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Low-emissivity coating (low-e)



By Jerry Kahn of A Plus Windows





Low-emissivity coating (low-e)


By Jerry Kahn of A Plus Windows





      Low-e is effective at keeping your home cooler.   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 (8 feet by 5 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 2nd floor bedroom with two large sun-facing windows had risen 100 degrees, while the rest of the house felt to be about 85 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 absorbs and reflects infrared light, which generates must of the solar heat gain.  Another advantage is that the coating upon the glass slows the thermal transmission through the glass, probably improving on a cold day the inner (room) surface temperature about 2 degrees.  

        There are several factors to consider when deciding if low-e coating is what you want.  One is coast for low-e2 per windows from A Plus averages $13.  The only other disadvantage is the reduction in visible light, about 25% when compared to dual-glaze clear glass.  We thus recommend the low-e2 on just those rooms which have a sunshine heat issue.  Of for all the windows if you wish for a uniform appearance, since the low-e2  has a green tint.  However, very few people will that only some of the windows have low-e2.  My experience is that when presented these facts, about 80% of the people will choose low-e2 on all the window, about 10% clear glass, and about 10% will select low-e2 only where needed.   (See bottom of article for more details).


Form our government’s website at http://www.energysavers.gov/your_home/windows_doors_skylights/index.cfm/mytopic=13320

Energy Performance Ratings for Windows, Doors, and Skylights

You can use the energy performance ratings of windows, doors, and skylights to tell you their potential for gaining and losing heat, as well as transmitting sunlight into your home.

Heat Gain and Loss

Windows, doors, skylights can gain and lose heat in the following ways:

  • Direct conduction through the glass or glazing, frame, and/or door
  • The radiation of heat into a house (typically from the sun) and out of a house from room-temperature objects, such as people, furniture, and interior walls
  • Air leakage through and around them.

These properties can be measured and rated according to the following energy performance characteristics:

  • U-factor

The rate at which a window, door, or skylight conducts non-solar heat flow. It's usually expressed in units of Btu/hr-ft2-ºF. For windows, skylights, and glass doors, a U-factor may refer to just the glass or glazing alone. But National Fenestration Rating Council U-factor ratings represent the entire window performance, including frame and spacer material. The lower the U-factor, the more energy-efficient the window, door, or skylight.

  • Solar heat gain coefficient (SHGC)

A fraction of solar radiation admitted through a window, door, or skylight—either transmitted directly and/or absorbed, and subsequently released as heat inside a home. The lower the SHGC, the less solar heat it transmits and the greater its shading ability. A product with a high SHGC rating is more effective at collecting solar heat gain during the winter. A product with a low SHGC rating is more effective at reducing cooling loads during the summer by blocking heat gained from the sun. Therefore, what SHGC you need for a window, door, or skylight should be determined by such factors as your climate, orientation, and external shading. For more information about SHGC and windows, see passive solar window design.  {The best choice is to use low-e for the windows whose rooms have a persistent heat gain issue during the summer, and clear glass for the remainder of rooms.  Clear glass allows about 30% more light than low-e--jk.}

  • Air leakage

The rate of air infiltration around a window, door, or skylight in the presence of a specific pressure difference across it. It's expressed in units of cubic feet per minute per square foot of frame area (cfm/ft2). A product with a low air leakage rating is tighter than one with a high air leakage rating.

Sunlight Transmittance

A window's, door's, or skylight's ability to transmit sunlight into a home can be measured and rated according to the following energy performance characteristics:

  • Visible transmittance (VT)

A fraction of the visible spectrum of sunlight (380 to 720 nanometers), weighted by the sensitivity of the human eye, that is transmitted through a window's, door's, or skylight's glazing. A product with a higher VT transmits more visible light. VT is expressed as a number between 0 and 1. The VT you need for a window, door, or skylight should be determined by your home's daylighting requirements and/or whether you need to reduce interior glare in a space.

  • Light-to-solar gain (LSG)

The ratio between the SHGC and VT. It provides a gauge of the relative efficiency of different glass or glazing types in transmitting daylight while blocking heat gains. The higher the number, the more light transmitted without adding excessive amounts of heat. This energy performance rating isn't always provided.

Energy Performance Testing, Certification and Labeling

The National Fenestration Rating Council (NFRC) operates a voluntary program that tests, certifies, and labels windows, doors, and skylights based on their energy performance ratings. The NFRC label provides a reliable way to determine a window's energy properties and to compare products.

The NFRC label can be found on all ENERGY STAR® qualified window, door, and skylight products, but ENERGY STAR bases its qualification only on U-factor and SHGC ratings.


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. 

Beware of manufacturer’s figures--they are created for to promote the sales of low-e and argon

I have corrected this site (Feb 2011) by eliminating the values listed by a manufacture in two of their brochures.  The values for the advantage of low-e2 were off by over 30% from the test results listed upon the NFRC labels on their windows. 


JKs test results using an infrared point and read temperature device* and an indoor/outdoor digital thermometer



Room 75.7, outside 42.6 no wind.  Time 3:15 AM.  Curtains were pulled back from the window at 2:05 AM.  A 93” x 46” Milgard vinyl Style Line double slider (XOX), dual glazed with low-e2.  Bathroom single glazed aluminum 20” x 35”  


Wall 12” from X (sliding) panel 74.1------X panel 72.7----O panel 71.1

Single glazed bathroom window 51.6


The wall was only 1.6 less than air, and thus is the standard for comparison.  The X panel was 1.4⁰ less than the wall, while the large O panel was 3⁰ less.   The small single glazed was 24.1 degrees less than the bedroom air temperature. 

Based on these results when not exposed to direct sunlight for this window without argon gas and low-e3 their addition would only gain about 1⁰ and the cost of about 25% light and $15 per window. 




Failed low-e the gold color on black background
Superior WIndows Inc.

This is rare; the low-e coating has become opaque.  The manufacturer, shipped free of charge, a new sash to replace this one.  (The picture was taken against an asphalt background in the sun.)  The two adjacent panels didn't fail, nor any of the over 50 panels throughout the home.    Failure of the low-e coating is quite rare.  Often moisture will initiate the process, then evaporate.  For the newer coatings, this problem has been solved--we hope.   

I have not been able to find ratings of the different low-e coatings as to how much infrared light they block.  If you have this information contact me at mailtoinfo@apluswindows.biz.  It is generally listed at 70%, but the source of this number is not stated. 


** The hard coat is much less effective than the soft coat, and is rarely used today with dual-pane units.  Thus Milgard’s comparison to hard coat is misleading.  (Hard coat is however applied to automotive vehicles and appears as a slight grey or greenish tinting.)  The typical soft coat two layers has a Solar Heat Gain Coefficient of about .27 and a U value of between .35 and .33.  Three coats of low-e (such as Milgard’s SUnCoatMAX) are available for an average of about $10/window.   Milgard is currently (07-08) using Cardinal 366;  IWC Guardian.  VPI windows for 3 coats is using Solar Ban 70.  There is little difference between Cardinal, Guardian, and Solar Ban low-e coatings. 

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