Choosing a Window based on Air Infiltration (CFM)

Picture this: you’re shopping for new windows and you’ve narrowed it down to three finalists. To the untrained eye they look pretty similar. Because so many window features are government regulated these days, the glass and and U-value (amount of heat transmitted through the window) are basically the same across the board. The material, appearance and price of the three windows are also very comparable. What other features will help you choose between these windows?

Installation

You can buy the best window in the world, but if it’s not installed properly it won’t perform properly. If you’re deciding between installation companies, one question to ask yourself is this: Do I want to choose a general construction company that only installs windows some of the time, or a specialized window company that installs windows every day?

Warranty

Another factor to consider is the warranty. There are two possible warranties a window may have: one from the manufacturer and one from the installer. Some companies only warranty their windows for 10 years against manufacturing defects, while others offer a lifetime warranty. Some companies also warranty against glass breakage. You will also want to ask the installer if they include a labor and service warranty on their installations.

Air Infiltration or CFM

According to Energy Star, “heat gain and loss through windows accounts for up to 50% of a home’s heating and cooling needs,” depending on climate. One final factor that can help you choose between comparable windows is air infiltration. This is measured by the CFM/sqft, or cubic feet per minute per square foot of window area. The CFM/sqft rating describes how much air would leak through your windows each minute at a standard windspeed. This feature is also regulated by the government, and the industry standard is set at .30 CFM/sqft. But what does that number mean in reality? And what are the factors that affect air infiltration?

It’s hard to understand what .30 CFM/sqft actually represents in your home. And, let’s face it, when companies test their windows for air infiltration, they’re doing it in a lab under specific conditions that won’t always match the conditions in your home. A common visual representation that you may see companies use is a graph that shows how many “soda cans” of air are leaked per minute at .30 CFM/sqft vs. whatever their window’s CFM rating is. In these graphs, .30 CFM/sqft = 24 soda cans/minute; .23 CFM/sqft = 18.4 soda cans/minute; .15 CFM/sqft = 12 soda cans/minute; and .04 CFM/sqft = 3.2 soda cans/minute. The lower the CFM rating, the better, but once a window gets below .10 you likely won’t feel a difference in your home.

Factors That Affect Air Infiltration

What factors most affect air infiltration? As we mentioned above, proper installation is critical to a window performing well. We recommend choosing a company whose main focus is windows and doors.

The climate and weather conditions where you live also determine how much air will leak through your windows. Here in New England we definitely want to choose windows with a lower CFM rating whenever possible.

Finally, the design of the window will play a major role in air infiltration. Windows that don’t open (i.e. picture) will have less air infiltration than those that have moving parts. All windows that move will have pile (weatherstripping material) between the moving parts to help prevent air leakage.  Rubber or felt are the most common pile choices amongst window manufacturers. No matter how many times you open or close your windows, rubber will bounce back to its original shape. Over time felt tends to stay compressed and may not provide the same level of protection against air leakage that rubber does. Window frame material is another design factor that impacts CFM ratings. Materials that expand or contract due to weather conditions will be more likely to sometimes have gaps between the frame and the sash. Finally, some windows are designed specifically to provide superior protection against air leakage. These windows will likely be the ones that are in the under .10 CFM/sqft category. Look for windows that have their CFM rating clearly stated in their brochure or on their website.

Danielle Cleary is with of United Better Homes in Pawtucket, RI.  United is a leading installer of replacement windows in the Rhode Island area and they’re our recommended company in the area.  If you’re anywhere near The Ocean State I’d recommend you give them a ring.