Consumer GuidesWindows & Doors

Thermal Replacement Windows

Many homeowners are bombarded with sales pitches about thermal replacement windows. Because the window replacement business is generally more profitable than other home service businesses, there are many companies pushing windows in the marketplace. Window firms, as a group, are among the most aggressive marketers, and are likely to rely on telemarketing and high-pressure sales tactics. As a consumer, you must be careful or you may find yourself fooled by some very smooth operators.

While many of these companies devote a lot of time and energy to marketing, the installation end of the business may get very little attention. This can be a big problem for you. If installed improperly, the best window in the world may yield you results no better than the old one you’re replacing.

Why Replace Your Windows?

The No. 1 reason people replace their windows is to realize substantial savings on utility bills. The second biggest reason is that good quality replacement windows deliver large savings in maintenance costs. Many people are simply tired of wrestling with old windows that are hard to open and close or that won’t stay open. When old windows need scraping, new putty and repainting, it’s an ideal time to consider replacement. Replacing windows is also an opportunity to upgrade your home in terms of style and resale value. In the rest of this guide, we’ll look at each of these issues in some detail.

Saving Money on Utility Bills

Window salespeople make many claims about energy savings. Are they true? Good quality windows, installed properly, can yield substantial energy savings. How much you save depends on the type of window you choose and the type and condition of the windows you are replacing.

In the average home, 38 percent of the heat loss is through windows and doors. If your home has drafty single-pane windows or single-pane aluminum sliders, the heat loss from windows may be as much as 50 percent. The poorer the performance of your old windows, the more dramatic the savings and the sooner energy savings alone will cover the cost of your investment.

Many technologies contribute to a window’s overall thermal performance. We’ll describe the main ones here and explain U-factor ratings. The U-factor rating allows you to make decisions without becoming an expert on all the latest technologies.

Opening Style

Many people don’t realize that you don’t have to replace your old windows with new ones that have the same opening style. Air leakage in and around windows is a big factor in the window’s overall thermal performance. In terms of air leakage, the best window is fixed; that is, can’t open or close. There simply aren’t any gaps and openings for air to find its way through.

While useless for ventilation or for escaping a fire, fixed windows may be an excellent option in certain locations. Nearly as good are “casements” or awning style windows that crank open and closed. Because the seals are compressed slightly when the window is closed, it is difficult for air leaks to develop.

The least efficient opening style is sliders. Many people select sliders simply because that is what they’ve always had. There are plenty of good sliders on the market, but they have to rely more on other features, such as low-E coatings (see below) and more glazing layers, to deliver top energy performance. Sliders, which have more joints and gaps, are also more susceptible to air leaks as the seals age and get worn or lose their resiliency.

Frame material and construction. As much heat can be lost through the frame as through the glazed (glass) portion of the window.

Both wood and vinyl perform well, whereas making a metal frame that contains heat well is difficult. Metal frames are generally stronger, which becomes a factor in commercial applications or with extra-large windows. For normal size residential windows, most homeowners choose vinyl or wood.


The frame is the key to the long-term structural integrity of the window. If the frame warps or cracks, the window’s performance can suffer enormously. With vinyl windows, avoid frames that are held together with screws because they tend to loosen over time. Instead, look for “welded” seams, where either heat or chemicals have been used to fuse the joints.

In terms of energy savings, wood and vinyl are comparable. Some people prefer the aesthetics of wood, though it lacks the maintenance-free aspect of vinyl.


Glazing layers. Most older windows are made with a single layer of glass. The most popular replacement windows are made with two panes of glass. Some people opt for triple-pane windows to maximize the improvement in thermal performance. Added layers improve performance in two ways.

First, enclosed air and other gases (see “Fill Gases” below) are effective insulators. The more enclosed spaces the window has, the less heat can escape. Each layer of glass also provides more surfaces for low-E coatings. Some windows use a plastic film suspended between two layers of glass to achieve a triple-pane effect at a lower cost.

Low-E coatings. Low-E coatings are almost invisible finishes that are usually applied to glass. While they let through most of the light humans can see, they block much of the heat-intensive infrared light, thus improving the window’s insulating value. Some windows have the low-E coating on a film suspended between two layers of glass, creating the effect of a triple-glazed product.

Fill gases. While plain air is a good insulator, some gases (like argon, krypton and carbon dioxide) are even better. The gas or combination of gases affects the window’s overall thermal performance.

NFRC Rating

The NFRC rating system. Fortunately, the National Fenestration Rating Council has developed a rating system that considers nearly all of these factors. One element the NFRC rating system doesn’t account for is the long-term durability of the product because it rates the overall window only when it is new.

The NFRC gives each window a U-factor rating. The U-factor is the inverse of the more familiar R-factor used in attic insulation. So a U-factor of .5 equals an R-factor of 2. The lower the U-factor the better.

When comparing windows, check for the NFRC label. Look for the U-factor. The first number after the words “U-factor” is the rating that’s appropriate for residential purposes. It will be marked “AA” or “Residential.” The U-factor marked “BB” or “Non-Residential” is for commercial window applications. Use the U-factor rating to make meaningful comparisons. Be wary of a window vendor who won’t provide this number.

How far should you go? Obviously, you need to get price comparisons to make a decision. Keep in mind that a good portion of your cost is installation, so it makes sense to leverage those costs by installing a better window. Here are some guidelines:

Buy a double-pane window with a low U-factor when:

1. You don’t expect to live in the home long.

2. You have less expensive gas or oil heat.

3. You expect energy prices to remain stable or drop.

4. You expect to have more income in 10-15 years.

Buy a triple-pane window with a very low U-factor when:

1. You expect to live in the home for 10 years or more.

2. You have more expensive electric heat.

3. You expect energy prices to rise sharply.

4. You expect to have less income in 10-15 years.


What about durability? The NFRC ratings don’t address durability directly. If the windows warp, leak or loosen over time, their U-factor ratings are likely to plummet. Your best resource for choosing a durable, problem-free window is to rely on the advice of a reputable installer. He or she will be interested in your long-term satisfaction and will quickly steer you clear of windows that don’t hold up well.

You can also inspect the window before buying. Look for a good fit between parts. Slip a business card between any slidable sashes and the frame. The card should slide but there should be some resistance.

Low U-factor ratings and durable construction are both determined by attention to details. In general, the better rated windows will be better made as well.

Saving Money on Maintenance

New windows can save you money on maintenance in two important ways. First, if you choose vinyl windows, you can eliminate the cost of painting the windows inside or out. In a typical painting project, the most expensive part is painting the windows. Eliminate the need for that and your painting costs will be reduced.

The outside of most modern replacement windows is designed to be cleaned easily from the inside. If you wash your own windows, you’ll save yourself a lot of time and eliminate a major safety risk. If you normally hire a pro, you may find you can do it yourself with these practical new windows.

Easier to Open and Close

Many older windows are tough to open or close. Wrestling with recalcitrant old windows is a common cause of back injury. In many old windows, the springs and weights that ease the load and allow the window to stay open (unsupported) no longer work properly. A new window should operate smoothly with little exertion.

Opportunity to Upgrade the Home

When considering replacing windows, recognize that this is an opportunity to improve your home in more observable ways than just lower fuel bills.

With better quality windows, you may be able to increase the overall window area, making your home lighter and more open. Special touches like curved windows or stained glass can give a room special appeal. Specialty products, like greenhouse windows and bow windows, can dramatically change the look and function of a room.

Windows can also come with films providing ultraviolet protection so that rugs, drapes, and upholstery are less susceptible to fading. Frosted windows are available to offer more privacy.

There are even windows that go from clear to frosted at the flick of a switch!

Many new windows come with better locks and special catches that allow you to leave a window ajar without permitting a burglar to open it far enough to climb through.

Windows, especially distinctive ones, add considerably to the resale value of a typical home.

While getting the entire cost back in increased resale value isn’t likely, the odds are good that half or more of the cost will be reflected in the selling price.

What to Look for in a Window Seller

For a window to actually perform at the levels indicated by the NFRC ratings, it must be installed correctly. The new window must be installed exactly level and must be exactly square in the frame. Any gaps around the window must be carefully filled with insulation. If all this isn’t done right, you may be wasting your money.

Therefore, choosing a reliable company to handle the installation is extremely important. A big advantage of buying the window from the company that installs it is that there can be no passing the buck if there are problems. Whether the trouble comes from the window or the installation isn’t something you have to worry about since one company is responsible for everything.

Many people are inspired to replace their windows when they see a great sale at a local home center store. Be careful! Usually, the great prices you see are for the bottom-of-the-line windows. Some well-known manufacturers make top, middle and “bargain” lines of windows. You’re better off avoiding the so-called bargain line. Many homeowners can’t get someone to install bargain windows because the pros don’t want to be associated with an inferior product.

If you want to find a company on your own, be aware that being well known or big isn’t always the same thing as competent. In the window field, in particular, there are a number of firms with high profile names that have difficulty delivering consistently high-quality work.

Beware of companies whose reputations come from ads rather than actual performance. Some of the large companies are more adept at generating sales than they are at managing installations.

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