Air Leakage - An In-Depth Look At Window Air Leakage From Window Joe
Last updated on 05/16/2022
In the window world, most shoppers expect their replacement or new construction to carry the Energy Star label. Eco-friendly agencies, company marketing campaigns, and big box stores are especially good at building the hype around these labels, but they're not doing such a great job of explaining why it's so important. In fact, one of the most critical aspects of qualifying for an Energy Star label, air leakage, doesn't even register for most window shoppers.
But what is air leakage, what causes it, who measures it, and how can shoppers find it? Keep reading to find out.
What is Air Leakage?
Air leakage is a term used to describe the volume of air that can pass through a window within a given time frame. Testing bodies define air leakage as the cubic feet of air that passes through a window in one minute, divided by the area of the window. To find this value, agencies perform tests in controlled environments with repeatable and specific environmental factors.
That sounds like a lot of window jargon, so think of it this way: It's the amount of air that can pass through the window itself, with the window shut and locked. It does not include the air that passes around the window frame.
When homeowners swap their old, drafty windows for new ones, they expect that their home will then be air-tight, but that's hardly the case. Air is a very tricky force to deal with, and even the best of the best windows experience air leakage to some degree. Windows include many different components and moving parts, so it's tough to seal air out 100%.
How is Air Leakage Quantified?
The formula mentioned above will typically result in a decimal value, such as .1 or .3,, though these numbers will vary based on the window's size and manufacturer. It's this decimal that shoppers can use to compare air leakage between windows.
Generally speaking, the lower the decimal, the less air leakage a window will allow. Ratings of .1 or less are considered to be excellent, while anything above .3 is considered to be subpar. In fact, Energy Star uses the .3 benchmark to determine whether a window is eligible for the label. Any window with an air leakage rating above .3 doesn't pass the qualifying test, though there are other factors as well.
Interestingly, there are manufacturers that produce windows with air leakage ratings below .1. While these low values are technically more energy efficient, many in the industry maintain that the difference between windows exhibiting air leakage levels below .1 is unnoticeable. So, while customers might be able to pay more for a window with a .01 air leakage rating, they might as well save money and buy the windows with the .1 air leakage rating.
Note: When comparing air leakage data between two windows, be sure that the windows are the same size. Windows of different sizes will naturally have different air leakage ratings, making their comparison a moot point. Some manufacturers take advantage of this to make their windows sound superior to the competition.
What Causes Air Leakage in Windows?
Before diving too deeply into the causes for air leakage, understand that there is a difference between air leakage from a new, intact window and drafts through a wall, around the window (through the framing), and older cracked or damaged windows. With that out of the way, let's take a look at what causes air leakage.
There Are a Lot of Moving Parts
Consider the average double-hung window. It consists of a frame made from several pieces of wood, vinyl, metal, or a composite, with tracks installed, welded, or molded in place. In these tracks sit the window sashes, and there needs to be enough space for the panes to slide up and down easily. Each sash also consists of several parts, including the glass (typically two or three panes) and a frame made from metal, wood, or vinyl.
With so many parts, it's nearly impossible for a manufacturer to build a functional window that is also completely air-tight. Weather stripping and sealants may help, but they don't solve the problem altogether. Air can pass through the tiny gaps between the pane and sash frame, through the tracks, or the large gap where the two sashes meet and lock. However, the tighter the tolerance during manufacturing, the less air that will pass through.
Materials and Installation
There are some other factors that contribute to air leakage as well. For one, materials matter. Wood and metal windows will experience worse air leakage ratings than vinyl windows, provided all the windows in question are the same size.
Also, the air passing through these gaps is quantified in a testing lab in a controlled environment, with ideal installation techniques used. For homeowners to experience the same degree of air leakage, the windows must be installed properly.
Air Leakage vs. Convection
Another interesting point that many homeowners might not realize is that not all noticeable air movement in front of a window is caused by leakage. In cold climates, the window glass inside the home can be considerably colder than the heated air in the space causing convection. As the heated air hits the window, it cools and falls to the floor, pushing warm air upward. This creates a cycle of convection, which can be quite noticeable (and uncomfortable) next to a window.
Where You Can Find a Window's Air Leakage Rating
For some shoppers, an Energy Star rating is enough. And, from a practical standpoint, this qualification does mean the window has an air leakage rating of .3 or less. But, Energy Star doesn't break the values down for consumers to understand. Luckily, there's another organization that will tell consumers exactly what they want to know.
Beyond Energy Star, there is an additional testing and certification body that helps consumers make better decisions. The National Fenestration Rating Council (NFRC) is a third-party nonprofit that tests and certifies windows, doors, and skylights.
For NFRC certification, the product must have:
- A U-Factor between .20 and 1.20
- A Solar Heat Gain Coefficient of 0 to 1
- A Visible Transmittance range of 0 to 1
- And an Air Leakage rating of less than .3
If the product does not pass these tests, the NFRC will not certify it. But here's the best part for consumers: the NFRC posts these values on an easily understood label for consumers to see, usually underneath the Energy Star logo.
That said, it is possible for a window to have an air leakage rating lower than .3 and fail one of the other 3 tests. In that case, the window won't meet certification or wear a label. The consumer can ask the sales rep or manufacturer for the air leakage data, but do understand that this product is probably missing the mark somehow.
Understanding Air Leakage Will Help You Shop Smarter
Air leakage isn't the only factor that should go into choosing the best windows for your home. However, it is a critical part of ensuring that a window replacement project results in comfort, energy savings, and value. While air leakage isn't entirely unavoidable, having the knowledge to choose the correct windows will minimize its effects on your home's comfort and energy bill.