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.

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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

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31 thoughts on “Air Infiltration or Air Leakage Rating Explained”

  1. Nice blog.

    Where does the NFRC state that rates below 0.10 are “accurate” below that point?

    We are looking at some windows with some lower numbers and trying to figure out the value proposition point.

    Do you email back or do I have to check here?


    1. Hi there Winston, from the quantity and scope of your other comments I don’t know if you’re really a window shopper, but I’ll answer this one for you. The NFRC 400 document outlines the standards for air leakage testing. It says on page 10 that all results will be measured to one decimal place. It’s a bit of a dry read.

      Just let us know if there are any other questions we can help with!

  2. Need to compare casements vs. doube hung windows. Which is better, Feldco told me thier Double hung windows are as effiecient as thier casements and said I could pay as high as $5,000 for a simple 3′ x3′ vinyl window casement with a laminate inside. Obvioulsy I laughed at that alleged fact, and he said that the more affluent places in our area (Milwaukee County, WI) would and do pay that price. We reside in a nice suburban area, not the City of Milwaukee, WI. Again I laughed and said just because you may be rich doesn’t make you stupid…..All the reseach I have done says casements and awning windows are the best for limiting air leakage.

    1. Ha, we love hearing the stories about interactions with window companies. You’re right that casement and awning windows typically have the best air infiltration ratings. The Feldco website looks a little cheesy, “It’s the biggest sale of the year!!”. I’d suggest getting another quote or two. A casement window typically costs a couple bucks more than a double hung. Maybe $50-$75 tops.

      If you want to keep dealing with them ask them for the air infiltration rating on their double hung and then ask for the CPD#. After he told you 1 casement window will cost $5,000 I would suggest saving your energy and looking elsewhere.

      As an aside, my favorite part about working in Milwaukee was learning the term doorwall. I grew up just down 94 in Chicago and I had never heard that before in my life. Wisconsin.

      1. CPD is Certified Product Directory from the NRFC (National Fenestration Rating Council). They do love their acronyms.

  3. I just replaced my house with Vinyl Max -Edison windows. I noticed that with the windows locked, there is up and down movement of 1/4 inch to 3/8 inch . That seems like too much movement to maintain the AI rating. Should a window have that much play after it is locked? This is easy to measure…lock window and pull sashes down, then place a pencil at frame against top sash and move the window up and down while locked. You can then open sash and measure movement. These are not as tight as I would expect. Feedback?

    Also, the sticker on the window states <=.3 for AI and not the rated .12 on its website. When I asked the salesman, the response was that they are required to put the min AI rating on the window sticker. Help!

    1. It’s true that most windows come with stickers that say <=0.30 as the air infiltration rate does vary based on the size of the window, but the stickers do not. 0.30 is the min for AAMA gold rating so they print that to say that they meet that standard regardless of the size. Regarding the movement, some windows do move a little more than others when they're locked. I'd have them check it out and see if they think it's acceptable. One simple option would be to order a sash a little taller, but it may get hard to lock if they get too tight.

  4. Would you suggest using the Vytex Georgetown series or the Simonton 5500 Reflection series. I live in the mid-atlantic region and both have similar prices, but I am looking at quality over a couple hundred bucks? Does the Reflection series offer an R-50 or higher alternative? Thank-you.

    1. I’m not a huge fan of either one. What’s your zip code? I may have an option for you in the mid-atlantic region.

  5. Can you advise on decent (efficient, attractive) double hung replacement windows available in the NE Georgia/Metro Atlanta area? We were offered Simonton Asure but saw your reviews of the Asure and the 5050 and 5500. Is the Impressions series any better? Or is there another mfr you like in this region? Thanks, we REALLY need help!

  6. Where do you go to find published air infiltration ratings? NFRC certified products directory doesn’t list them, it just shows <=.3 or nothing for most manufacturers. I'm trying to decide between Okna Elegante and Sunrise patio doors and would like to see how their air infiltration numbers compare.

    1. Most smaller manufacturers like the ones you mentioned don’t publish their ratings publicly which I always thought was a real sign of weakness. My guess would be they do it that way to avoid scrutiny or because some of their dealers salespeople might be making questionable claims out there. Larger more well established manufacturers will typically have all of that info available so you can compare. I think that is a real sign of strength.

      You can ask the salespeople for their ratings, but you may need to just take their word for it which can be a little risky. Reputable dealers should be able to get you all of that info without any issue.

      1. I am comparing Okna to Pella and the Okna brochure does have the U value (.16), Design Pressure (80) and Air infiltration (.01) ratings on it, but I’ve had a hard time finding the Pella air infiltration ratings. I’ve asked our sales guy a few times, and he has sent me some documents but maybe the air infiltration rating is buried somewhere. I was looking for anything that said cfm or AL or AI but they show a “Performance Class and Grade” and “Hallmark Rating.” Could the AL be part of these ratings? The wood windows we wanted from Pella don’t meet the Energy Star U Value for our area, and we can’t go to vinyl without changing our large double hung dining room windows into something else. I think their design pressure is only 30-40. It seems like Okna is the way to go and I feel comfortable with the people at HomeSealed here in Milwaukee area but my husband had his heart set on Pella and wood windows so I’m trying to get all the ratings so we can compare apples to apples. Thanks.

        1. The air infiltration on a Pella wood window will be quite a bit higher. It’s a pretty strong sign that they don’t publish it. If it were info they were proud of they’d be shouting it from the rooftops. Good luck with your project.

  7. Great blog/website. Very helpful. I moved to the Denver metro area 2 yrs ago and need to start upgrading our current 20 yr old builders grade vinyl windows. I can’t do them all so once so need product/company that isn’t going to change in next few years. I’ve read lots of reviews and talked to a few companies. Do you have a recommendation for a vinyl window product for the altitude, range in temps and string winds? When have one side of the house with a large window that gets pounded by winds on a regular basis. Thank you

  8. Looking for replacement windows for my 17 Silverline builder grade windows.
    The condensation on the windows is horrible and one seal has fogged up.
    White vinyl or fiberglass is fine.
    Suggestions in the 53051 zip code? Suburb of Milwaukee.

  9. Looking at Anlin Del Mar windows in Phoenix and am concerned about vinyl durability as well as air infiltration.
    Can you help?

    1. All nicer vinyl windows will hold up just fine. The only person who would tell you otherwise would be a fiberglass or composite window salesman. Those guys seem to only be able to pay their bills with tall tales sometimes. I think Anlin windows would hold up fine. My company also offers windows in Phoenix. You can find us here if you’d like an alternative.

  10. I’ve used your site a lot to make decisions, now I’m posting, because I need help.

    OKNA 500 Dx vs ProVia Ecolite.

    OKNA: U Factor: .25, SHGC: .29, Visible Trans: .53, Air Leakage: .3 Air Infiltration: .02 Condensation: 62

    EcoLite: U Fact: .29, SHGC .31, Visible trans: .56 Air Leakage: .3 Air Infiltration: .13 Condensation: 56

    Does the 5.5% performance increase, justify the extra 20% in cost?

    OKNA is a great window, I have them already, but I can’t justify spending thousands extra (20% more) on a 5.5% performance increase. Even after spending a few hours going back and forth, it’s just too hard to justify it.

    They’re nearly identical in build. The OKNA likes to tout their .02 AI rating, but as this article stated, it’s not really that big of a deal.

    If you could skim the first 14 posts, probably would take about 5 minutes and tell me if I am truly wrong? All my life I’ve double guessed myself, but refuse to do so unless I get more people to say so when it comes to spending thousands extra.

    1. I should mention the prices.

      Okna 500DX: $723, each, doesn’t matter size.
      ProVia Ecolite: $583 each, doesn’t matter size.

      Thank you!

  11. Hello, I live in Minnesota and was wondering if you know of a manufacturer who makes windows or storm windows with an STC of 40? We recently purchased a home which has Alside Mezzo windows which have a 26 STC, I believe, and it is so noisy in our house that my husband cannot get any quality sleep. We had no idea it was going to be this bad because the model home is more than a block away from the street we live on and no homes were on this block and we selected the location from a map. I have no idea how to proceed. Do you have any suggestions for me?

    1. Hi Maryann, one challenge you’ll face is that sound is also coming through the walls and the roof. Imagine if there were no windows in the bedroom at all. You’d still hear sound if the area is noisy. Windows typically make up a pretty small percentage of the total wall area.

      That said, you can get different sashes for the Alside Mezzo windows to improve, or reduce, sound transmission. It’s a little more involved that just ordering new sashes as they’ll also be heavier than the old sashes which will mean you’ll need new balances too. That’s not a hard thing to replace, just an extra step.

      If you wanted to go that route you’d want to look at triple pane or laminated glass. Just keep in mind that no windows will eliminate sound transmission. In my experience in situations like this setting expectations is important.

      Beyond that you can get higher STC windows that are specifically designed for projects in loud areas, near a freeway or an airport. I just don’t know how much of a benefit that will provide if the walls and roof weren’t built with extra sound reducing insulation.

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