Cantilevered floor systems are frequently used in residential architecture. They provide an opportunity to capture additional square footage without enlarging the house’s footprint. But they can be problematic if not detailed well—leading to moisture issues and occupant discomfort. Correctly positioning the water, air, vapor, and thermal control layers is crucial.
In the control layer hierarchy, bulk water management comes first. An overhanging floor system inherently protects the wall assembly below but we should still strive for appropriate water management within the wall assembly itself. In this detail, we used Zip R-6 sheathing, a type of OSB backed with 1 in. of laminated rigid-foam insulation with an integrated weather-resistant barrier (WRB). Using Zip R means there is no need for an additional weather barrier. Once installed, all the joints should be wiped cleaned, sealed with Zip tape, and rolled with adequate pressure. (Click here for “Zip Sheathing Tips.”) The taped Zip R will handle the field of water management.
To reduce stress on the WRB, a drainage/ventilation plane is created on the face of the Zip R sheathing using 1×3 wood furring strips spaced 24 in. on center to align with the 2×6 stud framing below. A corrugated vent strip runs along the bottom of the gap, allowing for proper drainage and adequate ventilation; it also keeps bugs and critters out. In this case, the exterior finish—cement board siding—is installed on top of the furring strips.
With the water management control layer positioned for success, the next consideration is air leakage. Air-sealing protects against the transfer of energy either by heat loss or heat gain through the building assembly. Restricting the flow of air diminishes the energy penalty. A good air-sealing strategy also prohibits moisture from moving along an air stream and hitting a cold surface, where it can condense and cause moisture-related problems.
In this detail, not only is the Zip R sheathing the WRB, it also acts as the primary air barrier. It is folded across the underside of the cantilever, thereby attaching the exterior wall of the first floor to the exterior wall of the displaced second floor. The same taping conventions used on the sheathing installation are applied to the inside and outside corners of the cantilever. (Fundamentally, the cantilevered floor is nothing more than a wall assembly laid on its side.) Air-sealing tape is also applied around the 2×10 blocking installed at each joist bay above the lower-wall top plate. This is a secondary barrier to restrict air flow into the floor frame.
With air leakage accounted for we move on to vapor control, which is pretty straightforward in this assembly. With the sheathing on the exterior, the laminated rigid foam limits the drying potential outward. Therefore, the wall cavity is insulated with a vapor-permeable blown insulation such as cellulose or fiberglass. With a drywall interior, the wall cavity can dry to the inside.
Thermal control is the last layer in order of priority. In this detail, the Zip R sheathing provides an R-6 continuous thermal barrier across the exterior of the wood-framed wall. Coupled with blown insulation in the 2×6 cavity, it’s an R-27 exterior wall assembly. With the aid of the 2×10 blocking above the top plate, the cantilevered cavity is insulated to full depth with the same cavity insulation used in the exterior wall. Together with the R-6 sheathing, this creates an R-42 floor assembly at the cantilever.
Depending on your climate zone, the Zip R-6 sheathing can be increased to R-9 or R-12, or conversely, decreased to R-3. This detail easily meets the code requirements in most locations. In fact, it probably exceeds most code minimums. As for structural integrity, a cantilever up to 24 in. is easily handled. Beyond that, consulting a structural engineer is recommended.
Alexandra Baczek is an associate at Steven Baczek Architect. She is a graduate with a Master of Architecture from Roger Williams University. Illustration by the author.