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Windows and doors are the second leading cause of heat loss and heat gain in a home (the attic comes first). Using efficient windows makes your home more comfortable and energy efficient. Here is information on how to judge the quality and efficiency of windows and tips on things to consider when buying windows.

Casement Windows Versus Double Hung Windows

Casement windows are more air tight than double hung windows. Double hung windows can have up to 4 times more air infiltration than casement windows. Also, double hung windows have wood going through the center of the window and this can obstruct views.

Window Ratings

The National Fenestration Rating Council (NFRC) is an organization that rates window efficiency. Look for a NFRC label on windows and use it to compare the efficiency of the windows you are considering. Here is a list of some of the terms used to rate window efficiency:

R-Value - To improve your home's efficiency, invest in windows that have the highest overall R-value and the lowest infiltration loss. Double paned glass with argon gas has an R-value of around 4. Single pane glass has an R-value of around 1. You should invest in better windows to reduce your energy costs.

U-Factor - The U-factor is a measurement of how well the window prevents interior heat from escaping from the room. A lower U-factor is better. Most U-factors fall between 0.2 and 1.2. A low U-factor is especially important in colder climates.

Solar Heat Gain Coefficient (SHGC) - The SHGC is a measurement of how well a window prevents solar heat from entering a room. A lower SHGC is better. SHGC is a number between 0 and 1. A lower SHGC is especially important in hot climates.

Visible Transmittance (VT) - This is a measurement of how much light can pass through a window. VT is a number between 0 and 1 and the lower the number the less light can pass through the window. Windows with heavy tinting tend to have a lower VT. The VT rating may be expressed 2 ways. The "glass only" VT indicates how much light can pass through the glass. A glass rating around 0.7 is typical for a window that allows sufficient daylight. The "total window" rating shows takes into account how much light is blocked by the window's frame (a total window VT of 0.5 is typical for sufficient daylight).

Air Leakage (AL) - This measurement shows how much air can leak through the cracks found in the window's assembly (a lower number is better). The measurement indicates how many cubic feet of air per minute can pass through a square foot of the window. The U-Factor and the SHGC are more important then the AL.

Low-E Glass (Low-Emittance Glass)

Use windows with low-emissivity (Low-E) glass. Low-E glass has transparent metal coatings that help make the glass more efficient. These coatings work by allowing light to pass through but reflect infrared heat radiation back into the room. Low-E coatings can make glass about 35% more efficient in the winter and around 40% more efficient in the summer. Some manufactures offer different formulas of Low-E coatings for different climates. For warmer climates it is better to have low-e glass that has a low heat gain coefficients because summer cooling is more important. In colder climates a high heat gain coefficient is better because it improves winter heat gain.

Ultra Violet Protection (UV)

Sunlight can cause carpet, drapes, furniture, pictures, and other items to fade and be damaged over time. Some window manufactures offer window coatings that block or reduce the effects of damaging light. If you are short on funds, than consider using the UV protection on the windows that receive the most light (South and West sides of the home). Some Low-E glass may provide UV protection.

Design Pressure Rating (DP rating)

The Window and Door Manufacturers Association (WDMA) gives a DP rating to windows. This rating is based on a window's structural integrity and resistance to wind. The rating is also based on how well a window prevents air and water infiltration. Windows with higher the DP ratings are more resist to the elements

Solar Orientation

The north side of the home is the coldest and it's best to avoid putting large windows on the north side of the home. Ideally, large windows, or walls of windows should be put on the south side of the home. When selecting types of windows you may want to consider different features for windows depending on the orientation of the window. For example, in a warm climate you may want to use windows that reduce solar gain and in colder climates you may want solar gain from south-facing windows. The north and east sides of a home typically can benefit from window that can reflect heat back into the room and keep cold air out. Ideally windows on the North side of your home should have a low U-factor. Windows on the south side of your home should have low-e glass. Don't assume you should just use the same windows and glass on all sides of your home. By balancing the features and capabilities of various windows according to solar orientation you can make you home more comfortable and more efficient.

Window to Floor Space Ratio

In general the surface are of your windows should not be more than 20% of the floor space of a room. If your window space is greater then that then the room may be difficult to cool during the summer.

Energy-Star® Certification

Energy Star is a program developed by branches of the US government to help consumers find energy efficient materials. A window can carry an energy star label if it meets or exceeds guidelines set by the government for the climate of the area you plan to build in. According to the EPA, energy efficient windows can save around 15% on your heating and cooling bill.

Other Certification

Several other organizations such as AAMA-WDMA, ANSI, or NFRC do certification on windows. These agencies establish guidelines a window needs to satisfy in order to receive their certification. If a manufacturer has a certification label on it's windows then it indicates that the manufacturer has taken the steps to get their windows tested and approved by a certification agency. Windows that have a certification label may be of better quality then windows that are not certified. Certification ratings can also makes it easier to compare different brands of windows.

Double Paned Windows

Windows that have 2 panes of glass are more energy efficient. Having inert gas such as argon or krypton between the panes will make the window more efficient. The biggest source of heat loss in a home is the windows. You should strongly consider having double paned windows.

Window Spacers

The spacer used to separate multiple panes of glass in a window can affect the window's efficiency. Spacers that heat up and cool faster make the window less efficient. In general, aluminum spacers are less efficient and plastic or steel spacers are more efficient.

Gas Fills

Windows that have multiple panes of glass sometimes have the space between glass filled with inert gas such as argon or krypton. Air can be used to fill the space if the space between glass is less than 1/2 inch. If the space gets much greater then that then the air will have more movement and this allows more heat to transfer through the glass by a process known as convection. Inert gasses (such as krypton or argon) are less likely to transfer heat and inert gasses are better insulators than air. Krypton works better than argon but krypton is more expensive. Some manufactures use a combination of krypton and argon gas. Typically argon gas needs glass to be spaced at around 1/2 inch. Krypton can work efficiently in less space and allows the spacing of the glass to be around 1/4 inch to 3/8 inch.


Glazings are the layers of film or coatings applied to a window. Glazings can help improve a window's efficiency and affect the amount of light and type of light allowed through the window.

Impact Resistant Glass

Impact resistant glass is designed to prevent the glass from shattering or allowing objects to be thrust through a window. During a storm if the windows of a home are destroyed then it significantly increases the amount of damage that can occur to a home. Once the homes envelope is penetrated, pressure changes inside the home can cause the home's roof to collapse. Impact resistant glass or shutters are required by building codes in some areas that get hurricanes. If your home is at risk to hurricanes and you do not have impact resistant glass then some insurance companies may require higher deductibles on your insurance policy.


Remember to look at the warrantee. Better quality windows will tend to have better warrantees. Also different parts of the window may be warranted for different lengths of time. For example the manufacture might warrant the glass for 20 years but moving parts for only 10 years. Also, is the warrantee transferable? Some manufactures give warrantees that are limited to the original buyer. A transferable warrantee is better because if you sell the home the new buyer can still benefit from the warrantee.

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