Balloon-frame Construction

I grew up in a large, vintage New England house with a high-ceiling attic that was every kid’s dream clubhouse. There was a 2-ft.-high kneewall around the perimeter of the attic, and studs protruded through the flooring. I didn’t know then that this type of framing, balloon-frame construction, meant that the wall studs ran uninterrupted from the top of the foundation to the bottom of the roof rafters. But I learned that if I got close to the 4-in.-wide chasm around the perimeter of the attic, I could feel a breeze blowing up from below (my first experience with the stack effect) and that if I dropped a marble in that chasm, I could hear it rattling all the way down through the wall and later find it waiting on the basement floor.

Balloon framing was a dying practice back then and is essentially extinct today. That’s because in the 1920s, builders began framing houses using a method called platform framing, by which the first floor is built and then used as a platform to erect the walls, which support the next floor, and so on. The reasons for the switch were many: easier framing without scaffolding, better resistance against fire jumping floors, and a general decline in the availability of tall, straight, quality framing lumber.

Nowadays, the challenge we face in building custom homes is in finding the most cost-effective way to build the highest-performing shell with the smallest renewable system necessary to provide 100% of the heat, cooling, and domestic hot water for our clients. Ironically, I’ve discovered that balloon- framed walls fit this need nicely, as long as the design is updated with a few modern construction details and allows enough depth for adequate insulation. In short, I prefer to build balloon-framed double-stud walls, which represent an excellent balance of cost, familiarity, and performance.

Old meets new to create an ideal thermal assembly 

Although double-stud walls are certainly not the norm in modern building, they are far from groundbreaking and have been used by progressive builders for decades. As with any building system, methods for their construction vary; in addition, your code offical may require an engineer’s stamp.

Our process starts with two independently framed 2x4 stud walls set with about a 41⁄4-in. space between. When we’re able to frame all the walls on-site, we build and stand each one separately. If  the framing happens in the dead of winter, as on this project, then we frame many of the walls off-site and truck them in for faster on-siteassembly. In those cases, we frame the inner and outer faces of the walls at the same time, using 2x12 plates at the top and bottom. On gable walls, we do some of the infill framing after the walls have been raised and the roof trusses installed, making it easier to air seal the connection between wall and roof.

The thing that pushes the envelope, though, is the integration of the balloon-frame concept. Rather than framing the double-stud walls in the platform fashion, we build the walls tall enough to run from the first-floor bottom plates to the second-floor top plates in a single push, with the floor system hung from a ledger. Now, other than the plywood gussets between studs (and 2x12 plates at the top and bottom, if included), thermal bridging between the exterior wall and interior wall is negligible. This approach also eliminates the weak hinge point of short-stud walls in the upstairs of a 1 1⁄2-story house.

Caveats to consider

When working with double-stud walls of any kind, you must know which wall is bearing the weight of the house. I prefer to have the interior wall be load bearing, but an exterior bearing wall is possible if you change the way the floor system attaches. This approach could also affect foundation details when using insulated concrete forms (ICFs).

You may also find that engineers get nervous when asked to approve a plan in which the floor joists are hung from a structural ledger attached to the wall studs. I always thought that engineers relied solely on math (and when done properly, the math works fine here), but I’m learning that intuition plays a role as well. In some cases, I have been asked to beef up the carrying studs to 4x4s instead of 2x4s. When I asked why, the engineer simply replied, “Because it makes me feel better.” Since the engineers are the ones who have to sign off in order for me to get a building permit, I just do what they tell me. For interior-design reasons, the floor joists on this project run the length of the building, but this unconventional floor-joist direction isn’t related to the double-stud or balloon-frame methods.

Wherever a double-frame wall encounters a window or door opening, I prefer to install 3⁄4-in. by11 1⁄4-in. plywood bucks around the inside of the rough opening. The plywood serves as a solid connection between both walls for installing the door or the window, thus providing a stable subplane for attach- ing the trim. Without it, the walls could move slightly over several years of drying, which would wreak havoc with jambs and casings. A word of caution: when framing door and window bucks using this system, you’ve got to tweak the size of each rough opening to allow for the additional thickness of the plywood.