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.

What to look for in a Window Warranty

Replacement Window Warranty Factors

When shopping for replacement windows, one of the majors factors that folks consider is the window warranty.  The warranty coverage on your new windows is significant for many reasons, not the least of which is that you want some security in knowing that you will have long term support on a purchase of thousands of dollars. Like many things in the marketplace, window warranties are part real concern, and part sales and marketing. I like to break warranties down to two primary areas: Product Warranty and Installation/Workmanship Warranty.

Product Warranty:

The product warranty on a replacement window is exactly that. It is the length of time that a manufacturer will provide warranty coverage for manufacturer defects. Generally that will not include items such as damage from abuse, failure to maintain, etc.  However, it does provide for replacement parts should anything fail due to a manufacturer defect. Common items include failed glass seals in an insulated glass unit, defective hardware, warpage, etc.  Some manufacturers offer accidental glass breakage as part of their coverage as well. Most vinyl window manufacturers offer a lifetime warranty, while wood windows generally have some variation of a 20/10 warranty, meaning 20 years on glass and 10 years on everything else including the wood.

Things to watch for:

Transferability  is important if you are considering selling the home any time soon. This means that you will be able to transfer the product warranty to one or more future owners of the home. Proration is an often overlooked factor. “Prorated” means that while there is coverage for the entire warranty term, it goes down over time. This is similar to a car battery or roofing shingle, where you have 100% coverage for say the first 10 years, then the coverage starts dropping in increments where you are paying an increasing portion of that as time goes on.  Ideally, a warranty that is fully transferrable and not prorated is best.

What product warranties typically do not cover:

Abuse, acts of God (ie: fires, storms, etc. that would be covered be homeowner’s insurance). Product warranties typically will not cover any labor needed to install the replacements parts. That falls on you as the homeowner, or on the installation company if they offer that service, whether it be paid or at no charge to you. The “fail to maintain” provision can be tricky on wood windows where condensation can damage or rot the wood before the 10 years and potentially be denied as a claim.

Workmanship/Labor/Installation Warranty:

The installation warranty on your replacement windows while important, can vary wildly from one dealer to the next. One company may offer one year on workmanship/installation, while the next offers lifetime, and a third still somewhere in between. The fact of the matter is that most installation related issues will manifest themselves visually within the first year or two after install, so something like a 5 or 10 year warranty is generally adequate. That said, many companies offer 20+ years all the way through lifetime. A lifetime workmanship warranty may sound great in marketing, but remember that the installation warranty is only as good as the company offering it. If all else is equal, certainly a longer term only benefits you as a consumer.

Things to watch for:

Service call/diagnosis charges, coverage to install the parts that are provided under the manufacturer’s warranty and for how long, a list of items that are included or excluded.

What installation warranties typically do not cover:

Installation warranties are intended to cover any poor or defective installation practices. Therefore common things that would be considered maintenance items like caulking, cleaning tracks and such which could hinder operation, etc., are typically not covered or the coverage is limited.


Every window warranty is not created equal. “Lifetime warranty” whether on product or installation may not mean the same thing from one company to the next, so take a look at the details and make sure that you are getting the protection that you are expecting. Also keep in mind that those warranties are only as strong as the companies offering them. Company history and reputation are important tools to look at. Lastly, “Lifetime” does have a quantifiable definition in most states. Here in Wisconsin, “Lifetime” for warranty purposes is defined as 30 years. It is recommended that you determine how this is specifically defined in your own location.

Brandon Erdmann is the owner of HomeSealed Exteriors in Milwaukee, WI.  He’s a window installation expert, a supporter of this site and an all around great guy.  If you’re in the Milwaukee area and you’re thinking about new windows you should give Brandon a ring.  You’ll be glad you did.