A Visit to a Passive House Panelizing Shop

Owner Chris Corson and designer Mike Maines of EcoCor discuss a construction detail in the “Ivory Tower” office above Corson’s panelizing shop in Searsmont, Maine.

Using a prototype, Chris Corson discusses the construction details and building science characteristics of his panelized wall system with architect Rachel Conly, visiting from Portland, Maine. Consisting of an OSB-sheathed 2x4 interior frame, then an insulation cavity formed by exterior-applied wood I-joists, the assembly is designed to be air-tight at the level of the OSB, but vapor-open to both the interior and exterior

EcoCor designer Mike Maines discusses a tricky construction detail with shop manager Steve Greenleaf. Maines generates a three-dimensional rendering of each panel to be built to guide the construction process on the shop floor. The rendering can be viewed from multiple angles.

On the shop floor, shop manager Steve Greenleaf sets studs and jacks in place on the precision framing table. At rear, carpenter George Reefer trims wood I-joists to length on the shop’s up-cut saw.

Steve Greenleaf nails OSB sheathing onto the exterior side of the interior stud frame of a Passive House wall panel. A shop table saw positioned near the wall assembly table is used to make precise sheathing cuts. Once seams are taped, the sheathing will serve as the air barrier for the house.

Steve Greenleaf applies ProClima Extoseal membrane tape to the underside of the 2x4 inner wall plate for the panel. Extoseal is designed as a moisture-barrier tape for junctures between wood members and concrete foundation elements or other moisture sources. This plate will contact the slab foundation when the wall is set. However, the slab itself will also be isolated from ground moisture by an EPS insulating form system and a sheet of heavy-duty polyethylene.

Steve Greenleaf applies ProClima Vana tape to the nailing lines and seams of the OSB wall sheathing for the inner 2x4 wall of the panel. This tape will seal against any leaks through nail penetrations in the OSB. It also serves to air-seal the screw penetrations created when wood I-joists are attached over the sheathing during the next step in the panel construction process.

Steve Greenleaf cuts engineered rim board to length for a window buck, using EcoCor’s Northtech 18-inch up-cut cutoff saw. Equipped with a digitally controlled RazorGage positioning system, the Northtech saw is used for fast precision cutting of all the lumber components of the Passive House walls, including studs, wood I-joists, engineered headers, and strapping.

Steve Greenleaf builds the inboard portion of a window buck into a framed window opening in the interior 2x4 portion of the wall panel, using 1?-by-9.5-inch rim board. Windows will be set flush with this inner buck.

Air-sealing the corners of the inner window buck with ProClima Tescon Vana tape. This integrates the window buck into the wall panel’s air barrier control layer, defined by the taped OSB sheathing.

Drilling pressure-relief holes in the wood I-joist webs before attaching the I-joists to the OSB face of the inner wall. These holes turned out to be necessary when dense-packing the I-joist cavities with blown cellulose — otherwise, back-pressure in the cavities impedes and slows the placement of the insulation.

Fastening the I-joist outer wall members to the inner stud wall with structural screws through the OSB structural air barrier and into the studs. ProClima Tescon Vana tape under the I-joists air-seals the screw penetration.

Framing out the outboard edge of the window buck, to bring the opening flush with the outer face of the wood I-joist assembly.

Attaching an OSB plate to the underside of the wood I-joist outer wall assembly. This portion of the wall base will extend out over the EPS foam perimeter insulation for the building’s reinforced raft slab, creating a thermal-bridge-free joint where the wall meets the foundation. (The site-built version of this detail is drawn and photographed in Chris Corson’s May, 2012, JLC feature, “An Affordable Passive House, Part 1”.)

Stapling mesh screen over the pressure-relief hole in the wood I-joist at the edge of the wall panel. This opening turned out to be necessary because the I-joist cavities, once covered with taped fabric, are air-tight enough to create a back-pressure the can impede the operation of the insulation blower unless relief is provided.

Rolling out ProClima Solitex Mento fabric over the wood I-joist outer section of the wall panel. Air-tight but highly vapor-permeable, the Mento fabric provides a weather barrier and serves to contain the cellulose insulation within the I-joist cavity while allowing the wall to dry in the outboard direction.

Stapling the Mento fabric to the underside edge of the wood I-joist cavity. This edge will be taped for air-tightness.

Taping the weather barrier fabric joints with ProClima Tescon Vana tape.

Two layers of strapping are screwed down in a crisscross pattern over the fabric-covered wood I-joist outer wall frame. This strapping provides nailing for wood siding, and creates a drainage plane and convection space so that the wall exterior is able to drain and dry if any incidental moisture gets behind the siding.

Shop manager Steve Greenleaf, owner Chris Corson, and designer Mike Maines roll a completed panel off the framing table onto the forks of a forklift. The framing table is now ready for the next panel, but Greenleaf still has to finish a few details on this piece of wall.

Steve Greenleaf applies ProClima Tescon Vana tape to the inside of the wall panel’s exterior window buck, integrating the weather-resistive barrier into the punched opening. The interior portion of the window buck will be detailed in the field before windows are installed. At this point, EcoCor does not build windows into wall panels before shipment because of shipping limitations.

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