Low-e Coating

Low-emissivity coating (low-e)

LOW-E COATING, DUAL PANE, & THERMAL EFFICENCY

By Jerry Kahn of A Plus Windows



FOR THE CONSUMER

      Low-e is effective at keeping your home cooler. Our government’s website writes that it filters out 40-70% of the heat normally transmitted through insulated window glass or glazing while allowing full amount of light to be transmitted. My own extensive experience of going to thousands of homes of those both with and without dual pane windows over the years confirms the effectiveness of low-e coating. My own bedroom is an excellent example. It has from 7AM to 2 PM in the summer direct sun; yet that room has not been more than 6 degrees above an adjacent room not receiving direct sun, nor has it been 6 degrees above the outdoors. I track the temperatures with an indoor/outdoor wireless thermometer. Not all the rise in temperature can be attributed to the large window (5 feet by 8 feet), since the wall and ceiling also contribute to heat gain. I have also been in home in El Cajon with single pane glass under similar conditions. The door was kept close to keep to the 2^nd floor bedroom whose temperature on a 85 degree day had risen within to over 100 degrees. I have also been in a home in North Park, where the dual pane windows we installed didn't have the low-e coating. The rooms were much warmer than I expected. Low-e works.

       Low-e, since it blocks infrared light, also keeps heat in—important in winter. This is similar to the green-house effect. The objects within your home (including you) radiate at a low level infrared light. The coated glass, blocks most of this radiation in your home, thus keeping the heat in. Another--often overlooked--advantage is that the coating upon the glass slows the thermal transmission through the glass. Low-e coating is of such value, that our federal government is now (2006-2008) offering an income-tax rebate for replacement windows with this coating. California from around 2000 until 2004 offered an even more substantial rebate.

       There are several factors to consider when deciding if low-e coating is what you want. One is coast: it adds at the wholesale end on an average $30 dollars/window. The only other disadvantage is the reduction in visible light, about 30% when compared to dual-glaze clear glass. It allows much more light than tinting, and you will not noticeable a change the visibility when looking out. Tinting now can only be justified for privacy. Even for windows without direct light, the energy savings will pay for its cost in a few years.

     So what can you expect from your upgrade?

MILGARD figures**

    -20 F

   +20 F

Inside glass

temperature

Inside glass

temperature

Solar heat

Gain*

Visible light

Transmission

Single pane clear

4

30

.86

90%

Double pane clear

36

49

.76

81%

Hard-coat low-e

50

61

.32

65%

     * Lower is better

As you can see from the above chart, comparing single pane to double pane, most of the thermal gain comes from going to a dual pane unit. Air is an excellent insulator, as long as there are no convection currents. It is also an excellent sound insulator. Typical street noise, such as talking and traffic, becomes barely noticeable. The gap, whether overall 3/4 inch or 1 inch does not affect either thermal or sound transmission properties significantly.

        The other decision is which brand of windows to buy. I would not base a purchase decision upon either the low-e coating or spacer bar, which according to the sales hype greatly affects the window’s thermal efficiency. Facts given in a sales presentation, including the printed materials from the manufacture, are, often not facts, but just sales hype. "Facts" are generated to sell the product.

If you want a moderate improvement in energy savings, ask for low-e cubed, which is three layers of coating. Low-e squared (2 layers) is generally standard now.



U-Factor

U-factor measures how well a product prevents heat from escaping and entering. U-Factor ratings generally fall between 0.20 and 1.20; lower being better.

WINTER U VLAUES are based on an outdoor temperature of 0 degrees F, and an indoor temperature of 70 degrees, and a 15 mph wind velocity with no sun.

SUMMER U VALUES are based on an outdoor temperature of 89 degrees F, an indoor temperature of 75, and 7 ½ mph wind velocity.

Solar Heat Gain Coefficient

Solar Heat Gain Coefficient (SHGC) measures how well a product blocks heat caused by sunlight. The SHGC is the fraction of incident solar radiation admitted through a window (both directly transmitted and absorbed) and subsequently released inward. SHGC is expressed as a number between 0 and 1. The lower a window's solar heat gain coefficient, the less solar heat it transmits in the house.

Visible Transmittance

Visible Transmittance (VT) measures how much light comes through a product. The visible transmittance is an optical property that indicates the amount of visible light transmitted. VT is expressed as a number between 0 and 1. The higher the VT, the more light is transmitted.

Shading coefficient (SC)

A measure of the ability of a window or skylight to transmit solar heat, relative to that ability for 1/8-inch clear, double- strength, single glass. It is being phased out in favor of the solar heat gain coefficient, and is approximately equal to the SHGC multiplied by 1.15. It is expressed as a number without units between 0 and 1. The lower a window's solar heat gain coefficient or shading coefficient, the less solar heat it transmits, and the greater is its shading ability.

Air Leakage

Air Leakage (AL) is indicated by an air leakage rating expressed as the equivalent cubic feet of air passing through a square foot of window area (cfm/sq ft). Heat loss and gain occur by infiltration through cracks in the window assembly. The lower the AL, the less air will pass through cracks in the window assembly.

Conclusion

Low-e coating will result in a very substantial savings, more than enough to cover the cost of the coating, which averages about $30 per window above the cost of clear glass. For the average home of 10 windows and a patio door, the additional net cost is about $350. (A Plus Windows, doesn’t add a markup above net cost for low-e coating). Moreover, the slight greenish tinting makes the windows more attractive. I strongly recommend getting this coating.