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Window Efficiency

What To Know, What To Look For

Window efficiency is a term that's bandied about in lots of window advertisements and "save energy" topics. Regardless of the motivation behind the discussion, it really is a key point to consider when buying new windows. If your windows aren't efficient you'll pay higher utility bills and your home won't be as comfortable as it can be.

How do you know if the windows you're buying are efficient? One smart way is to find out what makes a window "efficient" in the first place. Education is your best tool and when it comes to the more sophisticated parts of your home, the more knowledgeable you are the better your decisions become.

By the end of this article you should have a good handle on what makes a window efficient and how to use that information along with other pieces of window knowledge to make good buying decisions. It might seem like "homework" but it's a little bit of time well invested.

Factors That Influence Window Efficiency

The first step in choosing an efficient window is becoming familiar with the things that impact a window's performance. There are several aspects to consider including more than just how the window is made.

In the most simplistic terms an efficient window should keep your home comfortable while minimizing the amount of energy your furnace or air conditioner has to expend to maintain that comfort. In trying to achieve this a window faces challenges from the outside air temperature, the impact of sunlight and the effects of wind. Precipitation (rain/snow) has an impact on a window's performance but not necessarily as it relates to it's energy efficiency.

Here's what impacts a window's efficiency:

Climate & Position

The location of your home as it relates to the prevailing climate is an important factor in the consideration of window efficiency. Homes in northern climates have a predominance of heating needs whereas homes in southern climates require more cooling than heating.

There is a middle ground so to speak involving temperate climates where heating and cooling needs are roughly equivalent.

How this relates to windows is that a window can be designed to offset the environmental challenges it faces, whether it's the cold or excess heat from the sun. By choosing a window that's made for your geographic region you'll ensure a certain level of window and home efficiency.

house orientation

The position of your home relative to the sun is another factor that impacts window performance. North-facing windows don't experience direct sunlight whereas windows facing east and west do. Windows that face the north don't need the same features (we'll get into those a bit later) that windows facing east, west or south do to lessen the impact of direct sunlight.

The amount of shade that's available also plays a role in window selection. If the windows are under a roof overhang or are shaded by trees they won't experience the same amount of direct sunlight as a window that doesn't share that scenario. That's a plus in cooling-dominated environments. In this particular scenario you may not require windows with the same measure of "solar heat gain" resistance (more on that later, too) as windows that aren't shaded. However in colder climates some passive heating from the sun is desired. Choosing a window with properties that can benefit from one scenario or the other is important.

Perhaps a simpler way of stating all this is to say that modern windows are made with various technologies and features that address differing environmental conditions. In essence, based on the particular setting of your home and its windows you can "tune" your window choices to take advantage of these varying conditions.

Window Materials

The material your window is made from is another important factor in how efficient it is. Before we go there however it might help to learn more about window construction (link opens in a new browser window) and the various window materials that are available to you, along with their pros and cons.

The Window Frame

The term "window material" is typically used to describe the material that the window's frame is made from. Because the frame makes up a portion of the overall window opening in your wall it plays a role in the efficiency equation. The material it's made from governs how well (or poorly) it does that job.

Think of it this way: the frame acts as a "bridge" of sorts that transfers heat and cold (lack of heat) between the inside of your home and the outside. The ease in which this transfer across the bridge (the window frame) occurs depends on the materials used in the frame itself.

window section

Ultimately you want a frame that's a good insulator, one that puts a roadblock across that bridge to prevent easy transfer of heat and cold across it. Window frame materials that are good insulators are fiberglass, wood (and vinyl or aluminum-clad wood) and vinyl that uses foam inside the frame.

On the other side of the spectrum is a purely metal frame such as aluminum (don't confuse this with an aluminum-clad wood frame). Metal is a good conductor of temperature which means it's not a good insulator. In hot and cold climates it wouldn't be considered an "efficient" window frame material.

The Glass

The window glass (or "glazing" as it's referred to in window-speak) is where all the fine tuning comes into play relative to window efficiency. The glass is where modern windows employ the latest technology to control heat gain and loss while at the same time allowing as much visible light to pass through as possible.

To start with, the number of panes of glass used in a window will define how efficient that window is, all other things being equal. Single glazed windows are the least efficient. Double and triple glazed windows provide the most efficiency because of the air gap that exists between the panes of glass, which acts as a good insulator.

Coatings like the low emissivity coatings used on many windows today are what give the glass it's ability to keep your house warm when it's cold outside and cool when it's hot. They have the ability to "discriminate", meaning they retain the heat energy that's needed to keep the inside warm during the cold season and prevent the hot solar energy from heating your house in warm climates.

The spacers used between the panes of glass in double and triple glazed windows also factor into a window's efficiency capability. Just as the window frame can act like a temperature bridge between the inside and outside, so can the spacer between the two or three panes of glass. The more efficient spacers use technology that reduces the transfer of heat and cold between the two panes. These kinds of spacers are often referred to as "warm edge spacers". Manufacturers have different means of making warm edge spacers but regardless of the specific technology used, a window is better with them than without.

The substance that fills the gap between double and triple glazed windows contributes to window efficiency too. However if it's just plain old air your window won't be as efficient as it could be. Gases like argon and krypton do a better job of insulating against heat loss. These gases are denser than air which slows down the process called "convection", or the transfer of heat through a fluid ("air" and gases are considered fluids).

If that's getting a bit to techy for you just remember that the most efficient windows use one of these two gases in between the panes of glass. Argon is typically used in double glazed windows and krypton in triple glazings.

Window "Tightness"

How tight your window seals against the outside elements directly affects the amount of air leakage it allows. A "tighter" the window is a better window because it won't leak as much air. Keep in mind that this doesn't mean its "tight" in that you can't open the window. Instead it relates to how well the window is designed so that it seals up well.

If you have drafty old windows you've experienced air leakage firsthand and can understand that a window that doesn't seal well won't do a good job keeping your home comfortable. Colder outside air will infiltrate the house during the cold months and hot air will find it's way into your air-conditioned space during the warm seasons. All the while your furnace and air conditioner will have to work harder to maintain the home's comfort level.

Factors that impact air leakage include the type of window it is (casement, double hung, etc.), how it's installed and the overall quality of the window assembly.

Installation

Even the most efficient window in the world won't be worth a darn efficiency-wise if it's not installed correctly. Remember: windows plug up the holes in the walls. If the installation isn't correct there's a likelihood that the window won't perform as it's designed to.

Proper installation as it relates to a window's efficiency means that air infiltration will be kept to an absolute minimum. As mentioned earlier, air leakage negatively impacts your utility bills so it literally pays to ensure your windows are installed properly.

Publisher's Comments

So how do you know if your window is installed correctly? For the average homeowner, that's the ten-thousand-dollar question. Most of us don't install windows for a living so validating whether our new windows are installed properly might be a challenge. While you might not be able to know with absolute uncertainty there are steps you can take to hedge your bets.

  • Use Certified Installers
    Some window manufacturers require that their windows be installed by certified installers; contractors that have installed their windows and may have received specific training on how to do it. You can usually find this information at the manufacturer's website under information that discusses installation.

    Ask/find out how much experience an installer has installing the brand and model of windows you want. If a contractor can't validate that he or she is trained and certified to install your particular brand of windows, or doesn't appear to have much experience with those windows, find another person to do the job.

  • Get Familiar With The Installation & Watch
    The installation instructions for a particular brand of windows isn't hard to find. Check the manufacturer's website or ask the dealer for the instructions. Then familiarize yourself with the procedure and observe the installation. That doesn't mean you have to look over the contractor's shoulder every minute but "be present" and observe to see if the windows are installed in accordance with the instructions as you reviewed them.

    Pay particular attention to the steps that involve making the window level and installing the required sealants and flashing. If something seems amiss don't be afraid to speak up and ask questions. This doesn't mean you have to verify that he window is level, but if the installer doesn't appear to take any steps similar to what the instruction procedure calls for, it's time to speak up.

  • Take Pictures Of The Installation
    Take some pictures as the installation progresses. I did this with my own remodel and it can be a big help if there are questions later on. If a contractor squawks about this simply mention to them that it's just your way of documenting the maintenance and repairs done on your house. If they're doing it correctly they should have nothing to complain about. In the worst-case scenario, if you ever have problems with your windows that have warranty implications, you'll have photo documentation to validate proper (or perhaps improper) installation. Let's just hope it's the former and not the latter.
  • If it's likely that your window installation is going to happen when you're at work or out of the house talk with your contractor to see if the job can be scheduled when you will be at home. Maybe even schedule a day off from work if that's what it takes to be around and monitor the installation.

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What To Look For When Buying Energy Efficient Windows

Now that you have a good understanding of what impacts a window's efficiency here's what you should look for when buying an energy efficient window (the links below will take you to more information on each topic).

ENERGY STAR Label

Buying windows that hold the ENERGY STAR label means you're buying a window that meets a certain standard of energy efficiency. ENERGY STAR establishes guidelines for window performance for the various climate zones in North America, including the U.S. and Canada. If you buy a window that's rated for your particular zone, you're ensured of having a window that meets an established level of efficiency.

The two examples below show ENERGY STAR climate zones for both the U.S. and Canada.

U.S. climate zone label canada climate zone label
Graphic sources: energystar.gov (US) / nrcan.gc.ca (Canada)

Energy Performance Ratings

A window's energy rating is where you can see the values for important performance parameters. It also allows you to make comparisons among various windows. The rating is noted on the NFRC label which should be affixed to every window you buy.

The label and the values represented on it are established by the National Fenestration Rating Council (NFRC). The NFRC is a non-profit organization chartered with administering the independent rating and labeling of window energy performance.

Here's an example of an NFRC label. An explanation of what's on the label is found below.

NFRC Label example

  • U-Factor
    U-factor represents a window's ability to prevent heat loss from your home. Technically it's the window's rate of heat transfer but from a homeowner's point of view, it tells you how "good" the window is at maintaining the comfortable temperature inside your home.

    U-values are expressed as a number that ranges anywhere from 0.10 to 1.20 although lower and higher numbers are possible. The lower the U-value, the better the window is at resisting heat loss. More typical numbers are in the 0.20 to 0.35 range.

    Be sure that any U-value number you see is cited for the entire window, frame included. In the past some manufacturers provided U-value numbers for the glass only, which skews the overall efficiency rating of the window.

  • Solar Heat Gain Coefficient (SHGC)
    This is just fancy language to express the amount of heat energy from the sun that the window allows into your home. This number is a "coefficient", another mathematical term, but the important thing to remember is that lower numbers mean less heat enters your house. On the flip side, higher SHGC numbers mean more of the sun's heat makes it through your window.

    Solar heat gain is one of those numbers that is climate and orientation-specific. There are situations, in northern climates for example, where it's desirable to let some of the sun's heat energy into your homes. That means buying windows with a higher SHGC is preferable.

    On the other hand, hot climates and/or windows that face intense direct sun, like those on the east or west side of your home, should "repel" the sun's heat energy. In these situations you want a window with a lower SHGC number. This will help keep the inside of your home cooler and means less energy used for air conditioning.

  • Visible Transmittance (VT)
    Visible transmittance is another techie term that really expresses the amount of light that comes through the window. It's not something that affects efficiency but it's related -- and here's how:

    The technology that helps keep the heat in and/or the sun's hot energy out is made possible by coatings on the glass surfaces. These coatings can limit the amount of light that's "transmitted" through the window, depending on the specific coatings used.

    Visible transmittance numbers are used to express how much actual light the window lets shine through. Higher numbers mean more light and lower numbers mean less light. Don't be put off by the seemingly low value of the number that you see. Instead, just know that a window with a VT of 0.51 allows more light than one with a VT of 0.35.

    In practical terms the glass in windows with lower VT may appear slightly darker or 'grayer' when you look through them. Depending on the magnitude it might impact your view to the outside somewhat.

Those are the window performance values that are "standard" on all NFRC window labels. As you might guess there are some tradeoffs that go along with these values. Windows with very low U-Factors and SHGC may drag down the Visible Transmittance number. Finding a happy medium between what will give you good performance while offering the best view to the outside should be the goal.

There are a few other window performance ratings that are important but aren't necessarily on all NFRC labels. That's because right now, there's no industry or government requirement to have them and manufacturers can opt to not include them. While that might change in the future, for now it's information that you might have to hunt for on a particular window you're interested in.

Here's an example of an NFRC label with additional performance parameters (shown in red):

NFRC Label Example

  • Condensation Resistance
    The condensation rating will tell you how well a window will resist forming condensation on the interior surface. It's measured on a scale of 1 to 100 with higher numbers representing better resistance to condensation.

    While the number itself might not mean much to you it offers benefit when you go to compare Window A against Window B. So if Window A has a CR of 76 whereas Window B has a CR of 51, you'll know that Window A is better at resisting condensation.

Publisher's Comments

A couple of comments here: Window condensation isn't necessarily the evil scourge that must be avoided at all costs. Sure, you want to avoid it because it can damage your windows long-term, particularly wood windows. Having moisture seep into the wood can cause rot and/or mold and mildew. However, there are probably going to be circumstances when your home's humidity will be high (lots of shower-taking, cooking, etc.) and the conditions are ripe for some condensation to form. So an occasional occurrence of some condensation doesn't mean your windows will fall apart next week.

Second, a window can only do so much to stave off the formation of condensation. If you don't take steps to control the humidity in your home the best window in the world will probably develop condensation when it's real cold outside and the inside of your house is as humid as a tropical rain forest.

My advice is to pay attention to the number, but put more focus on getting a window that's efficient (good U-Factor and SHGC for your climate) along with good air leakage values.

  • Air Leakage (Air Infiltration)
    Air leakage is another window performance parameter that's not a required part of the NFRC label. You may see it on some windows but not others. This is an important aspect of window efficiency and deserves it's own discussion. So let's get to know a little more about. . .

Air Leakage (AL)

A window's air leakage value (AL) is the amount of air that passes through and around the window when air pressure is applied to it. Air leakage is sometimes referred to as air infiltration.

The reason this value is important is because the leakier a window is the less efficient it is. Obviously if a window allows a significant amount of air to leak past it that window won't do a very good job keeping your warm air inside or the cold air outside. It won't really matter how fantastic the window's U-Factor or SHGC value may be.

Window air leakage values are stated as a decimal number and the lower the value, the better or "less leaky" the window assembly is.

Keep in mind that windows are an "assembly". They're made up of various parts and pieces and while they're designed to be tight, there will still be seams and very small crevices that are simply the nature of an assembled product. The better a manufacturer's design and assembly process (along with proper installation in your house) the less air leakage a window will experience.

If you don't see an air leakage limit on the NFRC label there are a couple of ways to find what the value is for a given window.

The first step is to visit the manufacturer's website and look for specifications on the type of window you're interested in. Results vary by manufacturer and you'll find some that provide the information whereas others don't.

If that doesn't work you can contact the window maker directly and ask them what the value is.

On some labels you might see something like "Meets or exceeds C.E.C Air Infiltration Standards". In this instance "C.E.C." means the California Energy Commission. The most recent standard states that air leakage for window products can't exceed 0.3 cfm/sq-ft. If that's what's on the label the window at least meets that standard. However what you don't know is whether it's right at the limit or half of that value.

Publisher's Comments

I'm not a thermodynamics engineer but I don't think it takes one to see that a window with higher air leakage values will be less efficient than one with a lower value. My personal opinion is that the NFRC should specify air leakage as a required performance rating. Otherwise you're not really sure how efficient your window is. Without an air leakage value, you're not getting the entire picture.


Low Emissivity (Low-E) Coatings

Low emissivity (or low emittance) coatings are very thin, transparent films applied to the glass that help reject unwanted solar heat energy and retain desirable indoor warmth.

On a double or triple glazed window heat is transferred by the process of radiation. More simply put, it means that the heat "travels" from one source to another without the need for surfaces to be in contact with one another. Heat from inside your home contacts the inside pane of glass and can cross the gap to the outside pane without those two panes of glass coming in contact. That's heat radiation at work.

"Emissivity" describes the quality of a material to help make that happen or to 'radiate' that heat energy. A material that has low emissive properties resists transferring that heat, essentially blocking the escape of heat from inside your house. Because these coatings have low emissivity properties they help reject the heat energy from traveling through the window.

Low-E films are not some form of plastic or similar material used to shade the window. Rather, they're a very thin metallic or metallic oxide coating that's applied to the glass when it's manufactured. The coatings are transparent but depending on the specific type of coating they can impact the amount of visible light (visible transmittance) that comes through the window.

Most insulated glass windows made today include some form of low-E coating technology though you may still find some that don't. Since it's a feature that gives a window a healthy portion of it's energy efficiency, it's not something you should do without.

Probably the most important point to understand about buying windows with low-E coatings is that there are various types of coatings available. This makes it possible to design windows with varying performance characteristics tailored to the specific needs of your climate and the location of your windows (see the section above on "Climate & Position").

Depending on the specific situation, low-E coatings allow a window to:

  1. Take in solar heat energy but reject internal heat loss (good in climates dominated by heating needs)
  2. Admit a moderate amount of solar heat energy while rejecting internal heat loss (good for temperate climates with both heating and cooling needs)
  3. Reject most of the solar heat energy and retain some internal warmth for occasional heating (for climates dominated by cooling needs)

If this all sounds like too much to keep track of don't fret. If you choose a an ENERGY STAR window that's rated for your specific geographic location you'll be assured of meeting current acceptable standards for window efficiency.

But if you're the type that likes to dig into the numbers you can tune your window choices to optimize your home's efficiency. You do this by working with a window manufacturer/dealer to order windows with the performance characteristics you want (U-Factor, SHGC, etc.).

Gas Filled Glass

Double and triple pane windows should be gas-filled. This means that what's between the panes of glass should be either argon or krypton gas and not just regular "air".

The reason for this is that these gases are better insulators than air. Argon is typically used in double glazed windows and krypton is normally used in triple glazed windows.

Most window manufacturers provide this feature on their insulated glass windows. However it's a good idea to make sure because these gases make your window more efficient than windows with regular air or nitrogen in between the panes of glass.

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Shopping For An Efficient Window

Shopping for windows might initially seem like shopping for cereal in the grocery store - there are simply too many choices be able to choose one brand. But it's not actually that hard to do and here's one reason why: there are only a handful of manufacturers that make glass for residential windows. The same glass that's in one brand of windows is also in other brands too.

So it's not as if you have to be concerned with choosing from hundreds of different kinds of insulated glass packages in order to get an efficient window. The main thing is to choose the type of window you want relative to functionality and style and then factor in the level of efficiency you need.

The beauty of the NFRC labeling system is that it allows you to compare windows on an apples-to-apples basis. You can see from the numbers on the label how the U-factor, SHGC and other parameters mentioned above stack up against other window products.

Find several window suppliers/dealers in your area and then research their track record (online reviews, Better Business Bureau, warranty coverage, past customer references, etc.). Compare similar products relative to window materials, performance parameters and cost (don't forget about installation cost too). Then make your decision knowing you've done your homework and due diligence.

Choosing an efficient window is an important part of your overall window decision but there other things to consider too. See the other sections of this website to help you in your window choices. The article on choosing new windows will help you understand the considerations that go into buying new windows such as features and options to think about, warranty implications and costs.

The article on window types and styles walks you through the various kinds of windows that are available along with their pros and cons. It also talks about window "style" and it's importance and impact on your home.

And don't forget to understand what you need to know about window construction. You've probably heard about "wood", "vinyl" and "clad" windows and this web page details what they're all about and when/where each one is a good fit. It also shows you the important parts of a window and what to look for in good window construction so that you can shop for windows with confidence.

Finding a local window dealer can sometimes be a challenge too. If you'd like help finding a window installer/dealer in your local area and obtain free quotes for comparison purposes, see this web page for more information.

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