a basic guide to low e windows

A Basic Guide to Low-E Windows


What Is a Low-E Window?

PART 1

Low-E is a big buzzword in window replacement, but what exactly does it mean and how can it benefit you and your home?



EMISSIVITY DEFINEDEmissivity refers to a material’s ability to emit energy in the form of thermal radiation. In the context of windows, “low-emissivity” or “low-E” means that the window glazing radiates or emits only a small fraction of the incident thermal radiation that strikes its surface.

ultraviolet light

infrared light

visible light

THE LINK BETWEEN LOW-E & ENERGY EFFICIENCYDepending on where the low-E coating is applied, a window can be configured to either:

a. minimize the amount of solar heat coming into a home or

b. maximize heat retention.

Take the case of the double-pane window shown below. In a cooling-dominated (hot) climate, the low-E coating should be on the inward-facing surface of the outer pane (Surface B). This allows the window to reflect most of the unwanted heat from sunlight, reducing the overall heat gain.



In a heating-dominated (cold) climate, the goal is to reflect or reradiate as much heat back into the home, so the low-E coating should be on the outward-facing surface of the inner pane (Surface C).

Outside

Inside

Because of its ability to minimize heat gain in hot climates and heat loss in cold climates, a low-E window can make any home more energy efficient. The frame and other components will also have a hand in determining overall energy efficiency, but because glazing takes up the biggest surface area in a window, its impact is the greatest.



HOW DO LOW-E WINDOWS COMPARETO STANDARD WINDOWS?

The emittance of standard clear glass is 0.84. This means that it absorbs 84% of the incident thermal energy and reflects only 16%. Low-E glass, on the other hand, can have an emittance as low as 0.04—4% of the incident heat energy absorbed, 96% reflected.

heat reflected

heat absorbed

84% 96%

4%

16%

STANDARD CLEAR GLASS LOW-E GLASS

Note that emittance isn’t explicitly included on the label that shows a window’s energy performance ratings. However, it is taken into consideration when computing U-factor, which is a measure of the window’s resistance to heat transfer. For optimum energy efficiency, choose a window with a low U-factor.



BEYOND LOW-E

Other energy performance ratings to consider:

Low-E window glazing is a step up from standard clear glass in more ways than one. Stick around for Part Two of this ebook to learn more about the benefits low-E windows can offer you and your home.

Solar Heat Gain Coefficient (SHGC) – fraction of solar heat transmitted through a window

Visible Transmittance (VT) – fraction of visible light transmitted through a window

Air Leakage (AL) – cubic feet of air passing through a square foot of window area

Condensation Resistance (CR) – a measure of a window’s resistance to condensation formation


Fringe Benefits of Low-E Windows

PART 2

A low-E window has a lot more to offer than a window with ordinary glass. While energy efficiency is its strongest suit, its benefits go

well beyond that.



IMPROVED DAYLIGHTINGOne of the best things about low-E windows is that they eliminate worries about excessive solar heat gain or winter heat loss but do not hinder visible light transmission. This means that whatever type of climate you live in, you’ll be able to allow as much natural light as you want into your home without the corresponding spike in your energy bill.

Low-E windows’ daylighting benefits also mean further energy savings because you won’t have to switch on artificial lighting to brighten up your indoor spaces during daytime.

BETTER UV PROTECTIONYou may have noticed that drapes, carpets, artwork and furniture that get plenty of sun exposure have a tendency to fade faster than those in the shade. This is a manifestation of damage caused primarily by ultraviolet (UV) light. Low-E glazing can help put a stop to this by blocking out most of the UV light hitting your windows. Where ordinary window glass only blocks about 25% of the sun’s UV rays, low-E glass can block up to 74%.



REDUCED CONDENSATIONDuring winter, beads of moisture form when warm, moist indoor air comes into contact with a cold surface such as window glass. Because low-E glazing doesn’t give up as much heat to the outside, the temperature difference isn’t as great, allowing it to minimize—if not eliminate—condensation problems during the heating season.

BETTER COMFORTYour exact heating and/or cooling needs will vary according to the time of the year and where you live. Because low-E windows control how much heat goes in or out of your home, indoor temperature is much easier to stabilize. A stable indoor environment means that you can stay comfortable no matter what the weather outside is like.

If you’re already sold on low-E windows and the many benefits they offer, stick around for Part Three of this ebook to learn how you can get the most out of your window investment.


Maximizing Low-E Windows

PART 3

Like most major home upgrades, having low-E windows installed is a real investment. Maximize the return on your home improvement dollar by

taking the following tips into consideration:



TIP #1Work with a reputable and experienced window contractor.A big part of ensuring your ability to reap all the benefits of low-E windows is having them installed properly. Never cut corners when it comes to workmanship. Keep in mind that even the best window products may fail to perform as advertised if left in the hands of a careless contractor.

TIP #2Know the ideal window ratings for your location.ENERGY STAR rates fenestration products according to their energy efficiency in specific climate zones. It pays to realize that there is no one-size-fits-all approach when it comes to “window shopping”. For instance, many homes in hot climates will benefit from low-SHGC windows, but a higher SHGC may be desirable in colder climates where solar heat gain can offset winter heating costs.

ENERGY STAR’s Windows, Doors, and Skylights Climate Zone Finder can help you pick window products best suited for wherever you live.

TIP #3Know your window styles.Low-E technology can be applied to any window style, so don’t let worries about aesthetics hinder you from having more energy-efficient windows installed. It’s fairly easy to find a low-E window in a style that complements your home’s architecture, so you can enjoy the best of form and function at the same time.

http://www.energystar.gov/index.cfm?fuseaction=windows_doors.search_climate

http://www.energystar.gov/index.cfm?fuseaction=windows_doors.search_climate



TIP #4Consider adding window treatments.Low-E windows work great on their own when installed properly, but you can improve their performance even further by adding window treatments. Window treatments are mostly used to block out sunlight and add a measure of privacy, but they can also have a hand in upgrading the look of your interior spaces.

TIP #5Learn basic window care.You can help extend your windows’ service life and maintain their beauty with regular care. Ask your contractor for tips on window cleaning and general TLC. Schedule routine inspections and maintenance so that your contractor will be able to catch problems while they’re small and easier to deal with.

Good luck on your next window replacement project!


RENEWAL BY ANDERSEN OF CONNECTICUT25 Meadow Rd. Windsor CT 06095

(860)-904-1931www.WorldsGreatestWindow.com

a basic guide to low e windows

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