Low-E glass

Low-Emissivity glass (or low-E glass) provides natural light transmission and a low U-value to help 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 code requirements.

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. 

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 hot 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. 

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.

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 the cooling 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.

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 hot 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.

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.

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.

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

Need to compare the performance of our products? Visit our product section to search, compare and filter through our wide range of glass solutions.