Differences in pressure can cause air to travel through small gaps in window assemblies. (You’ll recognize the physics at work here if you’ve ever had a door slam shut to a room that had an open window.) Air can travel into your home or out of it, altering its temperature and exacerbating its heating and cooling needs. That means less energy efficiency and higher energy bills.

Window air infiltration (or air leakage) is commonly overlooked. When considering new windows, homeowners tend to focus upon features like its U-Factor and Solar Heat Gain Coefficient. Although these ratings are significant, it’s important to take all aspects of a window into account when assessing its potential value to your home.

Excess window air infiltration will detract from your home’s comfort level and energy efficiency. By giving air leakage careful consideration and investing in energy efficiency, you will keep your home comfortable and your energy costs down.

Understanding Window Air Infiltration

Window air infiltration occurs for one of two reasons: (1) the window’s structural design allows air to leak through it, or (2) the window has been poorly installed or maintained. 

Nearly all windows (except those that are fixed) are susceptible to window air infiltration simply by design. It would be almost impossible, after all, to design an operable window with no gaps or spaces through which air could pass.

The National Fenestration Rating Council (NFRC) sometimes assigns these types of windows an air leakage rating (AL). Ranging from 0.1 to 0.3, AL is expressed in units of cubic feet of air that passes through a square foot of window frame area. The lower the rating, the less air the window allows through it.

Testing of this rating is currently optional, but if a manufacturer chooses to have a product tested, the window cannot have a rating that exceeds 0.3. Nevertheless, this rating is a requisite for Energy Star certification.

Reducing Window Air Infiltration

Eliminating window air infiltration entirely is unrealistic, but reducing the amount of air leakage your windows are allowing will make your home more comfortable and energy efficient.

The first step in reducing window air infiltration is recognizing that it is occurring. If you suspect that it is, check the caulking and weather stripping around your windows for gaps and cracks that air could pass through. If you’re able to see daylight passing through the edges of your window frames, that’s a sign that air is leaking through. If you can’t be sure one way or the other, hire a technician to perform an energy audit.

If you determine that window air infiltration is an issue for your home, there are two routes you can take. You can make energy-efficient additions to the windows you already have, or purchase completely new windows that are more energy-efficient.

If you would prefer to stick to additions, you might consider adding storm windows, which help defend against air infiltration. Caulking and weather stripping are also helpful in preventing air leakage. Even window coverings like shades, drapes, curtains, and blinds can help your windows resist airflow.

Choosing Energy-Efficient Windows

If you decide to go with new energy-efficient windows, you’ll want to carefully consider the different types of windows available to you. Certain windows have better air leakage ratings than others as a sole result of their structures.

Fixed windows are your best bet as far as air leakage goes. As long as they are installed correctly, they will remain airtight. Since they don’t open or provide ventilation, however, they’re not always the most practical option.

Casement, hopper, and awning windows tend to have lower air leakage ratings because they close securely and effectively block out airflow.

Single and double-hung windows, as well as single and double-sliding windows, generally have the highest air leakage ratings, making them the least energy-efficient in terms of window air infiltration.

Reducing window air infiltration in any way possible is a valuable investment in your home’s comfort and energy efficiency.