WHAT IS LOW-E GLASS ?
Glass is known to be one of the most popular and adaptable building materials used today, due in part to the constant improvement of the solar and thermal performance, One way this performance is achieved is through the use of passive and solar control low-e coatings.
So, the question often asked is : What is Low-E Glass?
In this blog post , we will provide you with the explanation, and a in-depth overview of the coatings. In order for our customers to understand coatings, it is important to understand the solar energy spectrum or energy from the sun. UV light also known as Ultra Violet light, visible light and IR aka Infrared Light, all occupy different parts of the solar spectrum — the differences between the three are determined by their wavelengths.
Details To Keep In Mind :
- Ultraviolet light – which is what causes interior materials such as fabrics and wall coverings to fade, has wavelengths of 310-380 nano meters when reporting glass performance.
- Visible light – occupies the part of the spectrum between wavelengths from about 380-780 nano meters.
- Infrared light aka heat energy – is transmitted as heat into a building, and begins at wavelengths of 780 nano meters. Solar infrared is commonly referred to as short-wave infrared energy, while heat radiating off of warm objects has higher wavelengths than the sun and referred to as long-wave infrared.
Low-E coatings have been developed in order to minimize the amount of ultraviolet and infrared light that can pass through glass without compromising the amount of visible light that is transmitted.
When heat or light energy is absorbed by glass, it causes it to shift away by moving air by the glass surface. The ability of a material to radiate energy is known as emissivity . High reflective material have a low emissivity and dull darker colored material have a higher emissivity. All materials, including windows, are known to radiate heat in the forms of long-wave, infrared energy depending on the emissivity and temperature of the surface. Radiant energy is one of the important ways heat transfer occurs with windows. Reducing the impassivity of one or more of the window glass surfaces improves a window’s insulating properties.
This is where or low-e glass coatings come into play. Low-E glass has a microscopically thin, transparent coating ( which is extremely thin, thinner than a human hair ) , that reflects long-wave heat. Some low-e’s also reflect significant amounts of short-wave solar infrared energy. When the interior heat energy tries to escape to the colder outside during the winter, the low-e coating reflects the heat back to the inside, reducing the radiant heat loss through the glass. The reverse happens during the summer. To use a simple analogy, low-e glass works the same way as a thermos. A thermos has a silver lining, which reflects the temperature of the drink it contains. The temperature is maintained because of the constant reflection that occurs, as well as the insulating benefits that the air space provides between the inner and outer shells of the thermos, similar to an insulating glass unit. Since low-e glass is comprised of extremely thin layers of silver or other low emissivity materials, the same theory applies. The silver low-e coating reflects the interior temperatures back inside, keeping the room warm or cold.
Low-e Coating Types & Manufacturing Processes
There are actually two different types of low-e coatings
- Passive Low-E Coatings – designed to maximize solar heat gain into a home or building to create the effect of ” passive ” heating and reducing reliance on artificial heating.
- Solar Control Low-E Coatings – designed to limit the amount of solar heat that passes into a home or building for the purpose of keeping buildings cooler and reducing energy consumption related to air conditioning.
Both of the low-e glass , are produced by two primary productions methods :
a) Pyrolytic Process – is applied to the glass ribbon while it is being produces on the float line. The coating then blends to the hot glass surface, creating a strong bond that is very durable for glass processing during fabrication. In the end, the glass is cut into stock sheets of multiple sizes for shipment to the fabricators.
b) Magnetron Sputter Vacuum Deposition – the coating is applied off-line to pre-cut glass in a vacuum chamber at room temperature.
We know we live in a world where technology advances more and more each day. Because of the coating technology, passive low-w coatings are sometimes associated with the pyrolytic process and solar control low-w coatings with MSVD, however this is no longer 100% accurate. Performance varies widely from product and manufacturer to manufacturer, but performance data tables are readily available and several online tools can be used to compare all low-e coatings on the market.
In a standard double panel, there are four potential surfaces to which coatings can be applied. The first, surface faces outdoors, the second and third surfaces face each other inside the insulating glass unit and are separated by and peripheral spacer which creates an insulating air space, and forth surface faces directly indoors.
Low-e Coating Performance Measures
Low-e coatings are applied to the various surfaces of insulating glass units. Whether a low-e coating is considered passive or solar control, they offer improvements in performance values. The following are used to measure the effectiveness of glass with low-e coatings:
- U-Value is the rating given to a window based on how much heat loss it allows.
- Visible Light Transmittance is a measure of how much light passes through a window.
- Solar Heat Gain Coefficient is the fraction of incident solar radiation admitted through a window, both directly transmitted and absorbed & re-radiated inward. The lower a window’s solar heat gain coefficient, the less solar heat it transmits.
- Light to Solar Gain is the ratio between the window’s Solar Heat Gain Coefficient (SHGC) and its visible light transmittance (VLT) rating.
Here is how coatings measure up by minimizing the amount of ultra violet and infrared light that can pass through glass without compromising the amount of visible light that is transmitted.
We hope this article helped you understand what low-e glass is , how it work and the important role it plays in a home. Also, the way it affects the performance of the window, the total heat, light and cooling of your home.