Replacement Windows and Air Infiltration - The Forgotten Window Rating

When deciding to purchase replacement windows

, there are many factors to consider to make the best purchase. Many window sales people will point out the obvious: aesthetics, "cool features" like tilt wash windows, and warranty. As consumers get more energy conscious, another factor comes to the forefront when choosing to replace windows: window ratings. While many window salespeople will focus on the big two: U values and Solar Heat Gain Coefficient, there is another important factor to consider when selecting windows: air infiltration.

Air infiltration does not get its due when considering replacement windows, but it is a major culprit for heat gain and loss in a home. An older window does not seal as tightly and creates the opportunity for air to enter into the house. People refer to this as a "drafty" window in the winter because the cold air is easily entering the room. Heat also enters the house in the summer through cracks, however, many people mistake this for the heat coming in through the inefficient glass.

There is a scientific way to determine air infiltration beyond just standing next to a window and calling it drafty. Air leakage is measured by the testing of a standard sized window. The window is installed in a testing wall. A vacuum is of 75 pascals (1.57 psf) is applied on one side of the test window. This corresponds to roughly a 25 mph wind blowing perpendicularly to the window. Flow meters are positioned to measure the rate of air leakage. This is divided by the total window area to get a reading (in the U.S) in cfm/ft2 (cubic feet per minute per foot squared) to the nearest tenth (i.e., a test value of 0.17 would be given a rating of 0.2). The best possible rating by the NFRC is 0.1, as they will not round to 0.

So why are more window salespeople not discussing air infiltration when touting the performance of a replacement window? Currently windows are not required to be tested for air infiltration performance. If a window company chooses to have its product tested, it cannot have more than 0.3 cfm/ft2 of air leakage. If this maximum value is not met, the energy loss through air leakage is likely considerably less than that through heat loss across the glass or window frame. Regardless, air infiltration has a major effect on perceived performance of the window. The reality is that even at the passing value of 0.3, a window will still exhibit noticeable drafts around the elements of the windows.

Certain window types are designed to seal tighter than others and thus provide lower air infiltration numbers. Casement and awning windows (think crank handle to open and close) offer the best performance because as the window is closed, the locks pull the sash tight against the frame. In addition, as the wind blows against the window, it pushes the sash tighter against the frame.

Double hung and single hung windows are not as air tight because they rely on gaskets and weatherstrip to prevent air infiltration around the sashes. Hung windows do not have any overlap or compression fit between the sashes so as the wind blows against the window, the air can get between the sashes and create a draft.

Horizontal sliding windows are the biggest air infiltration culprit because the operable sash is simply sitting in the window frame. While many modern sliders have weather strip and gaskets to try and reduce air infiltration, the windows are still more prone to movement in the wind.

Reducing the air infiltration around a window is critical if you want to maximize the energy performance of the replacement windows in your home. Just having the energy efficient glass is not enough when it comes to selecting the best window possible. Many manufacturers today are making windows that are designed to seal tight so chances are you will get a window that meets or exceeds the basic testing standard of .3 for windows.

While it is not a required measure for window performance, air infiltration can not be ignored when selecting replacement windows for your home. It does not matter how great the low e is, or how insulated the window frame is if the window leaks like a sieve, the energy efficiency problem has not really been solved. Pay attention to the air infiltration, see how well the windows are sealed at the joints, know which windows are best for your climate, and you will make the best purchase for your replacement window project.

Replacement Windows and Air Infiltration - The Forgotten Window Rating

By: Justin M Howe

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