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?


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?


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.

Top Factors to Consider When Choosing a Window

With over 35 years combined experience in the window business, we’ve worked with many customers to find the right window for their needs. As you move forward with your research, ask yourself what factors are most important to you as a homeowner.  Is matching a particular design aesthetic your primary goal?  Do you prize extreme energy efficiency above all other considerations?  Do you prefer a window with a lifetime warranty that will last for many years to come?  How important is price in making your final decision?  We’ve created a list below of the most common factors homeowners consider when choosing new windows.  The clearer you are about what’s most important to you at the beginning of the window shopping process, the easier it will be to choose the window that best meets your needs.


For many customers, their spending plan is an important factor in determining which replacement window or door they choose. Some window companies offer only one type of replacement window; others offer a variety of brands and materials at a range of price points that work with many different budgets.  Knowing what you’re able to spend on your project will help you rule out some companies and focus on others.


Some window companies offer a variety of choices while others have only one window material or brand to offer. The goal of a one-size-fits-all window company will be to convince you that their window is the best and that it’s exactly the right window for you.  Although this may be true in some cases, it’s certainly not true in all.  There are four main materials used in residential replacement windows today:  wood, vinyl, fiberglass, and composite (a blend of more than one material, i.e. wood and vinyl).  The wider the selection of window materials to choose from, the more likely you will end up with the window that’s the best fit for your needs.

Window Quality

Let’s face it: all building materials are not created equal, and windows are no exception.  For some clients, a premium quality window is of the utmost importance; for others, not so much.  In many cases, the higher the quality, the higher the initial price tag.  Higher quality windows will often outlast other brands, so your long-term savings could certainly be worth the up-front investment.  Will you live in the home for many years to come, or are you focused on a short-term solution?  These are all things to consider when looking at the different window brands available.


Some homeowners will start their window research with a specific design aesthetic in mind.  Owners of condominiums and historic homes will usually have to follow certain guidelines when choosing which window to install.  Alternately, your home style or location may demand that you work within specific architectural parameters in order to maintain the integrity of the design; such is the case with Craftsman style homes, or regions like Cape Cod.  Only certain types of windows will look right in these instances, and you want to be sure you’re working with a window installer who can match your home’s design needs.


Energy efficiency, U-factor, Solar heat gain coefficient, double-glazed, impact resistant, egress windows, condensation resistance, air leakage, visible transmittance…are you confused yet? These are all words that can be used to describe how a window will perform.  Sometimes online research can leave you with more questions than answers.  All residential replacement windows installed today will need to meet certain basic standards for energy efficiency.  Some homes require specific performance features based on their location, like coastal impact windows for homes located within a certain distance from the ocean.  Additionally, some homeowners may opt for certain features based on what’s important to them (i.e. triple-glazed windows for more energy efficiency; windows with more sound proofing for busy urban areas).  A good window professional will explain your options in language that’s easy to understand, and will make recommendations based on the needs and wants of both home and homeowner.


The terms of a warranty can mean the difference between a good purchase and a great purchase. When choosing a window for your home, be sure to explore the different warranties available.  Common warranty lengths are:  10 year, 20 year, and lifetime.  Many companies have different warranty lengths for different parts of the window (glass seal, frame, hardware, or moving parts).  Not all warranties cover glass breakage, but you can sometimes add that at the time of purchase.  Labor and installation warranties are another aspect to watch out for.  These would usually be offered by the company that installed your windows, and are often a sign that the installer is very confident in their work.

Customer Reviews

Before making any final decisions, we HIGHLY recommend you read reviews by previous customers as part of your research. Angie’s List (which now has a free membership option), Better Business Bureau, Consumer Reports, HomeAdvisor, The Window Dog…there are many websites out there that can help you see who it is that you’ll be inviting into your home.  Read the responses from each company to get a balanced view of complaints and to see how they handle issues.  No company is perfect, and seeing how they handle problems is important should any unforeseen issues arise with your installation.

 Buying Experience

It often doesn’t take more than a couple of appointments for customers to realize that ‘buying experience’ may be one of the most important factors in choosing a window company.  Some window companies employ hard-core sales tactics or focus on bashing other window brands to make their own look better.  Their salesmen may wear you down over the course of several hours or offer a “sign now for huge savings that go away tomorrow!” sort of deal that makes you feel like you have no choice.  Other companies will aim to educate you rather than scare you, and will honor their price quote for a full 30 days after your consultation.  These companies are interested in providing a professional buying experience backed with exceptional customer service.  We encourage you to meet with more than one company so you can see the difference for yourself.

 Company Features

A final question for homeowners to ask themselves is “Who do I want to install my windows?” Do you prefer to work with a large chain business, or would you rather work with a locally-owned company?  What, if any, kind of labor and installation warranty does the company offer?  Are window installations their specialty, or is the company more of a jack-of-all trades?  Do they employ their own installers or do they subcontract their installations out?  Choose your windows and your installer wisely and you will be satisfied with your purchase for years to come.

Nicole Spano is the CEO of United Better Homes in Pawtucket, RI.