Air Leakage Rating for Windows

There are many factors to consider when purchasing new or replacement windows for your home. In addition to window colors, sizes, shapes, and styles you also need to consider the window’s energy ratings. When you order your windows you’ll need to make sure that they meet your local energy requirements. Windows from reputable companies have certified ratings provided by the National Fenestration Rating Council↗ (NFRC).

These ratings include:

U-Factor
Solar Heat Gain Coefficient (SHGC)
Air Leakage
Visible Transmittance
Condensation Resistance

In this post, we’ll introduce you to air leakage ratings for windows and explain why air leakage matters.

What Are Air Leakage Ratings for Windows?

Air leakage ratings tell us how much air is passing through the window unit. You might be wondering why air leakage would be a concern with new windows, new windows are supposed to resolve the problem of drafty windows right? Well, yes, for the most part new windows that are properly installed should resolve your drafty window problem but air can sneak in through anything including cracks in walls, windows, and doors.

Air leakage is better described as air infiltration. The measurement of air that passes through the window unit is determined before the window is installed, to maximize the window’s value it should be installed by an experienced professional.

Air-Leakage Rating Defined

A measure of the rate of air-leakage around a window, door, or skylight in the presence of a specific pressure difference. It is expressed in units of cubic feet per minute per square foot of frame area (cfm/sq ft). Formerly expressed as cubic feet per minute per foot of window perimeter length (cfm/ft) but not now in use. The lower a window’s air leakage rating, the better its airtightness.¹

What’s the Range of Window Air Leakage Values?

The range of window air leakage values is between 0.1 to 0.3. The lowest rating on the scale is a 0.1, this represents the least amount of air that passes through the window. Air leakage ratings can technically go below 0.1 but the value isn’t recognized by the NFRC.

Air leakage ratings are calculated in cubic feet per minute per square foot. The measure describes the amount of air in cubic feet that can pass through a window per minute at a constant rate of 25 miles per hour, divided by the total area of the window.

What is a Good Air Leakage Rating?

A good air leakage rating is one below 0.3. Ratings below 0.3 are recognized as energy efficient by both the NFRC and ENERGY STAR. Windows with ratings closer to 0.1 are the most efficient.

As you search for windows for your home remember that air leakage is not a required rating on NFRC labels. While it may not be visible on the label you can ask the window dealer to tell you if the know what the window’s air leakage rating is.

Questions to Ask Your Window Dealer About Air Leakage

What is the air leakage rating of the window?
Do you know how the window performs in windy conditions?
Have you received any complaints about these windows being drafty?

Cold Glass is Not an Air Leak

If you live in a cold area and feel uncomfortable drafts near your window on a cold day it may not be a draft from a window air leak. According to the Efficient Window Collective↗, cold glass can create drafts by cooling the air next to the window. When the air cools it drops to the floor in a convective loop. High-performance windows are less likely to experience this type of draft.

Brennan Enterprises offers window sales and installation services to homeowners and businesses in North Texas, give us a call or fill out a form to schedule your free in-home or virtual consultation.

¹ Carmody, J., Selkowitz, S., Arasteh, D., & Heschong, L. (2007). Residential windows. New York: W.W. Norton.