Passive House Wall Section Development, Part III
Having covered the basics of our previous wall designs, we should probably start to tell you all about what we’re building now. What follows is good for all: a tonic for the energy nerd, free advice for the builder, and the promise of reduced cost and complexity for the homeowner.
In the prefab wall panels for our residential Passive Houses, we now use a 2X8 structural wood frame filled with blown-in cellulose, with an air-sealed layer of ZIP sheathing, wrapped—a little redundantly perhaps—with a layer of drainable house wrap. Still sounds familiar, right? It should. Well, stop right there. Sort of. That sheathing is where the structure stops, anyway. Outboard of the sheathing, there’s a warm, dense, continuous 6” thick blanket of rockwool insulation. A thing of beauty, really: vapor open, hydrophobic rockwool insulation. We tether that to the structure with strapping, fastened back to the stud wall with long structural screws. Driving these screws in an alternating fashion—one horizontal, then one upward at 45°, then one horizontal again, and so on— makes a sort of Z truss. This keeps the strapping from sagging under the weight of the siding, and your walls from looking sad and droopy. What this gives us is an airtight, vapor-permeable wall, with a structure that is protected from the ravages of weather, temperature fluctuations, and mischievous gnawing critters. It is lighter in weight, faster to build, has redundant systems for moisture control, and uses less material and fewer different materials in its construction than our previous panel designs.
What we started with was nixing the outer layer of 2X8s in favor of furring strips/strapping applied over the exterior insulation. This requires a great deal of attention to detail on the part of the architects, because there is virtually no structure on the exterior half of the wall; any blocking for lights, plumbing penetrations, etc. must be provided before we seal up the panels.
The Advantech rims and window bucks have been eliminated, effectively flattening the air seal to a continuous uninterrupted plane. We’ve also changed around our construction approach to take advantage of how we build a prefab panel, which is flat on the floor. Or, to phrase that in less flattering terms: we finally realized that gravity’s charming ability to hold heavy things still against the ground could be used to our advantage.
Since we build our panels from start to finish with them laying horizontally, we added the simple step of attaching a plywood “box” around each window frame before dropping it, (very softly, mind you), into the rough opening in the wall. This box/flange/extension/whatever-you-want-to-call-it extends inward the depth of the structural stud wall, and since the wall is laying flat on the ground, the window is held up perfectly in plane with the sheathing. We just peel the backing off the split-release tape already attached to the window and flop it down onto the sheathing. Simple. Dignified. No sticky origami required. Not only that, but our plywood box provides a perfectly square surface for drywall returns on the interior, with no more finicky shimming.
With the Advantech gone and no material protruding out around the windows, we’ve taken to mounting the exterior window trim on simple metal brackets, fabricated in-house. We fasten these brackets to the trim first, which means that applying the window trim with perfect reveals all around is a simple matter of setting it in position and screwing the brackets to the sheathing. Not one of the builders has yet expressed nostalgia for the days of wrestling trim on to an Advantech buck that’s been forced out of square by Mobius-strip-like lumber. No rim and no window buck also means that thermal bridges have been eliminated at every window and panel connection; again, there is virtually no structure penetrating the external insulation layer.
These measures are saving no small amount of time and money. So much of both, in fact, that when the foremen revealed at a company meeting how much per window we were now saving, the response was disbelief, derision and outright denial. Having always wished I could have been witness to the riots accompanying the debut performance of Stravinsky’s The Rite of Spring, I now feel like I’ve had a fair taste.
So, to sum up our new wall in no uncertain terms: we present to you a 2X8 stud wall filled with dense pack cellulose, a flat air seal at the sheathing layer, a layer of house wrap, 6” monolithic layer of rigid rockwool insulation, and some strapping to hold your siding of choice. This wall puts us comfortably in the R50 ballpark, R-53 to be exact. Finish inside and out as you (and your budget) see fit.
There you have our hard-won lessons of making affordable, durable North American Passive House prefab construction. And we’re happy to share more as we continue developing new techniques and new market opportunities for passive house in the U.S. After all, we are still looking for any possible improvement, every day.
-Andrew Frederick, Foreman
