator, reducing heat loss in winter and heat gain in summer.

The demand for energy-efficient buildings has increased in recent years due to rising energy costs and stricter building codes focused on reducing energy consumption. Using energy-saving glass is one of the most effective ways to improve the insulation of the building envelope. It can reduce heating and cooling loads, helping lower energy bills while maintaining occupant comfort.

Compared to standard single-pane windows, energy-saving glass provides much higher insulation. It improves insulation by trapping air or other gases between the glass panes, reducing convective heat flow. The more layers of glass, the better the insulation performance. Many modern energy-efficient buildings utilize triple-pane insulating glass to maximize energy savings.

As energy codes continue to evolve, there is a need for glass with even higher insulation values. This is driving innovations in glazing technologies and the development of more thermally efficient window frame materials. Energy-saving glass has become an essential component in constructing sustainable, high-performance buildings.

Types of Glass Used

There are several types of glass that can be used in energy-saving windows. These include:

Heat Absorbing Glass

Heat absorbing glass is tinted with minerals that selectively absorb heat. It reduces solar heat gain by absorbing more solar radiation. Common tints for heat absorbing glass include gray, bronze, and green. The depth of the tint affects the degree of solar heat gain reduction. A darker tint absorbs more solar radiation. Heat absorbing glass has no effect on insulating value or conductivity.

Low-E Glass

Low-emissivity (Low-E) glass has a special metallic coating that reflects long-wave infrared radiation. This reduces the transfer of heat through the glass. Low-E coatings can reduce the U-factor, or rate of heat loss, by up to 30-50% compared to clear glass. There are two types of Low-E coatings:

  • Hard coat Low-E – More durable, applied via pyrolytic process
  • Soft coat Low-E – More effective, applied via sputtering process

Low-E glass provides better insulation than tinted and reflective glass. It also transmits visible light well.

Solar Control Coated Glass

Solar control coated glass has a microscopically thin coating that reduces solar heat gain. The coating reflects and absorbs solar radiation. Different coatings can be tuned to different parts of the solar spectrum. Solar control coated glass provides shading without greatly reducing visible transmittance.

Compared to clear glass, all of these specialty glasses reduce solar heat gain. Low-E glass also improves insulating value. Tints and coatings can be combined for optimal energy performance. The best energy savings are achieved with Low-E coatings on tinted glass.

Impact of Glass Thickness

The thickness of glass used in windows has a direct impact on its insulating properties and energy efficiency. Thicker glass provides better insulation and reduces heat transfer through the window. Here are some key points on how glass thickness affects energy savings:

  • The thicker the glass, the better it insulates. Double or triple pane windows with thicker glass panes have much lower U-factors (rate of heat transfer) than single pane windows.
  • Most energy efficient windows today use double or triple glazing with glass thicknesses between 3-6mm per pane. Going beyond 6mm provides diminishing returns on better insulation.
  • In colder climates, choosing windows with thicker glass (low-E coated) gives better insulation against heat loss and lower heating bills. The optimal thickness is around 4-6mm per pane.
  • In warmer climates, thicker glass also reduces solar heat gain and cooling costs. 5-6mm thickness is ideal for better insulation without sacrificing too much natural light.
  • The gap between panes also affects insulation. Wider gaps (up to 20mm) filled with argon/krypton gas improve insulation efficiency in multi-pane windows.
  • Balancing thickness, number of panes, and gas fills optimizes the energy savings from windows. 6mm triple glazed with argon fill is an efficient combination for most climates.

So in summary, while single pane 3mm glass offers little insulation, 4-6mm double or triple glazing dramatically improves energy efficiency. Choosing the right thickness and number of panes for your climate optimizes energy savings from windows.

Impact of Tint and Color

The tint or color of glass affects both the amount of light that passes through (visible transmittance) as well as how much solar heat is transmitted (solar heat gain coefficient). Darker tints block more visible light from entering, but also more solar heat.

Tints are added to glass through either a surface coating or mixed directly into the glass during manufacturing. Common tints include gray, bronze, green, and blue. These tints reduce both light and heat transmittance compared to clear glass.

For example, a gray tinted glass may have a visible transmittance of 50% compared to 90% for clear glass. The tint cuts the passage of light in half. The solar heat gain coefficient is also reduced from 0.86 to 0.46. This means only 46% of solar radiation passes through gray tinted glass versus 86% for clear.

The tradeoff is less natural lighting, but also less heat gain in summer. Tints can help control glare and fading, while improving comfort and energy efficiency. Darker tints have a greater impact, but also block more desirable visible light.

Light colors like pale blue and green strike a better balance. They reduce solar gain significantly, while still transmitting 60-70% of visible light. This improves energy savings, while still allowing ample natural lighting.

Proper tint selection depends on climate and orientation. In hot, sunny climates, darker tints help cut cooling costs. In cloudier northern locations, lighter tints maintain sufficient daylighting. East and west facing windows, which get more direct sun, also benefit from darker tints. The optimal balance improves comfort and efficiency.

Impact of Air vs Argon Fill

One of the most impactful factors on the insulating value of a window is the gas fill between the glass panes. Typical options are air or argon gas.

Air has a thermal conductivity of 0.024 W/mK, while argon has a much lower thermal conductivity of 0.016 W/mK. This means argon provides significantly better insulation.

Studies have shown that replacing the air fill with argon gas can improve the U-factor (insulating value) of a window by 10-15%. For example, a double pane window filled with air may have a U-factor of 0.35, while switching to argon fill could bring that down to 0.30.

The improved insulation leads to lower heat loss through the window, reducing energy usage for heating in the winter. Argon-filled windows also reduce condensation on the glass by keeping the inner pane warmer.

The downside is that argon fill adds cost to the window. However, the energy savings can provide a return on investment in 5-10 years. For cold climates, argon fill is often recommended as one of the most cost-effective efficiency upgrades. Additionally, argon can leak out of the window pane unit quickly, meaning the added cost and efficiency is lost.

Impact of Number of Panes

The number of panes in an insulated glass unit impacts its energy efficiency and cost. The more panes, the better the insulation, but also the higher the cost.

Double pane windows are the most common. They have two panes of glass separated by an air gap, usually around 1/2″ wide. The air gap creates an insulating layer that reduces heat transfer through the window. Double pane windows have a U-factor (measure of heat transfer) of around 0.30-0.50.

Triple pane windows add another pane and air gap, creating two insulating layers. The extra pane improves the U-factor to around 0.20-0.30. This significantly reduces heat loss in winter. Triple pane windows are recommended for colder climates.

The optimal number of panes depends on climate and budget. In moderate climates, double pane is usually sufficient. In very cold climates, triple pane provides noticeably better insulation. Going beyond triple pane provides diminishing returns and is usually not cost-effective.

The choice between double and triple pane windows involves tradeoffs between upfront cost and long-term energy savings. Triple pane costs 15-30% more but can reduce heating/cooling costs by around 10-15% compared to double pane. Payback period is around 10 years.

Impact of Low-Emissivity Coatings

Low-emissivity (low-E) coatings are microscopically thin, virtually invisible, metal or metallic oxide layers deposited on a window or skylight glazing surface primarily to reduce the U-factor by suppressing radiative heat flow. They are spectrally selective, which means they reflect infrared energy (heat) while transmitting visible light.

There are two categories of low-E coatings:

  • Hard-coat low-E – More durable pyrolitic coating applied to the glass at high temperatures. They typically have an emissivity between 0.15 and 0.4.
  • Soft-coat low-E – Less durable sputter-coated layer applied in a vacuum chamber. They have a lower emissivity, typically between 0.01 and 0.1.

Compared to uncoated glass, low-E coatings can reduce the U-factor by as much as 0.3-0.5. This can significantly improve the insulating value of the window and reduce heat loss in winter and heat gain in summer. Low-E coatings are now standard on most energy efficient windows.

The lower the emissivity, the lower the U-factor. However, lower emissivity also results in lower solar heat gain coefficient (SHGC). There is usually an optimum emissivity that balances sufficient solar heat gain in winter against excessive heat gain in summer.

Impact of Frame Material

The type of frame used for insulating glass windows can impact their energy efficiency. This is because the frame serves as the edge spacer between the glass panes and influences heat flow. Common frame materials include:

  • Aluminum – Aluminum frames have high conductivity, which allows heat to easily transfer through the frame. This reduces the insulation capability of the window. However, aluminum is durable and affordable.
  • Wood – Wood has much lower conductivity than aluminum, creating a better insulated edge spacer. Wood frames improve the energy efficiency of windows compared to aluminum. However, wood requires more maintenance and has a shorter lifespan.
  • Vinyl – Vinyl frames have very low conductivity, similar to wood. This makes vinyl one of the most energy efficient frame options. Vinyl also has the benefits of being affordable and durable.
  • Fiberglass – Fiberglass frames have insulation properties comparable to wood and vinyl. Fiberglass combines good insulation with strength and dimensional stability.
  • Composite – Composite frames blend materials like vinyl, wood, and fiberglass to achieve optimal energy efficiency, durability, and cost. Advanced composites provide excellent thermal performance.

When selecting window frames, the choice often involves tradeoffs between insulation, durability, maintenance, and cost. Comparing conductivity and energy efficiency ratings can help identify the best option for a specific climate and budget. High-performance frames enhance the energy savings of efficient glass types.

Impact of Window Orientation

The orientation of windows in a building has a significant impact on their energy efficiency and performance. The direction a window faces determines how much direct sunlight it receives, which affects solar heat gain. Strategically orienting windows can maximize benefits of passive solar heating in winter and minimize overheating from sunlight in summer.

Effect of Direction Faced

  • South-facing windows receive the most direct sun exposure. In cold climates, placing more glass area on the south side allows solar radiation to warm the building passively during winter. However, south-facing windows can cause overheating issues in summer if not properly shaded.
  • East-facing windows receive morning sun, which provides some solar heat gain in winter. However, the low morning sun can cause glare issues. East-facing glass should be limited in hot climates to prevent overheating.
  • West-facing windows receive intense afternoon sun in summer, leading to overheating issues. Minimizing west-facing glass helps control heat gain in hot climates. However, the afternoon sun can supplement heating in winter.
  • North-facing windows receive the least direct sunlight. North-facing glass maximizes daylighting while minimizing solar heat gain, making it ideal for hot climates. However, minimal solar gain can increase heating loads in cold climates.

Strategies for Optimal Orientation

  • Place the majority of window area on the south facade in cold climates for passive solar heating. Use overhangs, tints, and low-E coatings to control summer heat gain.
  • Limit west-facing glass in hot climates to control afternoon solar heat gain. Optimize north and east-facing glass for daylighting.
  • Balance glass distribution for east, south, and west exposures in temperate climates. Use shading, tints, and low-E coatings to tune seasonal solar gain.
  • Orient primary living spaces with more glass to the south and daily use spaces to the north in mixed climates.
  • Consider adjacent buildings and site obstructions that may impact solar access when orienting windows.
  • Follow solar design guidelines and utilize solar modeling tools to optimize orientation for local climate and building parameters.


The various factors that impact the energy efficiency of glass all interact with each other in complex ways. Here’s a summary of the key factors:

  • Glass thickness – Thicker glass provides more insulation. Going from single pane to double pane provides a big jump in efficiency. Further gains from triple pane are smaller.
  • Tint/color – Tints like green and blue reduce solar heat gain compared to clear glass. This helps in hot climates.
  • Gas fill – Using argon gas instead of air provides a modest improvement in insulation.
  • Low-E coatings – A metallic low-emissivity coating is the most impactful upgrade. It prevents radiant heat transfer through the glass.
  • Frame material – Frames with thermal breaks, like fiberglass or vinyl, outperform metal frames.
  • Orientation – South-facing windows get more solar heat gain than other orientations. East/west facing are next. North is least.

For optimal energy efficiency in colder climates, choose double pane windows with Low-E coatings and argon fill, with a fiberglass or vinyl frame. Avoid excessive south and west facing glass.

In hotter climates, also look for tinted glass to reduce solar heat gain. Triple pane windows provide diminishing returns except in extremely cold locations. Carefully weigh the costs vs benefits for your climate and project goals.