Passive House Wall Section Development, Part II
When a client stepped into our office looking for a cost-effective micro house for their property, we knew our first prefab project had just walked through the door. The program called for a single level, 600 sq. ft. open floor plan. A simple layout meant that the house could be constructed with only five panels; it was the perfect stepping stone to test the efficiency of prefab. With this project, we shifted from SIP-based construction to what—in hindsight—was a rather elaborate stud wall construction. The panel assembly was as follows: a 2X6 structural wall sheathed with Zip system, with 2X8s outboard of the sheathing spaced at 24”. Dense-pack cellulose filled the 2X6 wall, and rockwool batts provided the insulation on the exterior. The idea was that we could use a relatively inexpensive and highly durable insulation on the exterior side of the air seal while maintaining a solid structure to catch the siding.
Each panel perimeter and every window and door opening was surrounded by an Advantech rim. The Advantech tightened up and trued the panels, and provided a clean face for applying sealant between panels. This panel construction made for a sturdy wall, but the amount of material in each panel was bordering on unnecessary.
As they say, old habits die-hard. Earlier this year we had the opportunity to prefab a larger, multi-story residence, and decided to produce the project with the same wall panel system. To be fair, we stuck with the system due to the unique demands of this project; panel sizes were constrained by the site (where maneuvering large panels would have been all but impossible), and at that time by our shop equipment. We overbuilt everything to eliminate any concerns with warping or deflection during handling, both by our shop gantry and by the crane on site. (We’ve found our new, lighter panels to be plenty durable during transport, but, y’know, better straight than sorry.) The production of this house was an important step in growing and developing our prefab system. The planning and fabrication of this house’s panels meant that every member of our team got a chance to see the wall section first hand. Again, the importance of that team can’t be understated: more eyes, hands, and minds on the wall panels just meant more recommendations on how to streamline, lighten, and simplify their construction. To wit, a few of the (now elegantly resolved) issues that we faced on a daily basis:
- Air sealing the myriad materials to one another involved quite a bit of tape. Most obvious among the improvements to be made was in eliminating as much of it as possible. After all, utilizing over half a dozen different varieties, each with its own unique purpose, could eventually lead to confusion on the part of the builders and difficulties with effective estimating and ordering. Tape was used in three vertical lines to carry the air seal out around the Advantech rim at each panel; and while accounting for that zigzag was simple, our eventual step of flattening out the air seal at that juncture would streamline our panel production, meaning fewer cuts to make and a smaller amount of tape. The air sealing results were impressive, however, as a blower door test logged 0.24 ACH50, meaning the house is more than two times as airtight as passivhaus certification requires.
- The window openings were only as straight as the lumber used to fabricate them, which left us in the unenviable position of meticulously shimming out each window opening for drywall returns on the interior and extension jambs on the exterior. Fixed window units were held in place with metal straps fastened to the buck, which pushed our options for window air-seal taping to the interior face of the window. We were getting far too talented at compound-angle tape origami in hard-to-reach places.
- Our panels were heavier than necessary, and had more material than they needed to get the simple job of “being a wall” done. It was time to trim the fat, and everyone, in the office, in the field, and in the shop, had their eyes on those non-structural outer 2X8s. But we’d no clue how that starting point—that is, the elimination of the outer layer of 2X8 studs—would snowball into one insight after another. Our wall was about to drop in cost, weight, and complexity, and that snowball would pick up some real clever details along the way.
Next up!: we’ll discuss our newest panel design and walk through the performance of a greatly simplified prefab system.
-Andrew Frederick, Foreman
