The crew here is nailing two pieces of lumber together to create mullions, which are used between window sections—similar to what you see on the far wall. In that example, the windows are spaced just three inches apart, and the lumber acts as the structural separation between them.
Right now, they’re also pre-drilling for the electrical chases. Typically, traditional electrical chases are drilled at 16 inches and 45 inches above the finished floor. However, in this case, they only need to drill at 16 inches because the 45-inch height would land inside the window opening.
For the connection, they’re applying sealant—one bead on each side and an extra ring of sealant around the electrical chase. The sealant application typically involves a "bull bead," about the diameter of a thumb, which helps fill any minor gaps between the lumber and the panel or lumber-to-lumber connections. You can see a slight 1/16-inch gap between the foam and lumber, but the sealant completely fills and seals it. The sealant is what truly prevents unwanted airflow between the interior and exterior of the structure.
Headers and Openings in SIP Construction
Some windows and door openings require structural headers; others do not. In cases like this one, where the window is cut out of the middle of the SIP panel, the laminated panel itself provides the necessary structural integrity—no additional header needed. The same is true for the other window sections down the wall.
However, for larger openings, a structural header is required. This is where an Insulbeam header comes in—an engineered beam made with LVLs (Laminated Veneer Lumber) on both sides and rigid insulation in between. For large window or door openings, these insulated headers are slid into recesses pre-cut into the SIPs, locking securely into place.
Wall Assembly with Mullions
On this particular wall:
There are two full-height SIP panels.
Three window fillers are installed between them.
Solid lumber mullions connect these sections together. To ensure everything planes out properly and matches the thickness of the SIP panels, the lumber mullions are surfaced with OSB sheets on both sides.
Factory Pre-Installation Options
Premier SIPs offers three manufacturing options for window and door openings:
Blank Panels: Panels with no pre-cut openings—you fabricate the windows and doors yourself on-site.
Cut and Recessed Panels: Premier manufactures the openings and recesses the foam for dimensional lumber, but the lumber is not installed.
Fully Pre-Installed Panels: Premier cuts the openings, recesses the foam, and installs the framing lumber at the factory—so when panels arrive, they’re ready to set directly into place, either individually or assembled into large wall sections like the team is doing here.
In this example, you can see a panel recessed by 1½ inches to allow for the insertion of structural lumber. There’s also a sill filler installed underneath the window opening—this sill piece is factory-installed into the panel. The adjoining panel carries the king studs, trimmers, and cripple studs needed to complete the framed opening.
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