Extremely Airtight Passive House Construction

Creating an Extremely Airtight Passive House Ceiling of a Truss Roof

I believe the subject of air tightness and how to actually construct an extremely airtight Passive House is probably my single most favorite topic of discussion.

Why?  Because it so bloody hard to accomplish!  This was going to be my version of summiting Mount-Everest.  Remember, air-tightness is not accomplished with insulation.

My goals with respect to constructing an extremely airtight passive house were relatively simple, but lofty, nonetheless: 

• Shoot for an ACH50 rating of 0.15 - 0.19 (three to four times as rigorous as PH's specification of 0.6)
• Completely eliminate air-leakage as much as humanly possible from the actual building itself, leaving only the windows and doors as sources of air-leaks 
• Accomplish this with a more conventionally appearing home--and one that has a lot of windows and doors--not a six-sided shoe-box
• Accomplish this while incorporating some recessed lighting into the ceiling below the unconditioned attic space 
• Accomplish this with the incorporation of two cantilevered balconies

More than one individual that I have worked with thought it was going to be absolutely impossible to achieve an airtight rating anywhere close to this, let alone even making PH's required 0.6 ACH50.

Unfazed by this, we began our pursuit...

Starting from the outside in and then from the top down, we had some specific challenges we needed to overcome.  The secondary interior stud wall and truss roof coupled with the use of some recessed lighting created issues that would absolutely need to be addressed. Taken what we have learned here into future projects, we could address some of these challenges earlier and more easily in the construction phase.


Fortunately for us, I had met up with Floris from Four Seven Five at a PHIUS conference.  Floris convinced me that their company and the products that they imported were more than up to the task of making the construction of our airtight passive house possible and, indeed, 475 provided everything we needed to accomplish our goals.

However, I do believe even Floris himself had some [serious] doubts, at least early on, as to whether or not we could actually attain them. 0.60 ACH50 he believed was possible.  0.20 ACH50?  Well that could be a bit of stretch there--given that only a relative handful of buildings certified around the world perform in this realm of extreme air tightness.   And of those, how many have been built with as many deviations from the simplified and minimalistic architectural designs that are so typical of passive house construction?

475's case study of our airtight passive house:

• Making the underside of truss roof in a SIP house airtight with the Pro Clima system