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Low e coated glass : what is it and how does it work?

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Low-e glass

The solution to solar control and thermal insulation needs

Low-emissivity glass (or low-E glass) provides natural light transmission while helping to limit heat gain and thermal energy transfer.

Due to continuous improvements in its thermal insulation and solar control performance, glass is a flexible building material that can help improve the energy efficiency of buildings. One way this performance can be achieved is through the use of low-E coatings, a critical part of the glazing, which enable building occupants to visually engage with the external environment from a comfortable interior.

What is low-E glass and why do we use it? 

Low-E coatings in architectural glazing can help control temperatures inside a building by providing varying levels of thermal insulation and/or solar control. Low-E glazing can allow plenty of natural light into interior spaces, helping to improve occupant comfort while contributing to the energy efficiency of the building. 

Low-E coatings also offer architects a wide range of aesthetic options in terms of colors, allowing them to select the most appropriate look to suit their project.

What are the benefits of low e glass?

Low-E coatings offer a range of contributions to architectural glazing.

In terms of aesthetics, the glazing can be made to look neutral or colored, with the desired level of reflection, light transmission and privacy. Architects have the freedom to create visually stunning glazed facades that help integrate the building with its environment by reflecting the surroundings or by showing what’s happening inside a building. In terms of energy performance, the routing of solar energy and the intensity of thermal insulation are impacted. Low-E coatings can help meet ENERGY STAR® program and other energy code requirements.

Thermal insulation

In terms of its thermal insulation performance, low-E glass offers the following benefits:

  • Can allow the sun’s rays to pass through the glass when desired.
  • Helps reflect indoor heat back into the room.
  • Can form a shield against the cold exterior.
  • Generally used in double or triple glazing.
  • Can help to reduce energy costs associated with indoor heating systems.

Solar control: low-E glazing lets the light in

Often specified for windows, roofs and glazed facades, solar control glass help optimize light transmission, solar control and thermal performance. The glass lets sunlight pass through while reflecting a large proportion of the sun’s heat.

Solar control low-E glass is ideal for maximizing natural daylight and reflecting the most solar heat back to its source. The indoor space remains bright and cooler compared to uncoated glass. Energy efficiency can also be improved as solar control glass helps reduce air conditioning loads during warmer months.

Spectral selectivity

Light coming inside the building, as well as coatings and their placement in the glazing, are crucial to support occupant comfort. The term “spectral selectivity” is used to address the amount of daylight transmission relative to solar energy blockage. Spectral selectivity is calculated by dividing the visible light transmission (VLT) by the SHGC or solar factor. Greater spectral selectivity is achieved when more visible light, and less overall solar energy, are transmitted. The United States Department of Energy has established a Light-to-Solar Gain (LSG) Ratio of 1.25 as the minimum measurement to be classified as a “spectral selective glazing”.

Helping to reduce destructive UV rays and glare

Depending on the location and orientation of the building, low-E glazing can help mitigate glare from the sun and increase the visual comfort of building occupants, particularly if a glazed façade is directly exposed to the sun and with a high window-to-wall ratio.

Ultraviolet (UV) light can be attributed to approximately 50% of interior furnishing and fittings fading. Using laminated glass can block up to 99% of UV light.

How does Low-E glass work?

In order to understand how low-E glass works, it is necessary to consider how glass interacts with the electromagnetic spectrum

Solar energy (or shortwave radiation) is received at the surface of the earth from the sun. It includes ultraviolet, visible, and near-infrared wavelengths – together ranging from 300 to 2,500 nm. Solar control coated glass can block a significant amount of this energy by reflecting and absorbing part of it.

Longwave radiation includes wavelengths from 5,000 to 50,000 nm. Low-E coatings on glass are designed to slow radiant heat transfer. Low-E coatings reflect longwave radiation (heat) back into the structure during cooler periods. 

Glass and the Electromagnetic Spectrum

 

Thermal insulating performance of low-E glass

Heat transfer takes place through three mechanisms: radiation, conduction and convection. All three types of heat transfer take place within an insulating glass unit (IGU). 

  • Radiation is heat transfer by electromagnetic energy, where longwave energy moves through the glazing assembly.
  • Conduction is the heat transfer within a medium (solid or gas) caused by temperature differences. This transfer is reduced by using argon inside the IGU because of its lower thermal conductivity compared to air. Along the edge of the IGU, conductive heat moves from glass to spacer and back to glass. This transfer can be slowed by using a warm edge spacer.
  • Convection is heat transfer through currents of slowly moving air or gas within the IGU.

Insulating performance refers to the reduction of heat transfer associated with exterior-versus-interior air temperature differences.

Hot and cold climates

In cold climates, insulating performance is a benefit. Thermal insulation glass can allow incoming shortwave energy to enter and improves the retention of heat inside of the building, with longwave energy reflected.

In warm climates, the glazing should reduce both incoming shortwave energy, as well as incidental incoming longwave energy, which can help lessen the need for air conditioning

Regardless of the climate, a glazing that lowers heat transfer is a benefit for energy performance.

The insulating performance of glazing can be improved with the use of a low-E coating that performs well in reflecting longwave energy. The number 3 surface placement of a thermal insulation low-E coating can be ideal by helping to improve the retention of heat inside of the building. This performance can also be improved by adding another pane of glass to the IGU so that multiple cavities and coated surfaces can contribute to insulating performance.

How is insulating performance measured?

Insulating performance is measured using a parameter called the “U-value”, which describes energy transfer per unit time, i.e. the duration required for energy transfer per unit of glazing area and per degree of temperature difference between the exterior and interior conditions.

If a glazing assembly has strong thermal insulating performance, only a small amount of energy will be transferred and so the U-value will be low.

Solar Energy Performance

How is solar heat gain measured? 

The solar heat gain coefficient (SHGC) and the solar factor (g-value) are used to measure the solar energy that transfers indoors. This includes direct transmission and indirect transmission because of absorption and inward re-radiation. The SHGC is the decimal share of the solar energy that is transmitted through a glazing composition. For example, if 31% of the incoming solar energy passes through the glazing, the SHGC is 0.31. 

 

Solar energy and its interaction with glass 

When a ray of electromagnetic energy strikes a glass pane, some of the energy may be reflected, some may be absorbed and any remaining energy will be transmitted.  

Hot and cold climates 

For a building project located in a warm or moderate climate, a low Solar Heat Gain Coefficient (SHGC) or g-value is preferred. The number 2 surface placement of a solar control low-E coating often facilitates the best performance because it reflects away part of the incoming solar energy before it can enter the glazing

In particularly cold climates, a higher SHGC or g-value could be beneficial to allow passive heat gain.

 

Low-e coating types and manufacturing processes

Low-E coatings can be divided into two categories: those that are applied through pyrolytic deposition (hard coatings), and those applied through sputter deposition (soft coatings). Guardian Glass production plants only use the sputter deposition process. 

Sputter coatings

Sputter coatings are applied in a large magnetron sputter vacuum deposition machine, offline from the float glass production process. The fully formed glass travels along a conveyor system through a long vacuum chamber where a range of materials are sequentially accumulated on the glass surface.  Together these materials measure approximately 1/500th of the thickness of a sheet of paper.   

Sputter coatings are applied with more precision than pyrolytic coatings. Current sputter-coated low-E coatings are multilayered complex designs engineered to provide high visible light transmission, low visible light reflection and reduce heat transfer.

Pyrolytic coatings 

Pyrolytic coatings are applied online during the float glass production process. The upper surface of the glass ribbon is sprayed with material, typically tin oxide. As the glass cools, the surface bond solidifies, creating a strong bond that is very durable for glass processing during fabrication. However, their properties are very limited due to their simple structure.

video-thumbnail

How is low-E glass coated? See the sputter coating process in this video animation.

The influence of silver

Depending on the performance requirement of the coating, as many as 15 layers may be needed to achieve the desired performance. 

Of the materials that comprise a sputter low-E coating, silver is the most influential in enhancing energy performance. As more silver layers are added to the composition of the coating, the better the spectral selectivity of the glass will be.

Want to know more about coated glass?

Guardian Glass offers you an intercative way to enhance your knowledge about glass. Connect to the Training center to start a CEU module, and more!

 

Applications and uses of low-E glass 

The applications for architectural low-E glazing are wide ranging. From windows and curtain walls to roofs and skylights, in fact, any application where glazing is a physical barrier between the inside and outside of a building, low-e glazing can be considered.

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Curtain walls

A curtain wall is a non-structural outer covering of a building that can be made of glass. Using low-E coated glass in a curtain wall helps designers to control the appearance (e.g. reflectivity, transparency, color) and performance of the glazing, including thermal insulation and solar protection.  

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Roofs and skylights 

Overhead glazing such as roof glazing can help reduce the need for artificial lighting, as well as provide a natural source of daylight to help brighten and open up the interior spaces of a building. Low-E coatings in the glazing can help address thermal insulation and solar control needs. Low-E glass can even be laminated for use in overhead safety glass applications

Curved glazing

In curved or bent glazing, low-E coatings have been developed that withstand the bending process without affecting the visual appearance of the glass. Low-E coatings are available for curved glass applications that still provide the required thermal and solar performance to contribute to the energy efficiency of the building.

Oversized glazing

Oversized low-E glass provides innovative possibilities for designers to create unique, striking designs. Using large panels of low-E glass can help minimize the use of structural elements, while creating a seamless glass façade that allows building interiors to be flooded with more natural daylight and which can provide spectacular views for occupants to enjoy, as well as meeting the solar and thermal performance needs of the project.

Combining tinted glass with low-E glass

Tinted glass is produced by small additions of metal oxides to the float glass composition. Tinted glass helps reduce both visible light and solar heat gain. 

Tinted glass colors can range from blue to grey with different contrasts created by using different metal oxides. Low-E coatings can be applied on tinted glass substrates to:

  • Further improve the solar performance of the glass.
  • Allow more aesthetic options by combining different substrates and coatings.
  • Select the right level of light transmission to help mitigate glare.
  • Help maintain the privacy of a home or commercial building by making it more difficult for people outside the building to see inside.

Glass is a critical, versatile building material that has many uses and benefits. Each build project will have its own unique requirements in terms of the environment, climate and how the glass should look, function and perform. Low-E glass can help meet these challenging requirements, particularly in terms of solar control, thermal insulation and how these affect the energy efficiency of buildings. With such a wide range of low-e coatings to choose from, architects and designers can find the most appropriate low-e coated glazing to suit their aesthetic and performance needs, while helping to enhance the comfort and wellbeing of building occupants and meeting their energy efficiency targets.

Some of our projects made possible with low-E glass:

Want to see more architectural projects made with low emissivity glass? Visit our project section.

Our low-E glass products:

Guardian SunGuard® glass

Our Guardian SunGuard® range of low-E products combine solar control, light transmission and low solar heat gain properties with a range of colors and appearances to help accommodate any application.

Discuss your project with our team!