What's the range of window air leakage values for good-quality US windows vs. garden-variety vs. Passive House-quality?

It's very hard to find published window air leakage rates (in cfm/ft2), harder still to get independent test reports with this information. So far I've found the following data points:
* a maximum of 0.3 for all of one US mfr's product line (in an internal manual);
* .02 for a tilt-turn unit by same mfr (from independent test report);
* .003 for a Canadian mfr's tilt-turn unit (from website literature).

I've also been told that the test apparatus doesn't yield a result smaller than .01.

What's the real story here? What is a typical value for a US good-quality (i.e., double-pane, low-e, wood or fiberglass) residential window? What is a typical value for an older, single-pane window? What is a typical value for a Passive House-worthy window?

Asked by Ann Edminster, GBA Advisor
Posted Sat, 09/19/2009 - 11:38

Tags: Green products and materials

Ann,
To get you started, I found this document from Wasco Windows to be helpful:
www.wascowindows.com/Documents/U-factor.pdf

Here's the relevant section:

"How is air infiltration measured?
Air leakage is measured by physical testing of a standard sized window. The test window is installed on a large wall. On one side, a vacuum is of 75 pascals (1.57 psf) is applied. This corresponds to approximately a 25 mph wind blowing perpendicularly to the window. Flow meters measure the rate of air leakage. This is divided by the total window area to get a reading (in the U.S) in cfm/ft2 to the nearest tenth (i.e., a test value of 0.17 would be given a rating of 0.2). The best possible rating by the NFRC is 0.1, as they will not round to 0. ...

Why doesn't manufacturer “X” have air leakage ratings on their windows?
The NFRC does not require manufacturers to test for air leakage. ...

Does air leakage have a large influence on energy bills?
If a manufacturer elects to have its product tested, it cannot have more than 0.3 cfm/ft2 of air leakage. If this maximum is met, the energy loss through air leakage is likely considerably less than that through heat loss (ASHRAE 2005 Fundamentals, P. 31.55). Nonetheless, air infiltration has a major effect on perceived performance of the window. Even at the passing value of 0.3, disturbing drafts may be felt. These drafts are often the very reason a customer is replacing a window!

Are some window designs better than others for air infiltration?
Yes. Casement and awning windows offer excellent air infiltration performance because pressure from the wind tightens up the seals. Tilt-and-turn windows, with their dual compression seals and multiple locking points around the perimeter offer equal or better performance. Traditional double-hung windows (sometimes called “vertical sliders”) cannot do as well due to their sliding surfaces where compression seals are not possible. And, horizontal sliders generally have the worst air infiltration performance of all window types. Beyond the sliding surfaces of the double hung, these windows have frames designed for sash removal. This feature, along with necessary provisions for drainage, make it impossible to seal these windows as tightly as a double hung, much less a casement."

Answered by Martin Holladay, GBA Advisor
Posted Mon, 09/21/2009 - 09:07

Thanks, Martin, this is very helpful!

Answered by Ann Edminster, GBA Advisor
Posted Mon, 09/21/2009 - 11:06

HA- HA. The gentleman who wrote this is definately one who any window replacement costumer should hear! I almost made the mistake of buying a double pane window from a company. They had all the low U-Factor ratings, and SHGC numbers, but when I operated their sample, you could easily see how when opening the upper, or lower window up & down, air could seep through the sides where they slide on each other !!!

Any single hung will provide for less dusting of your home. Always check how tight, and the micro- fibers that seal the moveable parts, and make up your own decision.

Personally, I don't want outside air comming in (drafts)... that's why I replaced my windows in the first place !!!

Don't get ripped off... make your own assessment of a window with your own eyes, not just words in a brochure, or numbers. AIR LEAKAGE between moveable parts is something to LOOK AT - EVEN IF not too many people have figured out it's importance regarding window replacements.

Answered by Frank Peters
Posted Sun, 03/14/2010 - 01:44

I've been considering this same issue as I look to replace my windows. After reading Martin's article on blower door basics on this site, I learned that a reasonable target for air leakage at 50 pascals is around 1000 -2000 cfm with older leaky homes measuring up to 5000 cfm. For my house, I have roughly 250 ft2 or window area, so a .3 cfm/ft2 window that meets the AAMA/NFRC standard tested at 75 pascals would likely contribute around 75 cfm of air leakage for double hung windows. Given that this is only 7.5% of the leakage at a 1000 cfm target, there are probably many other larger contributors to overall air infiltration than the windows. Hopefully this performance is maintained over the life of the window so durability for the weather stripping something to consider.

http://www.nfrc.org/documents/ReplacementWindows.pdf states that air leakage numbers typically range from .1 to .3 and this is consistent with values I found on the Andersen, Pella, and Paradigm sites. The casement number are down to .01.

Answered by Robert Hagood
Posted Sun, 03/14/2010 - 22:20

Answered by Robert Hagood
Posted Sun, 03/14/2010 - 22:25

Robert,

A leakage target must take into account the volume of the heated space. 1000 CFM50 alone is meaningless. In a 2,000 SF house, that would amount to 3.75 air changes per hour (ACH50), but in a 1200 SF house that rate would equate with 6.25 ACH50.

Air changes per hour under 50 pascals pressure is a more useful number. Typical new homes leak about 3.9 ACH50. A tight house will leak 1-2 ACH50. How that relates to a home's natural leakage rate, under normal stack effect and wind pressures, depends on the climate zone and the height of the building.

IECC standards for maximum fenestration air leakage are:

Operable windows, skylights, and sliding glass doors: 0.34 cfm per lineal foot of operable sash crack or 0.30 cfm per square foot of window area
Residential doors, swinging: 0.5 cfm per square foot of door area
Residential doors, sliding: 0.37 cfm per square foot of door area

Answered by Riversong
Posted Sun, 03/14/2010 - 23:35

Also remember that, when assessing the value of window replacement, existing old weight-and-chain double-hung window sash will perform dramatically less well than the worst case new unit rated at 0.3. This is compounded by the leakage at the weight pocket and casing, which were never designed for air sealing. Replacement sash will not be able to perform well either. The only window replacement scenario that succeeds in the goal of air sealing (not the sole goal, we understand) is a new full-frame unit with modern air and water sealing at the building's air barrier. You can easily verify this yourself with a blower door. Enjoy. Also remember that the NFRC test is at 75 pascals, not 50.

Answered by Jamie Leef
Posted Wed, 05/19/2010 - 15:07

What's the range of window air leakage values for good-quality US windows vs. garden-variety vs. Passive House-quality?

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