Glass Types

1st generation Low E (Low Emissivity) coating. Infused onto the glass making up surface 4 of a dual pane window. Surface 4 is a surface you can touch so it has to be a hard coat. This coating blocks infra-red and helps improve the U-rating of a window.

A thin soft coat of usually tin, titanium or silver oxide applied onto the backside of the outer pane (surface 2) of a double pane glass pack or IG unit. Because it is a soft coat, it has to be applied to a surface you can’t touch, or it would smudge. It helps reflect Infra-red and Ultra Violet light from passing through the glass. It improves both the U-Rating & SHG rating of the glass.

This glass can look grey or bronze. This glass consists of 1 coat of Low E combined with a tint applied to the glass to reduce the “Visible Light Transmittance” to 45% (center-of-glass measurement). Combining both of these elements together helped improve the U-rating & SHG #’s. The tint does improve the SHG #’s but also counter acts some of the benefits of the Low E on the U-rating. There are better ways to address both compared to combining a tint with a Low E.

2 Layers of Low E applied to surface 2 of a glass pack & a tint has been applied to darken the glass. The darker glass reduces the visible light transmission to 40% (center-of-glass measurement). Darker glass improves the SHG rating because less total light penetrates the glass but, the darker color absorbs more heat so more heat is transferred to the interior which makes the U Rating worse.

2 Layers of Low E applied to surface 2 of a glass pack while still allowing 72% (center-of-glass measurement) of visible light to pass through the glass. Greatly improves both the U-rating & SHG.

3 layers of Low-E on surface 2 with a 65% visible light transmittance. The visible light transmittance is calculated on center of glass measurement. When you add the frame to the measurement more light is blocked. 65% is virtually clear when viewed from the inside. From the outside especially in sunlight you will see a greenish hue. It’s not really a tint but a wave length of light that is reflected has a greenish hue to the eye. The glass appears greener when there is something white behind the glass, like wood shutters or blinds. This is the current technology used as state of the art energy glass used in windows that are trying to meet the highest standards.

It first came out in 2008. New home construction contractors still don’t use it in new homes and most of them don’t even know what it is. The standard in the industry is to use 20-year-old technology and call it cutting edge Low E glass.

When you use this glass in conjunction with the following 3 factors, you can meet current energy star requirements in all zones throughout the United States.

• This glass must be filled with argon gas and not have capillary tubes, so it doesn’t leak out. Argon is useless if capillary tubes are installed in the glass (most of them have this).
• Use a warm edge spacer that can maintain a partial vacuum.
• The frame of the window is not all aluminum. It can have an aluminum outer skin (aluminum clad frame), but the inside frame must be a non-conductive material.

3 layers of Low-E on surface 2, made to cut glare and the sun’s heat. It lets 39% of visible light through (center-of-glass measurement). Its solar heat gain coefficient (SHGC) is 0.18, the lowest of the glass types on this page, so it lets the least of the sun’s heat into the house. It also blocks 98% of UV light, the part of sunlight that fades floors and furniture.

The trade-off is daylight. At 39%, a room gets a lot less natural light than it would with Low-E 366 at 65%. The glass also has a blue tint that can cast a bluish hue on white drywall or blinds inside the house. The U-factor is 0.25, about the same as Low-E 366. It can make sense on a wall that takes hard afternoon sun, but most homes don’t need glass this dark on every window. Low-E 452 with argon lets in more light (52%) and still blocks most of the sun’s heat (SHGC 0.22).

These numbers are for the glass alone, at the center of the glass: dual-pane, 3 mm glass, a half-inch argon-filled gap, measured to the NFRC 100 standard. A whole window, frame included, will have different numbers.

Low-E 452 with argon is a step up from Low-E 366 for blocking the Arizona sun. It lets 52% of visible light through (center-of-glass measurement). Its solar heat gain coefficient (SHGC) is 0.22. SHGC is the share of the sun’s heat that gets through the glass, so lower is better here in the Valley. Low-E 366 measured the same way is 0.27.

It also blocks 99% of UV light, compared with 95% for Low-E 366. UV is what fades floors, furniture and blinds. The U-factor, which measures how fast heat moves through the glass, is 0.24 for both. So the gain is in keeping the sun’s heat out, not in insulation.

The trade-off is daylight. At 52%, it lets in less light than Low-E 366 at 65%. On a west- or south-facing wall in Phoenix, less glare and less heat is usually a trade worth making.

All of these numbers are for the glass alone, at the center of the glass: dual-pane, 3 mm glass, a half-inch argon-filled gap, measured to the NFRC 100 standard. A whole window, frame included, will have different numbers. See energy efficient windows for how the window as a whole is rated.

Sometimes called safety glass, is not tempered. It isn’t made for the purpose of energy efficiency, it is made for impact resistance. It consists of 3 components. An outer sheet of regular clear glass, a center sheet of polybutyl, and an inner sheet of regular clear glass all sandwiched together to make one sheet of extremely hard to break glass. It is used to make hurricane glass because you can shoot a 2×4 out of a cannon at it and it won’t go through. A version of this is used in car windshields.

Regular glass is called annealed glass. When it breaks it breaks into large shards that are deadly. Glass that is low to the ground where people can walk or fall through it or glass in a shower, pool area, doorway, on a staircase landing etc… where people can slip is tempered. Tempered glass is 4 times as hard to break & when it does break, it breaks into little cube looking pieces that may scratch you but it usually doesn’t cut deep.

Tempered glass is also called safe glass. Regular glass shatters at 350°. Annealed glass can be cut into any size easily, which makes custom sizing not too much problem. Once glass is tempered it can’t be cut without the glass shattering. This makes custom sizing more expensive. To custom size tempered glass 1st you must cut a piece of regular glass to the size you need. Then you put the glass in a pressure furnace increase the temperature & pressure on the glass until it reaches approx. 1,150°F (about 620°C). The glass doesn’t shatter at 350 degrees because the pressure keeps the glass together. Then it is cooled rapidly with jets of air.

The result – the heat increases the strength of the glass but if there is a significant impact or something with a small point (ex: ice pick) the glass will shatter into tiny little pieces. Basically, tempered glass is strong, pre-broken tiny pieces of glass created in high heat and pressure. An example is the driver side window in your car.