Airtight Windows for Passive House Construction

Airtight Windows for Passive House Construction

About as good as it gets under pressure of 50 Pa:  Airtight casement window when properly sealed, installed, and closed.
When it comes to creating an airtight passive house, I am relentless in my pursuits.

I would argue the most important performance attribute of any high energy efficiency window is how airtight the window is.  Furthermore, I believe the single most challenging (and most important) aspect of Passive House construction, in general, is airtightness. Even small levels of air-leakage can decimate insulating performance (and in the case of windows, solar heat gains).

With other aspects of Passive House construction, such as insulation or thermal breaking, weaknesses in certain areas can be offset by strengths in others.

However, this really isn't the case with airtightness.  Since Passivehaus requires an absolute limit of 0.60 air changes per hour at 50 Pascals of pressure (0.60 ACH50), there is no margin for error or sloppiness.  Even in our home of nearly 6000sf TFA and 60000cf of interior volume, that would equate to a maximum of 600cfm of air-leakage.

In our case we were striving to far exceed that number.  We were shooting for 0.15-0.17 ACH50 which equates to only 150cfm-170cfm.  I knew this was an extremely ambitious target and, in fact, would put us at one of the most air tight buildings ever certified.  With so many windows in our home (about 800 linear feet of window frame!), total air leakage, of all the windows and doors combined, could easily blow our chances of hitting this target or even the Passivehaus minimum of 0.6 ACH50 and therefore cause us to not pass Passive House certification.

It is difficult enough to hit the airtight standards of Passivehaus, especially with the complexity of our building envelope and the number of walls we had (again another rule that we "bent").  Remember, we didn't want to live in a shoebox.  For us, airtightness was, by far, the most important factor in our construction.

For windows to be highly airtight, they must be designed to exceptional levels of tolerance with functionally designed weather stripping and sealing.  Any penetrations of the frame, like the cranks and the locking levers most also be designed with utmost attention to detail.  Windows and doors should open and close like a vault door and you should be able to feel that. 

Perhaps even more important is the selection of window type because not only do varying window types affect thermal performance, but they can greatly affect airtightness.

The most airtight window types are tilt-and-turns (common abroad) and casements.  The least airtight window types are double-hungs and sliders.  Don't even think about using double-hung windows in a Passive House!  Yes, windows of casement or tilt-and-turn type are more expensive, but their designs will go along way to helping you achieve the airtight levels that are required for Passive House construction.

Unfortunately finding published airtight specifications from window manufacturers can be difficult, at best, to find.  I would suggest gravitating to fenestration makers who readily provide their airtightness performance ratings in their marketing and/or technical materials.  Those companies who really intend to market to the Passive House community and have been successful in their designs, I believe, would be proud to vocally pronounce their airtightness ratings.

IR Image of Air-Leakage of Awning/Casement During our 2nd Blower Door Test

Airtightness testing is generally performed to a standardized procedural test of ASTM E283.  This test basically pressurizes the window at 75 Pascals of pressure to scientifically measure air-leakage rates.  One should be concerned with both negative (pulling) and positive (pushing) pressure.  Casement types, as one may expect, would perform better when depressurized with negative air pressure as the window panel would tend to be "sucked" tighter into its frame than when positively pressurized when the window panel would tend to be pushed outward from its closed position.

Loewen provided me with the following airtight performance ratings at 75 Pa (ASTM E283)
  • Casement: 0.1 cfm/ft^2
  • Direct Set (Picture): 0.01 cfm/ft^2
  • Patio Door: 0.1 cfm/ft^2

Ultimately airtightness of your building envelope will be empirically measured by a blower door test.  This is an absolute test. There is no place to hide, no way to fudge, unless one outright cheats and falsifies the tests/results for (a bogus) certification. You will either succeed or fail, there is no in between, no grey area.  (Can I be clear enough?)

Actual IR image during our 2nd blower door test visually showing air-leakage occurring in realtime
For all the effort and aggravation to build a Passive House, don't let it be the latter.  One of the ways you can avoid failing here is to make certain that you get the airtightness ratings for each window and door type from you chosen window manufacturer.  Perform the calculations of either linear footage or square footage (however the windows are rated) and calculate the "theoretical" aggregate air-leakage of all windows and doors.  This should serve as a rough baseline as to the best performance you could achieve in your building with the assumption that there was NO air-leakage in any other area (an impossibility, by the way).

IR Image Taken During our 3rd/4th Blower Door Tests Dedicated Solely for our Window Manufacturer
After performing these calculations, if the aggregate air leakage rates are unacceptable (treat the 75 Pascal tests as if they were performed at only 50 Pascals) then select another window/door manufacturer that provides you the margin you seek and/or reduce the amount of window glazings.  Remember, when it comes to making a building envelope air-tight all the taping, caulking, foaming of the rest of your building will not make up for leaky windows (and doors) whether these shortcomings are by design, manufacturing, assembly, or installation.

To conclude, if you can not get those performance ratings or the manufacturer doesn't specifically warranty their windows and doors to clearly specified airtightness numbers, select another manufacturer which does.  I can not stress this last point enough.