There used to be pretty much one way to frame a wall: 2x4 studs spaced 16 inches on center with plywood or OSB sheathing. No more. With construction codes demanding higher insulation levels, the market share for this old standby has taken it on the chin from alternatives such as 2x6s with foam sheathing, structural insulated panels, and insulated concrete forms. But the lowly 2x4 is by no means down for the count. Some builders get as good performance from 2x4s as from competing systems–and for less cost–by doubling up the frame.
A big appeal is that 2x4 platform framing is familiar to every carpenter and is easily taught to an unskilled crew. Even Habitat for Humanity chose it for a Wheatridge, Colo., home because of its "volunteer-friendly construction techniques."
The system consists of inner and outer 2x4 frames separated by an air space. The studs are laid out on two-foot centers, with the outer wall bearing the entire structural load. R-values depend on the distance between the inner and outer walls and the type of insulation. For instance, a 2 1/2-inch gap between the inner and outer frames creates a total 10-inch cavity that, when filled with dense pack cellulose, achieves a nominal insulating value of around R-31. Because the space between the inner and outer walls creates a thermal break that reduces heat loss through the framing members, the actual R-value will be closer to the nominal than in a standard 2x4 wall. The gap also muffles sound transmission, a real plus if the home is sited on a busy road.
Another plus for double 2x4s is flexibility. Gary Higginbotham at Alan Mascord Design Associates in Portland, Ore., chose double 2x4 walls for some recent projects. He likes the system because it lets the builder choose the wall depth. "Splitting the plate lets [a builder] change the depth of the wall on the fly, depending on how much interior space and R-value he needs," Higginbotham says. "You can't do that with 2x6s and foam."
Is It Really Better?
Not everyone is sold on this system. Building Science Corporation (BSC) in Westford, Mass., did a comparison between standard 2x4 single-wall construction and two of the most familiar high-performance alternatives: double 2x4 walls with OSB Sheathing, and 2x6 Advanced Framing with extruded polystyrene foam sheathing (XPS). The comparison looked at thermal control, durability, buildability, cost, and material use. On a 25-point scale, the 2x6 wall scored 20 and the double 2x4 only 15. One reason for the low number: BSC believes the double 2x4 is at risk for moisture-related durability issues.
Of course, that could be said of any wall with uninsulated sheathing. BSC principal Joseph Lstiburek is right that a 2x6 wall with 2 inches of foam on the outside will have a lower condensation potential, but foam isn't always acceptable.
Such was the case with South Mountain Co. (SMC), a West Tisbury, Mass., design/build company that strives to reduce its homes' environmental impact. SMC chose double 2x4 walls for a series of small Cape style homes it was building, partly because the carpenters preferred this system but also because the designers see XPS as environmentally unfriendly. "We like to minimize the use of rigid foam," says company president John Abrams.
The company's designers point out that condensation worries can be addressed with good air sealing. To achieve this, they frame their homes with flush eaves and rakes, install Zip System panels with taped seams for sheathing, and only then add the overhangs. The result: Airtight shells that have yet to experience condensation problems.
Builders considering this system should keep in mind some details. First, damp spray cellulose can take too long to dry in a thick wall. That makes dense pack cellulose behind netting a better choice here.
Second, with drywall on just one side of the inner wall, there is concern the wall could flex and crack the drywall finish. Most builders address this by adding plywood gussets. Some use them on each stud, some on every other stud.
Third, keeping labor costs down will require the framers to be more efficient. For instance, some builders save time by having the supplier pre-cut all lumber to the exact lengths needed. This also reduces waste.
There may be structural issues. Glenn Mathewson, a Westminster, Colo., code official, says that under the International Residential Code, it's OK to space 2x4s 16 inches on-center in a one-story house, or in one with a habitable attic where the second floor is entirely under the roof pitch. But once you add second-story walls, you need to reduce the spacing or use 2x6s.
Deep window openings can reduce the amount of light getting into the house. The typical solution is to angle the sides of window openings 45 degrees into the room. Doing so can even save time and money on trim work. For instance, SMC's crews install wood sills on all the windows before hanging the drywall, then use drywall returns on the sides and top of the opening. When the drywallers leave, the opening is finished.