By Matthew Power. The longer you build homes, the more you understand why construction techniques take so long to change. As a real-world exercise in combining new and old materials and methods, the framing of Builder's 2002 show home, Homelink, began with a plethora of unconventional ideas. Not all of them wound up in the finished house.
Rick Foster, project manager for Morrison Homes in Atlanta, has just completed framing on Homelink. As part of that process, Foster tried to follow many of the energy-efficient engineering guidelines laid out by Joe Lstiburek of the Building Science Corp., a home building think tank based in Westford, Mass. The result: Some worked like a charm. Others followed what he calls the law of diminishing returns. Then there were the ones he simply rejected as a matter of prudence--for fear of alarming the building inspector.
Among all of the unusual framing details, says Foster; shear panels get the highest marks.
"Those were the neatest thing," he explains. "They're basically simple, site-built 2x4 panels with plywood over the face, but they set into the 2x6 wall. What you do is leave out a 2x6 stud occasionally, and you insert one of these shear panels. They act as intermittent strengthening devices."
The primary purpose of the panels, however, is to make way for better insulating.
"One of the thing's Lstiburek wants to do is get rid of as much wood in the house as you can," Foster continues, "to replace it with insulation. By using these panels, we were able to run unbroken 1-inch foam insulation on the exterior."
To finish the exterior, Foster says, his crews taped all of the tongue-in-groove seams of the EPS (expanded polystyrene) foam and added building wrap only around window and door penetrations. As a result of that tight outer shell, Lstiburek has suggested that interior wall cavities be filled with unfaced fiberglass insulation batts--to ward off any potential problems with trapped moisture.
Expecting the inspector
Lstiburek's cookbook of wood-saving framing instructions includes numerous other ideas. Some details were easily adopted no-brainers, such as pushing headers to be flush with the exterior sheathing or creating 'L' type corners to allow for better insulating.
In addition, by building a 2x4 "insert" at intervals, the engineer provided enough lateral strength in the exterior walls to do away with the need for plywood sheathing. As a result, EPS foam could be bonded directly to the outer frame.
"Most of the exterior is 2x6, 2-foot on-center framing," says Foster. "But every 20 feet or so, we would leave a 4-foot opening. Then, this panel would be recessed back into the 2x6 frame to give it strength."
Another complicated adjustment to the builder's modus operandi was the use of stack framing. This method requires that all structural members line up vertically, from the foundation wall to the roof rafters. The result: a frame with an evenly distributed load that can get by with single 2x4 top plates.
But Foster wonders whether the added complexity in this case was worth what was gained. "I think Joe would argue that we gained insulating space by getting rid of the top plates," he says, "but wood is a pretty good insulator, and lining up all of those verticals perfectly took a lot of effort. It's an idea that might work great on a rectangular house in a laboratory but not on a custom-type house. You could put down a double top plate and not have to worry about every single piece lining up perfectly--or what the building inspector might say."
The concern about the building inspector's reaction extended to many other framing details. Foster's concern: A building inspector may not have either the knowledge or flexibility to embrace new ideas--no matter how good they are.
In response, he "overbuilt" many of Lstiburek's details. For example, he added cripple studs beneath window bottom plates and doorway headers and added blocks above single headers in interior walls.
Despite the little extras, Foster says, he used about the same amount of framing material as a standard house--but gained a higher energy dividend.
"If you're able to produce a house using the same board feet of lumber, but you get a much more efficient house, that's a good trade," he says. "That's also something you can market and use to sell houses."