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.  

Air Infiltration or Air Leakage Rating Explained

Air infiltration or air leakage ratings are an important factor to consider when comparing energy efficient replacement windows.  In this post we’ll look into how these ratings are often misinterpreted, what is considered to be a pretty good rating and what to watch out for as you’re comparing the options.

To understand the rating you’ll first need to understand what the number actually represents.  The unit of measure for air infiltration is cubic feet per minute per square foot.  That is how many cubic feet of air can pass through the window in a minute at a constant 25 mph wind divided by the total window area.  I know, the unit itself is a little tricky.

best replacement windows of 2015

How are air infiltration and air leakage ratings misinterpreted?

Manufacturers are always looking for a way to tell folks that their products are “better” than another.  Focusing on air infiltration ratings is an increasingly common way to do that.

For example, the manufacturer Soft-lite uses a chart in their Elements window brochure that some customers could easily misinterpret.  It tries to put air infiltration in terms of how many milk jugs or pop cans of air can get through the widow.  Unfortunately, it clearly leaves out the size and the style of the window being tested.

Since the unit of measure for these ratings has the total window area in the denominator the size of the window affects the rating.  There are also several options offered by manufacturers that do change the ratings.  For one company to say that they’re rating is XX is oversimplifying the situation.

It’s also worthwhile to consider how often your home is hit with a constant 25 mph wind.  I’m not much of a sailor, but I’m aware of the wind enough to know that doesn’t happen by my house very often.

Manufacturers do this to try to separate themselves from their competition and there’s noting wrong with that, but trying to say a 0.01 difference in anything will make or break a project is pretty silly.

So what is a good air leakage or air infiltration rating?

Obviously the rating of how much air can get through the window is important.  The American Architectural Manufacturers Association (AAMA) says that any residential window that achieves a rating below 0.30 cfm/sqft meets the requirements for their gold rating.  This is the best rating they offer.  This is also the minimum air infiltration rating for the energy star program.  In fact, many of us in this business expected this minimum rating to be reduced with the 2015 energy star program, but it was not.  I expect we will see changes to this standard in the years to come.

High quality vinyl replacement windows consistently achieve ratings MUCH lower than this standard.  For a vinyl double hung window anything below 0.05 is pretty darn good.  Casement windows and picture windows are typically even lower while sliding windows are often the worst of the bunch.

On the other hand, wood, fiberglass and composite windows typically cannot achieve as tight of a seal.  These products typically have much higher air infiltration ratings.  The fact that wood, fiberglass and composite windows typically have substantially worse air infiltration or air leakage ratings should be weighed when considering those options.

What should I watch out for when looking at air infiltration ratings?

Typically it is wise to avoid putting too much weight into statements from folks who are drastically over simplifying things.  For example if a salesperson tells you his windows achieve a rating of 0.01 and that everything else is worse and any other window will make your house drafty he’s not telling you the truth (he’s probably the guy using the chart above).

Remember for vinyl windows anything below 0.05 is pretty darn good any you’ll likely never know the difference.  In fact, the National Fenestration Rating Council or NFRC doesn’t even give ratings below 0.10 because they don’t believe the air infiltration measurements are accurate below that point.  If the major independent testing body in our industry doesn’t recognize ratings below 0.10 that’s probably a good sign that you don’t need to worry about it below that point.

Important points to remember:

  • Air infiltration or air leakage is just one factor to consider as you sort through window ratings
  • It’s pretty easy to compare the differences between models
  • Lower is better, but below a certain point there is not much difference
  • Wood, composite and fiberglass options will typically have higher ratings than high quality vinyl
  • All types of windows are available below the 0.30 threshold
  • Not all types are available below the 0.10 level

If you’re looking for a window company right now, the best advice we have is to join Angie’s List.  For just a couple bucks you can get a 1-month membership and it’ll be worth much more than a caramel macchiato in the long run.  You can find the best pricing for Angie’s List on the internet right here

If you’re already a member of Angie’s List or if you’re just not going to join (despite my ringing endorsement), you can find our suggestions for the best companies in your area right here.