Why to choose Low-E window glass?

1. What is low-E window glass

Low-e window glass is a specially treated glass with low emissivity. Low emissivity refers to a surface condition that emits low levels of radiant thermal (heat) energy Because the window glass surface is coated with silver and metal oxide film of Low radiation material, making the window glass appear different colors. This special thermal coating has the characteristics of high transmission to visible light and high reflection to far infrared ray, which makes it have excellent heat insulation effect and good light transmission compared with ordinary window glass and traditional coating window glass for building. It keeps heat on the right side of the window glass. In the winter,low-e window glass keeps the heat inside, and in the summer, low-e window glass keeps solar heat outside. Low-e window glass acts as a high performance thermal insulator, reducing heating and cooling costs, improving energy efficiency, and reducing the carbon footprint of a building. What makes low-e window glass so special is its ability to dramatically improve the comfort and energy efficiency of your building while enabling visible light to pass through the window glass. 

2. How low-E window glass works

In order to understand how the Low-E coating works, it is important to understand the source of heat energy. The maximum heat energy in the natural environment is solar radiant energy, in which the energy of visible light accounts for only about 1/3, and the remaining 2/3 is mainly thermal radiant energy. Another form of heat energy in nature is far-infrared thermal radiant energy (wherein the thermal effects of infrared thermal radiant energy on the human body are significant). This part of the heat is radiated by the sun after being absorbed by the object and then radiated out. Therefore in summer, it becomes one of the main heat sources from the outdoors . This part of the heat is also generated by heating, household appliances, furniture after the sunshine and the human body, therefore in winter, it becomes the main source of heat from the indoors.

Solar radiation is projected onto the  window glass, one part is absorbed or reflected by the window glass, and the other part is transmitted directly through the window glass. The solar energy absorbed by window glass raise glass’s temperature, then the heat was transmited to indoors by convection and outward radiation. For the heating from the far-infrared heat radiation, the window glass cannot pass through directly, only reflects or absorbs it, and finally some heat passes through the window glass only in the form of conduction, radiation and convection.

The strength of window glass absorption is directly related to the blocking effect of window glass on far infrared heat. The window glass with low emissivity is not easy to absorb the external heat radiation energy, so the heat energy transmitted by the window glass through conduction, radiation and convection is less. The low-E window glass limits this part of heat to transfer.

If making window glass as a boundary, we hope to reduce the outdoor solar thermal radiation energy in the summer or at low latitudes, and block the far-infrared radiation energy outside; in winter or at high latitudes, we hope that outdoor solar thermal radiation energy comes in, and the infrared radiation energy in the room should not be leaked. Therefore, choosing a window glass with lower U-Factor (thermal transmittance) is the key.

The transmission range of ordinary transparent window glass coincides with the range of the solar radiation spectrum. Therefore, while transmitting visible light, the infrared heat energy in the sunlight also passes through the window glass in a large amount, and the thermal energy in the middle and infrared bands of 3 to 5 μm is absorbed in a large amount. , which causes it to not effectively block solar radiation energy.

However, Low-E window glass has a microscopically thin, transparent coating—it is much thinner than a human hair—that reflects long-wave infrared energy (or 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 window glass. The reverse happens during the summer. To use a simple analogy, low-e window 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 window glass unit. Since low-e window 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.

3. How to choose low-E window glass

Glass is one of the most popular and versatile building materials used today, due in part to its constantly improving solar and thermal performance. One way this performance is achieved is through the use of passive and solar control low-e coatings.

Solar control Low-E window glass, shading coefficient Sc <0.5. Features: 1.it can block the strong light from outdoors, but the visible light can pass properly. 2. The solar transmittance is relatively moderate, but it can greatly reduce the solar heat radiation into the room; 3. the far-field infrared reflectivity is high, effectively preventing the secondary heat radiation from outdoors to entering the room. Therefore, this is extremely suitable for the southern region where use the air conditioner. In the summer, the solar energy can be restricted from entering the room to the maximum extent, and the far-infrared heat radiation from the outdoor is blocked, and the extravasation of the air-conditioning cold air can be reduced to save the use cost of the air conditioner.

Passive Low-E window glass, shading coefficient Sc≥0.5, characteristics: 1. solar energy transmittance is higher than solar control low-E window glass, most of the solar energy can be radiated into the room through the glass; less attenuation of the transmitted solar energy. 2. COSCO infrared rays can be reflected back indoors well, so they have good thermal insulation properties. Therefore, this is extremely suitable for the northern region where heating is dominant. In the winter, the solar radiation can enter the room through the Low-E window glass, and after being absorbed by the indoor objects, it becomes the far-infrared heat radiation that the Low-E glass cannot penetrate. And it is confined indoors together with the far-infrared heat radiation emitted by the indoor heating, so that the indoor temperature keeps in a high temperature level, and the energy saving effect is achieved. In addition, it is also suitable for buildings with high requirements for light. For example, office buildings, shopping malls, etc.

4. How to place different low-E window glass for windows with insulated glass unit(IGU)?

Factors that may influence your low-e coating selection and placement strategy include:

• Heating or Cooling dominated climate

• Energy performance

▪ Building codes

▪ Project HVAC requirements

• Aesthetic objectives

• Site characteristics

• Additional design factors

Solar control Low-E Coatings Placement Strategy

A) Double-Pane Solar Control Low-E Coating Placement Strategy

• Apply solar control low-e coating to #2 surface of the IGU to maximize solarcontrol performance.

• A second low-e coating (engineered for "interior surface”) can be placed on surface #4 to optimize insulating performance.

• Only one low-e coating should be in an airspace for best performance.

B) Triple-Pane Solar Control Low-E Coating Placement Strategy

• Apply a solar control low-e coating to the #2 surface and a second low-e coating to the #4 surface of the IGU to optimize solar control performance.

• Placing a third low-e coating engineered for "interior surface”(surface #6) of a triple-pane IGU will further enhance insulating performance.

Passive Low-E Coatings Placement Strategy

Unlike solar control low-e coatings, passive low-e coatings allow some of the sun’s short-wave infrared energy to pass through and help heat building interiors.

A) Double-Pane Passive Low-E Coatings

• Apply passive low-e coatings to the surface of the innermost lite of glass, which are the #3 or #4 surfaces.

• The further away from the sun a passive low-e coating is placed, the more solar heat will be transmitted into the building

B) Triple-Pane Passive Low-E Coatings

• For optimal performance, place the low-e coating on surface #5.

▪ For additional insulating performance, the low-e coating can be moved to surface #4 and a second low-e coating engineered for "interior surface”can be added to surface #6.

• Marginally improve insulating performance by adding a second passive low-e coating to surface #3

Relevant parameters.

1.U factor—The thermal heat transfer coefficient of glass, which reflects the ability of glass to conduct heat.

2.SHGC -----the ratio of the amount of solar radiation energy entering the room through the window glass under the same conditions of solar radiation comparing with the amount of solar heat entering the room through the same size but no glass opening. It directly affects indoor heating energy consumption and cooling energy consumption

3.Sc—the shading coefficient of glass (based on the total solar transmittance of 3mm transparent glass as the benchmark, the total solar transmittance of other glass types compared with it), the value range is 0~1, which reflects the glass pair The shielding effect of direct solar radiation.

4.SC = SHGC ÷ 0.87 (or 0.889). The smaller the shading coefficient, the better the performance of blocking sunlight from radiating into the room. However, only in the hot climate area and the large window wall, the low shading coefficient glass is more conducive to energy saving. In the cold area and the small window wall ratio, the high shading coefficient glass is more conducive to the use of solar heat to reduce heating energy consumption and achieve energy saving