Choosing a Window for All Seasons

How will your new windows affect your home’s comfort and energy use?

This guide will help you select windows that are right for your house and your climate.

When you're at home, windows connect you to the outside world. They allow you to see beyond your walls, to enjoy the trees and sky and sunlight, and to bring fresh air into the house when you need it. But too often, windows also connect you to outside weather that you're trying to avoid. They can let in drafts, draw out heat in winter, and bake a home in the summer sun. Selecting new windows today can be confusing. Windows used to all be very similar, but now there are dozens of types to choose from. There is metal, wood, vinyl, fiberglass, and composite frames; and single, double, and triple glass panes, with gas fills, tints, and low-e coatings. All these windows allow different amounts of heat, light, and air to pass through.

How do you find the right windows to make your house comfortable?

The first step is to think about what you need from the window. Consider your climate, what direction the window will face, and what functions it will serve.

 Will it be opened often for ventilation?

 Will it replace another window in a historically cold or hot room?

 Does it look out on an incredible view?

To choose the right windows for your house, it helps to understand the ways that heat enters or leaves a house through windows. The best windows have high insulating values and good summer performance, and they don't allow unwanted air to infiltrate in winter or summer.

Choosing a Well-Insulated Window

Windows that have good insulating values will make your home more comfortable, particularly in winter. This is partly because they allow less heat to pass through, but there's another reason - the inside surface of a better insulated window will be warmer. When you stand or sit by it, your body won't lose as much heat to the window as it would to a colder surface. Sometimes the draftiness that people feel from windows isn't due to air movement at all, but rather to the fact that we radiate body heat to the cold window surface.

In addition to improving comfort, windows with high insulating values are less likely to have problems with condensation. Condensation occurs when warm, moist indoor air comes in contact with a cold surface, such as a poorly insulated window. Better insulating values are most valuable in cold climates. The bigger the difference in temperature between outside and inside, the faster heat will move through the window. You'll notice the difference if you replace an old window with a better-insulating one. For instance, when it is 0°F outside, the inside surface temperature of a double-pane glass window is about 44°F, but for a high-performance window it jumps to about 56°F. High-performance windows will help in the summer as well, particularly if you are trying to cool the house to 78°F as the outside temperature climbs toward 100°F.

Understanding U-Factor

You can tell how much heat a window allows through by its U-factor, which measures thermal conductivity. A lower U-factor means a better-insulating window. The more common term Rvalue refers to the resistance of the window to heat conduction, and it is the inverse of the Ufactor (that is, R-value = 1/U-factor). Better windows have high R-values and low U-factors (see Table 1).

Since the different parts of a window all have different U-factors, you should look at the U-factor for the whole window. The frame and the edge of the glass usually have higher U-factors than the center of the glass. If they don't specify-and they often do not-manufacturers or dealers may refer to a window's center-of-glass U-factor, which is almost always lower than the U-factor for the window as a whole.

Fortunately, many new windows are labeled with an energy information sticker from the National Fenestration Rating Council (NFRC). The U-factor on the NFRC label always refers to the whole window. To make sure you are comparing apples to apples, ask for the NFRC ratings even when there is no label on the window Also, be sure to use the same size windows for comparison, as the ratio of glass to framing affects the result.

Double Panes

The first step to improving a window is usually to add a second pane of glass. This traps a layer of still air, a good insulator, between the panes. Double-pane windows insulate about twice as well as single-pane windows, so only half as much heat passes through the window.

Gas Fill

The space between the two panes can also be filled with argon or, less often, krypton gas, which insulate better than air. Krypton is somewhat more effective in windows with less space between the panes (1/4 inch to 3/8 inch), so it is often used in windows with multiple air spaces (such as triple-pane windows) to keep the thickness down. Windows filled with air or argon work best when the space is about 1/2 inch. Windows with krypton are usually more expensive, both because krypton itself is expensive and because the designs tend to be upper scale. Argon is nearly as effective and does not add much to the cost of a double-pane window.

Improved Frames and Spacers

Window frames are made from aluminum, wood, vinyl (polyvinyl chloride), or fiberglass. There are also composites of two materials (for instance, vinyl and wood) mixed together and formed or extruded like plastic. To achieve a certain look, manufacturers also offer vinyl or fiberglass frames with a thin veneer of wood on the inside (wood-clad vinyl). Others offer wood frames with a cladding of vinyl or aluminum on the outside for increased durability. The frame can account for about 15% of the energy loss through a window. Aluminum frames have high Ufactors, unless they include a thermal break-a strip of urethane that interrupts the transfer of heat through the metal.

Wood, vinyl, and fiberglass are much better insulators than standard aluminum frames (without the thermal break). Of these, fiberglass performs slightly better than the rest and is also the most durable. You'll find that vinyl and wood frames generally have similar U-factors. Some very expensive vinyl frames are filled with urethane foam insulation.

In double- and triple-pane windows, the panes of glass are separated by spacers. The spacers are traditionally made of aluminum, even in wood, vinyl, or fiberglass frames, creating greater conductivity around the window edges. This makes the windows colder at the edges in winter, and water vapor may condense there as it hits the cold surface. New warm-edge spacers are made from better-insulating materials, and are recommended for cold climates. The biggest

advantage to warm edge spacers is that they reduce condensation around the edge of the window.

Storm Windows

Many people put up storm windows in the winter, and they do help. But storm windows are typically fairly leaky. If you are deciding whether to buy storm windows or replace your existing ones, you're probably better off putting your money into new double-pane windows.

Frequently Asked Questions

How much energy can I save by replacing my windows?

The average home can attribute about 25% of the energy required for heating and up to 40% of the energy required for cooling to windows. Replacing the windows may save a significant fraction of this amount. But be skeptical of salespeople who tell you that you'll save 40%-50% of your energy bills. If your heating bill is half of your total winter energy bill, and you replace the windows with ones that are twice as efficient, you may save about 6% of your total utility bill, or 13% of your heating bill. Also remember that every home is different. Energy use varies by the home's climate, size, orientation to the sun, amount of window area, residents' comfort needs, and many other factors.

How important is a low-conductivity frame?

A typical frame can account for about 15%-30% of the heat loss or gain through a window. In effect, the actual reduction in your energy bills due to a better frame will probably be small. But the frame can have a large effect on condensation. Replacing an old aluminum frame with wood, vinyl, fiberglass, or a composite can significantly reduce problems with condensation.

How wide should the space between the panes be in a double-glazed window?

The air space between the two panes of glass varies between 1/4 inch and 1 inch thick. The most effective thickness between the panes of glass is 1/2 inch if it's filled with air or argon, or 1/4 inch to 3/8 inch if it's filled with krypton.

Are low-e coatings a good solution for reducing cooling bills in summer?

If the window has only the standard low-e coating it will only help a little in the summer. However, spectrally selective low-e coatings are designed to block much of the heat from the sun, and reflect interior heat back into the room in winter.

Do low-e coatings change the color of the window glass?

Very slightly. Most people do not notice the color difference. Low-e products are faintly reflective and have a slight green tint. Look at a sample first to see if you like the color and visibility through the window.

How can I replace my existing windows without damaging the stucco or siding of my home?

Many new window products install over the old window frame. A window contractor removes the existing glass and sash and installs a new window custom-made to the exact size directly over the existing window frame. This process makes the window opening slightly smaller, so you should be sure that the finished window size still meets building code egress requirements. A complete set of windows in a typical home can usually be installed in a day.

What does it mean when there is condensation between the two panes of glass in a window?

This indicates that the seal that joins the window together between the panes has an air leak. The condensation is caused when warm moist air from inside your home enters the broken seal between the two panes of glass and condenses on the cold inside of the outer pane of glass. To make matters worse, when the condensation evaporates away, a mineral film residue is left behind on the glass. This film looks foggy and is difficult to see through. If the window has argon or krypton gas fill, the gas will leak out, and even if it's just air inside, the insulating value will be reduced. When the seal fails in this way, the window must be replaced.

Are the argon or krypton gases in some double paned windows toxic? No. These inert gases are part of our atmosphere and are not toxic.

Source: https://dticorp.com/choosing-a-window-for-all-seasons/