Windows let in a lot more than light. In fact, they are almost always a house's largest contributor to heat loss in winter and gain in summer. The problem with windows is twofold.

First, windows have a very low insulation value compared to a wall. The window's U-value is a measure of the rate of heat loss for the entire window unit. The lower the U-value, the lower the rate of heat loss. Even an excellent window has twice the heat loss rate of a barely adequate wall, while older windows usually have five, ten, or more times the heat loss. If your house is a canoe, your windows are holes in the bottom as far as heat loss is concerned!

The second problem is air infiltration. All the insulation in the world doesn't help if the wind is whistling right into the room. Air can leak in around the window frame and through the seals around a closed window. While every new window will have a published U-value, you often have to ask about the air leakage rates, abbreviated AL. When shopping for my replacement windows, I found two companies that utilized the same insulation technology, so their U-values were almost identical. However, they had different latch designs, leading to an air leakage value for one that was more than half the other.

So how do enjoy what your windows have to offer without losing your shirt?

If you are planning to build, consider following passive solar construction guidelines to turn your windows into a heating asset instead of a liability. Always choose your window sizes carefully, balancing light and aesthetics with cost. Keep in mind that picture windows have the lowest air leakage rates, followed by casements. Similarly, French doors are better for efficiency than patio sliders.

What about replacing existing windows? The key is research! Replacing your windows does not result in automatic savings. Most window replacements are poorly planned and do not pay for themselves in the time that the average family owns their home. The best way to determine whether replacing your windows will pay you back is to run heat load calculations on your house to determine what your windows are costing you now. If you don't know what U-value your current windows have, you can use these descriptions of various window assemblies to estimate. Too technical?  You can also hire a contractor to do a full scale audit, and some utility companies offer free energy assessments.

Let me dash your romantic visions of wood casements now. Vinyl windows are the only type that will yield a good return on your investment, so seek out estimates and specifications from local vinyl window contractors. Quotes will vary widely. Companies with large sales and marketing forces will often cost more than a company that primarily relies on word of mouth.

Use the quotes in your energy load calculations to determine how much you will save. Divide the windows' cost by your projected total yearly savings to get the number of years it will take to earn back your money. In general, any investment with a return in five years or less is considered good. Given the state of the economy and energy prices, you may choose to use a threshold of ten years instead.

If the numbers don't work out, don't despair. For many homeowners, the installation of storm windows is a better bet than window replacement. Storm windows are almost as effective for improving insulation and can significantly improve air leakage, as well. The price of storm windows ranges from 10-50 percent of the cost of replacement windows, making it far more likely to yield a good return on your investment.

Once you select your window, installation is key. Foam insulation, caulking, and proper measurement are crucial to prevent air infiltration, so make sure you find out how the contractor plans on installing your windows.

This has been a guest post by Genevieve from Bowie, MD
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