Low E, the glass for the future

Introduction

Low-e glass stands for low emissivity glass. This glass varies from normal clear glass in that one side of the glass has a special metal coating, technically known as a low emissitivity, or Low E. coating. Low-E glass is a type of insulating glass, which increases the energy efficiency of windows by reducing the transfer of heat or cold through glass. That means in the winter your house stays warmer, and in the summer it stays cooler.

There are two types of Low-E (low emissivity) glass available - Pyrolytic (hard coat) is considered to be a medium performer, and sputtered (vacuum deposition or soft coat) is considered to be the highest performer.

As energy saving will become more and more important in our daily lives, implementing low-e glass in our houses will soon be a standard we all have to comply to. 

Low-e glass, how does it work?

The radiation coming from your heating system and your furniture and furnishings is long wave radiation. This type of radiation should be contained in your room as best as possible, while the radiation from the sun should be shielded and reflected back outside.

Winter Time

The sun's energy is "SHORT WAVE RADIATION" which passes through the window and is absorbed by carpet, furniture, etc. The energy is then transformed into long wave radiation. The long wave radiation wants to flow from warm to cool. Naturally, it will try to escape from the glass. At night the heat produced from radiators, wood stoves, etc., will also want to escape out through the glazing. The Low-E coating prevents this when the radiant room-side heat is reflected back into the building. This results in a lower winter U-value. For winter comfort, the higher the indoor glass temperature, the better the product is for comfort.

Summer Time

The Low-E coating manages the sun's heat in the summer by reducing the amount of heat transferred through the window in the summer. The Low-E coating filters the sun's short-wave radiation, which cuts down on the amount of solar heat gain into your building. For summer comfort, the lower the indoor glass temperature, the better the product is for comfort. 

More on the two types of Low-e glass

Hard Coat Low E

Hard coat Low E, or pyrolytic coating, is a coating applied at high temperatures and is sprayed onto the glass surface during the float glass process. 

Advantages

• The advantage is that the coating is relatively durable, which allows for ease of handling and tempering.
• Can be tempered after coating application.
• Can be used in single glazing applications.
• Utilizes passive solar heat gain.

Disadvantages

• Higher U-values compared to soft coat Low-E products
• Slightly higher haze levels
• Higher solar heat gain coefficient compared to soft coat Low-E products.
• Hard coat glass also has the possibility of a slight haze, which can be visible under certain angles.

Suppliers include www.pilkington.com

NOTE: Pilkington do supply a type of soft coat low E glass also (Pilkington Optitherm™ SN) - but in the UK most of their Low E supply is in the form of Pilkington K Glass - a hard coat version of Low E.

Soft Coat Low E

Soft coat Low E, or sputter coating, is applied in multiple layers of optically transparent silver sandwiched between layers of metal oxide in a vacuum chamber. This process provides the highest level of performance and a nearly invisible coating.

Advantages

• High visible light transmission
• Ultra-low emissivities giving optimum winter U-values
• Up to 70% less UV transmission compared with standard clear glazing
• Optical clarity - minimal color haze

Disadvantages

• Soft coat Low E must be used in a double glazed unit; the soft coating is sensitive to handling.
• Most soft coat Low-E products require tempering the glass prior to the coating application.
• Edge deletion of the coating is required to insure a proper seal in an insulated unit
• There can be slight color variations of coating.
• Generally speaking, a more expensive alternative than Hard Coat Low e glass.