FOR
THE CONSUMER
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:
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.}
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. |