Fast Floors With Structural Insulated Panels - Continued
In the house we're showing here, the transition got a little complicated where the floor above the garage met the first-floor frame of the main house. A 6-inch panel provided a convenient stepped-down floor for the home's entryway, but there was some fussy framing involved in joining the different thicknesses of panel and connecting them to the conventionally framed floor.
Panelizing Wood I-Joist Floors
Whether panelizing a wood floor or framing it on site, you have to consider the wall structure that will hold up the floor. The floors for a SIP building can sit on top of the walls, just as they do in conventional platform framing. But then you lose the SIP wall's insulating value at the floor perimeter. So we like to hang the floor systems inside the wall, using top-nailing joist hangers. (Some designers use a system like balloon framing, with a ledger board face-attached to the panel's interior skin, but that brings up some tricky structural issues we prefer to avoid.)
Point loads. SIP walls can carry the distributed load of a "hung floor" even better than a typical stick wall. But for point loads, like the end of a floor's midspan girder, SIPs need structural reinforcement. Where a girder meets a SIP wall, we usually bury a post inside the wall to catch the beam. If there's a window or door opening in the wall where the post would fall, we sleeve an LVL header into the panel above the opening; posts at either side support the header. A beam can either sit on top of the header or attach to the face of it with a beam hanger.
We rely on load tables from the engineered-wood suppliers and hanger manufacturers when we spec this type of connection.
Wood I-Joist Floor Panels
Panelizing the floors as well as the walls has several advantages. First of all, it lets us take full advantage of the crane we bring to the site. I'll ask my crews to manhandle wall sections as big as 8 by 10 feet without equipment, but we need a crane to set larger wall assemblies, engineered beams, and roof panels. If we use the crane for floors, too, we can maximize the value of our investment in equipment.
Prebuilding also lets us compress the schedule by accomplishing two phases at the same time: Our crews can be at our shop framing the floors while the foundation contractor is still on site forming and pouring the basement. Saving days from the site schedule has particular benefits for us: My crews live near our Keene, N.H., shop, but they often have to travel to jobs around the region, staying overnight in motels. Framing the floors in the shop lets them have more time at home.
When we get a set of plans to panelize, a designer here in our shop analyzes the floor loads to determine where point loads will need to be supported within panels, and where wall openings may need headers. If we're planning to panelize the floors, the designer breaks up floor systems into 8-foot sections (the section width is limited by the length of the floor sheathing). The crew assembles the floor sections in our shop. On site, they use the crane to "fly" the pieces into place.
The total time it takes to frame the floor probably isn't much different with this system than it would be with on-site framing. Whether we work in the shop or in the field, we still have to put all the pieces together. But building the floors in the shop is safer: There's less fatigue; there's less time when the crew is exposed to the risk of falling; and help is nearby if there's an accident. It's also easier to keep tools and materials organized and available, and if there's any confusion about the plans, the designers are right in the building.
That doesn't mean things always go perfectly. With panelizing, you have to be careful to double-check everything. It's easy to make a mistake in dimensions or load paths when you're building the house in pieces, and we've made a few. But with each house we panelize, we get a little better, and we're finding that the added productivity — along with the convenience we can offer our customers — makes that learning curve worth the climb.
Jim LeRoyowns and operates Panel Pros, Inc., in Keene, N.H.